Psychophysiology, 50 (2013), 1120–1132. Wiley Periodicals, Inc. Printed in the USA. Copyright © 2013 Society for Psychophysiological Research DOI: 10.1111/psyp.12125

Intentional forgetting reduces the semantic processing of to-be-forgotten items: An ERP study of item-method directed forgetting

WEN-JING LIN,a YU-CHING KUO,b TZU-LING LIU,a YI-JHONG HAN,a and SHIH-KUEN CHENGa a

Institute of Cognitive Neuroscience, National Central University, Jhongli City, Taiwan Department of Special Education, Taipei Municipal University of Education, Taipei, Taiwan

b

Abstract In two ERP experiments, we examined whether active inhibition is involved in intentional forgetting. Both experiments consisted of a nondirected-forgetting (nDF) and a directed-forgetting (DF) block. Participants were sequentially presented with a prime, an R/F (remember/forget) cue, and a target. Participants made lexical decisions to both the primes and targets (Experiment 1) or only to the targets (Experiment 2). They were also instructed to remember or to forget the primes in response to the R/F cues in the DF block but to ignore these cues in the nDF block. The N400 semantic priming effect was observed when comparing the ERPs elicited by semantically unrelated and related targets in the DF block. In comparison to the nDF block, the N400 effect was greatly reduced for targets preceded by F cues in the DF block. These findings suggest that semantic processing is reduced by the instruction to forget and active inhibition is involved in intentional forgetting. Descriptors: ERPs, Directed forgetting, N400, Semantic priming opposed to R cues (e.g., Fawcett & Taylor, 2008; Hourihan & Taylor, 2006; Taylor, 2005). Additionally, the neural signatures of active inhibition were observed when the encoding of TBF items was successfully terminated. A number of event-related potential (ERP) studies have linked a frontally distributed positivity elicited by F cues to the inhibitory process on TBF items (Cheng, Liu, Lee, Hung, & Tzeng, 2012; Hsieh, Hung, Tzeng, Lee, & Cheng, 2009; Paz-Caballero, Menor, & Jiménez, 2004), as this positivity correlated with whether TBF items were subsequently forgotten (Hauswald, Schulz, Iordanov, & Kissler, 2011; van Hooff & Ford, 2011). An fMRI study reported that the right inferior prefrontal gyrus, an area related to active inhibition (Aron, Robbins, & Poldrack, 2004; Hampshire, Chamberlain, Monti, Duncan, & Owen, 2010), was strongly activated when TBF items were successfully forgotten (Wylie, Foxe, & Taylor, 2008). The inhibitionrelated activities in the right frontal cortex for TBF items were also found to correlate negatively with activations in the medial temporal lobe (Rizio & Dennis, 2013), an area known to be essential for memory encoding (Scoville & Milner, 1957; Wagner et al., 1998). There is, however, evidence suggesting that TBF items might not be actively inhibited. Marks and Dulaney (2001) combined item-method directed forgetting and semantic priming to test the active inhibition account. In their experiment, each study trial consisted of the sequential presentations of a prime, an R/F cue, and a target. Participants made lexical decisions to both primes and targets upon their presentations. They were also instructed to memorize or forget the prime in response to the R/F cues. It was argued that the semantic priming effect, that is, faster responses to semantically related targets than unrelated ones, should be

In the item method of directed forgetting, a “remember” (R) or “forget” (F) instruction is presented after each study item. Participants are later asked to recognize or to recall all study items, regardless of whether an item was initially cued to be remembered (TBR) or forgotten (TBF). For both recognition and recall tests, TBF items are found to be less well remembered than TBR ones (Basden, Basden, & Gargano, 1993). To construe this “itemmethod directed forgetting effect,” the encoding difference account argues that TBR items are better remembered because they receive more elaborative processing after the presentation of R cues, whereas TBF items only receive maintenance rehearsal prior to the presentation of F cues (Gardiner, Gawlik, & Richardson-Klavehn, 1994; Woodward & Bjork, 1971; Woodward, Bjork, & Jongeward, 1973). The active inhibition account additionally argues that TBF items are actively inhibited after the presentation of F cues (Zacks, Radvansky, & Hasher, 1996). Supportive evidence for the involvement of active inhibition in item-method directed forgetting comes from behavioral studies showing performance reduction on attentional-demanding responses to stimuli that are concurrently presented with F cues as

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This research was supported by grants from Academia Sinica and National Science Council, Taiwan to Shih-kuen Cheng (NSC 98-2410-H008-011, NSC 98-2517-S-004-001, NSC 101-2410-H-008-034, AS-102TP-C06) and Yu-Ching Kuo (NSC 99-2410-H-133-010). Address correspondence to: Shih-kuen Cheng, Institute of Cognitive Neuroscience, National Central University, No. 300, Jhongda Rd., Jhongli City, Taoyuan County 32001, Taiwan. E-mail: [email protected] 1120

ERPs and directed forgetting attenuated if F cues led to active inhibition on the TBF primes. Nevertheless, equivalent semantic priming effect was found for targets preceded by R and F cues. The null effect of Marks and Dulaney (2001) does not necessarily suggest that TBR and TBF items receive equivalent semantic processing. As noted by Neely (1991), there are multiple mechanisms underlying semantic priming. The semantic priming effects elicited by the TBR and TBF primes in Marks and Delaney’s study could involve nonequivalent mechanisms that reaction time was not sensitive enough to detect (Heil, Rolke, & Pecchinenda, 2004; Neely & Kahan, 2001; Rolke, Heil, Streb, & Hennighausen, 2001). Cheng et al. (2012) tested this possibility by incorporating ERP recordings into the procedure of Marks and Dulaney (2001). The N400 wave (Kutas & Hillyard, 1980), a negative deflection peaking at around 400 ms after stimulus onset over the central-parietal scalp and known to be sensitive to semantic processing (Kutas & Federmeier, 2000; Kutas & Hillyard, 1984), elicited by the targets was used to examine whether TBR and TBF primes were processed to different levels of representations. They found that targets preceded by the two types of cues yielded N400 effects that were of different topographical distributions. They concluded that intentional forgetting could result in different semantic processing than remembering. One caveat of this conclusion was that the different ERP effects elicited by the targets preceded by R and F cues could reflect selective rehearsal of the TBR items rather than inhibition of the TBF items. A prime might have been more elaborated following the presentation of R cues, having received semantic processing qualitatively different from that for TBF primes, hence yielding different patterns of N400 effect. The current study clarified this caveat by including a nondirected forgetting (nDF) block prior to the directed forgetting (DF) task. During the nDF block, participants were instructed to make lexical decisions to both words of prime-target pairs (Experiment 1) or only to the targets (Experiment 2) but were not informed about the meanings of R/F cues. They were instructed to memorize (Experiment 1) or received no mnemonic instructions (Experiment 2) to the primes. We predicted that, in comparison to targets in the nDF block, there should be a reduced N400 effect for targets preceded by F cues in the DF block if TBF primes were inhibited. In addition to the targets, we also examined the ERPs elicited by the R/F cues because a posterior positivity elicited by R cues has been consistently reported (e.g., Hauswald et al. 2011; Hsieh et al., 2009; van Hooff & Ford, 2011). Other experiments include larger late positive potentials for words later recalled or recognized than those not remembered, and the authors have attributed the enhanced positivity to greater attention or more extensive memory reorganization when the words are initially presented, both leading to superior encoding (Azizian & Polich, 2007; Fabiani & Donchin, 1995; Kok, 2001; see also Van Petten & Senkfor, 1996). The resemblance between the R versus F cue effect and the subsequent memory effect for the actual items on a study list may suggest similar explanations. However, it is unclear whether F cues similarly elicit the P3b-like activity and how to relate the positive waves associated with R/F cues to the processing of TBR/TBF items. We therefore compared the ERPs elicited by the cues in the DF and nDF blocks. The R and F cues in the nDF block should not differ if participants indeed ignored the R/F signs. On the other hand, R cues in the DF block should yield the P3b-like activity when compared with F cues. We also hypothesized that F cues in the DF block would elicit the P3b-like activity when compared with cues in the nDF block if F cues in the DF block elicited active processing

1121 of the TBF primes. The processes elicited by F cues, however, should not be identical to those elicited by R cues. Although both types of cues provide information about how items held in working memory should be processed, R cues rendered the items relevant to encoding and initiated elaboration of TBR items. F cues should make the items irrelevant and signal that they should be removed from working memory (see Donchin & Coles, 1988, for the proposal that the P3b indexes the updating of working memory). This hypothesis was tested by comparing the topographies of the P3blike waves elicited by the R/F cues and how their magnitudes relate to the N400 effect elicited by the targets as well as the memory performance of the TBR and TBF items. Experiment 1 Method Participants. Thirty college students (aged between 18 and 24) participated in the experiment. All participants were right-handed, native Mandarin Chinese speakers with normal or corrected-tonormal vision. They were paid at the rate of 250 New Taiwan Dollars ($8.50 US) per hour. The experiments were approved by the local ethical committee, and written consent was obtained from all participants. Data from eight participants were excluded because they contributed insufficient (< 16) valid ERP trials to at least one critical experimental condition. Among the remaining 22 participants, 9 were females. Stimuli. The critical stimuli were 180 word triplets, each consisting of a prime, a semantically related target, and an unrelated target. The primes were Chinese two-character nouns selected from the Academia Sinica balanced corpus (Huang & Chen, 1998) with the mean frequency of 256 per million. Both related and unrelated targets were also two-character nouns. The related targets were selected from the free association reactions to the primes obtained from 20 undergraduate students who did not participate in the experiment. The unrelated targets were selected from the corpus and were not found in the free association reactions to the primes. There was no significant difference, t (179) = .07, p = .94, between the mean frequencies of the related and unrelated targets (494 and 495 per million, respectively). Among the 180 triplets, 60 were used as stimuli in the nDF block and 120 in the DF block. In both blocks, the triplets were evenly assigned to one of the following four conditions, in which the prime was followed by an R cue and the related target (hereafter abbreviated to “wRr”), an R cue and the unrelated target (“wRu”), an F cue and the related target (“wFr”), or an F cue and the unrelated target (“wFu”). The trial numbers for each of the four types of stimuli were 15 and 30, respectively, in the nDF and DF blocks. The allocations of the triplets to the four conditions and to the nDF/DF blocks were counterbalanced across participants. In addition to the critical stimuli, the DF block contained 20 “wRn” pairs (the prime was followed by an R cue and a nonword target), 20 “wFn” pairs (the prime was followed by an F cue and a nonword target), 40 nonword-real word pairs, and 40 nonwordnonword pairs (no R/F cues for the latter two pair types). The corresponding numbers in the nDF blocks were 10 for wRn, 10 for wFn, 20 for nonword-real word, and 20 for nonword-nonword pairs. The nonwords were generated by rearranging real twocharacter words that were not used in the experiment. None of the nonwords were listed in the corpus. There were therefore 360 trials across the experiment with 120 in the nDF block and 240 in the DF

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Figure 1. The structure of study trials in Experiments 1 and 2.

block. The R/F cues were delivered in trials whose primes were real words. The presentation orders of the 120 trials in the nDF block and the 240 trials in the DF block were randomly assigned for each participant. Procedure. Participants engaged in two study-test blocks, with a study phase followed by a test phase in each block. At study, participants made lexical decisions to pairs of primes and targets. Each study trial (see Figure 1) started with a cross fixation shown on the center of the screen for 500 ms, followed by the presentation of the prime. The prime was displayed for 1,000 ms, during which participants judged whether the item was a real word or a nonword. The prime was then replaced by an R cue (*RRRRRR*) or an F cue (*FFFFFF*) when it was a real word, and by a neutral cue (********) when it was a nonword. The duration of the cue was 500 ms, after which the screen went blank for 250 ms. The target was then presented for 1,000 ms, during which participants made the second lexical decision. Immediately after the target item, a neutral cue (********) was shown for 1,500 ms followed by a blank screen of 500 ms. Responses of the lexical decisions were made by pressing one of two response keys with the index finger of each hand. The mapping of the hand to response category (word vs. nonword) was counterbalanced across participants. Before the study phase of the first block, participants were instructed to memorize the real-word primes and were told that the R/F cues were irrelevant to the task. In the second block, participants were instructed to remember the real-word primes when an R cue was presented but to forget the primes if it was an F cue. They were also told that nonwords and targets in the lexical decision task would not be included in the subsequent memory test. After the study phase, participants engaged in a backward counting task for 5 min. They were later instructed to recall the real-word primes. In the DF block, it was emphasized that participants should report all primes they could remember whether the word was followed by an R cue or an F cue in the lexical decision task. The recall test was conducted by writing down the words on a sheet of paper. There was no time limit for the recall test. The experiment always started with the nDF block followed by the DF block because otherwise participants might have doubts on the instruction of ignoring the R/F cues in the nDF block. There was a 15-min break between the two blocks. ERP recording. Electroencephalography (EEG) was continuously recorded during the study phase from 64 Ag/AgCl electrodes, 62 of which were embedded in an elastic cap (Quick-Cap, Neuromedical Supplies, Sterling, TX). The remaining two electrodes were placed on the right and left mastoids. All channels were referenced to a channel located between Cz and CPz, and were rereferenced offline to the average of the two mastoids. A ground

electrode was placed on the forehead anterior to the Fz electrode. Vertical and horizontal electrooculograms (EOG) were recorded bipolarly from electrodes placed above and below the right eye and on the outer canthi of each eye, respectively. Data were sampled at 256 Hz and digitized with 24-bit resolution. All channels were amplified by SYNAMPS2 (Neuroscan, Inc., El Paso, TX) with a band-pass of 0.05–70 Hz (3 dB points). Interelectrode impedance was kept below 5 kΩ. ERPs were computed for epochs of 850 ms for the R/F cues and 1,100 ms for the targets in the lexical decision task, both containing a 100-ms prestimulus baseline period. Linear regression was used to estimate and correct the contribution of blink artifacts to the EEG (Semlitsch, Anderer, Schuster, & Presslich, 1986). Trials containing horizontal eye movement, nonblink vertical eye movement, A/D saturation, or with a baseline drift exceeding 70 microvolts in any channel were rejected. A low-pass filter with cutoff frequency at 30 Hz was applied to the epoched data. Behavioral Results For both the behavioral and ERP results reported below, the Greenhouse-Geisser correction for nonsphericity was applied when necessary. F ratios are reported with Greenhouse-Geisser epsilon values (ε) and adjusted p levels. Trials with lexical decision times longer or shorter than two standard deviations from the means of each individual participant in each condition were removed prior to the behavioral and ERP analyses of lexical decisions to the targets. Free recall of the primes. Table 1 displays the recall rates of the primes. A repeated measures analysis of variance (ANOVA) employing the factors of directed forgetting instruction (DF instruction, nDF vs. DF), R/F cue (remember vs. forget), and target type (related vs. unrelated) found a significant interaction between DF instruction and R/F cue, F(1,21) = 22.65, p < .001. Primes

Table 1. Mean Proportions (SD) of Correct Recall of the Primes and Intrusions of Targets in Experiment 1 Correct recall of primes

Intrusion of targets

Prime-target relationship R/F cue

Related

Remember Forget

.14 (.11) .16 (.14)

Remember Forget

.25 (.16) .03 (.04)

Unrelated Related Nondirected forgetting block .08 (.08) .06 (.07) .07 (.07) .05 (.05) Directed forgetting block .23 (.17) .03 (.06) .01 (.03) .01 (.02)

Unrelated .01 (.01) .01 (.01) .01 (.01) .01 (.01)

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Table 2. Mean Response Latencies (ms) in the Lexical Decisions to Targets in Experiment 1 Prime-target relationship R/F cue

Related

Remember Forget

Nondirected forgetting block 546 (72) 577 (69) 547 (66) 576 (71)

Unrelated

Remember Forget

619 (100) 579 (71)

Directed forgetting block 648 (89) 604 (77)

Priming 31 29 29 25

Note. SD are in parentheses.

followed by R cues were better recalled than those followed by F cues in the DF block, t(21) = 6.06, p < .001, but not in the nDF block, t(21) = −.343, p = .735, confirming that the directed forgetting effect was observed in the DF block only. Taking another perspective on the interaction effect, the recall rate for primes followed by R cues was higher in the DF block than that in the nDF block, t(21) = 3.46, p = .002. In contrast, the recall rate for primes followed by F cues was lower in the DF block than that in the nDF block, t(21) = −4.36, p < .001. Intrusion of targets in the free recall of primes. The analysis on the intrusion rates of targets (see Table 1) found a significant main effect of target type, F(1,21) = 18.59, p < .001, reflecting more intrusions for related targets than unrelated ones. The main effect of DF instruction was also significant, F(1,21) = 7.32, p = .013, reflecting a higher intrusion rate in the nDF block than in the DF block. Reaction times of lexical decisions to the targets. Table 2 displays the reaction times of correct trials in the lexical decisions to the targets. An ANOVA employing the factors of DF instruction, R/F cue, and target type showed that the main effect of target type was significant, F(1,21) = 62.09, p < .001, reflecting the semantic priming effect that lexical decision was faster for related targets than unrelated ones. The main effect of DF instruction was significant, F(1,21) = 26.13, p < .001, as was its interaction with R/F cue, F(1,21) = 17.15, p < .001. Follow-up analyses found that, in the DF block, the reaction time was longer for targets preceded by R cues than those preceded by F cues, t(21) = 5.5, p < .001. In the nDF block, there was no significant difference between the reaction times for targets preceded by the two types of cues, t(21) = −.17, p = .871. No other interaction effects were significant, revealing no evidence that the semantic priming effect was modulated by the R/F cues in either the nDF or the DF block. Accuracy rates of lexical decisions to the targets. The accuracy rates of lexical decisions to the target items are displayed in Table 3. The main effect of DF instruction was significant, F(1,21) = 29.86, p < .001, reflecting a higher accuracy rate in the nDF block than in the DF block. The main effect of target type was also significant, F(1,21) = 19.27, p < .001, revealing that related targets received more correct lexical decision judgments than unrelated ones.

in Figure 2. The mean trial numbers (range in brackets) were 26 (20–28) and 24 (21–27) for R and F cues in the nDF blocks and 51 (42–56) and 52 (47–55) in the DF block. ERPs were quantified by measuring the mean amplitudes of 250–600 ms time period, roughly corresponding to the time windows used in our previous study (Hsieh et al., 2009). A repeated measures ANOVA employing the factors of DF instruction and R/F cue was conducted on the data from 27 electrode sites over nine scalp regions: left anterior (F7, F5, F3), medial anterior (F1, Fz, F2), right anterior (F4, F6, F8), left central (T7, C5, C3), medialcentral (C1, Cz, C2), right central (C4, C6, T8), left posterior (P7, P5, P3), medial-posterior (P1, Pz, P2), and right posterior (P4, P7, P8). These electrodes were categorized into three levels of caudality (anterior, central, and posterior) and laterality (left, medial, and right). The main effects of DF instruction and R/F cue were both significant, F(1,21) = 35.34, p < .001 and F(1,21) = 9.37, p < .001, respectively. Importantly, the interaction between DF instruction and R/F cue was significant, F(1,21) = 15.79, p = .001, as was the four-way interaction between DF instruction, R/F cue, laterality, and caudality, F(4,84) = 3.37, p = .025, ε = .594. Follow-up analyses in the nDF block found no significant effects involving the factor of R/F cue (all Fs < 1). In the DF block, the main effect of R/F cue was significant, F(1,21) = 23.1, p < .001, as were its interactions with laterality, F(2,42) = 20.99, p < .001, ε = 0.956, and with caudality, F(2,42) = 11.87, p = .001, ε = 0.592. The waveforms associated with R cues were more positive-going than those associated with F cues over the central and posterior scalp regions, F(1,21) = 20.34, p < .001, and F(1,21) = 47.71, p < .001. Over the anterior sites, the effect of R/F cue was not significant. Nevertheless, in a more limited temporal interval (450–600 ms) where previous studies (Cheng et al., 2012; Hsieh et al., 2009; Paz-Caballero et al., 2004) reported a positivity associated with F cues, the waveforms associated with F cues were more positive than R cues over the anterior sites, F(1,21) = 5.43, p = .03. A topographical analysis was conducted to compare the distributions of the DF instruction effects for the R and F cues (see upper panel of Figure 3). Two sets of difference waveforms, respectively, for the R and F cues, were obtained by subtracting the waveforms associated with R/F cues in the nDF block from those in the DF block. The mean amplitudes of the difference waveforms in the 250–600 ms time windows were range-normalized with the maxmin method (McCarthy & Wood, 1985). The range-normalized data were then analyzed with the factors of R/F cue and recording sites (62 levels, i.e., all the scalp electrodes). The interactions between R/F cue and recording sites were significant, F(61,1281) = 4.89, p = .002, ε = .059. A separate analysis on the 27 sites used in the waveform analysis showed that the R/F Cue × Caudality interaction was significant, F(2,42) = 4.36, p = .038, ε = .925, as was the interaction between R/F cue, caudality, and laterality, F(4,84) = 4.42, p = .012, ε = .605. Subsidiary analyses found a significant R/F

Table 3. Mean Accuracy Rates (SD) in the Lexical Decisions to Targets in Experiment 1 Prime-target relationship Nondirected forgetting block

ERP Results ERPs elicited by the R/F cues.ERPs elicited by the R/F cues were separately averaged for the DF and nDF blocks, as shown

R/F cue Remember Forget

Related .96 (.08) .96 (.06)

Unrelated .94 (.08) .92 (.08)

Directed forgetting block Related .86 (.11) .94 (.06)

Unrelated .84 (.11) .90 (.07)

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F5

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P5

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F6

100 200 300 400 500 600 700ms

P6

R cues in the nDF Block

R cues in the DF Block

F cues in the nDF Block

F cues in the DF Block

Figure 2. Grand-average ERPs elicited by the R/F cues in Experiment 1.

Cue × Caudality interaction over lateral sites, F(2,42) = 7.39, p = .009, ε = .591, reflecting that the positivity associated with F cues was more pronounced and widespread over anterior scalp region whereas the positivity for R cues was greater over the posterior scalp region.

Rcue(DF)-Rcue(nDF)

ERPs elicited by the targets. ERPs elicited by the targets in the DF block were averaged for the wRr, wRu, wFr, and wFu trials with the mean trial numbers of 23 (16–29), 22 (17–29), 24 (16–30), and 23 (16–28), respectively. In the nDF block, because there would be insufficient valid trials if ERPs elicited by related and

Fcue(DF)-Fcue(nDF)

Exp. 1

Exp. 2

Figure 3. The topographies of the positive-going effects elicited by the R and F cues, derived by subtracting the waveforms associated with the cues in the nDF block from those in the DF block, in the 250–600 ms time window of Experiments 1 (upper) and 2 (lower).

ERPs and directed forgetting

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CZ

CZ

PZ

PZ

PZ

–100

FZ

FZ

FZ

related targets unrelated targets

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300

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nDF Block

900 ms

related targets preceded by F cues unrelated targets preceded by F cues related targets preceded by R cues unrelated targets preceded by R cues

targets in the nDF block targets preceded by F cues in the DF block targets preceded by R cues in the DF block

Figure 4. Grand-average ERPs elicited by related and unrelated target words in the nDF block (left), DF block (middle), and their difference waves (right) generated by subtracting the waveforms associated with unrelated targets from those associated with related ones in Experiment 1.

unrelated targets were separately averaged for both R and F cues, the ERPs associated with wRr and wFr trials were collapsed into one category and those associated with wRu and wFu trials into another. The trial numbers were 25 (18–30) and 24 (17–29) for these two collapsed categories. The grand-average ERP waveforms elicited by targets are shown in Figure 4. ERPs were quantified by measuring the mean amplitudes of 300–550 ms time periods. A repeated measures ANOVA was conducted on the 27 sites listed above with the factors of R/F cue (R, F, and nDF), target type, laterality, and caudality. The main effect of target type and its interaction with laterality were significant, F(1,21) = 38.56, p < .001, and F(2,42) = 11.55, p < .001, ε = .97, suggesting that the waveforms elicited by unrelated targets were more negative-going than those elicited by related ones, and this negative-going effect was most pronounced over the midline areas. Importantly, there was a significant interaction between target type and R/F cue, F(2,42) = 3.87, p = .03, ε = .89. Subsidiary analyses showed that the effect of target type was significant for targets in the nDF block, F(1,21) = 30.76, p < .001, and those preceded by R cues in the DF block, F(1,21) = 22.86, p < .001. There was, however, no difference between related and unrelated targets in the DF block when the targets were preceded by F cues, F(1,21) = 2.74, p = .11. Correlation analysis. Correlational analyses were conducted to examine how the processing depth of the primes relates to the R/F instructions in the DF block. As shown in the upper panel of Figure 5, the magnitude of the positive effect elicited by the R cues over the midposterior region (P1, Pz, and P2 sites), derived by subtracting the R cue waveforms in the nDF block from those in the DF block, was positively correlated with the magnitude of the N400 effect elicited by targets preceded by R cues over the midcentral scalp region (C1, Cz, and C2 sites), which was derived by subtracting the waveforms associated with unrelated targets from those

associated with related ones (r = .49, p = .022). In contrast, the magnitude of the positive-going effect elicited by the F cues in the DF block was negatively correlated with the N400 effect elicited by targets preceded by F cues. This negative correlation was observed not only over the midposterior scalp region (r = −.49, p = .022) but also in the left anterior sites (F7, F5, and F3 sites; r = −.44, p = .041). We also examined whether the effects of DF instructions on the memory performance for the primes, defined by subtracting the recall rates for TBR and TBF primes in the DF block from those in the nDF block, correlate with the magnitude of the positive-going effects elicited by the R/F cues in the DF block. As shown in the lower panel of Figure 5, the magnitude of the positive-going effects elicited by the R cues over the midposterior scalp region (P1, Pz, and P2 sites) positively correlated with the effect of R cues on the recall of the primes in the DF block (r = .5, p = .017). In contrast, the magnitude of the positive-going effect elicited by the F cues over the left anterior scalp region (Fz, F5, and F3 sites) was negatively correlated with the recall performance of TBF items in the DF block (r = −.6, p = .003). Experiment 2 The inclusion of an nDF block was aiming to clarify whether the different N400 effect between preceded R and F cues was due to enhanced semantic processing of the TBR primes or reduced processing of TBF ones. Nevertheless, the nDF block in Experiment 1 might not be an appropriate baseline to which the modulation of intentional forgetting on the semantic processing of TBF items could be compared. Firstly, although participants were not informed about the meanings of the R/F cues in the nDF block, they were instructed to memorize the primes, which effectively made all primes in the nDF block TBR items. Secondly, following the design of Marks and Dulaney (2001), participants also made lexical

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F cue

Effects of the R/F cues on the recall rates of the primes in the DF block in comparison to those in the nDF block

Magnitude of the N400 effect (uV) during the 300-550 ms time window over the middle central (C1, Cz, C2) scalp region

R cue

R cue (left) and F cue (left) amplitude differences (uV) between DF and nDF blocks over the middle posterior (P1, Pz, P2) scalp region in 250-600 ms time window.

Figure 5. Upper: The relationship between the magnitude of the positive-going effect elicited by the R/F cues and the magnitude of the N400 effect elicited by the targets in the DF block of Experiment 1. Lower: The relationship between the magnitude of the positive-going effect elicited by the R/F cues and the magnitude of the effect of directed-forgetting instructions on the recall performance of the primes in Experiment 1.

decisions to the primes. These two designs might have resulted in elaborative processing of the primes and made the N400 effect observed in the nDF block a “raised” baseline condition. Finally, the trial numbers in the nDF block was half of that in the DF block. The unequal trial numbers in the two blocks might compromise the interpretation of the recall data. Additionally, the targets preceded by R and F cues in the nDF block were not separated for the N400 analysis mainly because the trials in each condition were too few to expect clean ERPs. These issues were addressed in Experiment 2 in which participants were not asked to memorize primes in the nDF block. In addition, participants were not required to make lexical judgments to the primes in either the nDF or DF blocks. We also included more stimuli to ensure that both nDF and DF blocks contained equal and sufficient trials based on which unequivocal interpretation of the recall and ERP data could be drawn. Method Participants. Nineteen college students (aged between 18 and 24) participated in the experiment. Data from three participants were excluded from all analyses because they contributed insufficient (< 16) valid ERP trials to at least one critical experimental condition. Among the remaining 16 participants, 10 were females.

Stimuli. The 180 word triplets used in Experiment 1 and another 60 triplets were used in Experiment 2. The critical stimuli were therefore 240 triplets, with 120 used in the nDF block and the other 120 in the DF block. In both blocks, the 120 triplets were evenly assigned to the conditions of wRr, wRu, wFr, and wFu. The trial numbers for each of the four conditions were 30 in both the nDF and DF blocks. The allocations of the triplets to the four conditions and to the nDF/DF blocks were counterbalanced across participants. Both blocks also contained 20 wRn, 20 wFn, 40 nonword-real word, and 40 nonword-nonword pairs. There were 480 trials across the experiment with 240 in the nDF block and 240 in the DF block. Procedure. The procedure was similar to Experiment 1, but participants were not aware of the upcoming recall test for the primes in the nDF block. The experiment started with the nDF block followed by the DF block with a 15-min break in between. Prior to the study phase of the nDF block, participants were simply told that it was a lexical decision task. They were required to pay attention to the prime and make lexical decisions to the target items. There was no mnemonic instruction for the primes. The recall of the primes came as a surprise test after the lexical decision task. In the study phase of the DF block, however, participants were instructed to remember or to forget real-word primes upon the presentations

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Table 4. Mean Proportions (SD) of Correct Recall of the Primes and Intrusions of Targets in Experiment 2 Correct recall of primes

Table 6. Mean Accuracy Rates (SD) in the Lexical Decisions to Targets in Experiment 2

Intrusion of targets

Prime-target relationship

Prime-target relationship R/F cue

Related

Remember Forget

.03 (.03) .06 (.07)

Remember Forget

.15 (.09) .02 (.02)

Unrelated

Related

Nondirected forgetting block .01 (.01) .05 (.05) .01 (.01) .08 (.08) Directed forgetting block .13 (.09) .04 (.06) .01 (.01) .01 (.02)

Nondirected forgetting block Unrelated .03 (.03) .04 (.02) .01 (.01) .01 (.01)

of the R and F cues. EEG was recorded, processed, and epoched in the same way as in Experiment 1. Behavioral Results Free recall of the primes. Table 3 displays the recall rates of the primes. The interaction between DF instruction and R/F cue was significant, F(1,15) = 40.06, p < .001, revealing that primes followed by R cues were better recalled than those followed by F cues in the DF block, t(15) = 6.6, p < .001, but not in the nDF block, t(15) = −1.273, p = .22. Taking another perspective on the interaction effect, the recall rate for primes followed by R cues was higher in the DF block than that in the nDF block, t(15) = 7.2, p < .001. The recall rate for primes followed by F cues was marginally lower in the DF block than that in the nDF block, t(21) = −2.14, p = .048. When compared with Experiment 1, the recall rate of the primes in the nDF block was lower in the current experiment, t(36) = −3.9, p < .001. The directed forgetting effect in the DF block was also of a smaller magnitude, t(36) = −2.02, p = .05, due to the fact that fewer TBR items were recalled in comparison to Experiment 1, t(36) = −2.3, p = .03. Intrusion of targets in the free recall of primes. The analyses on the target intrusion rates (see Table 4) found a significant main effect of target type, F(1,15) = 5.51, p = .03, reflecting a higher intrusion rate for related targets as opposed to unrelated ones. The main effect of DF instruction was also significant, F(1,15) = 21.58, p < .001, reflecting more intrusions in the DF block than in the nDF block. Reaction times of lexical decisions to the targets. The analyses on the lexical decision times (see Table 5) found a significant main

Table 5. Mean Response Latencies (ms) in the Lexical Decisions in Experiment 2 Prime-target relationship R/F cue Remember Forget Remember Forget

Related

Unrelated Nondirected forgetting block 518 (45) 578 (49) 511 (42) 580 (48) Directed forgetting block 650 (63) 669 (56) 633 (56) 654 (59)

Note. SD are in parentheses.

Priming 60 69 20 21

R/F cue Remember Forget

Related .99 (.01) .99 (.01)

Unrelated .94 (.04) .96 (.04)

Directed forgetting block Related .98 (.05) .95 (.05)

Unrelated .93 (.07) .92 (.10)

effect for DF instruction, F(1,15) = 122.27, p < .001, reflecting faster lexical decisions to targets in the nDF block than in the DF block. The main effect of target type was significant, F(1,15) = 85.47, p < .001, revealing the semantic priming effect of faster lexical decisions to related targets than unrelated ones. The interaction between DF instruction and target type was also significant, F(1,15) = 32.49, p < .001, suggesting that the semantic priming effect was greater in the nDF block. No other interaction effects were significant, revealing no evidence that the semantic priming effect was modulated by the R/F cue in either the nDF or the DF block. Accuracy rates of lexical decisions to the targets. The accuracy rates of lexical decisions are displayed in Table 6. The main effect of target type was significant, F(1,15) = 39.46, p < .001, reflecting that the accuracy rate was higher for related targets as opposed to unrelated ones. The interaction between DF instruction and R/F cue, F(1,1) = 4.96, p = .04, was significant, suggesting that the accuracy of lexical decision was modulated by the R/F cues to a greater extent in the DF condition than in the nDF condition. ERP Results ERPs elicited by the R/F cues. The grand-average waveforms elicited by the R/F cues are displayed in Figure 6. The mean trial numbers (range in brackets) for R and F cues were 51 (35–59) and 48 (36–56) in the nDF block and 47 (25–58) and 45 (36–55) in the DF block. The analysis in the 250–600 ms time window found a significant interaction between DF instruction and R/F cue, F(1,15) = 13.36, p = .002. There were no significant effects involving the factor of R/F cue in the nDF block (all Fs < 1). In the DF block, the main effect of R/F cue was significant, F(1,15) = 23.78, p < .001, as was its interaction with laterality, F(2,30) = 8.49, p = .002, ε = 0.94, and the three-way interaction between R/F cue, caudality, and laterality, F(4,60) = 3.35, p = .032, ε = 0.68, reflecting the positivity associated with R cues as opposed to F ones over the midcentral and midposterior recording sites. Similar to Experiment 1, in the 450–600 ms time window, F cue-eliciting waveforms tended to be more positive-going than R cues over the left anterior scalp region (F7, F5, and F3 sites; F(1,15) = 4.1, p = .06). The topographic analysis, similar to Experiment 1, found a significant interaction between R/F cue and recording site, F(61,915) = 2.94, p = .02, ε = .079. A separate analysis on the 27 sites used in the waveform analysis found a significant R/F Cue × Caudality interaction over left scalp regions, F(2,30) = 5.17, p = .018, ε = .812, reflecting that the positivity associated with F cues was more pronounced and widespread over anterior scalp region whereas the positivity for R cues was greater over the posterior scalp region (see Figure 3, lower panel). ERPs elicited by the targets. Figure 7 shows the waveforms elicited by the targets. Different from Experiment 1, ERPs elicited

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F6

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+5 uV –100 0

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R cues in the nDF Block

R cues in the DF Block

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Figure 6. Grand-average ERPs elicited by the R and F cues in Experiment 2.

the nDF block. The corresponding numbers were 26 (19–28), 23 (18–29), 24 (18–28), and 23 (17–29) in the DF block. The N400 semantic priming effect was analyzed with a repeated measures ANOVA with the factors of DF instruction, R/F cue,

by the target items of the wRr, wRu, wFr, and wFu trials were separately averaged for both the DF and nDF blocks. The mean trial numbers (range in brackets) were 27 (21–30), 26 (21–29), 26 (19–30), and 25 (20–30) for the wRr, wRu, wFr, and wFu trials in

Difference Waves

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related targets preceded by F cues unrelated targets preceded by F cues related targets preceded by R cues unrelated targets preceded by R cues

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related targets preceded by F cues unrelated targets preceded by F cues related targets preceded by R cues unrelated targets preceded by R cues

targets preceded by F cues in the nDF block targets preceded by R cues in the nDF block targets preceded by F cues in the DF block targets preceded by R cues in the DF block

+5 uV –100

100

300

500

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Figure 7. Grand-average ERPs elicited by related and unrelated target words in the nDF block (left), DF block (middle), and their difference waves (right) generated by subtracting the waveforms associated with unrelated targets from those associated with related ones in Experiment 2.

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F cue

Effects of the R/F cues on the recall rates of the primes in the DF block in comparison to those in the nDF block

Magnitude of the N400 effect (uV) during the 300-550 ms time window over the middle central (C1, Cz, C2) scalp region

R cue

R cue (left) and F cue (left) amplitude differences (uV) between DF and nDF blocks over the middle posterior (P1, Pz, P2) scalp region in 250-600 ms time window.

Figure 8. Upper: The relationship between the magnitude of the positive-going effect elicited by the R/F cues and the magnitude of the N400 effect elicited by the targets in the DF block of Experiment 2. Lower: The relationship between the magnitude of the positive-going effect elicited by the R/F cues and the magnitude of the effect of directed-forgetting instructions on the recall performance of the primes in Experiment 2.

target type, caudality, and laterality. The three-way interaction between DF instruction, R/F cue, and target type was significant, F(1,15) = 6.98, p = .02. Separate ANOVAs were conducted in the nDF and DF blocks. In the nDF block, the main effect of target type was significant, F(1,15) = 57.27, p < .001, as was its interactions with caudality, F(2,30) = 6.36, p = .02, and with laterality, F(2,30) = 12.11, p =

Intentional forgetting reduces the semantic processing of to-be-forgotten items: an ERP study of item-method directed forgetting.

In two ERP experiments, we examined whether active inhibition is involved in intentional forgetting. Both experiments consisted of a nondirected-forge...
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