J Psycholinguist Res DOI 10.1007/s10936-017-9496-9
Native and Non-native Speakers’ Brain Responses to Filled Indirect Object Gaps Anna Jessen1 · Julia Festman1 · Oliver Boxell1 · Claudia Felser1
© Springer Science+Business Media New York 2017
Abstract We examined native and non-native English speakers’ processing of indirect object wh-dependencies using a filled-gap paradigm while recording event-related potentials (ERPs). The non-native group was comprised of native German-speaking, proficient non-native speakers of English. Both participant groups showed evidence of linking fronted indirect objects to the subcategorizing verb when this was encountered, reflected in an N400 component. Evidence for continued filler activation beyond the verb was seen only in the non-native group, in the shape of a prolonged left-anterior negativity. Both participant groups showed sensitivity to filled indirect object gaps reflected in a P600 response, which was more pronounced and more globally distributed in our non-native group. Taken together, our results indicate that resolving indirect object dependencies is a two-step process in both native and non-native sentence comprehension, with greater processing cost incurred in non-native compared to native comprehension. Keywords Sentence processing · Wh-movement · Filled gaps · ERPs
Introduction A growing body of research has investigated whether native (L1) and non-native (L2) comprehenders process non-canonical word orders in the same way (see Dallas and Kaan 2008,
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Claudia Felser [email protected] Anna Jessen [email protected] Julia Festman [email protected] Oliver Boxell [email protected]
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Potsdam Research Institute for Multilingualism, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany
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for a review). In wh-movement languages like English, readers or listeners need to hold a fronted constituent in memory until it can be integrated with its licenser (such as a verb or a preposition) further downstream. During the processing of sentences such as (1) below, for instance, the fronted object wh-phrase which book cannot be fully interpreted in situ and thus must be carried forward until it can be linked to its thematic role assigner, the verb read. (1) Which book did you say you would like to read __ ? In the sentence processing literature a fronted constituent is usually referred to as a “filler” and its canonical structural position as a corresponding “gap” (indicated by underscores). Encountering a filler triggers the search for an associated gap or lexical licenser while the parser must simultaneously process and integrate new incoming words and phrases (Clifton and Frazier 1989). Besides filler storage, resolving filler-gap dependencies (FGDs) requires identifying potential gaps in the input stream and evaluating the filler’s semantic fit as a verbal argument or event participant (e.g. Gibson 1998). Filler storage and filler integration processes have been shown to be dissociable at the neuro-cognitive level (e.g. Felser et al. 2003; Fiebach et al. 2002; Phillips et al. 2005). Empirically dissociating verb-based semantic integration from position-specific gap filling is more difficult, however, especially for sentences such as (1) above in which the filler’s canonical structural position is immediately adjacent to its lexical licenser. In verb-initial languages like English, one way of teasing these processes apart is to investigate indirect object dependencies. Using the cross-modal priming technique, Nicol (1993) found that for sentences containing indirect object wh-fillers such as (2), native English comprehenders showed filler reactivation effects both at the main verb (give) and at the canonical indirect object position further downstream. (2) To which butcher did the woman who had just inherited a large sum of money give the very expensive gift __ the other day? This, according to Nicol, indicates that in sentences such as (2), the processing system first evaluates the filler’s suitability as a potential argument of the verb, possibly assigning it a preliminary thematic role, but then continues to hold the filler in working memory (WM) until it comes across the indirect object gap, at which point the thematic role is confirmed. The cognitive processes involved in non-native (L2) speakers’ processing of FGDs are as yet only partially understood. Previous research has shown that proficient L2 comprehenders are able to link a filler to a potential lexical licenser when this is encountered, as witnessed by their ability to detect semantic or pragmatic incongruencies between the two during real-time processing (e.g. Cunnings et al. 2010; Dallas et al. 2013; Omaki and Schulz 2011; Williams 2006; Williams et al. 2001). L1/L2 differences have been observed, however, for gaps that are non-adjacent to their lexical licenser (Marinis et al. 2005; Felser and Roberts 2007; Miller 2015—but cf. Pliatsikas and Marinis 2013). Felser and Roberts (2007), for example, used a cross-modal priming task to examine whether proficient Greek/English bilinguals would mentally reactivate indirect object fillers at the filler’s canonical position in sentences such as (3). (3) John saw the peacock to which the small penguin gave the nice birthday present ___ in the garden last weekend. Unlike native speakers, who showed evidence of selectively reactivating a representation of the understood indirect object the peacock when coming across the indirect object gap (Roberts et al. 2007), the bilinguals tested by Felser and Roberts showed priming effects both at the gap and at an earlier, non-gap control position, but no evidence for position-specific
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filler retrieval at the gap site (see also Miller 2015, for parallel findings for bilinguals in French). It is conceivable, however, that the relatively high cognitive resource demands made by the cross-modal priming technique prevented the L2 comprehenders from processing the auditory experimental stimuli properly, as has been suggested by Miller (2015). Another possibility is that L2 comprehenders have a reduced ability, compared to native comprehenders, to predict upcoming sentence material, including indirect object gaps (see Kaan 2014, for review and discussion), in which case filler reactivation effects might be delayed in non-native processing. Neither Felser and Roberts (2007) nor Miller (2015) tested for potential filler reactivation effects at the subcategorizing verb itself (such as give in example 2 above) or for potentially delayed gap-related priming effects. Taking the above findings from cross-modal priming as a point of departure, the current study used event-related potentials (ERPs) to examine the processing of indirect object dependencies. Using a less resource-demanding experimental task compared to cross-modal priming, we specifically sought to examine whether establishing indirect object dependencies also involves a two-step process in L2 processing, and whether wh-fillers are being kept activated in WM beyond the point at which they can be associated with their lexical subcategorizer. Given earlier findings suggesting that L2 comprehenders may have difficulty identifying covert linguistic structure in the input stream (e.g. Marinis et al. 2005; Felser and Roberts 2007), the current study uses filled gaps as an experimental diagnostic for filler reactivation at potential gap sites.
Sensitivity to Filled Gaps in Native and Non-Native Language Processing So-called “filled-gap” effects are observed when a structural position that could potentially host a filler is discovered to be occupied by another constituent. In reading-time studies, encountering a filled gap usually gives rise to elevated reading times in comparison to a control condition (e.g. Stowe 1986). Hestvik et al. (2007) used ERPs to investigate filled-gap effects. They presented native English-speaking listeners with ungrammatical filled-gap sentences such as (4a) below, in which the zebra could initially be assumed to be the object of the verb kissed. However, the postverbal position turns out to be filled by another noun phrase (the camel) here. (4) a. *The zebra that the hippo kissed the camel on the nose ran far away. b. The zebra said that the hippo kissed the camel on the nose and then ran far away. Encountering the filled gap in (4a) elicited a bilaterally distributed anterior negativity compared to encountering a grammatical object (4b), which Hestvik et al. (2007) interpreted as an early left-anterior negativity (ELAN). A later positive-going wave (P600) was also observed but failed to reach full significance. As an ELAN response is frequently elicited by phrase-structure violations (Friederici 2002), Hestvik et al. (2007) concluded that a filled direct object gap constitutes a phrase-structure violation, with two noun phrases competing for the same structural slot. Using similar materials as in Hestvik et al. (2007), Hestvik et al. (2012) found filled direct object gaps triggering a bilateral LAN followed by a P600, with both effects being delayed by about 200ms in participants with a relatively low WM score. In an attempt to replicate Hestvik and colleagues’ findings, Schremm (2013), in as yet unpublished work, observed that filled direct object gaps triggered a P600 response but no negativity.
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Few studies to date have investigated non-native comprehenders’ sensitivity to filled gaps during real-time processing. Previous studies using behavioral tasks (Williams 2006; Williams et al. 2001; Dallas 2008), eye-movement measures (Felser et al. 2012) or ERPs (Schremm 2012) have reported filled-gap effects to be delayed in L2 compared to L1 processing (but cf. Aldwayan et al. 2010). Delayed sensitivity to filled gaps in non-native comprehension may potentially reflect (i) difficulty identifying possible gap positions, (ii) difficulty keeping a representation of the filler active during processing and/or retrieving it from WM when a potential gap position is identified, (iii) generally slower syntactic or semantic integration processes, or (iv) some combination of these. Because in previous studies the filled gap always immediately followed the verb, it is difficult to distinguish semantic integration effects triggered by encountering the verb from effects triggered by finding a potential gap already occupied. The materials for the current study were designed to allow us to tease apart verb-driven and filled-gap driven ERP effects by testing filled indirect object gaps.
The Current Study The present study investigates whether L1 and L2 comprehenders process indirect object dependencies in the same way. We specifically examined whether gaps are predicted beyond the verb, whether L1 and L2 comprehenders show the same types of brain responses to both the filler’s semantic licenser and to filled gaps, and whether the two groups’ brain responses differ in their relative timing. To this end, we recorded participants’ ERP responses while they were reading sentences containing indirect object relative clauses.
Participants Twenty native speakers of English (13 female, mean age 29.0 years, range 18–59 years) served as our control group. Fifteen of them lived and worked in Potsdam or Berlin, five were students at the University of Potsdam (Germany). The L2 group consisted of 21 native speakers of German with English as L2 (10 female, mean age 25.4 years, range 20–34 years), recruited from the student population of the University of Potsdam and from the surrounding area. The L2 speakers’ general level of English grammar was assessed using part II of the grammar section of the Oxford Placement Test (OPT; Allan 2004). They achieved a mean score of 39.9 out of a possible maximum score of 50 (range 29–47), which places our non-native participants within the B2-C2 proficiency range according to the Common European Framework for Reference for Language (B2 = “upper intermediate”, C2 = “proficiency”). Our German-speaking participants had started learning English at a mean age of 10.3 years (range 6–13 years) and none of them reported having grown up bilingual. All participants were right-handed, had normal or corrected to normal vision, and were offered a small fee for their participation.
Materials Our experimental materials were originally designed so as to be suitable for young children. They typically described events within a fairy-tale kind of scenario, with animal characters being endowed with human qualities or cognitive abilities. Participants were briefed accordingly prior to the main experiment. The materials for this experiment included 48 ungrammatical and grammatical sentence pairs structurally parallel to (5a,b) below.
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(5) a. *Sarah tickled the monkey for which Peter arranged some classes for it after the vacation. b. Sarah tickled the monkey while Peter arranged some classes for it after the vacation.
Sentences in our extraction condition (5a) contained a relative clause modifying the main clause object. These were introduced by a prepositional phrase (PP) that functioned as an indirect object (e.g. for which). They were rendered ungrammatical by inserting a “resumptive” indirect object phrase (e.g. for it) at the filler’s canonical position following the direct object (e.g. after some classes). Half of the ungrammatical filled-gap sentences contained for which and half contained to which (e.g. James watched the penguin to which Adam supplied some sandwiches to it after the school run). Fronting the preposition along with the wh-pronoun (rather than stranding it) should prevent the fronted constituent being mistaken for either a subject or direct object. Our grammatical (non-extraction) control sentences (5b) did not involve FGDs but instead contained an adjunct clause introduced by while. Our materials were selected based on the results from two offline pretests, a sentence continuation questionnaire and a plausibility norming questionnaire. The primary purpose of the sentence continuation task was to confirm that the embedded verbs used in the ERP experiment could also appear in a monotransitive NP–V–NP frame. This was important in order to prevent participants from anticipating the indirect object (e.g. for it) in our grammatical control condition (5b). A set of 40 common dative verbs (i.e. those admitting a to-PP) and 40 common benefactive verbs (i.e. those admitting a for-PP) were selected from Levin (1993). From these, 80 short sentences of the form NP–V–NP (e.g. Peter arranged some classes) were constructed and mixed with 40 intransitive and monotransitive distractor items. 20 adult native speakers of British English (who did not participate any further in this study) were asked to indicate whether each sentence formed a complete, natural-sounding sentence as it stands. If they felt that a given fragment should better be continued, participants had to write down the first appropriate continuation that came to mind. Verbs that elicited more than 20% to-PP or for-PP completions and fewer than 70% “complete” responses were excluded from the initial set. The remaining 74 verbs were considered to be fine without an additional indirect object (mean: 93.9%, SD: 7.6, range: 70 − 100%). The purpose of the plausibility norming questionnaire was to make sure that all the experimental items to be used in the ERP study were deemed syntactically acceptable and plausible, and to match the to/for which–while pairs as closely as possible. A total of 80 critical sentence pairs, all of the form in (5a,b) above, were created using the 74 pre-selected verbs (with six of these used twice). The total materials comprised 240 sentences including 40 to which– while pairs, 40 for which–while pairs, and 80 implausible fillers of various structural types, of which 20 were also syntactically ill-formed. The critical items were distributed across two presentation lists such that no participant saw both versions of the same sentence. A total of 31 adult native speakers of British English (who did not participate any further in this study) completed the questionnaire. They were instructed to judge the naturalness of each of the sentences on a five-point scale from 1 (= “completely natural”) to 5 (= “completely unnatural”). As the materials were created bearing in mind that they might later be used with children, the participants were asked to imagine a kind of Animal Farm scenario where animals could show human-like behavior and to judge the sentences as to whether or not they sounded good within this kind of scenario. On the basis of participants’ plausibility scores, 24 closely matched to which–while and 24 for which–while pairs were selected for the ERP experiment (for which items mean: 1.98, SD: 0.17, range: 1.5–2.31; to which items mean: 1.99, SD: 0.23, range: 1.56–2.47; while items mean: 1.67, SD: 0.3, range: 1.27–2.53).
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The 48 experimental sentence pairs were distributed across two presentation lists in a Latin Square design and mixed with 48 fillers of similar length and structural type which served as the materials for another experiment, and 60 additional grammatical or ungrammatical fillers of similar length which represented a range of different structural types. To prevent participants from developing specific expectations or strategies when encountering relative clauses introduced by to/for which, we included ten grammatical indirect object gap fillers that were structurally similar to our experimental sentences (e.g. The pony for which Belinda had bought a new saddle jumped over the fence).
Predictions Based on previous findings on the processing of wh-dependencies we made the following predictions: 1. In previous ERP studies on FGDs a sustained left-anterior negativity (LAN) has often been observed, which has been suggested to reflect the processing cost incurred by temporarily storing a filler in WM (e.g. Felser et al. 2003; Fiebach et al. 2002; King and Kutas 1995; Kluender and Kutas 1993; Phillips et al. 2005). If encountering an indirect object filler triggers the prediction of a gap, filler storage cost should be reflected in a LAN following the presentation of the filler (for/to which) and extending beyond the subcategorizing verb until the wh-dependency can be completed. 2. If indirect object fillers are initially evaluated at the subcategorizing verb (Nicol 1993), we should see effects of semantic integration in the shape of an N400 component (compare e.g. Garnsey et al. 1989) following the presentation of the verb. 3. Surprisal at finding the filler’s canonical structural position filled by another constituent should be reflected in a biphasic N400–P600 response if a filled gap is perceived as an argument structure violation (e.g. Friederici and Frisch 2000), or in an (E)LAN-P600 response if it is perceived as a phrase structure violation (e.g. Hestvik et al. 2007).
Procedures Prior to the experimental session, participants filled out a biographical questionnaire; the L2 group also completed grammar part II of the OPT. The experimental session took place in a quiet laboratory at the University of Potsdam. Participants first signed a consent form and were informed about the EEG procedure. They were then prepared for the EEG-recording and seated in front of a computer screen at a distance of approximately 100 cm. The experiment was presented on a 61 cm wide monitor using Presentation version 14.9 (Neurobehavioral Systems). Sentences were presented in 12 presentation segments, with for which/to which and for it/to it always presented as single segments. Each segment was presented for 550ms in black letters against a white background in the middle of the screen. Each participant saw 156 sentences in total. At the start of each trial a fixation cross appeared at the center of the screen for 1250ms followed by the sentences displayed in Times New Roman, 96-point size font. Overall, 52 of all the trials (i.e. one third) were followed by a verification question presented in green letters against white background (40-point size font), which remained on the screen until the participant pressed one of two designated response buttons (“yes” or “no”). Questions were either repeating part of the stimulus sentence, such as Peter arranged some classes for the monkey? or contained a small change where a negative answer was required. The appearance of the questions was fully randomized between items and participants. A question was followed either by the appearance of a “smiley face” for a correct response or by a
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blank screen for an incorrect answer, each presented for 1000ms in the center of the screen. Participants were advised to relax before the next trial. The screen went blank for 500ms before the next sentence started with the fixation cross. Before the experiment proper, participants practiced the task with five practice trials, each followed by a comprehension question. Experimental trials were fully randomized and distributed over four blocks of 39 sentences each, with blocks counterbalanced amongst participants. After the first block, participants were allowed to take a short break. Participants were asked to minimize eye and muscle movements until presentation of the “smiley face” or blank screen after each trial. The experiment itself took approximately 30 min to complete, and an entire experimental session lasted for approximately 90 min.
Electrophysiological Recording The EEG was recorded continuously using Brain Products Vision Recorder acquisition software from 31-electrode sites (FP1, FP2, F7, F3, Fz, F4, F8, FC5, FC1, FC2, FC6, T7, C3, Cz, C4, T8, TP9, CP5, CP1, CP2, CP6, TP10, P7, P3, Pz, P4, P8, PO9, O1, O2, PO10) according to the international 10-20 system using active electrodes embedded in an elastic cap (ActiCap, Brain Products). Additionally, vertical electro-oculograms (VEOGs) were recorded for artifact rejection purposes. Signals were recorded continuously with an on-line band-pass filter between 0.016–70 Hz and digitized at 2500 Hz. Electrode impedances were kept below 20 k (according to the guidelines for using ActiCaps). All recordings were rereferenced to the average of the left and right mastoid electrodes offline. Recordings were offline band pass filtered between 0.1 and 30 Hz. Offline recordings were down-sampled to 250 Hz. EEG data were processed with Vision Analyzer 2. For single-segment analysis, epochs were extracted from 200 ms before the presentation of the critical word or segment up to 1000 ms after the presentation onset resulting in 1200 ms epochs (−200 to 1000 ms) and were baseline-corrected using a 200ms pre-stimulus baseline. For multiword ERPs we followed Fiebach et al. (2002) such that epochs were extracted after the presentation of the segment that distinguished between both sentence conditions (i.e. for it / while), until the end of the sentence, resulting in 4700 ms epochs (−200 to 4500 ms) and were baseline-corrected using a 200 ms pre-stimulus baseline.
Data Cleaning Two participants, one from each group, had to be excluded because of data loss due to technical problems. To remove typical muscle and eye-movement artifacts from the scalp recordings, an independent component analysis (ICA) algorithm (Infomax) was applied to the data. Epochs containing additional artifacts were identified with the semi-automatic rejection option in the Analyzer and rejected after visual inspection. In sum, 98% of all trials from the remaining participants were included in the statistical analysis.
Statistical Analysis We examined local ERPs both at the subcategorizing verb and at the filled indirect object gap, as well as clause-level ERPs following the presentation of the filler. For the statistical analysis of local ERPs, 27 electrodes were grouped into nine regions of interest (ROIs), eight of which following the division in Hestvik et al. (2007): left anterior inferior (F7, T7, FC5), left anterior superior (F3, FC1, C3), right anterior inferior (F8, FC6, T8), right anterior superior
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(F4, FC2, C4), left posterior inferior (TP9, P7, PO9), left posterior superior (CP5, CP1, P3), right posterior inferior (CP6, CP2, P4), right posterior superior (TP10, P8, PO10), and an additional mid posterior ROI (Pz, O1, O2). Time windows of interest were chosen based on visual inspection and a 50ms time-line analysis (for the verb and filled-gap site), conducted for the two groups separately. If interactions including the factors Condition and ROI reached significance in at least three consecutive time windows for both groups, we followed this up by analyzing larger time intervals. Mean ERP amplitudes were statistically analyzed using repeated measures ANOVAs. Preliminary mixed three-way ANOVAs included Condition (grammatical, ungrammatical) and ROI as within-participants factors and Group (L1, L2) as a between-participants factor. Follow-up per-group ANOVAs included the factors Condition and ROI as within-subject factors. To correct for multiple comparisons, we applied a simple sequentially rejective multiple test (Holm 1979). Further stepping-down analyses used oneway ANOVAs with the factor Condition to examine differences within individual ROIs, p-values were again adjusted with the Holm test. For effect sizes, Cohen’s d (standardized mean difference) was calculated based on the F-values and sample sizes of the two participant groups, with a 95% confidence interval. The results are reported in parentheses following the ANOVA results. All statistical analyses were carried out with RStudio, Version 3.0.2, with the packages ezANOVA (for the ANOVAs) and compute.es (for effect sizes).
Results Response Accuracy Participants’ average accuracy rate for responses to the end-of-trial comprehension questions was 83.3% (range 63.5–92.3%) for the L1 group and 82.5% (range 65.9–90.4%) for the L2 group. This indicates that participants were attending to the stimulus materials and actively read them for comprehension.
Clause-level ERPs Encountering the wh-filler elicited a sustained negativity in the L2 group at the left anterior inferior region (F7, FC5, T7) and a very small and short-lived negativity for the L1 group visible only at FC5 (see Fig. 1a, b). Visual inspection of the L2 group’s grand average ERPs at left-frontal electrodes revealed that this negativity started at about 300 ms at FC5 after the embedded subject (e.g. Peter in 5a,b) and was present until about 3000 ms, i.e. until shortly after the filled gap was encountered. Based on visual inspection, five time windows were chosen for analysis: 300–800, 800–1400, 1400–1900, 1900–2500 and 2500–3000 ms. The first three time windows showed significant or marginally significant interactions of Group × ROI × Condition: 300–800 ms F(8, 296) = 2.47, adjusted p = .03; 800–1400 ms F(8, 296) = 2.52, adjusted p = .03; 1400–1900 ms F(8, 296) = 1.8, adjusted p=.07. Subsequent per-group analyses for these time windows showed significant interactions of ROI and Condition in the L2 group only: 300–800 ms: F(8, 152) = 2.84, adjusted p = .018; 800– 1400 ms F(8, 152) = 3.53, adjusted p = .004; 1400–1900 ms: F(8, 152) = 2.8, adjusted p = .018. Follow-up analyses revealed significant differences between the extraction and non-extraction conditions in all three time windows in the left anterior inferior region for the L2 group: 300–800 ms F(1, 19) = 11.67, adjusted p = .008, [d = 1.08]; 800–1400 ms
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a
… Peter /arranged/ some / classes / for it / aer / the / vacaon
Fig. 1 a Grand average of electrodes F7, FC5 and T7 for the L2 group following the presentation of the wh-filler (e.g. for/to which). Extraction condition = dotted line. b Grand average of electrodes F7, FC5 and T7 for the L1 group following the presentation of the wh-filler (e.g. for/to which). Extraction condition = dotted line
F(1, 19) = 13.32, adjusted p = .005, [d = 1.15]; 1400–1900 ms F(1, 19) = 12.43, adjusted p = .008, [d = 1.11]. According to visual inspection and the effect sizes of the condition differences in the L2 group, the negativity increases during the first three time windows and then decreases after participants encountered the word preceding the expected gap (e.g. classes in example 5) at approximately 1700 ms. An interaction of the first three time windows as a continuous predictor with the factor Condition confirms this: F(2, 38) = 4.34, p = .02.
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b
… Peter /arranged/ some / classes / for it / aer / the / vacaon
Fig. 1 continued
Verb-driven Effects We examined ERPs following the presentation of the verb, the filler’s semantic licenser (Fig. 2a, b). Note that at this point, sentences in both the extraction and non-extraction conditions are still fully grammatical. In the time window from 350 to 500 ms after word onset we found a significant three-way interaction of Group × ROI × Condition: F(8,296)=2.46, p=.01. Subsequent per-group analyses revealed interactions of the factors ROI and Condition for both groups: L1 F(8, 144) = 3.15, adjusted p = .003; L2 F(8, 152) = 1.97, adjusted p = .05. Follow-up analyses showed a negativity for the extraction condition at posterior regions for both groups: mid-posterior for the L2 group F(1, 19) = 5.03, adjusted p = .039,
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a
Fig. 2 a Grand averages of the L2 group at the verb site for all ROIs. Extraction condition = dotted line. Negativity indicated by arrows. b Grand averages of the L1 group at the verb site for all ROIs. Extraction condition = dotted line. Negativity indicated by arrows
[d = 0.71] and right-posterior inferior for the L1 group F(1, 18) = 5.34, adjusted p = .04 [d = 0.75]. Additionally we found a significant negativity in the L2 data at the left anterior inferior region F(1, 19) = 6.93, adjusted p = .02. This negativity is most likely part of the sustained LAN reported above, which was present only in the L2 group. It is also likely that this difference caused the observed three-way interaction of ROI, Group and Condition.
Filled-gap Effects We then examined ERPs time-locked to the onset of the critical PP (i.e. for it / to it), comparing the (now ungrammatical) extraction condition to the non-extraction one. Mean amplitudes were extracted for each participant at each electrode site (Fig. 3a, b). During a time window of 600–800 ms after stimulus onset there was a main effect of Condition: F(1, 37) = 13.4, p < .001, with a more positive-going wave form for the extraction condition, a significant interaction of the factors ROI and Group: F(8, 296) = 2.79, p = .005, and of ROI and Condition: F(8, 296) = 5.55, p < .001, but no three-way interaction of ROI, Group and
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b
Fig. 2 continued
Condition. Step-down analyses by ROI revealed significant differences between the two conditions for both groups combined in the following ROIs: right anterior inferior: F(1, 37) = 9.93, adjusted p = .009 [d = 0.73]; right anterior superior: F(1, 37) = 12.75, adjusted p = .006 [d = 0.83]; left anterior superior: F(1, 37) = 13.39, adjusted p = .005 [d = 0.85]; left anterior inferior: F(1, 37) = 5.58, adjusted p = .02 [d = 0.55]; left posterior superior: F(1, 37) = 8.85, adjusted p = .01 [d = 0.69]; right posterior superior: F(1, 37) = 10.87, adjusted p = .006 [d = 0.77]. The ROI by Group interaction reported above, as well as visual inspection, indicated that the two groups differed in the distribution of the positivity, which was more frontal for the L1 but global for the L2 group, with the largest amplitude in posterior regions. The waves as well as the topographical maps (Fig. 4) also show between-group differences in amplitude, with lower amplitudes for the L1 group. The fact that both groups showed positivities for the extraction condition accounts for the absence of a three-way interaction of ROI, Group and condition. To examine possible between-group differences that had not given rise to a significant three-way interaction earlier, we carried out additional analyses for the two groups separately. These per-group analyses showed significant interactions of ROI and Condition in both groups: L1 F(8, 144) = 2.84, adjusted p = .006; L2 F(8, 152) = 3.62,
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a
Fig. 3 a Grand averages of the L2 group at the gap site for all ROIs. Extraction condition = dotted line. Positivity indicated by arrows. b Grand averages of the L1 group at the gap site for all ROIs. Extraction condition = dotted line. Positivity indicated by arrows
adjusted p = .002. Follow-up analyses revealed significant differences between the two conditions in both groups, with more positive-going waveforms for the extraction condition. The differences between the two conditions approached or reached significance for both groups in the right anterior superior and left anterior superior regions as well as the right anterior inferior region: L1: right anterior superior: F(1, 18) = 5.6, adjusted p = .06 [d = 0.77]; left anterior superior: F(1, 18) = 5.33, adjusted p = .06 [d = 0.75]; right anterior inferior: F(1, 18) = 8.13, adjusted p = .03 [d = 0.93]. L2: right anterior superior: F(1, 19) = 7.99, adjusted p = .04 [d = 0.89]; left anterior superior: F(1, 19) = 8.4, adjusted p = .04 [d = 0.92]; right anterior inferior: F(1, 19) = 4.7, adjusted p = .04 [d = 0.69]. Only the L2 group additionally showed significant differences at posterior regions: right posterior superiorF(1,19)=10.13, adjusted p = .024 [d = 1.01], left posterior superior F(1, 19) = 7.04, adjusted p = .04 [d = 0.84] and mid posterior F(1, 19) = 9.07, adjusted p = .04 [d = 0.95]. These results show that, as suspected, the groups differed regarding the scalp distribution of the condition effect but not in its direction: in both cases a positive-going wave form
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b
Fig. 3 continued
Fig. 4 Topographical maps illustrating the distribution of the positive-going ERPs observed at the filled gap for the L2 (left) and the L1 (right) group
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was seen for the extraction condition, which overlapped between the two groups in frontal areas—another possible explanation for the missing three-way interaction.
Discussion We used ERPs to investigate how L1 and L2 comprehenders process indirect object dependencies. We specifically examined whether gaps are predicted beyond the verb, whether L1 and L2 comprehenders show the same types of brain responses to filled gaps and whether these differ in their timing. Our ERP data revealed both similarities and differences between the two participant groups. Examining both local and clause-level ERPs, we found that sentences containing a wh-dependency elicited three different types of brain response relative to non-extraction control sentences. The first was a sustained LAN which was present in the L2 group only and which extended beyond the subcategorizing verb. The second was a posterior negativity that appeared after the presentation of the verb in both participant groups, and the third was a late positive-going deflection that was observed at the point at which the filled indirect object gap was encountered, with a broader distribution and higher amplitude for the L2 than for the L1 group. In the following, we will discuss each of these ERP effects in turn.
Storage Effects Sustained LAN effects have been argued to reflect the processing cost incurred by storing a fronted constituent in WM until the dependency can be completed. The long negativity found in the L2 group in the present study was more localized than, for example, the LAN reported by Fiebach et al. (2002), where it was present at most electrodes between 1000 and 3400 ms. In our study, the negativity lasted from 300 to 1900 ms after the embedded subject noun phrase (e.g. Peter) and until approximately 500ms after the word preceding the gap (e.g. classes) was encountered, and was restricted to the left anterior inferior ROI (i.e. F7, T7, FC5). Unlike in previous studies on direct object filled gaps (e.g. Hestvik et al. 2007, 2012), the design of our materials allowed us to examine whether and for how long the sustained negativity extended beyond the verb. The extended LAN seen in our L2 group indicates that displaced indirect objects were held in WM even after the verb had been processed. On the assumption that the LAN reflects the processing cost of filler storage, its presence in the L2 data indicates that our non-native comprehenders continued searching for a gap in order to have the filler’s thematic role confirmed. No sustained LAN was found for our native English-speaking control group, however. One possible explanation for this between-group difference is that L2 comprehenders kept the filler highly active in WM throughout the length of the dependency whereas L1 comprehenders mentally reactivated the filler at the point where a gap was expected. This interpretation would be in line with the position-specific (L1) versus across-the-board (L2) priming effects observed by Felser and Roberts (2007). Alternatively, it may be that our L1 comprehenders’ processing of indirect object dependencies was so highly automatized that effects of filler storage were barely detectable for this group. Recall also that the wh-fillers used in our study already contained information about the filler’s likely thematic role (for which, to which), which may have reduced its activation level in comparison to more ambiguous fillers. According to Gouvea et al. (2010), using PP fillers should have made it possible in principle for comprehenders to terminate dependency formation at the verb. Although this possibility cannot be ruled out for the L1 group, the presence of a sustained LAN extending beyond the verb in the L2 data is not consistent
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with this hypothesis. Unlike Gouvea et al. (2010), we also found no P600 effects (which the authors claim are indicative of creating syntactic relations) at the verb in either of our participant groups.
Verb-driven Effects Examining participants’ ERP responses to the subcategorizing verb—a point at which our extraction sentences were still fully grammatical—should reveal effects of semantic integration and preliminary thematic role assignment (compare Nicol 1993). Both participant groups showed a posterior negativity which reached significance in a time window from 350 to 500 ms after verb onset. According to its polarity, latency and distribution this negativity can clearly be identified as an N400, an ERP component which is typically (albeit not exclusively) associated with semantic integration difficulty (Kutas and Hillyard 1980). In studies on FGDs, verb-driven N400 effects have previously been observed in response to implausible direct object fillers (Dallas et al. 2013; Garnsey et al. 1989) and to fillers that do not meet the verb’s argument structure requirements (Phillips et al. 2005). Note that in all our experimental items an animal character (e.g. the monkey in 5a,b) served as the understood indirect object or Goal argument. Even though our participants were briefed about our stimulus items being intended for use with children and describing fairy-tale scenarios, this might have led to plausibility issues at the verb, many of which described an action that normally required the Goal argument to be human (e.g. confess, dictate, read). We therefore interpret the N400 as indicating an anomaly that results from an attempt to semantically integrate a pragmatically unusual filler with its subcategorizer in our extraction condition. No obvious between-group differences were found for the N400, except for a slightly more right-lateralized distribution in the L1 group. This finding is unsurprising since several previous ERP studies on L2 processing have reported native-like N400 brain responses to semantic anomalies, even for less proficient L2 speakers (e.g. Ardal et al. 1990; Hahne and Friederici 2001—but cf. Dallas et al. 2013). It is also in line with previous findings from behavioral and eye-movement studies showing that L2 comprehenders can semantically link a filler to a potential lexical licenser as quickly as native speakers. The absence of P600 effects at the verb might seem surprising given that several previous studies have reported such effects at the verb (e.g. Felser et al. 2003; Fiebach et al. 2002; Gouvea et al. 2010; Hagiwara et al. 2007; Phillips et al. 2005; Ueno and Kluender 2003). However, note that unlike in most of these studies, the predicted structural gap in our stimulus sentences was non-adjacent to the verb. On the assumption that a P600 can reflect the completion of a syntactic dependency at structural gap positions, which in our materials could only be accomplished after the direct object has been processed, the absence of P600 effects at the verb does not seem particularly surprising.
Filled-gap Effects At filled indirect object gaps, a fronto-central (for L1) or more globally distributed (for L2) positivity was seen which was significant in the 600–800 ms time window. Although its scalp distribution, at least for the L1 group, differs from the centro-parietal P600 effects that are typically seen in studies on morphosyntactic anomalies (e.g. Hagoort et al. 1993), garden-path sentences (Osterhout and Holcomb 1992) or on the processing of grammatical wh-dependencies (e.g. Felser et al. 2003; Fiebach et al. 2002; Kaan et al. 2000), the positivities observed in the current study may be interpretable as P600 responses. There was no evidence of the P600 effect being delayed in our L2 group.
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In both Hestvik et al.’s (2012) and Schremm’s (2012; 2013) studies (although not in Hestvik et al. 2007), encountering a filled direct object gap elicited a more typical posterior positivity of the kind that has frequently been associated with grammatical violations. Note that there is a crucial difference, however, between these authors’ materials and ours in that encountering a filled direct object gap in the above studies rendered the sentence both ungrammatical and uninterpretable. That is, the postverbal noun phrase the camel in a sentence like The zebra that the hippo kissed the camel on the nose ran far away adds a new argument which competes with the filler the zebra in that it is impossible for both of them to be legitimately integrated into the current sentence representation. In our materials, on the other hand, it is possible for the PP occupying the indirect object position to be integrated into the current sentence representation semantically if it is construed as being resumptive, with e.g. for it taking the filler for which as its antecedent. This possibility would render our ungrammatical filled-gap sentences globally interpretable. We should be cautious about interpreting the observed P600 effect as evidence that filled indirect object gaps were perceived as pure phrase-structure violations, however. Unlike the (E)LAN effects observed e.g. by Hestvik and colleagues (2007; 2012), the P600 is a comparatively late brain response which cannot easily be linked to any specific syntactic or semantic processes (Van Petten and Luka 2012). The monophasic positivity we observed was smaller in amplitude and distributed more fronto-centrally in our L1 group (compared to the larger and more global effect seen in the L2 group), which suggests that encountering a potentially resumptive filled gap disrupted processing less in L1 than in L2 comprehension. It may also indicate that the observed positivities index somewhat different processes (compare e.g. Friederici et al. 2002; Kaan and Swaab 2003). Frontal positivities have been linked, for example, to syntactic ambiguity resolution (Osterhout and Holcomb 1992), syntactic integration difficulty (Friederici et al. 2002), increased discourse complexity (Kaan and Swaab 2003) and disconfirmed lexical predictions (Van Petten and Luka 2012). Centro-parietal P600 components, on the other hand, have been associated with structural repair processes triggered by encountering downright grammatical violations (e.g. Friederici et al. 2002). The positivity that we saw in the L2 group, although globally visible, was more pronounced over posterior than over anterior regions and therefore more within the normal range of the latter kind of P600. As the differences between the two groups regarding amplitude and distribution were not confirmed by reliable three-way interactions, any such conclusions are merely speculative, though. Our L2 group ostensibly showing stronger effects regarding the positivity’s amplitude and distribution is surprising given that previous ERP studies on L2 sentence processing have often found P600 responses to syntactic anomalies to be smaller than native speakers’ or altogether absent, especially in late or less proficient L2 learners (see e.g. Bowden et al. 2013; Weber-Fox and Neville 1996). Recall that our L2 participants had started learning English during elementary school or the first year of high school and also had a relatively high proficiency level. For early or highly proficient learners, P600 effects in response to syntactic anomaly have frequently been reported in the L2 literature (e.g. Hahne 2001; Kotz et al. 2008), and Rossi et al. (2006) found an even stronger P600 response to category violations for highly proficient L2 speakers than for native speakers. Our results are thus in line with Rossi et al.’s (2006) findings as well as with those from fMRI studies which report increased activation of certain brain areas (e.g. Rüschemeyer et al. 2006) and more areas being engaged (Wartenburger et al. 2003) in L2 compared to L1 sentence processing. Neither of our participant groups showed a biphasic N400–P600 response to filled gaps of the kind elicited by argument structure violations in previous studies (e.g. Friederici and Frisch 2000). This is surprising on the assumption that subcategorization violations resulting
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from the presence of an unlicensed extra argument phrase constitute both a grammatical and semantic violation (Hagoort et al. 1993). N400 effects were also absent in both Hestvik and colleagues’ and Schremm’s (2012; 2013) earlier studies on filled direct object gaps, however. This may be due to the fact that in both ours and the above filled-gap ERP studies, the extra argument phrase was always semantically compatible with the preceding verb. Finally, note that finding both a sustained LAN indicative of filler storage and nativelike brain responses to filled gaps in our L2 group fails to support claims to the effect that L2 comprehenders have a reduced ability to predict in comparison to L1 comprehenders (e.g. Grüter et al. 2016). Given that our experimental sentences were potentially salvageable semantically by interpreting the extra PP as resumptive, the observation that our L2 group experienced more processing difficulty at this point than the L1 group might indicate that the L2 speakers had more difficulty ’rescuing’ the sentence by construing the two competing PPs as referring to the same event participant.
Conclusion The combination of ERP effects that we observed support the hypothesis that processing indirect object dependencies involves a two-step process. There is an initial attempt to integrate the filler into the verb’s lexical-semantic argument structure frame when the verb is encountered. The filler is held in WM until its thematic role can be confirmed, although in the current study statistically robust evidence of maintained filler activation was seen only in our non-native participants. Encountering an unexpected competitor phrase at the filler’s canonical position gave rise to P600 effects in both native and non-native comprehenders. Unlike what has been reported in previous L2 processing studies, we found no evidence of filled-gap effects being delayed in L2 relative to L1 processing. Taken together, our results indicate that proficient L2 comprehenders process indirect object wh-dependencies in a similar way to L1 comprehenders. L2 comprehenders appear to spend more processing resources than native ones on keeping a filler active in WM, and also find it harder to integrate an unexpected, redundant constituent into the current sentence representation. Acknowledgements We are grateful to Jennifer Meyer for programming assistance, and to Caroline Beese for help with the data collection.
Funding This study was funded by an Alexander-von-Humboldt Professorship to Harald Clahsen. Compliance with Ethical Standards Conflict of interest The authors declare that they have no conflict of interest.
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Native and Non-native Speakers’ Brain Responses to Filled Indirect Object Gaps Anna Jessen1 · Julia Festman1 · Oliver Boxell1 · Claudia Felser1
© Springer Science+Business Media New York 2017
Abstract We examined native and non-native English speakers’ processing of indirect object wh-dependencies using a filled-gap paradigm while recording event-related potentials (ERPs). The non-native group was comprised of native German-speaking, proficient non-native speakers of English. Both participant groups showed evidence of linking fronted indirect objects to the subcategorizing verb when this was encountered, reflected in an N400 component. Evidence for continued filler activation beyond the verb was seen only in the non-native group, in the shape of a prolonged left-anterior negativity. Both participant groups showed sensitivity to filled indirect object gaps reflected in a P600 response, which was more pronounced and more globally distributed in our non-native group. Taken together, our results indicate that resolving indirect object dependencies is a two-step process in both native and non-native sentence comprehension, with greater processing cost incurred in non-native compared to native comprehension. Keywords Sentence processing · Wh-movement · Filled gaps · ERPs
Introduction A growing body of research has investigated whether native (L1) and non-native (L2) comprehenders process non-canonical word orders in the same way (see Dallas and Kaan 2008,
B
Claudia Felser [email protected] Anna Jessen [email protected] Julia Festman [email protected] Oliver Boxell [email protected]
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Potsdam Research Institute for Multilingualism, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany
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for a review). In wh-movement languages like English, readers or listeners need to hold a fronted constituent in memory until it can be integrated with its licenser (such as a verb or a preposition) further downstream. During the processing of sentences such as (1) below, for instance, the fronted object wh-phrase which book cannot be fully interpreted in situ and thus must be carried forward until it can be linked to its thematic role assigner, the verb read. (1) Which book did you say you would like to read __ ? In the sentence processing literature a fronted constituent is usually referred to as a “filler” and its canonical structural position as a corresponding “gap” (indicated by underscores). Encountering a filler triggers the search for an associated gap or lexical licenser while the parser must simultaneously process and integrate new incoming words and phrases (Clifton and Frazier 1989). Besides filler storage, resolving filler-gap dependencies (FGDs) requires identifying potential gaps in the input stream and evaluating the filler’s semantic fit as a verbal argument or event participant (e.g. Gibson 1998). Filler storage and filler integration processes have been shown to be dissociable at the neuro-cognitive level (e.g. Felser et al. 2003; Fiebach et al. 2002; Phillips et al. 2005). Empirically dissociating verb-based semantic integration from position-specific gap filling is more difficult, however, especially for sentences such as (1) above in which the filler’s canonical structural position is immediately adjacent to its lexical licenser. In verb-initial languages like English, one way of teasing these processes apart is to investigate indirect object dependencies. Using the cross-modal priming technique, Nicol (1993) found that for sentences containing indirect object wh-fillers such as (2), native English comprehenders showed filler reactivation effects both at the main verb (give) and at the canonical indirect object position further downstream. (2) To which butcher did the woman who had just inherited a large sum of money give the very expensive gift __ the other day? This, according to Nicol, indicates that in sentences such as (2), the processing system first evaluates the filler’s suitability as a potential argument of the verb, possibly assigning it a preliminary thematic role, but then continues to hold the filler in working memory (WM) until it comes across the indirect object gap, at which point the thematic role is confirmed. The cognitive processes involved in non-native (L2) speakers’ processing of FGDs are as yet only partially understood. Previous research has shown that proficient L2 comprehenders are able to link a filler to a potential lexical licenser when this is encountered, as witnessed by their ability to detect semantic or pragmatic incongruencies between the two during real-time processing (e.g. Cunnings et al. 2010; Dallas et al. 2013; Omaki and Schulz 2011; Williams 2006; Williams et al. 2001). L1/L2 differences have been observed, however, for gaps that are non-adjacent to their lexical licenser (Marinis et al. 2005; Felser and Roberts 2007; Miller 2015—but cf. Pliatsikas and Marinis 2013). Felser and Roberts (2007), for example, used a cross-modal priming task to examine whether proficient Greek/English bilinguals would mentally reactivate indirect object fillers at the filler’s canonical position in sentences such as (3). (3) John saw the peacock to which the small penguin gave the nice birthday present ___ in the garden last weekend. Unlike native speakers, who showed evidence of selectively reactivating a representation of the understood indirect object the peacock when coming across the indirect object gap (Roberts et al. 2007), the bilinguals tested by Felser and Roberts showed priming effects both at the gap and at an earlier, non-gap control position, but no evidence for position-specific
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filler retrieval at the gap site (see also Miller 2015, for parallel findings for bilinguals in French). It is conceivable, however, that the relatively high cognitive resource demands made by the cross-modal priming technique prevented the L2 comprehenders from processing the auditory experimental stimuli properly, as has been suggested by Miller (2015). Another possibility is that L2 comprehenders have a reduced ability, compared to native comprehenders, to predict upcoming sentence material, including indirect object gaps (see Kaan 2014, for review and discussion), in which case filler reactivation effects might be delayed in non-native processing. Neither Felser and Roberts (2007) nor Miller (2015) tested for potential filler reactivation effects at the subcategorizing verb itself (such as give in example 2 above) or for potentially delayed gap-related priming effects. Taking the above findings from cross-modal priming as a point of departure, the current study used event-related potentials (ERPs) to examine the processing of indirect object dependencies. Using a less resource-demanding experimental task compared to cross-modal priming, we specifically sought to examine whether establishing indirect object dependencies also involves a two-step process in L2 processing, and whether wh-fillers are being kept activated in WM beyond the point at which they can be associated with their lexical subcategorizer. Given earlier findings suggesting that L2 comprehenders may have difficulty identifying covert linguistic structure in the input stream (e.g. Marinis et al. 2005; Felser and Roberts 2007), the current study uses filled gaps as an experimental diagnostic for filler reactivation at potential gap sites.
Sensitivity to Filled Gaps in Native and Non-Native Language Processing So-called “filled-gap” effects are observed when a structural position that could potentially host a filler is discovered to be occupied by another constituent. In reading-time studies, encountering a filled gap usually gives rise to elevated reading times in comparison to a control condition (e.g. Stowe 1986). Hestvik et al. (2007) used ERPs to investigate filled-gap effects. They presented native English-speaking listeners with ungrammatical filled-gap sentences such as (4a) below, in which the zebra could initially be assumed to be the object of the verb kissed. However, the postverbal position turns out to be filled by another noun phrase (the camel) here. (4) a. *The zebra that the hippo kissed the camel on the nose ran far away. b. The zebra said that the hippo kissed the camel on the nose and then ran far away. Encountering the filled gap in (4a) elicited a bilaterally distributed anterior negativity compared to encountering a grammatical object (4b), which Hestvik et al. (2007) interpreted as an early left-anterior negativity (ELAN). A later positive-going wave (P600) was also observed but failed to reach full significance. As an ELAN response is frequently elicited by phrase-structure violations (Friederici 2002), Hestvik et al. (2007) concluded that a filled direct object gap constitutes a phrase-structure violation, with two noun phrases competing for the same structural slot. Using similar materials as in Hestvik et al. (2007), Hestvik et al. (2012) found filled direct object gaps triggering a bilateral LAN followed by a P600, with both effects being delayed by about 200ms in participants with a relatively low WM score. In an attempt to replicate Hestvik and colleagues’ findings, Schremm (2013), in as yet unpublished work, observed that filled direct object gaps triggered a P600 response but no negativity.
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Few studies to date have investigated non-native comprehenders’ sensitivity to filled gaps during real-time processing. Previous studies using behavioral tasks (Williams 2006; Williams et al. 2001; Dallas 2008), eye-movement measures (Felser et al. 2012) or ERPs (Schremm 2012) have reported filled-gap effects to be delayed in L2 compared to L1 processing (but cf. Aldwayan et al. 2010). Delayed sensitivity to filled gaps in non-native comprehension may potentially reflect (i) difficulty identifying possible gap positions, (ii) difficulty keeping a representation of the filler active during processing and/or retrieving it from WM when a potential gap position is identified, (iii) generally slower syntactic or semantic integration processes, or (iv) some combination of these. Because in previous studies the filled gap always immediately followed the verb, it is difficult to distinguish semantic integration effects triggered by encountering the verb from effects triggered by finding a potential gap already occupied. The materials for the current study were designed to allow us to tease apart verb-driven and filled-gap driven ERP effects by testing filled indirect object gaps.
The Current Study The present study investigates whether L1 and L2 comprehenders process indirect object dependencies in the same way. We specifically examined whether gaps are predicted beyond the verb, whether L1 and L2 comprehenders show the same types of brain responses to both the filler’s semantic licenser and to filled gaps, and whether the two groups’ brain responses differ in their relative timing. To this end, we recorded participants’ ERP responses while they were reading sentences containing indirect object relative clauses.
Participants Twenty native speakers of English (13 female, mean age 29.0 years, range 18–59 years) served as our control group. Fifteen of them lived and worked in Potsdam or Berlin, five were students at the University of Potsdam (Germany). The L2 group consisted of 21 native speakers of German with English as L2 (10 female, mean age 25.4 years, range 20–34 years), recruited from the student population of the University of Potsdam and from the surrounding area. The L2 speakers’ general level of English grammar was assessed using part II of the grammar section of the Oxford Placement Test (OPT; Allan 2004). They achieved a mean score of 39.9 out of a possible maximum score of 50 (range 29–47), which places our non-native participants within the B2-C2 proficiency range according to the Common European Framework for Reference for Language (B2 = “upper intermediate”, C2 = “proficiency”). Our German-speaking participants had started learning English at a mean age of 10.3 years (range 6–13 years) and none of them reported having grown up bilingual. All participants were right-handed, had normal or corrected to normal vision, and were offered a small fee for their participation.
Materials Our experimental materials were originally designed so as to be suitable for young children. They typically described events within a fairy-tale kind of scenario, with animal characters being endowed with human qualities or cognitive abilities. Participants were briefed accordingly prior to the main experiment. The materials for this experiment included 48 ungrammatical and grammatical sentence pairs structurally parallel to (5a,b) below.
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(5) a. *Sarah tickled the monkey for which Peter arranged some classes for it after the vacation. b. Sarah tickled the monkey while Peter arranged some classes for it after the vacation.
Sentences in our extraction condition (5a) contained a relative clause modifying the main clause object. These were introduced by a prepositional phrase (PP) that functioned as an indirect object (e.g. for which). They were rendered ungrammatical by inserting a “resumptive” indirect object phrase (e.g. for it) at the filler’s canonical position following the direct object (e.g. after some classes). Half of the ungrammatical filled-gap sentences contained for which and half contained to which (e.g. James watched the penguin to which Adam supplied some sandwiches to it after the school run). Fronting the preposition along with the wh-pronoun (rather than stranding it) should prevent the fronted constituent being mistaken for either a subject or direct object. Our grammatical (non-extraction) control sentences (5b) did not involve FGDs but instead contained an adjunct clause introduced by while. Our materials were selected based on the results from two offline pretests, a sentence continuation questionnaire and a plausibility norming questionnaire. The primary purpose of the sentence continuation task was to confirm that the embedded verbs used in the ERP experiment could also appear in a monotransitive NP–V–NP frame. This was important in order to prevent participants from anticipating the indirect object (e.g. for it) in our grammatical control condition (5b). A set of 40 common dative verbs (i.e. those admitting a to-PP) and 40 common benefactive verbs (i.e. those admitting a for-PP) were selected from Levin (1993). From these, 80 short sentences of the form NP–V–NP (e.g. Peter arranged some classes) were constructed and mixed with 40 intransitive and monotransitive distractor items. 20 adult native speakers of British English (who did not participate any further in this study) were asked to indicate whether each sentence formed a complete, natural-sounding sentence as it stands. If they felt that a given fragment should better be continued, participants had to write down the first appropriate continuation that came to mind. Verbs that elicited more than 20% to-PP or for-PP completions and fewer than 70% “complete” responses were excluded from the initial set. The remaining 74 verbs were considered to be fine without an additional indirect object (mean: 93.9%, SD: 7.6, range: 70 − 100%). The purpose of the plausibility norming questionnaire was to make sure that all the experimental items to be used in the ERP study were deemed syntactically acceptable and plausible, and to match the to/for which–while pairs as closely as possible. A total of 80 critical sentence pairs, all of the form in (5a,b) above, were created using the 74 pre-selected verbs (with six of these used twice). The total materials comprised 240 sentences including 40 to which– while pairs, 40 for which–while pairs, and 80 implausible fillers of various structural types, of which 20 were also syntactically ill-formed. The critical items were distributed across two presentation lists such that no participant saw both versions of the same sentence. A total of 31 adult native speakers of British English (who did not participate any further in this study) completed the questionnaire. They were instructed to judge the naturalness of each of the sentences on a five-point scale from 1 (= “completely natural”) to 5 (= “completely unnatural”). As the materials were created bearing in mind that they might later be used with children, the participants were asked to imagine a kind of Animal Farm scenario where animals could show human-like behavior and to judge the sentences as to whether or not they sounded good within this kind of scenario. On the basis of participants’ plausibility scores, 24 closely matched to which–while and 24 for which–while pairs were selected for the ERP experiment (for which items mean: 1.98, SD: 0.17, range: 1.5–2.31; to which items mean: 1.99, SD: 0.23, range: 1.56–2.47; while items mean: 1.67, SD: 0.3, range: 1.27–2.53).
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The 48 experimental sentence pairs were distributed across two presentation lists in a Latin Square design and mixed with 48 fillers of similar length and structural type which served as the materials for another experiment, and 60 additional grammatical or ungrammatical fillers of similar length which represented a range of different structural types. To prevent participants from developing specific expectations or strategies when encountering relative clauses introduced by to/for which, we included ten grammatical indirect object gap fillers that were structurally similar to our experimental sentences (e.g. The pony for which Belinda had bought a new saddle jumped over the fence).
Predictions Based on previous findings on the processing of wh-dependencies we made the following predictions: 1. In previous ERP studies on FGDs a sustained left-anterior negativity (LAN) has often been observed, which has been suggested to reflect the processing cost incurred by temporarily storing a filler in WM (e.g. Felser et al. 2003; Fiebach et al. 2002; King and Kutas 1995; Kluender and Kutas 1993; Phillips et al. 2005). If encountering an indirect object filler triggers the prediction of a gap, filler storage cost should be reflected in a LAN following the presentation of the filler (for/to which) and extending beyond the subcategorizing verb until the wh-dependency can be completed. 2. If indirect object fillers are initially evaluated at the subcategorizing verb (Nicol 1993), we should see effects of semantic integration in the shape of an N400 component (compare e.g. Garnsey et al. 1989) following the presentation of the verb. 3. Surprisal at finding the filler’s canonical structural position filled by another constituent should be reflected in a biphasic N400–P600 response if a filled gap is perceived as an argument structure violation (e.g. Friederici and Frisch 2000), or in an (E)LAN-P600 response if it is perceived as a phrase structure violation (e.g. Hestvik et al. 2007).
Procedures Prior to the experimental session, participants filled out a biographical questionnaire; the L2 group also completed grammar part II of the OPT. The experimental session took place in a quiet laboratory at the University of Potsdam. Participants first signed a consent form and were informed about the EEG procedure. They were then prepared for the EEG-recording and seated in front of a computer screen at a distance of approximately 100 cm. The experiment was presented on a 61 cm wide monitor using Presentation version 14.9 (Neurobehavioral Systems). Sentences were presented in 12 presentation segments, with for which/to which and for it/to it always presented as single segments. Each segment was presented for 550ms in black letters against a white background in the middle of the screen. Each participant saw 156 sentences in total. At the start of each trial a fixation cross appeared at the center of the screen for 1250ms followed by the sentences displayed in Times New Roman, 96-point size font. Overall, 52 of all the trials (i.e. one third) were followed by a verification question presented in green letters against white background (40-point size font), which remained on the screen until the participant pressed one of two designated response buttons (“yes” or “no”). Questions were either repeating part of the stimulus sentence, such as Peter arranged some classes for the monkey? or contained a small change where a negative answer was required. The appearance of the questions was fully randomized between items and participants. A question was followed either by the appearance of a “smiley face” for a correct response or by a
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blank screen for an incorrect answer, each presented for 1000ms in the center of the screen. Participants were advised to relax before the next trial. The screen went blank for 500ms before the next sentence started with the fixation cross. Before the experiment proper, participants practiced the task with five practice trials, each followed by a comprehension question. Experimental trials were fully randomized and distributed over four blocks of 39 sentences each, with blocks counterbalanced amongst participants. After the first block, participants were allowed to take a short break. Participants were asked to minimize eye and muscle movements until presentation of the “smiley face” or blank screen after each trial. The experiment itself took approximately 30 min to complete, and an entire experimental session lasted for approximately 90 min.
Electrophysiological Recording The EEG was recorded continuously using Brain Products Vision Recorder acquisition software from 31-electrode sites (FP1, FP2, F7, F3, Fz, F4, F8, FC5, FC1, FC2, FC6, T7, C3, Cz, C4, T8, TP9, CP5, CP1, CP2, CP6, TP10, P7, P3, Pz, P4, P8, PO9, O1, O2, PO10) according to the international 10-20 system using active electrodes embedded in an elastic cap (ActiCap, Brain Products). Additionally, vertical electro-oculograms (VEOGs) were recorded for artifact rejection purposes. Signals were recorded continuously with an on-line band-pass filter between 0.016–70 Hz and digitized at 2500 Hz. Electrode impedances were kept below 20 k (according to the guidelines for using ActiCaps). All recordings were rereferenced to the average of the left and right mastoid electrodes offline. Recordings were offline band pass filtered between 0.1 and 30 Hz. Offline recordings were down-sampled to 250 Hz. EEG data were processed with Vision Analyzer 2. For single-segment analysis, epochs were extracted from 200 ms before the presentation of the critical word or segment up to 1000 ms after the presentation onset resulting in 1200 ms epochs (−200 to 1000 ms) and were baseline-corrected using a 200ms pre-stimulus baseline. For multiword ERPs we followed Fiebach et al. (2002) such that epochs were extracted after the presentation of the segment that distinguished between both sentence conditions (i.e. for it / while), until the end of the sentence, resulting in 4700 ms epochs (−200 to 4500 ms) and were baseline-corrected using a 200 ms pre-stimulus baseline.
Data Cleaning Two participants, one from each group, had to be excluded because of data loss due to technical problems. To remove typical muscle and eye-movement artifacts from the scalp recordings, an independent component analysis (ICA) algorithm (Infomax) was applied to the data. Epochs containing additional artifacts were identified with the semi-automatic rejection option in the Analyzer and rejected after visual inspection. In sum, 98% of all trials from the remaining participants were included in the statistical analysis.
Statistical Analysis We examined local ERPs both at the subcategorizing verb and at the filled indirect object gap, as well as clause-level ERPs following the presentation of the filler. For the statistical analysis of local ERPs, 27 electrodes were grouped into nine regions of interest (ROIs), eight of which following the division in Hestvik et al. (2007): left anterior inferior (F7, T7, FC5), left anterior superior (F3, FC1, C3), right anterior inferior (F8, FC6, T8), right anterior superior
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(F4, FC2, C4), left posterior inferior (TP9, P7, PO9), left posterior superior (CP5, CP1, P3), right posterior inferior (CP6, CP2, P4), right posterior superior (TP10, P8, PO10), and an additional mid posterior ROI (Pz, O1, O2). Time windows of interest were chosen based on visual inspection and a 50ms time-line analysis (for the verb and filled-gap site), conducted for the two groups separately. If interactions including the factors Condition and ROI reached significance in at least three consecutive time windows for both groups, we followed this up by analyzing larger time intervals. Mean ERP amplitudes were statistically analyzed using repeated measures ANOVAs. Preliminary mixed three-way ANOVAs included Condition (grammatical, ungrammatical) and ROI as within-participants factors and Group (L1, L2) as a between-participants factor. Follow-up per-group ANOVAs included the factors Condition and ROI as within-subject factors. To correct for multiple comparisons, we applied a simple sequentially rejective multiple test (Holm 1979). Further stepping-down analyses used oneway ANOVAs with the factor Condition to examine differences within individual ROIs, p-values were again adjusted with the Holm test. For effect sizes, Cohen’s d (standardized mean difference) was calculated based on the F-values and sample sizes of the two participant groups, with a 95% confidence interval. The results are reported in parentheses following the ANOVA results. All statistical analyses were carried out with RStudio, Version 3.0.2, with the packages ezANOVA (for the ANOVAs) and compute.es (for effect sizes).
Results Response Accuracy Participants’ average accuracy rate for responses to the end-of-trial comprehension questions was 83.3% (range 63.5–92.3%) for the L1 group and 82.5% (range 65.9–90.4%) for the L2 group. This indicates that participants were attending to the stimulus materials and actively read them for comprehension.
Clause-level ERPs Encountering the wh-filler elicited a sustained negativity in the L2 group at the left anterior inferior region (F7, FC5, T7) and a very small and short-lived negativity for the L1 group visible only at FC5 (see Fig. 1a, b). Visual inspection of the L2 group’s grand average ERPs at left-frontal electrodes revealed that this negativity started at about 300 ms at FC5 after the embedded subject (e.g. Peter in 5a,b) and was present until about 3000 ms, i.e. until shortly after the filled gap was encountered. Based on visual inspection, five time windows were chosen for analysis: 300–800, 800–1400, 1400–1900, 1900–2500 and 2500–3000 ms. The first three time windows showed significant or marginally significant interactions of Group × ROI × Condition: 300–800 ms F(8, 296) = 2.47, adjusted p = .03; 800–1400 ms F(8, 296) = 2.52, adjusted p = .03; 1400–1900 ms F(8, 296) = 1.8, adjusted p=.07. Subsequent per-group analyses for these time windows showed significant interactions of ROI and Condition in the L2 group only: 300–800 ms: F(8, 152) = 2.84, adjusted p = .018; 800– 1400 ms F(8, 152) = 3.53, adjusted p = .004; 1400–1900 ms: F(8, 152) = 2.8, adjusted p = .018. Follow-up analyses revealed significant differences between the extraction and non-extraction conditions in all three time windows in the left anterior inferior region for the L2 group: 300–800 ms F(1, 19) = 11.67, adjusted p = .008, [d = 1.08]; 800–1400 ms
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a
… Peter /arranged/ some / classes / for it / aer / the / vacaon
Fig. 1 a Grand average of electrodes F7, FC5 and T7 for the L2 group following the presentation of the wh-filler (e.g. for/to which). Extraction condition = dotted line. b Grand average of electrodes F7, FC5 and T7 for the L1 group following the presentation of the wh-filler (e.g. for/to which). Extraction condition = dotted line
F(1, 19) = 13.32, adjusted p = .005, [d = 1.15]; 1400–1900 ms F(1, 19) = 12.43, adjusted p = .008, [d = 1.11]. According to visual inspection and the effect sizes of the condition differences in the L2 group, the negativity increases during the first three time windows and then decreases after participants encountered the word preceding the expected gap (e.g. classes in example 5) at approximately 1700 ms. An interaction of the first three time windows as a continuous predictor with the factor Condition confirms this: F(2, 38) = 4.34, p = .02.
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b
… Peter /arranged/ some / classes / for it / aer / the / vacaon
Fig. 1 continued
Verb-driven Effects We examined ERPs following the presentation of the verb, the filler’s semantic licenser (Fig. 2a, b). Note that at this point, sentences in both the extraction and non-extraction conditions are still fully grammatical. In the time window from 350 to 500 ms after word onset we found a significant three-way interaction of Group × ROI × Condition: F(8,296)=2.46, p=.01. Subsequent per-group analyses revealed interactions of the factors ROI and Condition for both groups: L1 F(8, 144) = 3.15, adjusted p = .003; L2 F(8, 152) = 1.97, adjusted p = .05. Follow-up analyses showed a negativity for the extraction condition at posterior regions for both groups: mid-posterior for the L2 group F(1, 19) = 5.03, adjusted p = .039,
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a
Fig. 2 a Grand averages of the L2 group at the verb site for all ROIs. Extraction condition = dotted line. Negativity indicated by arrows. b Grand averages of the L1 group at the verb site for all ROIs. Extraction condition = dotted line. Negativity indicated by arrows
[d = 0.71] and right-posterior inferior for the L1 group F(1, 18) = 5.34, adjusted p = .04 [d = 0.75]. Additionally we found a significant negativity in the L2 data at the left anterior inferior region F(1, 19) = 6.93, adjusted p = .02. This negativity is most likely part of the sustained LAN reported above, which was present only in the L2 group. It is also likely that this difference caused the observed three-way interaction of ROI, Group and Condition.
Filled-gap Effects We then examined ERPs time-locked to the onset of the critical PP (i.e. for it / to it), comparing the (now ungrammatical) extraction condition to the non-extraction one. Mean amplitudes were extracted for each participant at each electrode site (Fig. 3a, b). During a time window of 600–800 ms after stimulus onset there was a main effect of Condition: F(1, 37) = 13.4, p < .001, with a more positive-going wave form for the extraction condition, a significant interaction of the factors ROI and Group: F(8, 296) = 2.79, p = .005, and of ROI and Condition: F(8, 296) = 5.55, p < .001, but no three-way interaction of ROI, Group and
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b
Fig. 2 continued
Condition. Step-down analyses by ROI revealed significant differences between the two conditions for both groups combined in the following ROIs: right anterior inferior: F(1, 37) = 9.93, adjusted p = .009 [d = 0.73]; right anterior superior: F(1, 37) = 12.75, adjusted p = .006 [d = 0.83]; left anterior superior: F(1, 37) = 13.39, adjusted p = .005 [d = 0.85]; left anterior inferior: F(1, 37) = 5.58, adjusted p = .02 [d = 0.55]; left posterior superior: F(1, 37) = 8.85, adjusted p = .01 [d = 0.69]; right posterior superior: F(1, 37) = 10.87, adjusted p = .006 [d = 0.77]. The ROI by Group interaction reported above, as well as visual inspection, indicated that the two groups differed in the distribution of the positivity, which was more frontal for the L1 but global for the L2 group, with the largest amplitude in posterior regions. The waves as well as the topographical maps (Fig. 4) also show between-group differences in amplitude, with lower amplitudes for the L1 group. The fact that both groups showed positivities for the extraction condition accounts for the absence of a three-way interaction of ROI, Group and condition. To examine possible between-group differences that had not given rise to a significant three-way interaction earlier, we carried out additional analyses for the two groups separately. These per-group analyses showed significant interactions of ROI and Condition in both groups: L1 F(8, 144) = 2.84, adjusted p = .006; L2 F(8, 152) = 3.62,
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a
Fig. 3 a Grand averages of the L2 group at the gap site for all ROIs. Extraction condition = dotted line. Positivity indicated by arrows. b Grand averages of the L1 group at the gap site for all ROIs. Extraction condition = dotted line. Positivity indicated by arrows
adjusted p = .002. Follow-up analyses revealed significant differences between the two conditions in both groups, with more positive-going waveforms for the extraction condition. The differences between the two conditions approached or reached significance for both groups in the right anterior superior and left anterior superior regions as well as the right anterior inferior region: L1: right anterior superior: F(1, 18) = 5.6, adjusted p = .06 [d = 0.77]; left anterior superior: F(1, 18) = 5.33, adjusted p = .06 [d = 0.75]; right anterior inferior: F(1, 18) = 8.13, adjusted p = .03 [d = 0.93]. L2: right anterior superior: F(1, 19) = 7.99, adjusted p = .04 [d = 0.89]; left anterior superior: F(1, 19) = 8.4, adjusted p = .04 [d = 0.92]; right anterior inferior: F(1, 19) = 4.7, adjusted p = .04 [d = 0.69]. Only the L2 group additionally showed significant differences at posterior regions: right posterior superiorF(1,19)=10.13, adjusted p = .024 [d = 1.01], left posterior superior F(1, 19) = 7.04, adjusted p = .04 [d = 0.84] and mid posterior F(1, 19) = 9.07, adjusted p = .04 [d = 0.95]. These results show that, as suspected, the groups differed regarding the scalp distribution of the condition effect but not in its direction: in both cases a positive-going wave form
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b
Fig. 3 continued
Fig. 4 Topographical maps illustrating the distribution of the positive-going ERPs observed at the filled gap for the L2 (left) and the L1 (right) group
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was seen for the extraction condition, which overlapped between the two groups in frontal areas—another possible explanation for the missing three-way interaction.
Discussion We used ERPs to investigate how L1 and L2 comprehenders process indirect object dependencies. We specifically examined whether gaps are predicted beyond the verb, whether L1 and L2 comprehenders show the same types of brain responses to filled gaps and whether these differ in their timing. Our ERP data revealed both similarities and differences between the two participant groups. Examining both local and clause-level ERPs, we found that sentences containing a wh-dependency elicited three different types of brain response relative to non-extraction control sentences. The first was a sustained LAN which was present in the L2 group only and which extended beyond the subcategorizing verb. The second was a posterior negativity that appeared after the presentation of the verb in both participant groups, and the third was a late positive-going deflection that was observed at the point at which the filled indirect object gap was encountered, with a broader distribution and higher amplitude for the L2 than for the L1 group. In the following, we will discuss each of these ERP effects in turn.
Storage Effects Sustained LAN effects have been argued to reflect the processing cost incurred by storing a fronted constituent in WM until the dependency can be completed. The long negativity found in the L2 group in the present study was more localized than, for example, the LAN reported by Fiebach et al. (2002), where it was present at most electrodes between 1000 and 3400 ms. In our study, the negativity lasted from 300 to 1900 ms after the embedded subject noun phrase (e.g. Peter) and until approximately 500ms after the word preceding the gap (e.g. classes) was encountered, and was restricted to the left anterior inferior ROI (i.e. F7, T7, FC5). Unlike in previous studies on direct object filled gaps (e.g. Hestvik et al. 2007, 2012), the design of our materials allowed us to examine whether and for how long the sustained negativity extended beyond the verb. The extended LAN seen in our L2 group indicates that displaced indirect objects were held in WM even after the verb had been processed. On the assumption that the LAN reflects the processing cost of filler storage, its presence in the L2 data indicates that our non-native comprehenders continued searching for a gap in order to have the filler’s thematic role confirmed. No sustained LAN was found for our native English-speaking control group, however. One possible explanation for this between-group difference is that L2 comprehenders kept the filler highly active in WM throughout the length of the dependency whereas L1 comprehenders mentally reactivated the filler at the point where a gap was expected. This interpretation would be in line with the position-specific (L1) versus across-the-board (L2) priming effects observed by Felser and Roberts (2007). Alternatively, it may be that our L1 comprehenders’ processing of indirect object dependencies was so highly automatized that effects of filler storage were barely detectable for this group. Recall also that the wh-fillers used in our study already contained information about the filler’s likely thematic role (for which, to which), which may have reduced its activation level in comparison to more ambiguous fillers. According to Gouvea et al. (2010), using PP fillers should have made it possible in principle for comprehenders to terminate dependency formation at the verb. Although this possibility cannot be ruled out for the L1 group, the presence of a sustained LAN extending beyond the verb in the L2 data is not consistent
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with this hypothesis. Unlike Gouvea et al. (2010), we also found no P600 effects (which the authors claim are indicative of creating syntactic relations) at the verb in either of our participant groups.
Verb-driven Effects Examining participants’ ERP responses to the subcategorizing verb—a point at which our extraction sentences were still fully grammatical—should reveal effects of semantic integration and preliminary thematic role assignment (compare Nicol 1993). Both participant groups showed a posterior negativity which reached significance in a time window from 350 to 500 ms after verb onset. According to its polarity, latency and distribution this negativity can clearly be identified as an N400, an ERP component which is typically (albeit not exclusively) associated with semantic integration difficulty (Kutas and Hillyard 1980). In studies on FGDs, verb-driven N400 effects have previously been observed in response to implausible direct object fillers (Dallas et al. 2013; Garnsey et al. 1989) and to fillers that do not meet the verb’s argument structure requirements (Phillips et al. 2005). Note that in all our experimental items an animal character (e.g. the monkey in 5a,b) served as the understood indirect object or Goal argument. Even though our participants were briefed about our stimulus items being intended for use with children and describing fairy-tale scenarios, this might have led to plausibility issues at the verb, many of which described an action that normally required the Goal argument to be human (e.g. confess, dictate, read). We therefore interpret the N400 as indicating an anomaly that results from an attempt to semantically integrate a pragmatically unusual filler with its subcategorizer in our extraction condition. No obvious between-group differences were found for the N400, except for a slightly more right-lateralized distribution in the L1 group. This finding is unsurprising since several previous ERP studies on L2 processing have reported native-like N400 brain responses to semantic anomalies, even for less proficient L2 speakers (e.g. Ardal et al. 1990; Hahne and Friederici 2001—but cf. Dallas et al. 2013). It is also in line with previous findings from behavioral and eye-movement studies showing that L2 comprehenders can semantically link a filler to a potential lexical licenser as quickly as native speakers. The absence of P600 effects at the verb might seem surprising given that several previous studies have reported such effects at the verb (e.g. Felser et al. 2003; Fiebach et al. 2002; Gouvea et al. 2010; Hagiwara et al. 2007; Phillips et al. 2005; Ueno and Kluender 2003). However, note that unlike in most of these studies, the predicted structural gap in our stimulus sentences was non-adjacent to the verb. On the assumption that a P600 can reflect the completion of a syntactic dependency at structural gap positions, which in our materials could only be accomplished after the direct object has been processed, the absence of P600 effects at the verb does not seem particularly surprising.
Filled-gap Effects At filled indirect object gaps, a fronto-central (for L1) or more globally distributed (for L2) positivity was seen which was significant in the 600–800 ms time window. Although its scalp distribution, at least for the L1 group, differs from the centro-parietal P600 effects that are typically seen in studies on morphosyntactic anomalies (e.g. Hagoort et al. 1993), garden-path sentences (Osterhout and Holcomb 1992) or on the processing of grammatical wh-dependencies (e.g. Felser et al. 2003; Fiebach et al. 2002; Kaan et al. 2000), the positivities observed in the current study may be interpretable as P600 responses. There was no evidence of the P600 effect being delayed in our L2 group.
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In both Hestvik et al.’s (2012) and Schremm’s (2012; 2013) studies (although not in Hestvik et al. 2007), encountering a filled direct object gap elicited a more typical posterior positivity of the kind that has frequently been associated with grammatical violations. Note that there is a crucial difference, however, between these authors’ materials and ours in that encountering a filled direct object gap in the above studies rendered the sentence both ungrammatical and uninterpretable. That is, the postverbal noun phrase the camel in a sentence like The zebra that the hippo kissed the camel on the nose ran far away adds a new argument which competes with the filler the zebra in that it is impossible for both of them to be legitimately integrated into the current sentence representation. In our materials, on the other hand, it is possible for the PP occupying the indirect object position to be integrated into the current sentence representation semantically if it is construed as being resumptive, with e.g. for it taking the filler for which as its antecedent. This possibility would render our ungrammatical filled-gap sentences globally interpretable. We should be cautious about interpreting the observed P600 effect as evidence that filled indirect object gaps were perceived as pure phrase-structure violations, however. Unlike the (E)LAN effects observed e.g. by Hestvik and colleagues (2007; 2012), the P600 is a comparatively late brain response which cannot easily be linked to any specific syntactic or semantic processes (Van Petten and Luka 2012). The monophasic positivity we observed was smaller in amplitude and distributed more fronto-centrally in our L1 group (compared to the larger and more global effect seen in the L2 group), which suggests that encountering a potentially resumptive filled gap disrupted processing less in L1 than in L2 comprehension. It may also indicate that the observed positivities index somewhat different processes (compare e.g. Friederici et al. 2002; Kaan and Swaab 2003). Frontal positivities have been linked, for example, to syntactic ambiguity resolution (Osterhout and Holcomb 1992), syntactic integration difficulty (Friederici et al. 2002), increased discourse complexity (Kaan and Swaab 2003) and disconfirmed lexical predictions (Van Petten and Luka 2012). Centro-parietal P600 components, on the other hand, have been associated with structural repair processes triggered by encountering downright grammatical violations (e.g. Friederici et al. 2002). The positivity that we saw in the L2 group, although globally visible, was more pronounced over posterior than over anterior regions and therefore more within the normal range of the latter kind of P600. As the differences between the two groups regarding amplitude and distribution were not confirmed by reliable three-way interactions, any such conclusions are merely speculative, though. Our L2 group ostensibly showing stronger effects regarding the positivity’s amplitude and distribution is surprising given that previous ERP studies on L2 sentence processing have often found P600 responses to syntactic anomalies to be smaller than native speakers’ or altogether absent, especially in late or less proficient L2 learners (see e.g. Bowden et al. 2013; Weber-Fox and Neville 1996). Recall that our L2 participants had started learning English during elementary school or the first year of high school and also had a relatively high proficiency level. For early or highly proficient learners, P600 effects in response to syntactic anomaly have frequently been reported in the L2 literature (e.g. Hahne 2001; Kotz et al. 2008), and Rossi et al. (2006) found an even stronger P600 response to category violations for highly proficient L2 speakers than for native speakers. Our results are thus in line with Rossi et al.’s (2006) findings as well as with those from fMRI studies which report increased activation of certain brain areas (e.g. Rüschemeyer et al. 2006) and more areas being engaged (Wartenburger et al. 2003) in L2 compared to L1 sentence processing. Neither of our participant groups showed a biphasic N400–P600 response to filled gaps of the kind elicited by argument structure violations in previous studies (e.g. Friederici and Frisch 2000). This is surprising on the assumption that subcategorization violations resulting
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from the presence of an unlicensed extra argument phrase constitute both a grammatical and semantic violation (Hagoort et al. 1993). N400 effects were also absent in both Hestvik and colleagues’ and Schremm’s (2012; 2013) earlier studies on filled direct object gaps, however. This may be due to the fact that in both ours and the above filled-gap ERP studies, the extra argument phrase was always semantically compatible with the preceding verb. Finally, note that finding both a sustained LAN indicative of filler storage and nativelike brain responses to filled gaps in our L2 group fails to support claims to the effect that L2 comprehenders have a reduced ability to predict in comparison to L1 comprehenders (e.g. Grüter et al. 2016). Given that our experimental sentences were potentially salvageable semantically by interpreting the extra PP as resumptive, the observation that our L2 group experienced more processing difficulty at this point than the L1 group might indicate that the L2 speakers had more difficulty ’rescuing’ the sentence by construing the two competing PPs as referring to the same event participant.
Conclusion The combination of ERP effects that we observed support the hypothesis that processing indirect object dependencies involves a two-step process. There is an initial attempt to integrate the filler into the verb’s lexical-semantic argument structure frame when the verb is encountered. The filler is held in WM until its thematic role can be confirmed, although in the current study statistically robust evidence of maintained filler activation was seen only in our non-native participants. Encountering an unexpected competitor phrase at the filler’s canonical position gave rise to P600 effects in both native and non-native comprehenders. Unlike what has been reported in previous L2 processing studies, we found no evidence of filled-gap effects being delayed in L2 relative to L1 processing. Taken together, our results indicate that proficient L2 comprehenders process indirect object wh-dependencies in a similar way to L1 comprehenders. L2 comprehenders appear to spend more processing resources than native ones on keeping a filler active in WM, and also find it harder to integrate an unexpected, redundant constituent into the current sentence representation. Acknowledgements We are grateful to Jennifer Meyer for programming assistance, and to Caroline Beese for help with the data collection.
Funding This study was funded by an Alexander-von-Humboldt Professorship to Harald Clahsen. Compliance with Ethical Standards Conflict of interest The authors declare that they have no conflict of interest.
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