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Modality-independence of order coding in working memory: Evidence from cross-modal order interference at recall a

André Vandierendonck a

Ghent University, Department of Experimental Psychology, Henri Dunantlaan 2, 9000 Ghent, Belgium Accepted author version posted online: 24 Mar 2015.

Click for updates To cite this article: André Vandierendonck (2015): Modality-independence of order coding in working memory: Evidence from cross-modal order interference at recall, The Quarterly Journal of Experimental Psychology, DOI: 10.1080/17470218.2015.1032987 To link to this article: http://dx.doi.org/10.1080/17470218.2015.1032987

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Publisher: Taylor & Francis & The Experimental Psychology Society Journal: The Quarterly Journal of Experimental Psychology DOI: 10.1080/17470218.2015.1032987 Modality-independence of order coding in working memory: Evidence from cross-modal order interference at recall

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Address Corresponding Author: André Vandierendonck

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Running Head: Modality-independence of order coding

Henri Dunantlaan 2

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9000 Ghent, Belgium.

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Ghent University , Department of Experimental Psychology

Author Note

This research was supported by Grant B/10451 of Ghent University Research Council to

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Ghent University, Belgium

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André Vandierendonck

the author. I am indebted to Robert H. Logie for extensive discussions about modality effects

in working memory and to Jean-Philippe Van Dijck for his comments on a previous version of the paper. Correspondence about the paper can be directed to: André Vandierendonck, Department of Experimental Psychology, Ghent University, Henri Dunantlaan 2, B-9000 Gent, Belgium. E-mail: [email protected].

Abstract Working memory researchers do no agree on whether order in serial recall is encoded by dedicated modality-specific systems or by a more general modality-independent system. Although previous research supports the existence of autonomous modality-specific systems, it has been shown that serial recognition memory is prone to cross-modal order-interference

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by concurrent tasks. The present study used a serial recall task which was performed in a

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single-task condition and in a dual-task condition with an embedded memory task in the

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embedded tasks were in the other modality and required either serial or item recall. Care was taken to avoid modality overlaps during presentation and recall. In Experiment 1, visuospatial

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but not verbal serial recall was more impaired when the embedded task was an order than when it was an item task. Using a more difficult verbal serial recall task, verbal serial recall was also more impaired by another order recall task in Experiment 2. These findings are consistent

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with the hypothesis of modality-independent order coding. The implications for views on

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short-term recall and the multicomponent view of working memory are discussed.

Keywords: working memory, serial recall, cross-modal interference

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retention interval. The modality of the serial task was either verbal or visuospatial and the

The present study addresses the question of how order is encoded in working memory

to achieve correct serial short-term recall. Development and testing of working memory models indeed strongly, but not exclusively, relies on the usage of short-term serial memory tasks, which play a central role not only in the typical dual-task studies (Baddeley & Hitch, 1974), but also in measurements of working memory capacity (Daneman & Carpenter, 1980). Which processes are responsible for successful serial recall is however still a matter of debate

among working memory researchers (for a recent review, see Hurlstone, Hitch, & Baddeley, 2014). On the one hand, some theoreticians assume that dedicated systems encode modalityspecific information (Baddeley, 1986; Baddeley & Hitch, 1974; Baddeley & Logie, 1999; Logie, 2011), whereas others believe that the information encoded for later recall consists of many features including input modality (Cowan, 1999, 2005; Engle, 1996; Engle, Tuholski, Laughlin, &

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Conway, 1999; Oberauer, 2009). The present study pursues this debate by investigating the

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extent to which serial recall is impaired when during retention additional order coding is

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Most of the research about the role of modality in immediate serial recall has been inspired by the multicomponent working memory framework (Baddeley, 1986, 2000, 2007;

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Baddeley & Hitch, 1974; Baddeley & Logie, 1999) which assumes that the working memory system comprises modality-specific subsystems such as the phonological loop (for verbal storage) and the visuospatial sketch pad (for visuospatial storage) as well as more integrative

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subsystems such as the central executive and the episodic buffer. Over the years an

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impressive body of evidence has been collected in favour of separate and autonomous verbal and visuospatial subsystems of the working memory system (Baddeley, Grant, Wight, &

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Thomson, 1975; Brooks, 1967; Logie, 1986, 1995, 2011; see several chapters in Vandierendonck & Szmalec, 2011). Dual-task studies using a variety of tasks converge on the finding that visuospatial serial memory is much more impaired by a concurrent task that

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required for another serial recall task.

requires visuospatial representations than by a concurrent task that requires verbal processing; similarly, verbal memory is much more impaired by a concurrent task that requires verbal representations than by a concurrent task that relies on visuospatial coding (e.g., Logie, Zucco, & Baddeley, 1990; see Parmentier, 2011, for a review). In short, the pattern of findings is one of strong within-modality interference, while cross-modal interference is very small or absent. In fact, the small cross-modality impairments that are usually observed are attributed

to a general cost due to the requirement to perform tasks concurrently. This general pattern of findings has been interpreted as strong evidence for a double dissociation between verbal and visuospatial working memory tasks in line with the multicomponent working memory framework (see also Baddeley, 2007). Notwithstanding this body of evidence, a few findings are not consistent with this

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pattern of dissociation. Jones, Farrand, Stuart and Morris (1995) observed cross-modal

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interference on serial recall when the concurrent task involved changing sequences rather

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findings has been confirmed by several other studies (e.g., Guerard, Tremblay, & Saint-Aubin, 2009; Tremblay, Macken, & Jones, 2001). Working within the time-based resource-sharing

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model (Barrouillet, Bernardin, & Camos, 2004), Vergauwe, Barrouillet and Camos (2010) presented a strictly timed series of choice tasks during the retention interval of a short-term memory serial recall task, and they observed impaired serial recall irrespective of the modality

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of the tasks in the retention interval.

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All these findings are primarily based on a dual-task methodology in which memory is impaired due to the concurrent execution of another task that taxes modality-specific

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resources but does not necessarily require serial representations or recall from working memory itself. A more direct test of the hypothesis can be achieved when one memory task is performed concurrently with another memory task, or when information of different

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than sequences of a steadily recurring event irrespective of the task modality. This pattern of

modalities has to be stored simultaneously. Only a few such studies have been reported in the literature. Saito, Logie, Morita, and Law (2008) combined verbal and visuospatial information in the same stimuli (kanji characters) and they varied independently the phonological and the visual similarity of the characters in a serial recall task with visual presentation of the characters, and written recall. Written recall was poorer in the phonologically similar than in the phonologically dissimilar condition, even with silent reading. This phonological similarity

effect disappeared when the task was performed under articulatory suppression. With or without articulatory suppression, serial recall was also poorer for visually similar than for visually dissimilar characters. With such materials, recall seems to rely on storage in both modalities (visual and phonological). On the basis of these findings, the authors suggest that models of serial order memory should incorporate domain specificity. In other words, this

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study supports the view that serial order is coded within the modality-specific working

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A study of Depoorter and Vandierendonck (2009) supports the opposite view, namely that although identity information may be stored in separate modality-specific systems, serial

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order is coded in a modality-independent way. The major advantage of this study is that it used a dual-task methodology in which one short-term memory task (secondary task) was embedded in the retention interval of another short-term serial memory task (primary task),

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so that the primary task elements had to be maintained while the items of the secondary

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memory task were presented and recalled. Over a series of experiments, several task features were varied, namely modality of the two memory tasks (verbal or visuospatial), type of

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embedded task (serial or item memory task), and load (single-task versus dual-task conditions). In order to equate as much as possible the difficulty of the memory tasks used, a recognition procedure was used which required a decision about the order of two elements in

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modalities so that order-based interference could not occur.

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memory modules. Note that in these conditions, the serial information is the same for both

the serial memory tasks and a decision about whether an element was among the presented ones or not in the item memory tasks. When the two tasks were in the same modality, performance on the primary task was impaired more when the embedded task was a serial task than when it was an item task. Interestingly, the same pattern of findings was observed when the two tasks were in different modalities and the size of the effect was comparable to

the effect size in the same-modality conditions. On the basis of these findings, these authors suggest that serial order information is not modality-specific. From the viewpoint of the proponents of the strong view on modality-specificity, the usage of a recognition procedure may be considered as a drawback: because memory was tested by the same recognition procedure (pressing a key when two proposed elements were

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in the same order) in both modalities, an output or response coding overlap occurs between

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the two memory tests in the same-modality as well as in the cross-modality condition.

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overlap.1

The present study was set up to replicate the findings of Depoorter and Vandierendonck

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(2009) now avoiding these potential overlaps. An experiment was designed following the same logic as the original study. Because only the cross-modal conditions are critical to decide whether or not serial order is coded in a modality-specific way, only these conditions were

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included in the present design. The primary task was either a serial verbal task or a serial

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visuospatial task. In the verbal condition, the embedded task was either the serial visuospatial task also used as visuospatial primary task or it was a visuospatial item task. In the visuospatial

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condition, the embedded verbal task was either the already mentioned verbal serial recall task or a verbal item recall task. The primary and embedded task were tested as well in single-task as in dual-task conditions. Table 1 displays the details of this design. The rows of this table

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Similarly, the usage of visual presentation for the tasks in both modalities may create an input

display the sequence of events in the different types of trials. For example, in the single-task condition with a visuospatial primary task (first row), first the sequence of locations is presented, immediately followed by recall of this sequence; next, the to-be-remembered verbal sequence is presented immediately followed by recall of the sequence. ——Table 1 about here —— 1

Footnote 1

Overlaps in presentation method of verbal and visuospatial memory tasks were avoided by using aural presentation for the verbal tasks and visual presentation for the visuospatial tasks. Similarly, overlaps in the recall procedure of the memory tasks were avoided by using oral recall in the verbal tasks and pointing and touching the screen in the visuospatial tasks. Thus there were no modality overlaps between verbal and visuospatial memory tasks, neither

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in presentation nor in recall procedures.

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The question now is whether in this design that remedies the alleged shortcomings of

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can be replicated. More precisely, does the degree of recall impairment in the primary serial task depend on the type of embedded task such that an embedded order task would result in

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more impairment than an embedded item recall task? On the basis of the earlier findings with a recognition task, it is expected that recall performance of the primary memory task will be more impaired when the embedded memory task of a different modality requires serial recall

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than when it requires only item recall. In contrast, researchers adhering to the view that the

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modality-specific working memory systems are responsible for order coding will expect no such differential cross-modal interference at all, especially when all potential modality

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overlaps are excluded.

Experiment 1

Method

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the procedure used by Depoorter and Vandierendonck (2009), cross-modal order interference

Care was taken to create conditions that were as similar as possible across modalities

and tasks. Although the average word span is about 6-7 and the visuospatial span is about 5, for both modalities sequences of 5 items were used. Apart from the intended procedural differences across conditions (aural presentation and oral recall in the verbal modality; visual presentation and manual responding at recall in the visuospatial modality), the tasks were kept as similar as possible both in terms of requirements and in terms of duration of presentation

and recall. Presentation duration was the same for the order and the item tasks in both modalities and the task requirements in the verbal and visuospatial order tasks were as similar as possible; the same applies to the verbal and visuospatial item tasks. Participants and Design Twenty-four first-year psychology students (age range 18-22 years; 22 female) at Ghent

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University participated voluntarily in return for course credit. All participants were fluent

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Dutch speakers, and all the testing was done in Dutch. They were randomly assigned to the

Materials

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signed an informed consent document.

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For the verbal memory task, 9 high-frequency one-syllable Dutch words were selected from the Celex database (Baayen, Piepenbrock, & van Rijn, 1993). The words were digitally recorded from a female voice and normalised. One-channel wav files were created for each

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spoken word filled up with silence for one-second total episode durations. The selected words

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were boek, deur, kerk, naam, plan, rol, stad, voet, zaak (book, door, church, name, plan, role, city, foot, thing). Fifty-six subsets of 5 words were randomly sampled without replacement for

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presentation in either the verbal primary or the verbal embedded task. A trial of the serial recall task (primary or embedded) consisted of a presentation of the five words of the trial’s sequence at a rate of one word per second, by playing the recorded word episodes on the

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two modality conditions: 11 to the visuospatial condition, 13 to the verbal condition. They all

computer’s speaker. When recall was invited, the participant recalled the words in their order of presentation. A trial of the verbal item task also presented the five words at a rate of one per second, and at recall, the words were presented again at the same rate in a different order. On half of the trials the same five words were presented; on the other half of the trials, one word was replaced by a different word from the set. Participants were required to name the changed word or to say “gelijk” (equal).

The visuospatial memory task was a variant of the dots task (Parmentier, Elford, & Maybery, 2005), in which a number of dots are presented at particular positions in an enclosed area. The area was 700 by 700 pixels representing a 10 x 10 matrix (not shown to the participants). Each dot (black circle) was shown centred in one of the matrix cells and was 36 pixels wide. Subsets of five dots were randomly selected for each trial such that dots never

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occurred in a position which was next to a position already used for another dot in the set. In

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order to keep recall difficulty constant over the trials, total path distance (euclidian distance

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of crossings M = 0.84, s = 0.74, range 0-3), and circumference of the pattern (euclidian distance over the matrix cells in pixels, M = 1848, s = 189, range 1512-2177) were kept as similar as

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possible. In the serial recall task, the five black dots were presented one by one at a rate of one dot per second (750 ms on; 250 ms off) on a touch screen. At the time of recall, all the presented dots were shown simultaneously in blue and were to be pointed (by touching) in the

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recalled order. In the visuospatial item task, five black dots were presented simultaneously for

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five seconds. At recall, a pattern of five simultaneous dots was shown in blue. On half of the trials, the pattern was identical to the one presented before; on the other half of the trials,

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one dot in the pattern appeared at a different but neighbouring position (i.e., one cell away in either direction from the original position). Participants were required to point and touch the changed dot or to touch a response key labeled “zelfde” (same) which was present on the right

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over the matrix cells in pixels, M = 1771, s = 238, range 1344-2205), path complexity (number

of the square containing the pattern. Procedure

The entire experiment was programmed in Tscope (Stevens, Lammertyn, Verbruggen, & Vandierendonck, 2006) and run on a personal computer with a 1024 x 768 pixel touch screen. Instructions were shown on the computer screen, black on white. When clarification was needed, the experimenter rephrased the instructions that were on the screen. The

experiment consisted of four blocks of trials, which were characterised by the combination of single-task versus dual-task condition and the type of embedded task (as shown in the rows of Table 1). The order of presentations of these blocks was based on a latin square design with the order of rows and columns randomised. Each block started with specific instructions concerning the tasks within the block. Each

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block contained two practice trials and 12 experimental trials. After the practice trials,

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another short instruction screen followed. In the verbal modality condition, the trial always

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followed by recall (single-task conditions) or by the start of the second memory task showing a black square outline in which the black dots for the visuospatial task were shown.

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Immediately after presentation finished, the test screen was shown. In the visuospatial modality condition, the trials always started with the presentation of the visuospatial serial task (a sequence of five dots). This was immediately followed either by recall of the dot

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sequence (single task) or by the presentation of the second memory task, namely a sequence

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of five words for serial or for item recall. All responses to the verbal tasks were noted on a special form by the experimenter; all responses to the visuospatial tasks were recorded by the

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computer.

Presentation duration was fixed at 5 s for the tasks in both modalities, recall duration

was fixed at 8 s, and 1 s was included between successive components of a trial. As single-task

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started with the presentation of five words, one per second. This was either immediately

and dual-task trials contain the same components, trial duration was the same in all cases (2 x 5 + 2 x 8 + 3 x 1 s). At the end of the session the participant was thanked and completed a post-

experimental questionnaire. A collective debriefing session followed 1-2 weeks later.

Results Because the research question concerns performance on the primary task as affected by the type of embedded task, the data analysis focuses on primary task performance, but embedded task performance will also be reported. Furthermore, the tasks used on each trial show some variability as they are sampled from a set of possible tasks. For this reason,

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memory tasks must be considered as an additional random factor besides subjects, as is

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required in language research (Clark, 1973), and nowadays has become common practice.

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linear-mixed effects (LME) modelling (Baayen, Davidson, & Bates, 2008; Pinheiro & Bates, 2000). In the present study, the data were analysed using LME modelling on the basis of the

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lme4 package (Bates, Maechler, Bolker, & Walker, 2014) and the lmerTest package (Kuznetsova, Brockhoff, & Christensen, 2014) for R (R_Core_Team, 2014). The lme4 package provides the basic calculations for estimation of the LME models while the lmerTest package

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extends it by providing additional testing procedures and estimation of the probabilities of the

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effects under the null hypothesis.

For all the data analyses reported here, all serial recall tasks were scored as the

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proportion of elements recalled in relative correct position, by taking the proportion of the number of pairs recalled in correct order to the total number of pairs presented2. This yields a scoring between 0 and 1. The item recall tasks were scored 0 (incorrect) or 1 (correct). The

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Present state-of-the-art data analysis of such mixed effects designs includes the usage of

design included random effects of the factors Subject (nested in the Modality conditions), and Task Pair3. The fixed effects were Modality (visuospatial or verbal primary task), Condition

(single vs. dual task), and Type of embedded task (order vs. item recall task) and their interactions. 2

Footnote 2

3

Footnote 3

Primary task performance. The present study uses a dual-task design to test whether the embedded task type affects primary task performance. In such dual-task designs, it is often assumed that the secondary (embedded) task must have been performed properly. Therefore, only trials with perfect embedded recall were included in the analysis, irrespective of whether single-task or dual-task trials were concerned. Thus 34% of the data points were

entire set of primary task trials.

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Figure 1 displays the mean primary recall proportions on the trials with perfect embedded recall as a function of the fixed factors of the design. Table 2 shows the means and

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standard deviations of primary task recall proportions when restricted to perfect embedded trials (top panel), for all trials (middle panel) and recall performance on the embedded task (bottom panel). Estimation of the best fitting LME model consisted of two stages. First, the

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parameters were estimated of an LME model including the factorial combinations of the fixed

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effects (Modality x Condition4 x Type), and the random intercepts of Subject and Task Pair. This model also included an estimate of a random slope of Subject with respect to Condition,

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because individual differences in coping with a working memory load are well documented (e.g., Engle, Kane, & Tuholski, 1999). In the second stage, the nonsignificant effects were stepwise removed until the model with the most optimal fit to the data was obtained (step

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—— Figure 1 about here ——

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lost. Because this is a large amount of data loss, the LME modelling was also applied to the

function of lmerTest package for R, Kuznetsova et al., 2014). This was achieved by means of maximum likelihood comparison of the full model to the restricted models that were obtained by stepwise deleting all the effects that did not result in a significant fit difference. The optimal model contained only the contributing effects and the nonsignificant effects that were part of a significant interaction.

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Footnote 4

—— Table 2 about here —— The effects retained in the resulting optimal model are displayed in the second column of Table 3. As expected, serial recall was higher in the single-task (M = 0.96) than in the dualtask condition (M = 0.84), and this was the only significant main effect. Condition interacted with Type revealing a larger dual-task effect for the embedded order task (difference of 0.15),

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than for the embedded item task (difference of 0.12). Condition interacted also with Modality,

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showing that the dual-task effect was larger in the visuospatial modality (difference of 0.21)

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Condition x Type was also significant. In order to clarify the latter interaction, LME models were estimated for the two modalities separately by splitting the data set on the basis of

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Modality.

—— Table 3 about here ——

Restricted to the visuospatial modality, the optimal LME model included a random

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2 intercept of Task Pair, χ (1) = 16.4, p < .001 and a random slope of Subject with respect to

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2 Condition, χ (1) = 20.6, p < .001. Serial recall was better in the single task (M = 0.97) than in

the dual-task condition (M = 0.75), F(1,25) = 45.23, p < .001 and this effect interacted with

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Type, F(1,325) = 6.96, p < .01, so that with an embedded order task, the dual-task effect was larger (difference = 0.27) than with an embedded item task (difference = 0.18). The optimal LME model in the verbal modality only contained a significant random intercept of Task Pair,

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than in the verbal modality (difference of 0.04). Finally, the triple interaction of Modality x

χ 2 (1) = 15.9, p < .001 and a random intercept as well as a random slope of Subject with

2 respect to Condition, χ (1) = 21.5, p < .001. Neither of the fixed effects nor their interactions

attained significance. These separate analyses confirm that the triple interaction in the overall LME estimation is due to different behaviour in the two modalities.

In order to check on the generality of the results obtained when only perfect embedded trials are considered, the LME modelling was also applied to all the trials. Embedded task scores were included as covariate in the LME model to control for variability in primary task performance due to embedded task performance. The modelling yielded completely similar results (see third column of Table 3). The averages per condition were only slightly lower than

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in the analysis based on perfect embedded trials only, as is shown in Table 2. Also LME

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analyses per modality with embedded recall as covariate were performed. In the spatial

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interaction were significant, F(1,472) = 5.56, p < .05. In the verbal modality, the covariate and

< .01, and F(1,12) = 5.33, p < .05.

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the effect of Condition were the only significant contributions, respectively, F(1,584) = 8.04, p

Embedded task performance. For completeness, also an analysis of embedded task performance is reported (see column 4 of Table 3). In the final LME model, the fixed effect of

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Type was significant with a higher recall score for the order tasks (M = 0.85) than for the item

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tasks (M = 0.74). Although the interaction with modality was not significant, this difference was mainly due to the difference between the visuospatial order (M = .86) and the visuospatial

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item task (M = .68), whereas the difference between the two verbal tasks was rather modest (M = .84 and .81). To better understand the significant interaction of modality and condition, separate model estimations were performed per modality. In the visuospatial modality, only

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modality, the effect of Condition, F(1,17) = 36.84, p < .001, and of the Condition by Type

2 the random intercept of Subject was significant, χ (1) = 13.8, p .001, and none of the fixed

effects attained significance (smallest p = .29). In the verbal modality, the random intercept of 2 Subject, and the main effects of Condition and Type were significant, respectively χ (1) =

13.3, p .001, F(1, 609) = 11.40, p .001, and F(1,609) = 42.14, p .01. The latter effect reflects some difficulties with the visuospatial item task which will be addressed in the discussion.

Discussion Whether all trials or only perfect embedded trials were considered, the findings show that visuospatial serial recall was poorer when the embedded verbal recall task during the retention interval was an order task than when it was an item task. Verbal serial recall did not vary as a function of the type of embedded visuospatial recall task. Verbal serial recall was

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sensitive only to the factor Condition, with better performance in single-task than in dual-task

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conditions when all trials were included.

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interference, whereas verbal serial recall was not affected by cross-modal order interference. As this is only partially consistent with the findings of Depoorter and Vandierendonck (2009),

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the question arises why no cross-modal order interference was observed in the verbal condition. Was there a lack of power? That does not seem to be the case, because the variability of the cell means (standard deviation per cell) was comparable in both modality

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conditions, as can be seen in Table 2. Furthermore, difficulty of the serial recall tasks was also

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similar in both modalities. Inspection of Table 2 indicates that the average primary task score in the single-task condition was at a similar level in the visuospatial and the verbal modalities

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(0.97 and 0.96).

However, when comparing the embedded single-task performance across the two

modalities, it appears that averaged over the two task types, verbal recall was better (0.84)

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These findings show that visuospatial serial recall was sensitive to cross-modal order

than visuospatial recall (0.82). The same comparison in the embedded dual-task conditions yields an even larger difference in favour of verbal recall (0.81 versus 0.72). In other words, embedded recall performance was poorer when the embedded tasks were in the visuospatial than in the verbal modality. Visuospatial recall performance was quite strongly disrupted under dual-task load as well when it was the embedded task as when it was the primary task. In contrast, the decrease of verbal recall performance under load was more modest. This is in

line with observations reported by Morey and Mall (2012), namely that interference from a verbal on a visuospatial memory task is stronger than in the reverse direction. Inspecting the serial position curves, these authors demonstrated that in verbal serial recall, only the first part of the serial position curve was affected whereas visuospatial serial recall was affected throughout the entire serial position continuum. Addition of a suffix to the verbal lists

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increased the amount of interference by extending it to later serial positions. This advantage

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of verbal recall may also be related to the possibility of using two sources of short-term

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available (Camos, Lagner, & Barrouillet, 2009). Another advantage of verbal recall concerns the ease with which elements can be grouped, effectively reducing the number of elements to

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be recalled.

In other words, the present design with rather short word lists may have created an advantage in line with the findings of Morey and Mall (2012). Furthermore, the usage of

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frequent concrete words may have stimulated the creation of groups or chunks (such as

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“church door”, “city plan”, “book name”, etc.). The features of the visuospatial item task may also have contributed to the results obtained in the verbal modality. Detection of the changed

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dot was quite difficult in some of the patterns, and this may have encouraged participants to use a guessing strategy instead of trying to identify the difference, at least on some of the trials. This can also be seen in the significant effect of Type in the embedded performance of

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memory encoding: in addition to activated long-term memory, the articulatory loop is also

the verbal condition, where the visuospatial item recall was much poorer than the visuospatial serial recall both in single-task and in dual-task conditions. Experiment 2 A new experiment was designed in which only the verbal condition was tested while trying to avoid these potential drawbacks. Considering that the average verbal span in the population is close to 7, in the next experiment, lists of 7 elements were presented in order to

make the verbal task more prone to disruption under dual-task load. The high frequency words were replaced by consonants to discourage grouping and chunking: concrete nouns as used in Experiment 1 can easily be grouped to form a meaningful word, while grouping of consonants is far less easy (although still possible). Also the visuospatial item task was amended. Two changes were made to discourage guessing behaviour. First, in order to make

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the task less difficult, the changes to the dot pattern were no longer restricted to a move to a

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nearby position; instead, the distance over which the dot could move was made variable.

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which invites to make a guess. Instead, all patterns were different at recall and the participant’s task was to point to the displaced dot. In order to help participants notice when

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their visuospatial recall was incorrect (which is often less obvious than with verbal recall) and to stress the importance of this task, error feedback was presented when the embedded trial was incorrect. Finally, instead of presenting the Condition x Type variations block-wise, they

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were presented in a random order so as to further reduce the possibility of developing

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condition-specific strategies. This required that all instructions were presented up front and a longer practice session was added.

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Method

Twenty-two students (12 female) from a new cohort of first-year psychology students

were recruited to participate voluntarily for partial course credit. Age varied between 18 and

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Second, participants were no longer requested to decide whether the pattern had changed,

32 years. They all served in the verbal modality condition. Except for some changes specified in what follows, materials and procedure were the

same as in Experiment 1. For the verbal task, recordings of 20 consonants (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, z) from a female voice were normalised and presented through the computer speakers at a rate of one per second. Random lists of 7 different consonants were constructed by selecting a consonant from 8 phonologically similar subsets (according to

standard Dutch consonant naming). Thus phonological similarity within the series was kept low, but over the series all consonants were presented. The visuospatial item task was adapted such that the changing dot was moved a variable number of matrix positions away from its original position in order to make the change more discriminable. On each trial of the item task, one dot was moved and the participants were requested to identify that dot by

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touching it on the touch screen.

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With respect to the procedure, the different types of trials of the Condition x Type

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each type) were presented before the experiment proper started. At the end of visuospatial recall, a 500 ms interval was inserted during which the square in which the dots were

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presented remained white (correct trial) or turned red (incorrect trial). Moreover, every 10 trials a feedback screen was presented showing the percentage of correct visuospatial serial and item recalls. All these changes were included to stress the importance of the embedded

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tasks. As in Experiment 1, timing of the trials was the same across conditions: 7 s for

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presentation of the letters, 5 s for presentation of the dots, and 8 s for each recall, with 1 s interspersed between the components of the trial.

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Results

The results are reported in the same way as in Experiment 1. First, embedded recall is

reported to check whether the visuospatial item recall was at an acceptable level. Next,

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combination were presented in a completely randomised order. First 12 practice trials (3 of

primary task performance on trials with perfect embedded recall, and primary recall on all trials are reported. Table 4 displays the means as a function of the Condition by Type combinations for primary task recall with perfect embedded trials (top), primary task recall for all trials (middle), and embedded task recall (bottom). —— Table 4 about here ——

Embedded recall. The estimated parameters and effects of the optimal LME model are shown in Table 5 (last column) and the cell means are displayed in the bottom panel of Table 4. Only an effect of Condition was observed. Both in single-task and in dual-task conditions, no difference in performance on the order and item recall tasks was observed, but the order recall task was less frequently completely correct (about 7 times out of 10) than the item task

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—— Table 5 about here ——

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was (about 9 out of 10).

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lost (number of incorrect trials per participant ranged from 0 to 16). The average proportion of consonants recalled in correct serial order (relative scoring) as a function of Condition x

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Type is displayed in Table 4 (top panel). The significant effects in the optimal LME model are displayed in the second column of Table 5. All the fixed effects, namely Condition, Type and their interaction were significant as can also be seen in Figure 2. Single-task recall (M = 0.68)

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was higher than dual-task recall (M = 0.57). The interaction of Condition and Type shows that

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the presence of an embedded order task had a larger impact on recall (difference of .17) than the presence of an embedded item task (difference of .05).

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—— Figure 2 about here ——

An LME modelling of primary task recall based on all trials was also performed. The

estimated parameters and effects are shown in the third column of Table 5. The modelling

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Primary task recall. Exclusion of incorrect embedded trials caused 21% of the data to be

included embedded recall performance as a covariate but this covariate was not retained in the final model. As appears from Table 5, the same significant effects were observed, with—as expected—an interaction of Condition and Type. The cell means based on all data were not substantially different from those including only perfect embedded trials (see Table 4). Extra analyses. As well the averages based on embedded-perfect only trials (Figure 2 and Table 4) as the all-trials averages (Table 4) of the single-task condition show a

performance difference between order and item recall, t(21) = 6.90, p .05. Such a difference is not at all expected, because at the moment of recall, the participant has no knowledge as to whether the secondary task that will follow within the same trial will be an order or an item recall task. Inspection of the data per participant showed that the performance difference was present in the majority of the participants: 16 (when all trials were included) or 18 (in the

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perfect-only trials data) out of 22 participants showed higher average single-task recall when a

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secondary order than a secondary item task was to follow. Although the difference is

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presence of an embedded order than from an embedded item task. This was confirmed by means of tests of the relevant contrasts in which performance on the two single-task

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conditions was collapsed (doBy package for R, Højsgaard & Halekoh, 2014, that allows to test contrasts within an LME model). In the data based on perfect-only embedded trials, dual-task verbal recall performance was poorer when the embedded task was an order task than when

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2 it required was an item task, χ (1) = 13.14, p < .001, while both were significantly worse than

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2 2 the average of the two single-task conditions, respectively χ (1) = 140.63, p < .001 and χ (1) =

18.36, p < .001. Similarly, in the data based on all trials, verbal recall was impaired more by an

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2 embedded order than an embedded item task, χ (1) = 12.38, p < .001, and both were 2 significantly different from the averaged single-task performance, respectively χ (1) = 142.08,

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unexpected, it does not invalidate the finding that verbal serial recall suffers more from the

2 p < .001 and χ (1) = 21.10, p < .001.

Nevertheless, it is important to explore the reasons why the difference between the two

single-task conditions occurred. Although several potential accounts for this difference were considered, none of them were confirmed by the data. Therefore, only a brief report of these analyses is given here. First, the possibility was considered that, on average, more difficult verbal items occurred more frequently when the trial contained a visuospatial item task than

when the trial contained a visuospatial order task in the single-task conditions. An analysis of the frequency with which each item occurred in the four cells of the Condition x Type 2 combination revealed no significant deviations from an equal-frequency distribution, χ (159) = 2 138.2, p = 0.88 in the perfect embedded trials analysis and χ (159) = 168.2, p = .29 in the all-

trials analysis. A second possibility considered was the hypothesis that the difference was due

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to the characteristics of the items. This was tested by means of an analysis of variance with item (the 40 items performed by all participants) as single random variable. Neither in the

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difference between the two single-task conditions was detected, F(1,39) = 2.10, p = .16, ηp =

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.05 and F(1,39) < 1, ηp = .02, respectively. A third possibility is that events on the previous trial affected recall performance on the current trial. The presence or absence of feedback on the previous trial did not correlate with performance in the single-task order or in the single-task item condition (respectively, r = .01, and r = -.02). Similarly, the actual performance on the

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previous trial did not correlate with performance in single-task order condition (r = .15, p =

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.36), or item condition (r = -.09, p = .57). A fourth alternative interpretation concerned the possibility that the pressure due to performing many dual-task trials would adversely affect

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performance as the session progressed. This was tested by entering the contrast between the first and the second half of the trials as another fixed effect, but performance did not depend on this contrast and it did not interact with any of the other fixed effects. The implication of all

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analysis restricted to perfect embedded trials nor in the analysis of all trials, a significant

these extra analyses will be addressed in the Discussion section. Discussion

In Experiment 2, verbal serial recall was more impaired when the embedded visuospatial task required order recall than when it required item recall. This pattern of findings is similar to the pattern of findings observed for visuospatial recall with verbal embedded tasks in

Experiment 1. Verbal serial recall thus seems to be also sensitive to cross-modal order interference. Clearly, the task modifications were effective in obtaining the cross-modal order interference effect. In comparison to the degree of disruption observed in visuospatial serial recall in Experiment 1 (a decrease of 0.27 from the single-task to the dual-task embedded

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order condition), in terms of measurement units the degree of disruption in the present

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experiment was much smaller (0.17).

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far less trials had to be excluded due to incorrect embedded task performance than in Experiment 1. This suggests that participants assigned more importance to the embedded

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task.

One unexpected aspect of the findings concerns the higher score in the single-task condition when the verbal task was followed by an order task than when it was followed by an

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item task. A number of possible explanations for this effect were explored, but none of them

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could account for this difference. Given that the distribution of the items over the cells of the design did not violate a uniform random allocation, it is clear that the randomisation of the

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items over the conditions has worked fine. Moreover, verbal recall performance in the singletask conditions was not accounted for by feedback or performance levels occurring at the previous trial. Furthermore, the observation that at the level of the items no significant

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The task modifications also resulted in better embedded task performance. As a result,

differences occurred in item performance when the item was followed by a secondary order task than when it was followed by a secondary item task provides no basis for suggesting that differences in item difficulty were driving this performance difference at the level of participants. The important point for the present study is that the predicted interaction of Condition and embedded item Type was confirmed also when the two single-task conditions were collapsed, and that indeed verbal serial recall was more impaired when the retention

interval was filled with a visuospatial order task than when it was filled with a visuospatial item task. General Discussion The results of Experiment 1 showed that compared to performing the same tasks in a single-task setting, visuospatial serial recall was more impaired when a serial verbal memory

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task was performed during the retention interval than when verbal item recall was required.

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Such cross-model order interference was not present for verbal serial recall in Experiment 1,

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visuospatial task was observed in one experimental setting and not in another restricts the generality of the observation. The present findings nevertheless show that cross-modal order

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interference also occurs in the verbal modality, at least when the memory tasks, as was the case in Experiment 2, are sufficiently taxing and are less favourable for a dominance of the verbal modality.

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One of the motivations for the present study was to test the suggestion that the cross-

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modal order interference effects reported by Depoorter and Vandierendonck (2009) in recognition tasks were due to illicit overlaps in the task settings. As these investigators used

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visual presentation for both the visuospatial and the verbal memory task, a potential “input” overlap was included, and because manual responses were used to select one of two choices in both modalities, a potential “output” overlap was created. After removing these potential

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but it was in Experiment 2. The fact that in the verbal modality, order interference from a

overlaps, the cross-modal order interference should no longer occur according to accounts that assume that order coding is part of the modality-specific encoding of the to-beremembered materials. As cross-modal interference did occur, the present results are not consistent with the latter expectation, but they corroborate the earlier findings of Depoorter and Vandierendonck and extend them from recognition to recall, thus ascertaining that the cross-modal order interference is a genuine phenomenon.

Nevertheless, the present findings also show that visuospatial serial recall is more sensitive to cross-modal order interference than verbal serial recall is. This observation is consistent with findings reported by Morey and colleagues (Morey & Mall, 2012; Morey, Morey, van der Reijden, & Holweg, 2013), who showed that verbal recall is in general much more robust to visuospatial interference than visuospatial recall is to verbal interference. In

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the present study, cross-modal order interference in the verbal task was obtained by making

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the verbal task more difficult and by including feedback to continuously stress the importance

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sensitive to cross-modal order interference but only under appropriate conditions. Theoretical Implications

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What do the present findings regarding cross-modal order interference imply for our theoretical views on the structure and operation of working memory? In discussing this, it is necessary to distinguish between two kinds of interference that characterise the present

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results. First, primary task recall is impaired by both types of embedded task. This is indicated

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by an overall effect of Condition which was present in both experiments. This general effect is due to the fact that two streams of information have to be processed and maintained, and can

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be considered as a general dual-task effect that in part also depends on recall delay which is different between single-task and dual-task conditions. There is, however, a second and more important aspect to the present findings, namely that in addition to this basic level of

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of the visuospatial task. The present findings hence show that verbal serial recall is also

impairment, primary recall performance is more impaired when the embedded task also calls on order (serial recall). This is cross-modal order interference, and its presence suggests that recalling the order of the items of the primary memory task is somehow disrupted by the presence of another memory task that also requires ordered recall. If order is completely and exclusively coded at the level of modality-specific systems, such a disruption is not expected to

happen. The present findings therefore suggest that order is at least in part coded at a different and modality-independent level of encoding. How could modality-independent order coding account for the finding of cross-modal order interference? Several types of models have been proposed to account for serial recall. These models basically belong to three types (Henson, 1999). Chaining models (e.g., TODAM,

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Murdock, 1976) and ordinal models (e.g., the primacy model, Page & Norris, 1998; see also

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Page & Norris, 2009) do only consider one modality for storage, and therefore they cannot

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models use a positional code to account for the serial order; the items are associated to their position in the list or to a contextual representation (Botvinick & Watanabe, 2007; Brown,

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Neath, & Chater, 2007; Brown, Preece, & Hulme, 2000; Burgess & Hitch, 1999; Henson, 1998; Oberauer, Lewandowsky, Farrell, Jarrold, & Greaves, 2012). Very often these models also consider only one encoding modality. However, the principle of positional coding at least

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provides a mechanism that could—if completed with appropriate assumptions about

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modality-specific item coding—explain cross-modal order interference. To that end, consider that each to be remembered element becomes associated to a modality-free ordered context

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code. When the first task finishes and a second serial task starts (in the same or in another modality) the modality-free ordered context continues to be linked to the to-be-remembered elements of this second sequence, so as to form one longer encoded series with a grouping

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distinguish between within-modality and cross-modality interference. The third type of

because of the difference in contents between the two lists. At recall in the dual-task condition, the second part of the ordered list has to be recalled first. The most difficult part is to find the start of the second group or list and starting from there. Next, recall of the first group is required. Quite likely the context of this first list is less easily accessible because it is less recent and has lost some encoding strength. Hence, in comparison to the quality of recall which is possible immediately after encoding, delayed recall after having learned and recalled

a second list is bound to be poorer, unless the series of items is also stored in the articulatory loop, thus providing a second basis for recalling the items in order, as is typically the case with short series of verbal items. An important question concerns whether the present findings are compatible with the multicomponent working memory model of Baddeley and colleagues (Baddeley, 1986, 2000;

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Baddeley, Allen, & Hitch, 2010; Baddeley & Hitch, 1974; Logie, 2011). Thus far, these

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researchers have assumed that the modality-specific systems encode order as well as identity

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are modality-specific but operate on the basis of the same set of principles (Hurlstone et al., 2014). The present findings are not compatible with such a view because with separate order

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encodings per modality even if these encodings are based on the same principles cross-modal interference is not expected to occur; therefore this version of a multicomponent model cannot account for the observed cross-modal interference.

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Nevertheless, a version of the multicomponent model based on adapted assumptions

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could be compatible with the present findings. If the assumption about modality-specific order encoding would be replaced by the assumption that a domain-independent component

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represents the item-position bindings postulated by many of the present-day models of serial recall (see Hurlstone et al., 2014, for an extensive review), the essentials of the multicomponent model could be retained. The model could account for the present findings

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information. They further assume that the order coding mechanisms used by these systems

by assuming as was already suggested by Depoorter and Vandierendonck (2009) that the episodic buffer would be suited to maintain such item-position bindings. However, if the encoding and decoding of item-position bindings as postulated by the positional models of serial recall do require some form of executive control, it may become problematic to attribute these bindings to the episodic buffer. Indeed, on the basis of recent findings, it has been suggested that the episodic buffer is a passive store (Allen, Hitch, Mate, & Baddeley, 2012). If

this conclusion stands and further research shows that order coding is indeed an attention demanding process, then the only solution is to attribute such order coding to the realm of the central executive, which again raises the issue of how to deal with this homunculus (e.g., Vandierendonck, in press). Conclusion

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Notwithstanding the solid body of evidence in support of modality-specific working

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memory representations (see reviews in Hamilton, 2011; Logie, 2003), the present findings

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level. According to the extant literature, the modality-specific memory systems remain useful devices for maintaining modality-based identity information. The present results, in contrast,

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further confirm the hypothesis that a modality-independent mechanism is used for coding the order of the elements. On the basis of the present findings it seems likely that the latter mechanism operates under attentional or executive control. The importance of the present

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findings is that they indicate some limits to the fractionation of working memory

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representations into separate modality-specific stores and that they invite a reformulation of a

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few basic assumptions of the multicomponent working memory model.

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support the hypothesis that order coding occurs at a more general modality-independent

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Figure Captions Figure 1. Proportion of correct serial primary-task recall (standard errors in the whiskers) when embedded recall was perfect, as a function of modality (visuospatial left panel; verbal right panel), single versus dual-task and type of task performed during the retention interval (order

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or item task).

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Figure 2. Proportion of correct serial primary-task recall (standard errors in the whiskers) when

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performed during the retention interval (order or item task).

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embedded recall was perfect, as a function of single versus dual-task and type of task

Footnotes I am indebted to Robert H. Logie for pointing this out in an oral communication. Nowadays serial recall is often scored as the number of items correctly recalled in their serial position. In the present experiments, the participants were not instructed to indicate the positions that were skipped in their recall. The visuospatial tasks were recorded by the

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computer program and no button was provided to indicate a forgotten position in the recall

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sequence. Similarly, in verbal serial recall, participants were not instructed to say “blank” for

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5-item sequences of the primary task was low in Experiment 1 (M = 0.19), the 7-item sequences in Experiment 2 provided more opportunity for skipping or forgetting (M = 0.84). It

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is important to note, though, that performance registration in the present study does not allow a clean distinction between skipped positions and forgotten items. Given these circumstances, a relative scoring scheme yields the best estimate of the serial recall

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performance. Due to the loss of some relevant information, an absolute scoring of the serial

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recall underestimates the serial recall performance in the present data. Nevertheless, using absolute scores instead of relative scores yields similar results as reported here, but with less

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statistical power.

As it happens, two memory tasks are presented per trial. Each task occurred only once

throughout the experiment, so that on each trial, a unique trial pair was used. Therefore it

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the forgotten positions. Whereas the average number of skipped or forgotten positions in the

suffices to include a random factor that represents the task pair on each trial. The contrast between single-task and dual-task condition is deliberately not labeled as

“Load” because the difference between single-task and dual-task recall it not only affected by the absence or presence of an embedded task but also depends on the duration of the retention interval (immediate in single-task recall and delayed in dual-task recall). In principle, it is possible to use retention intervals of equal duration in the two conditions. However,

when the single-task recall is delayed and the recall interval is unfilled, recall is not expected to differ much from an immediate recall (because rehearsal is possible). Imposing rehearsalprevention during the single-task retention interval would turn the single-task condition into a dual-task condition, and this would obscure the contrast between single-task and dual-task

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recall.

Table 1. Design of the study in the verbal and visuospatial conditions. Each row

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namely presentation of the primary task, recall of the primary task, presentation of

Condition

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the embedded task, and recall of the embedded task. Event 1

Visuospatial condition

Event 2

Event 3

Event 4

VS serial recall

Vb ordered list

Vb serial recall

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VS ordered list

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Single Task

VS serial recall

Vb ordered list

Vb item recall

VS ordered list

Vb ordered list

Vb serial recall

VS serial recall

Vb ordered list

Vb item recall

VS serial recall

Vb ordered list

Vb serial recall

VS ordered list

VS serial recall

Vb ordered list

Vb serial recall

VS pattern

VS item recall

Vb ordered list

VS ordered list

VS serial recall

Vb serial recall

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VS ordered list

Dual Task

VS ordered list

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specifies the events in one of the trial types. Four events are present in each trial,

Verbal condition

Single Task

Dual Task

Vb ordered list

VS pattern

VS item recall

Vb serial recall

Note. VS = visuospatial; Vb = verbal

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Table 2. Mean recall scores (standard deviations between parentheses) on the

primary task and on the embedded task as a function of primary task modality,

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separately for the subset of the data with only perfect embedded trials, and for all

Condition

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trials. Single Task

Embedded Task

Order

Dual Task

Item

Order

Item

Visuospatial

0.98 (0.06)

0.94 (0.12)

0.71 (0.28)

0.77 (0.25)

0.95 (0.15)

0.96 (0.10)

0.91 (0.16)

0.91 (0.14)

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Verbal

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Primary Task Scores, only perfect Embedded Trials

Primary Task Scores, all Trials

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condition and embedded task type. The primary task means are presented

Visuospatial

0.98 (0.07)

0.94 (0.13)

0.71 (0.31)

0.75 (0.27)

Verbal

0.93 (0.19)

0.92 (0.17)

0.85 (0.24)

0.84 (0.24)

0.81 (0.39)

0.81 (0.24)

0.81 (0.39)

Embedded Task Scores Verbala

0.86 (0.21)

Visuospatiala a

0.90 (0.18)

0.73 (0.44)

0.83 (0.24)

0.62 (0.48)

The labels “verbal” and “visuospatial” refer here to the modality of the embedded

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task.

Table 3. Random and fixed effects included in the optimal LME model based on the data of Experiment 1, for primary task scores when only perfect embedded trials are included (Perfect Only), for all primary task scores (All) and for the embedded task scores (Embedded). Perfect Only

All

Embedded

1|S: χ2(1) = 3.6

1|S: χ2(1) = 4.9

1|S: χ2(1) = 26.6

p = .06

p < .05

p < .001

C+0|S: χ2(1) =

C+0|S: χ2(1) =

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13.8

48.5

p < .001

p < .001.

1|P: χ2(1) = 16.5

1|P: χ2(1) = 15.6

p < .001

p < .001

Fixed Effects

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Task Pair (P)

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Embedded Score

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Subject (S)

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Random Effects

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Effects

Modality (M)

Condition (C)

F(1,1100) = 9.94

NAa

NAb (p < .01)

F(1,57) = 1.48

F(1,66) = 3.06

F(1,35) < 1

(p = .23)

(p = .09)

(p = .59)

F(1,47) = 68.90

F(1,33) = 44.65

F(1,1125) = 10.32

(p < .001)

(p