Journal of Experimental Psychology: Learning, Memory, and Cognition

Journal of Experimental Psychology: Learning, Memory, and Cognition Dissociating Working Memory Updating and Automatic Updating: The Reference-Back Paradigm Rachel Rac-Lubashevsky and Yoav Kessler Online First Publication, November 30, 2015. http://dx.doi.org/10.1037/xlm0000219

CITATION Rac-Lubashevsky, R., & Kessler, Y. (2015, November 30). Dissociating Working Memory Updating and Automatic Updating: The Reference-Back Paradigm. Journal of Experimental Psychology: Learning, Memory, and Cognition. Advance online publication. http:// dx.doi.org/10.1037/xlm0000219

Journal of Experimental Psychology: Learning, Memory, and Cognition 2015, Vol. 42, No. 1, 000

© 2015 American Psychological Association 0278-7393/15/$12.00 http://dx.doi.org/10.1037/xlm0000219

Dissociating Working Memory Updating and Automatic Updating: The Reference-Back Paradigm Rachel Rac-Lubashevsky and Yoav Kessler

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

Ben-Gurion University of the Negev Working memory (WM) updating is a controlled process through which relevant information in the environment is selected to enter the gate to WM and substitute its contents. We suggest that there is also an automatic form of updating, which influences performance in many tasks and is primarily manifested in reaction time sequential effects. The goal of the present study was to dissociate WM updating and automatic updating, characterize the nature of these operations and identify the memory system responsible for each. In addition, we investigated the relationship between WM updating and the P3 eventrelated potential component. In Experiment 1, we compared the sequential processes in 1-back and 2-alternative forced choice tasks. These results indicated differential sources of sequential processes in the 2 tasks. We proposed that automatic updating operates in long-term memory on representations separate from WM representations. In addition, the event-related potential results of Experiment 1 are inconsistent with the idea that P3 is triggered through WM updating. Subsequently, in Experiments 2–3, we decomposed the 1-back task to major subprocesses. To this end, a new paradigm is introduced: the reference-back task. This paradigm facilitated the empirical distinction between automatic updating, comparison processes, gating and WM updating, within the same task. The results replicated the separate effects of WM updating and automatic updating on performance, and they provided behavioral evidence for a gating mechanism that separates WM from long-term memory. Keywords: working memory updating, automatic updating, gating, ERP, P3

external world. Second, updating WM with information stemming from the internal world is crucial for high-level cognition, such as arithmetic operation, comprehension, and reasoning (e.g., Carretti, Cornoldi, De Beni, & Romanò, 2005; Deschuyteneer, Vandierendonck, & Muyllaert, 2006; Palladino, Cornoldi, De Beni, & Pazzaglia, 2001; Schmiedek, Hildebrandt, Lövdén, Wilhelm, & Lindenberger, 2009). Third, updating WM with a goal or relevant task-context is essential for goal-directed behavior, in which the required action cannot rely on overlearned long-term memory (LTM) routines (Braver & Cohen, 2000; D’Ardenne et al., 2012; Miller & Cohen, 2001). Fourth, cognitive training based on WM updating tasks (primarily n-back) has been proposed to produce long-term and wide-ranging cognitive benefits (Jaeggi, Buschkuehl, Jonides, & Perrig, 2008). This last option, which bears important educational and clinical implications, is currently at the center of a heated debate in the literature (Redick et al., 2013; Shipstead, Redick, & Engle, 2012; von Bastian & Oberauer, 2014). Moreover, due to the role of WM in maintaining accessible and conscious information, maladaptive WM updating has been associated with cognitive impairments in a wide range of situations, from normal aging (De Beni & Palladino, 2004; Pelegrina, Borella, Carretti, & Lechuga, 2012) to psychopathology (Joormann & Gotlib, 2008; Levens & Gotlib, 2010; Meiran, Diamond, Toder, & Nemets, 2011; Segal, Kessler, & Anholt, 2015).

Most daily tasks, from making coffee to crossing the road, require the storage of information to guide future actions, and this information is updated with newer information as required. Working memory (WM) is the cognitive system that facilitates these abilities. WM updating is defined as the modification of stored information (Morris & Jones, 1990), achieved through the removal of outdated items (Ecker, Lewandowsky, & Oberauer, 2014; Ecker, Oberauer, & Lewandowsky, 2014; Oberauer, 2001), the addition of new items, and/or the modification of the stored information (Kessler, Baruchin & Bouhsira-Sabag, 2015; Kessler & Meiran, 2006; Kessler & Meiran, 2008; Kessler & Oberauer, 2014, 2015; Kessler et al., 2015; Oberauer, 2002). WM updating is important for many reasons. First, updating WM with information from the external environment is essential to maintain correspondence between internal representations and the

Rachel Rac-Lubashevsky, Department of Brain and Cognitive Sciences and Zlotowski Center for Neuroscience, Ben-Gurion University of the Negev; Yoav Kessler, Department of Psychology and Zlotowski Center for Neuroscience, Ben-Gurion University of the Negev. This research was funded through Grant Number PCIG09-GA-2011293832) from the European Union Seventh Framework Program (FP7/ 2007–2013) and Grant Number I-2319-1088.4/2012 from the GermanIsraeli Foundation awarded to Yoav Kessler. We thank Tal Yatziv for helpful comments on the manuscript. Correspondence concerning this article should be addressed to Rachel Rac-Lubashevsky, Department of Brain and Cognitive Sciences and Zlotowski Center for Neuroscience, Ben-Gurion University of the Negev, Beer-Sheva, Israel 84105. E-mail: [email protected]

The n-Back Paradigm The n-back paradigm has primarily been utilized for the manipulation and measurement of WM updating. In the most typical variant of the n-back task, participants are presented with a sequence of stimuli, one in each trial, and are required to indicate 1


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whether each stimulus is the same or different from that presented n (typically, 1, 2, or 3) trials before (e.g., Cohen et al., 1994; Jonides et al., 1997; Kirchner, 1958). Notably, this task has been extensively used to investigate the neural basis of online WM processing (for review, see Owen, McMillan, Laird, & Bullmore, 2005). Performing the n-back task relies on a complex set of cognitive processes (Jonides et al., 1997; Chatham et al., 2011), including (a) encoding the incoming stimulus and associating (“binding”) it with a serial position within the remembered set; (b) comparing the incoming stimulus to the one stored in the correct position in WM (Chen, Mitra, & Schlaghecken, 2008), presumably while (c) inhibiting items in irrelevant positions to avoid intrusion effects; (d) updating the association of each stored item to a new serial position (e.g., the item in position n – 1 should be moved to n – 2, and so forth); and (e) removing representations in positions that fall beyond the minimal memory set needed to perform the task (namely, in positions greater than n). The complexity of the n-back task make it difficult to isolate the contributions of these subprocesses. Moreover, it is also unclear which of these processes occur serially and which occur in parallel. For example, is the updating operation dependent on the outcome of a comparison decision, or are these operations performed independently and in parallel? (We will return to this issue following the results of Experiments 2.) In addition, strategy might also affect the order of these subprocesses (Braver, 2012). For example, a proactive strategy would involve the retrieval of the relevant representation in position n prior to the presentation of the new stimulus. Alternatively, a more reactive strategy would involve the use of the incoming stimulus as a cue for retrieval. The choice of strategy might dictate the temporal organization of the processes and might be affected through individual preference, ability and task difficulty. In this study we introduce a novel variant of the n-back, the reference-back paradigm, which facilitates the separation of some of the subprocesses mentioned above.

Working Memory Updating and Gating Theoretical models of WM emphasize the conflict between its maintenance and updating functions (e.g., Frank, Loughry, & O’Reilly, 2001; Miller & Cohen, 2001; O’Reilly, 2006; O’Reilly & Frank, 2006). Specifically, WM facilitates the robust maintenance of information in a highly accessible state, while shielding the maintained information from interference from distracting or conflicting information. In contrast, selective updating involves the modification of the maintained information when needed (Badre, 2012), for example, to update WM with task-relevant information (often termed “context” or “set”). Therefore, updating must counteract maintenance. Regulation of the processes of maintenance and updating requires control. To account for this control, the aforementioned theoretical models (e.g., Miller & Cohen, 2001; O’Reilly & Frank, 2006) assume a gating system, which buffers between perceptual input and WM and hence regulates the flow of information to WM. Keeping the gate closed facilitates robust maintenance within WM, which is shielded from perceptually represented information. In contrast, opening the gate facilitates the updating of the WM with available perceptual input. In the following section, we extend this view to include not only gating perceptual input but also activated representations in LTM.

The two states of the gate to WM, open and closed, correspond to two discrete modes of WM operation: updating and maintenance (Kessler & Oberauer, 2014). Controlling the gate states and WM operation modes is critical for the optimal function of WM, and is needed in experimental tasks that require maintenance and updating of the goal, such as the continuous performance task (Braver & Cohen, 2000; D’Ardenne et al., 2012) and taskswitching (cf. Mayr, Kuhns, & Hubbard, 2014). Impaired gating function results in increased distractibility or maladaptive rigidity (Miller & Cohen, 2001).

Automatic Updating In contrast to the controlled nature of WM updating described earlier, some effects of automatic processing suggest another form of memory updating, that does not require passing information throughout the WM gate and hence occurs outside WM. This type of updating will be termed automatic updating. Numerous studies have indicated that some phenomena attributed to automatic processing are based on internal representations showing sensitivity to recent changes in the environment, even when external stimulation is removed, thereby reflecting memory updating phenomena. These effects include spatial updating (Medendorp, Goltz, Vilis, & Crawford, 2003), repetition priming (Jacoby & Dallas, 1981; Tulving & Schacter, 1990), sequential effects in choice reaction time (RT) tasks (Jones, Curran, Mozer, & Wilder, 2013; Soetens, Boer, & Hueting, 1985), recency effects (Baddeley & Hitch, 1977; Murdock, 1962), and the generation and updating of object files (Kahneman, Treisman, & Gibbs, 1992) and event files (Hommel, 1998). The automaticity of updating in these cases is reflected in the fact that memory updating of the past events is spontaneously achieved, although not being part of the task requirements (Tzelgov, 1997) or even while interfering with the task at hand. Thus, we defined automatic updating as taking place in an effortless and obligatory manner, without cognitive control or intention. Unlike the selective manner in which WM representations are retrieved (Chatham, Frank & Badre, 2014), the retrieval of automatically updated representations is spontaneous (cf. Logan, 1988) and does not, thus, depend on goal-driven control over WM gating. Rather, memory formation through this route is likely an emergent property of perception and action as “a byproduct of perceptual analysis” (Craik & Lockhart, 1972, p. 671). Importantly to our distinction, WM load does not affect automatic updating effects, such as recency and repetition priming (Baddeley & Hitch, 1974; Baqués, Sáiz, & Bowers, 2004), supporting the view that these effects occur outside WM. One of the most prevalent instances where automatic updating, as defined earlier, is observed, is observed in sequential effects in choice-RT tasks.1 Here, we narrow the discussion to the twoalternative forced choice (2AFC) paradigm. In this task, one of two potential stimuli appears on the screen in each trial. Participants are required to respond according to a predefined stimulusresponse (S-R) mapping, where each of the stimuli is mapped to a different response. Many studies have suggested that although the memory of previous trials is not part of the task requirements, the 1 The treatment of the sequential effects in this study is limited to the history of stimuli (namely, memoranda). Therefore, sequential congruency effects remain outside the scope of the present study.


WORKING MEMORY UPDATING

performance in each trial depends on the stimulus and response history in previous trials, lasting for at least five trials (e.g., Jones et al., 2013; Laming, 1968; Soetens et al., 1985). Specifically, the influence of past events on RT is accumulated along the trial history with a recency gradient, so that the influence of each trial on performance is greater than that of earlier trials. Trial history generates the “first-degree repetition effect,” which refers to the alternation versus repetition of the stimulus along successive trials. A decrease in RT in Trial N as a function of the number of previous trials with the same stimuli and responses. An additional effect is the “second-degree effect” (Jones et al., 2013), which refers to the sensitivity of RT to repetitions versus alternations of the condition. For example, an S-R alternation trial preceded by an alternation (as in the last trial in the sequence XOX) constitutes a first-degree alternation (of the S-R mapping), but a second-degree repetition (of the alternation condition). In the present study, we will examine the role of automatic sequential effects in n-back performance, and will attempt to dissociate it from the contribution of WM processes.

The Difference Between Automatic and Controlled Updating From a theoretical perspective, the two forms of updating, automatic versus controlled, reflect the operation of two different systems. These systems are complementary in two ways. First, the automatically updated system can be used to guide automatic behavior (Cowan, 1988; Unsworth & Engle, 2007). Without this system, WM would be required for every information-processing activity, and hence, robust maintenance of intact information could not occur. In other words, robust maintenance within WM is made possible due to the fact that a different system is responsible for perceiving and reacting to the outer world. Second, the internal decision to update WM with new information must be based on automatically updated representations. Otherwise, assuming WM as the locus of such a decision would lead to a chicken-and-egg infinite regress problem (Braver & Cohen, 2000). The gating model provides a conceptual framework for understanding the interplay between automatic and controlled updating. Accordingly, perceptual (“posterior,” “lower level”) representations are automatically and continuously updated as a result of changes in the environment. When the gate is closed, the automatically updated representations are different from those stored in WM. Goal-driven gate opening facilitates the mobilization of information to WM to protect against overriding through new input, reflecting automatic updating. Based on the idea that memory is a by-product of perception, we extended the gating model to also account for the influence of information stemming from memory, rather than from perception per se. Gating is thus suggested to occur between LTM and WM. The term LTM is used to describe a single-store memory system (that is, LTM as opposed to WM, not to short-term memory).

P3 Building on the distinction between WM updating and other related processes, Experiment 1 further investigated the relationship between WM updating and the P3 event-related potential (ERP) component. P3 is presumably the most extensively studied

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ERP component, peaking at approximately 300 ms poststimulus at central scalp electrodes (but occasionally observed between 250 to 500 ms, depending on the task). P3 is commonly associated with the oddball paradigm. In this paradigm, either a rare event (20% of the trials; “target”) or a frequent event (80%, “nontarget”) is presented in each trial. The typical finding is a larger P3 observed in rare target events (Squires, Wickens, Squires & Donchin, 1976). In addition, P3 is also sensitive to task-relevant dimensions; thus, the effect of frequency on P3 is larger in task-relevant compared with task-irrelevant dimensions (Donchin & Coles, 1988; Squires, Wickens, Squires & Donchin, 1976). P3 is not a monolithic component but can rather be divided into two subcomponents, P3a and P3b (Polich, 2007). P3a is observed at frontocentral scalp electrodes and is typically detected approximately 250 –300 ms poststimulus, and prior to the P3b. P3a is also termed novelty P3, reflecting the sensitivity of this component to the salience and frequency of the stimulus. This component presumably reflects the orientation response or involuntary capture of attention through the deviant stimulus (Friedman, Cycowicz, & Gaeta, 2001; Kok, 2001; Schröger, 1996). In contrast, P3b is typically detected at centroparietal regions, peaking at approximately 350 ms poststimulus. This component is also known as target P3, as it is produced through task relevant, yet infrequent stimuli requiring an overt or covert response (Squires, Donchin, Herning & McCarthy, 1977). P3b is typically associated with memory because its amplitude is decreased when the WM load is increased (García-Larrea & Cézanne, 1998; Strayer & Kramer, 1990; Watter, Geffen & Geffen, 2001). Because the focus of the present study is WM and not orienting attention, we will focus on the mechanism that underlies P3b.

P3b-Eliciting Mechanism The information-processing mechanism that elicits P3b is not fully understood. The most prominent model of the P3b is the context updating model, which suggests that the P3b component detects a mismatch between the “schema” in WM and the incoming input (Donchin, 1981; Donchin & Coles, 1988). This model is based on the finding that the P3b amplitude increases when relevant and unpredictable stimuli are detected. The occurrence of such events is surprising and unexpected, leading to the updating of the WM contents to maintain the most current information about the environment (Donchin, 1981). Specifically, after perceptual encoding, attentional evaluation is used to compare the new event to the previous event maintained in WM. When repeated, the representation of the event remains unchanged, and only sensory ERP components (N100, P200, and N200) are observed. However, if the new event differs from the previous event, then the internal representation is modified according to the new information, resulting in a larger P3b (Polich, 2007). According to this model, the P3b increases with increasing changes in the internal schema (Polich, 2007). The prediction of the context-updating model regarding the effect of WM updating on P3b in the n-back task would be a larger P3b amplitude in alternation trials compared with repetition trials, as WM updating only occurs in the former.

The Present Study The goal of the present study was to understand WM updating. Thus, the aims were threefold. First, to provide empirical support



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for the distinction between automatic and controlled updating. To this end, we examined the pattern of alternation and repetition trial history in 1-back and 2AFC. The second aim was to examine the subprocesses of the n-back task, and identify the role of WM updating in these processes. To this end, we developed a novel variant of the n-back paradigm, the reference-back task. Using this paradigm, in Experiments 2 and 3, we separated WM updating from automatic updating, the comparison process and the effects of gate opening and closing. The third aim was to clarify the nature of the relationship between P3b and WM updating. While the P3b complex has been extensively studied, the association of this component with WM updating has not been directly examined. In Experiment 1, we examined the effects of WM updating on P3b using the n-back task, which manipulates WM updating in a direct manner.

Experiment 1 As previously described, every trial in the n-back task involves updating the associations between the n items in WM and the serial positions (namely, the context of these items). In 1-back however, WM updating is only required when the presented stimuli are different from those presented in Trial N – 1. The RT difference between the mismatch (“different”) and match (“same”) conditions in 1-back however, does not only reflect the pure insertion of a WM updating process. Rather, mismatch trials in 1-back involve three confounding components: (a) the automatic updating effect, resulting from the fact that a mismatch trial is a first-degree alternation; (b) a process of comparison between the stimuli in Trial N and Trial N – 1, manifested in longer RT in mismatch compared with match trials (Farell, 1985); and (c) WM updating, specifically substitution which is the replacement of an old representation with a new one (Ecker, Lewandowsky, Oberauer, & Chee, 2010), is required in mismatch, but not in match trials. The sequential effects in 1-back resulting from automatic updating, comparison and WM updating were compared with the sequential effects in 2AFC, which only reflect automatic updating. We hypothesized that different sequential patterns will emerge in the two tasks. Specifically, we expected that the cost of WM updating in 1-back will be additive to the sequential effects from previous trials. This pattern is consistent with the idea that automatic and controlled updating occurs in separate memory systems, and affect different stages of processing.

Method Participants. Fifteen undergraduate students from BenGurion University of the Negev participated in the experiment (nine males; age: M ⫽ 24.71, SD ⫽ 1.47). All participants were right-handed. Informed consent was obtained from all participants, and these individuals were paid for participation. Three participants were removed from the analysis due to a high EEG artifact rate (⬎40% in one or more of the conditions). Stimuli and apparatus. Stimuli presentation and behavioral data were collected using E-Prime v2.0 (Psychology Software Tools, Pittsburgh, PA). The stimuli are indicated with the letters “X” and “O” in black against a light gray background. The responses were collected using a serial response box. Procedure. Each trial was initiated with a fixation screen presented for 800 ms, followed by a blank, jittered display main-

tained for 400⫺700 ms. The stimulus “X” or “O” was then presented until a response was indicated or until 1,500 ms had elapsed. The participants were instructed to keep their eyes fixated on the center of the screen throughout the experimental blocks. The experiment comprised two parts: a 1-back task, followed by a 2AFC task. In the 1-back task, the participants were required to decide whether each stimulus was the same as that presented in the previous trial. In the 2AFC task, the participants were asked to indicate whether the present stimulus was an “X” or an “O.” Responses for both tasks were indicated using the right and left index fingers, using a serial response box. The key mappings for both tasks were counterbalanced between participants. Each task comprised 20 blocks, including 40 trials each. A practice block containing 40 trials was provided at the beginning of each part. In both tasks, first-degree alternations and repetitions appeared with equal probabilities. That is, in 50% of the trials, the stimulus was identical to that presented in the previous trial, and in 50% of the trials the stimulus was different. The sequences of more than four repetitions or more than four alternations in a row were not permitted. This constraint limited the number of repeated stimuli in a row to four. Electroencephalogram (EEG) recordings and analysis. EEG recordings were obtained using a BioSemi Active Two 64-electrode system. Additional electrodes were placed at the outer left and right canthus and below the left eye to measure the eye movements (electrooculography). The data were acquired using a 0.01- to 100-Hz bandpass filter. The sampling rate was 512 Hz. The signal was digitized using a 24-bit A/D converter. The EEG data were processed using EEGLAB (Delorme & Makeig, 2004) and ERPLAB (Lopez-Calderon & Luck, 2014). The EEG data was offline filtered at 0.01 Hz high-pass and 30 Hz low-pass (infinite impulse response Butterworth filter, attenuation slope of 12 dB/octave), then it was subjected to automatic bad electrodes, eyeblinks, or movement-detection procedures, followed by manual verification. Bad electrodes were interpolated. The EEG was subsequently segmented at 200 ms prestimulus up to 600 ms poststimulus. The segments were averaged and referenced to a linked mastoid electrode and were subsequently submitted to a baseline correction relative to a 200-ms prestimulus baseline.

Results RT. The following analyses included only correct trials preceded by four correct trials in a row. In addition, the outliers were not trimmed, as a 1,500-ms response deadline was used. We initially examined whether the four-deep trial history pattern, namely, the first- and second-degree alternations versus repetition in trials N – 3 through N, differed between the tasks. A two-way analysis of variance (ANOVA) was conducted on mean RT with task (1-back 2AFC) and history (RRRR through AAAA, 16 levels in total; the rightmost letter reflects the most recent trial, namely Trial N) as within subject independent variables. The main effects of task, F(1, 11) ⫽ 89.73, MSE ⫽ 16,826.14, p ⬍ .001, ␩p2 ⫽ .89, and history, F(15, 165) ⫽ 42.17, MSE ⫽ 1,749.71, p ⬍ .001, ␩p2 ⫽ .79, were significant. The two-way interaction was also significant, F(15, 165) ⫽ 18.42, MSE ⫽ 1,497.52, p ⬍ .001, ␩p2 ⫽ .63, indicating that the overall sequential RT pattern differed between the tasks (see Figure 1).


–2 –3 –3 ⫻N–2 ⫻N–3 –2⫻N–3 ⫻N–2⫻N–3

Note. A four-way ANOVA was conducted for each task separately (2AFC, 1-back), with the conditions (repetition vs. alternation) in Trial N, in Trial N – 1, in Trial N – 2, and in Trial N – 3 as within-subject independent variables. Significant effects are indicated in bold.

F(1, 11) ⴝ 73.81, MSE ⴝ 6,305.32, p < .001, ␩p2 ⴝ .87 F(1, 11) ⴝ 84.38, MSE ⴝ 10,328.01, p < .001, ␩p2 ⴝ .88 F(1, 11) ⫽ .13, MSE ⫽ 2,684.57, p ⫽ .73, ␩2p ⫽ .01 F(1, 11) ⫽ .61, MSE ⫽ 1,183.16, p ⫽ .45, ␩2p ⫽ .05 F(1, 11) ⫽ 1.34, MSE ⫽ 2,187.70, p ⫽ .27, ␩2p ⫽ .11 F(1, 11) ⫽ .09, MSE ⫽ 2,367.90, p ⫽ .77, ␩2p ⫽ .01 F(1, 11) ⫽ .02, MSE ⫽ 907.28, p ⫽ .90, ␩2p ⫽ .00 F(1, 11) ⴝ 18.62, MSE ⴝ 2,875.42, p ⴝ .001, ␩p2 ⴝ .63 F(1, 11) ⫽ –.08, MSE ⫽ 2,683.50, p ⫽ .79, ␩2p ⫽ .01 F(1, 11) ⫽ .69, MSE ⫽ 960.70, p ⫽ .42, ␩2p ⫽ .06 F(1, 11) ⫽ 1.43, MSE ⫽ 2,188.06, p ⫽ .26, ␩2p ⫽ .12 F(1, 11) ⫽ .60, MSE ⫽ 1,318.20, p ⫽ .45, ␩2p ⫽ .05 F(1, 11) ⫽ .05, MSE ⫽ 1,308.98, p ⫽ .83, ␩2p ⫽ .00 F(1, 11) ⫽ .67, MSE ⫽ 2,404.95, p ⫽ .43, ␩2p ⫽ .06 F(1, 11) ⴝ 32.09, MSE ⴝ 2,212.62, p < .001, ⴝ .74 F(1, 11) ⴝ 21.94, MSE ⴝ 1,397.42, p < .001, ⴝ .67 F(1, 11) ⴝ 8.90, MSE ⴝ 239.35, p ⴝ .01, ␩p2 ⴝ .45 F(1, 11) ⫽ 1.56, MSE ⫽ 366.96, p ⫽ .24, ␩2p ⫽ .12 F(1, 11) ⴝ 5.85, MSE ⴝ 726.74, p ⴝ .03, ␩p2 ⴝ .35 F(1, 11) ⴝ 9.61, MSE ⴝ 381.82, p ⴝ .01, ␩p2 ⴝ .47 F(1, 11) ⫽ 4.74, MSE ⫽ 225.81, p ⫽ .052, ␩2p ⫽ .30 F(1, 11) ⫽ .92, MSE ⫽ 337.60, p ⫽ .36, ␩2p ⫽ .08 F(1, 11) ⫽ .25, MSE ⫽ 300.52, p ⫽ .62, ␩2p ⫽ .02 F(1, 11) ⫽ 1.10, MSE ⫽ 239.86, p ⫽ .32, ␩2p ⫽ .09 F(1, 11) ⫽ .14, MSE ⫽ 591.41, p ⫽ .71, ␩2p ⫽ .01 F(1, 11) ⫽ 3.28, MSE ⫽ 346.81, p ⫽ .10, ␩2p ⫽ .23 F(1, 11) ⫽ 1.52, MSE ⫽ 286.09, p ⫽ .24, ␩2p ⫽ .12 F(1, 11) ⫽ 1.47, MSE ⫽ 222.24, p ⫽ .25, ␩2p ⫽ .12 –1 –2 –3 ⫻N–1 ⫻N–2 ⫻N–3 –1⫻N –1⫻N –2⫻N ⫻N–1 ⫻N–1 –1⫻N ⫻N–1 N N N N N N N N N N N N N N Trial Trial Trial Trial Trial Trial Trial Trial Trial Trial Trial Trial Trial Trial

Two-alternative forced choice Effect

Next, a more detailed analysis of the sequential effects in each task was performed. A four-way ANOVA was conducted for each task separately, with the first-degree conditions (repetition vs. alternation) in Trials N, N – 1, N – 2, and N – 3 as within-subject independent variables (see Table 1). A main effect for Trial N was observed in both tasks. However, this trial interacted with the conditions in the previous trials only in the 2AFC task (Table 1 and Figure 2). The underadditive interaction in 2AFC, observed in Figure 2, results from the first- and second-order alternation effects in Trials N and N – 1. The error proportion (PE) data did not compromise the RT data. Two four-way ANOVAs were conducted as done with the RT data. No significant effects were observed in 2AFC. In 1-back the only significant effect was for Trial N – 1, F(1, 11) ⫽ 23.00, MSE ⫽ 0.01, p ⬍ .001, ␩p2 ⫽ .68. No other effect reached significance. ERP results. Analysis of the ERP data included only correct trials preceded by two correct trials in a row. Consistent with previous studies (e.g., Comerchero, & Polich, 1999; Squires, Squires, & Hillyard, 1975), the peak P3b activity was observed in the central parietal regions (Figure 3a). Accordingly, the analysis was conducted using the Pz electrode. The analysis focused on a 300 – 400-ms poststimulus window, in which maximal amplitude was observed. The mean amplitude and the 50% fractional area latency measures were used because these measures are resistant to latency variability and latency jitter problems (Luck, 2014). Amplitude. The mean amplitude in electrode Pz between 300 and 400 ms was subjected to a three-way ANOVA with task (1-back 2AFC), Trial N (alternation, repetition), and Trial N – 1 (alternation, repetition) as within-subject independent variables (Figures 3b, 3c). Significant main effects were observed for Trial N, F(1, 11) ⫽ 17.12, MSE ⫽ 1.74, p ⫽ .002, ␩p2 ⫽ .61, and Trial N – 1, F(1,

␩p2 ␩p2

Figure 1. Analysis of variance results of the sequential effects of both tasks are compared. The x-axis includes all 16 combinations of trial history in the past four trials. Due to the large reaction time (RT) difference in baseline performance between the tasks (namely, the main effect of task), mean RTs were transformed to z scores within each task to enable a more direct comparison. In both tasks, alternation in Trial N was slower than repetition. However, this effect interacted with previous trials. See text for details. 2AFC ⫽ two-alternative forced choice.

Table 1 Reaction Time Results of Experiment 1


1-back

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Figure 2. The two-way interaction between Trial N and Trial N – 1 in both tasks is presented. Reaction time (RT) results are presented on top, PE are depicted at the bottom. In all the figures, error bars represent 95% confidence intervals (Masson & Loftus, 2003). On the left, an under additive interaction is observed in two-alternative forced choice (2AFC), on the right, an additive interaction is observed in 1-back.

11) ⫽ 13.94, MSE ⫽ 4.67, p ⫽ .003, ␩p2 ⫽ .56. The main effect of task was nonsignificant, F(1, 11) ⫽ .17, MSE ⫽ 32.06, p ⫽ .69, ␩p2 ⫽ .01. The only interaction that reached significance was the two-way interaction between task and Trial N, F(1, 11) ⫽ 13.73, MSE ⫽ 0.93, p ⫽ .003, ␩p2 ⫽ .55. The simple effect of Trial N was significant in 1-back F(1, 11) ⫽ 35.20, MSE ⫽ 1.16, p ⬍ .001, ␩p2 ⫽ .76, reflecting a larger mean amplitude for repetition compared with alternation (11.45 vs. 9.61 ␮V, respectively). The simple effect of Trial N in 2AFC was nonsignificant, F(1, 11) ⫽ 1.17, MSE ⫽ 1.51, p ⫽ .30, ␩p2 ⫽ .10. To summarize, the P3b amplitude is sensitive to sequential effects, evidenced as the main effect of Trial N – 1. However, a difference between the tasks was only observed in Trial N. Latency. A parallel three-way ANOVA was conducted on a 50% fractional area latency in electrode Pz between 300 and 400 ms. This method identifies the time point in the aforementioned latency range, which divides the area under the ERP waveform into two equal areas. Significant main effects were observed for task, F(1, 11) ⫽ 5.90, MSE ⫽ 15.94, p ⫽ .03, ␩p2 ⫽ .35 (354 vs. 356 ms for 2AFC vs. 1-back respectively), and for Trial N, F(1, 11) ⫽ 11.66, MSE ⫽ 55.41, p ⫽ .006, ␩p2 ⫽ .51 (352 vs. 358 ms for repetition vs. alternation). None of the other effects was significant (all Fs ⬍ 2).

Discussion The RT results revealed different sequential patterns in the 2AFC task and in the 1-back task (Figures 1–3). In 2AFC, an accumulative effect was observed along trials N – 4 to N. Specifically, the RT in Trial N increased as a function of the number and recency of past alternations. The only exception to this observation was Condition AAAA, in which RT benefits from second-degree repetitions (i.e., of the condition, rather than the S-R mapping).

Thus, alternations that are preceded by alternations gained from second-order repetition, and the difference between AR and AA is reduced. Based on the additive factors logic (Sternberg, 1969), the interactions observed between the condition in Trial N and the condition in earlier trials reflect the fact that both Trial N and earlier trials affected the same processing stage. Because WM updating is not required in 2AFC, the effect of both first- and second-degree repetitions/alternations in all trials, including Trial N, solely reflected automatic updating, likely attributed to LTM. In contrast, performance on the 1-back task revealed a different pattern, suggesting a different architecture. The effect of Trial N in 1-back was additive to the effects of the conditions in the previous trials (see Figure 2). Based on the additive factors logic, this finding suggests two independent sources of contribution to overall RT (see Figure 4). The first source is WM updating, as reflected by the effect of Trial N. Longer RTs for alternations compared with repetitions at least partially reflect a pure insertion of WM updating. In addition, the accumulative sequential history from the previous trials affected RT independently of Trial N, reflecting the inevitable effect of automatic updating. While it is currently difficult to determine the specific processing stages affected by each updating process, we suggested that automatic updating might affect “early” perceptual stages and “late” response-related processing stages. In contrast, the condition in Trial N affected WM updating and response selection, that is, the decision whether to respond in the “same” or “different” manner. We attempted to distinguish between these two processes, updating and decision (“matching”), in Experiments 2 and 3. Another important difference between 1-back and 2AFC reflects inconsistent S-R associations in the former compared with consistent mappings in the latter (Szmalec, Verbruggen, Vandierendonck & Kemps, 2011). For example a stimulus repetition in 1-back



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Figure 3. (a) A scalp map of interpolated mean potential between 350 and 380 ms, for both tasks in four conditions: RR (repetition in both Trial N and Trial N – 1), AR (repetition in Trial N and alternation in Trial N ⫺ 1), RA (alternation in Trial N and repetition in Trial N ⫺ 1), AA (alternation in both Trial N and Trial N – 1). Positive potentials are observed around electrode Pz in both tasks, with larger positive potentials in 1-back. ERP waveforms describe Pz electrode activity in two-alternative forced choice (2AFC; Panel b) and 1-back (Panel c) in the four conditions. In both tasks, larger amplitudes were found for repetitions compared to alternations in Trial N ⫺ 1 (solid vs. dashed lines, respectively, in both colors). However, an effect for Trial N was only observed in 1-back, demonstrating larger amplitudes for repetition compared to alternation (red vs. blue lines, respectively). See the online article for the color version of this figure.

might indicate either a response repetition (e.g., XXX) or a response alternation (e.g., OXX), depending on the stimulus in Trial N – 2. Arguably, replacing the relevant S-R association alone is a form of memory updating (Hommel, 1998), which might contribute to the sequential effect pattern in 1-back. Thus, we refer to local inconsistency, namely the persistence of a stable S-R association from one trial to another. Local inconsistency was observed in two situations. The first situation involved repetition in Trial N and alternation in Trial N – 1 (Condition AR; e.g., the sequence XOO or OXX), the stimulus in Trial N was repeated, and the

response (“same”) required alternation compared with the previous trial (“different”). The second situation was when the stimulus in Trial N was alternated compared with Trial N – 1, but the response was repeated (Condition AA; e.g., sequence XOX or OXO). In contrast to these conditions, in two other situations, the S-R associations remained locally consistent. This finding results when a repetition in Trial N is preceded by a prior repetition (Condition RR; as in the sequence XXX or OOO), and also when both the stimulus and the response in Trial N are alternated (Condition RA; e.g., the sequence XXO or OOX). Because locally inconsistent


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Figure 4. The theoretical description of the process in both tasks is presented. The upper part is the description of the two-alternative forced choice (2AFC) task and at the bottom is the description of the 1-back task. The content of working memory (WM) and long-term memory (LTM) is described in each task. In LTM, decaying effect of previous trials (N ⫺ 1, N ⫺ 2, N ⫺ 3) on performance is described by the decaying coloring. The arrows between previous trials represents interactions between them. In 2AFC, the LTM contains previous stimuli, responses and their binding. Binding is depicted by the two-way arrow. The effect of each previous trial on performance is accumulative. WM is empty during this task. In 1-back the LTM contains previous stimuli, responses, conditions and their bindings. Previous trials only interact between themselves in LTM while Trial N is represented in WM. WM contains the target stimulus and its serial position. The gate is set between LTM and WM.

conditions require updating the S-R associations, the RT under these conditions was slower than the RT in locally consistent conditions. Another source for this slowing is the need to overcome proactive interference from previously active S-R associations (Szmalec et al., 2011). The RT results obtained in the 1-back task are consistent with this line of reasoning (see Figure 2). Specifically, in every condition in Trial N, consistent trials were faster than inconsistent trials (RR vs. AR and RA vs. AA, respectively). In other words, S-R consistency was completely correlated with the condition in Trial N – 1, N – 1 repetitions consistently lead to locally consistent mapping, and N – 1 alternations generated locally inconsistent mappings. Because this finding was orthogonal to the condition in Trial N, the effects of local-consistency cannot account for the difference observed between the two paradigms. Rather, local-consistency most likely reflects the automatic updating effects observed in 1-back. Comparing the P3b in 2AFC and 1-back allowed to examine the link between the P3b component and updating/alternation. Note that only two stimuli are used in each of these tasks, as typically done in the oddball paradigm. Here, instead of manipulating the frequency of the stimuli, we examined the effects of alternation (in 2AFC) versus updating (in 1-back) on P3b amplitude. The most relevant ERP finding in these tasks was the observation of a

significant effect of Trial N on P3b amplitude in 1-back but not in 2AFC. But, the direction of this effect was in the opposite direction to that predicted by the context updating model. Specifically, the P3b amplitude was smaller in alternation than in repetition in Trial N, although alternation (rather than repetition) requires WM updating. These results can be explained through the event categorization model (Kok, 1997, 2001). This model regards the P3b as reflecting a target categorization mechanism, which shares processing capacity with WM. This mechanism compares the perceived stimulus to an internal representation and categorizes this stimulus as a match or a mismatch to the target. Thus, novelty, saliency, and task-relevance facilitate categorization and lead to an increase in amplitude. However, under high WM load, where WM operation is more demanding, the reallocation of capacity between WM and the categorization process was required, resulting in less attentional resources for categorization and leading to a decrease in amplitude (Kok, 2001; Watter, Geffen, & Geffen, 2001). The P3b amplitude in 1-back was smaller for alternation than for repetition, reflecting the reduced resources for categorization. Alternation trials impose greater processing load on WM, due to WM updating in these trials, thereby leading to the reallocation of resources. In summary, the ERP results of Experiment 1 did not support the association of P3b with WM updating, but did support the notion that comparison is a relevant mechanism for this component. Note that attentional models (Friedman, Cycowicz, & Gaeta, 2001; Kok, 2001; Schröger, 1996) are not discussed because these models primarily explain the P3a or the “novelty” element of the P3 complex, and in either case, these models undermine the relationship between P3 and WM updating. Based on these results, and the fact that the primary focus in the present study was understanding WM updating, we did not measure ERP in the following experiments. A possible limitation to this argument is that the difference between the two tasks cannot be solely attributed to the insertion of controlled updating in 1-back. Another possibility is that the difference between the tasks is due to the comparison process which only takes place in 1-back. The source of the differences between the tasks should be obtained using a paradigm in which the comparison process is orthogonally manipulated to stimulus alternation/repetition and WM updating. Experiment 2 was designed to examine the role of the comparison process in 1-back.

Experiment 2 Experiment 1 demonstrated that the condition in Trial N was additive to those of the previous trials in 1-back but not in 2AFC. However, as explained earlier, this difference could reflect either WM updating (specifically, substituting the maintained item in WM) or the comparison process, both take place only in 1-back. The goal of Experiment 2 was to decide among these options, by deconfounding the processes in the experimental paradigm. To separate WM updating and comparison processes in the 1-back paradigm, we designed a novel variant: the reference-back task (see Figure 5). In each trial of this task, one of the letters “X” or “O” is presented. The stimulus might either appear in blue or red. Critically, the participants were required to decide whether each stimulus was the same as or different from the previous red letter (the colors described here are only for illustration. In practice, the color-condition mapping was counterbalanced across participants). This task is different from the standard 1-back task, in



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Figure 5. The reference-back task. In each trial, participants are required to indicate whether the presented letter is same or different from the most recent red letter (the bold black letter in the figure). Consequentially, trials with blue stimuli (the empty black letters in the figure) involve only the comparison process (comparison trials). Trials with red stimuli (reference trials) require both comparison and working memory updating in addition.

which each stimulus was compared with the immediately preceding stimulus. Stimuli that appear in blue have been compared with the most recent red item, which is stored in WM. However, blue stimuli do not need storage in WM, and hence they do not trigger WM updating. Accordingly, blue stimuli only involve a comparison process, but do not involve WM updating. By contrast, red stimuli require both comparison and updating because the presented red letter would serve as a reference to which the future stimuli will be compared, until a new red letter appears. Thus, the reference-back task included two trial types: comparison trials (e.g., blue) and reference trials (e.g., red). Similarly to common versions of the 1-back task, only one reference stimulus was maintained in WM in each trial. Comparison of behavioral performance between reference and comparison trials facilitated the isolation of the contribution of the WM updating process (specifically, substitution), which only occurs in the former condition.

Method Participants. Twenty-one undergraduate students from BenGurion University of the Negev participated in the experiment (11 males; age M ⫽ 25.67 years, SD ⫽ 1.85). All participants were right-handed. Informed consent was obtained and all individuals were paid for participation. Stimuli and apparatus. The apparatus was the same as in Experiment 1. The stimuli are indicated with the letters “X” and “O,” presented in either blue or red against a light gray background. Procedure. Each trial was initiated with a fixation screen presented for 800 ms, followed by a blank display whose duration was jittered between 400 and 700 ms. Subsequently, the stimulus “X” or “O” was presented, in either red or blue, until a response was indicated or until 1,500 ms had elapsed. The reference-back task comprised two trial types, reference and comparison (see Figure 5). The sequence of trial types was fixed,

thus each reference trial was followed by three comparison trials, and so forth. The stimulus in each trial was selected at random. For half of the participants, the stimuli appeared in red in reference trials and in blue in comparison trials, and vice versa for the other half. In each trial, the participants had to indicate whether the stimulus was the same as that appearing in the most recent reference trial. “Same” and “different” responses were indicated using the right and left index fingers, using a serial response box. The response keys were counterbalanced between participants. The experiment comprised 15 blocks, including 60 trials each.

Results and Discussion We examined whether the trial type affected the differences between the match and mismatch conditions. Specifically, a larger difference was predicted in reference trials than in comparison trials, reflecting an additional process of substitution that only takes place in reference trials. There are two viable possibilities as to whether a substitution process is involved in every reference trial. One option is that substitution occurs in all reference trials, but it is slower in the mismatch than in the match condition. This possibility suggests that substitution always takes place, but substituting one item with the exact item is faster than substituting this item with a different item (Oberauer, Souza, Druey, & Gade, 2013). The other possibility is that substitution only occurs in mismatches in reference trials. According to this possibility, whether the gate to WM opens or not is dependent on the stimulus and not only on the trial type. To address this issue, a two-way ANOVA was conducted on the RT data with trial type and matching (match, mismatch) as withinsubject independent variables (see Figure 6). Only correct trials preceded by two correct trials of the same trial type were included in this analysis. For example, accurate reference trials were analyzed only if the two preceding reference trials were accurate as well, rather than the immediately previous comparison trials. Both


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Figure 6. The two-way interaction between matching and trial type. RT ⫽ reaction time; PE ⫽ error proportion.

main effects were significant, F(1, 20) ⫽ 70.85, MSE ⫽ 3,503.12, p ⬍ .001, ␩p2 ⫽ .78, for trial type and, F(1, 20) ⫽ 42.59, MSE ⫽ 4,192.61, p ⬍ .001, ␩p2 ⫽ .68, matching. The two-way interaction was also significant, F(1, 20) ⫽ 18.16, MSE ⫽ 1,311.88, p ⬍ .001, ␩p2 ⫽ .48. The interaction effect indicated a larger matching effect (mismatch minus match) in reference trials than in comparison trials (125 vs. 58 ms, respectively). As predicted, this difference reflected the additional substitution process in reference trials. To test the two possibilities discussed earlier, we examined the simple effect of matching within each trial type. This effect was significant in both mismatch trials, F(1, 20) ⫽ 81.03, MSE ⫽ 2,627.46, p ⬍ .001, ␩p2 ⫽ .80, and in match trials, F(1, 20) ⫽ 27.02, MSE ⫽ 2,187.54, p ⬍ .001, ␩p2 ⫽ .57. The latter finding is consistent with the idea that substitution occurs in all reference trials and not only in reference trials with a mismatch, confirming the idea that the state of the gate, rather than the identity of the stimulus, dictates whether updating will occur (see Kessler et al., 2015). Moreover, substitution does not necessarily depend on the result of the matching decision. The PE results were consistent with the RT analysis. An ANOVA was conducted with trial type and matching as independent variables. Both main effects were significant, F(1, 20) ⫽ 24.50, MSE ⫽ .0004, p ⬍ .001, ␩p2 ⫽ .55, for trial type and, F(1,

20) ⫽ 6.75, MSE ⫽ .001, p ⫽ .017, ␩p2 ⫽ .25, for matching. The two-way interaction was also significant, F(1, 20) ⫽ 7.78, MSE ⫽ .001, p ⫽ . 01, ␩p2 ⫽ .28. We also examined the sequential effects in each trial type separately. In each trial type, performance was examined as a function of the alternation and repetition history of the same trial type. For example, performance in reference trials was analyzed as a function of the alternations and repetitions in the preceding reference trials, rather than in the immediately previous comparison trials. This analysis facilitated the extraction of the unique sequential effect pattern of each trial type independently, similar to Experiment 1. We restricted these analyses to two-deep sequential effects to ensure a sufficient number of trials per condition. Because the effect of alternation/repetition in Trial N is orthogonal to the same/different decision in comparison trials, but not in reference trials, the two trial types were analyzed separately. Comparison trials. For this analysis, we examined performance in the third comparison trial in a row (recall that each reference trial was followed by three comparison trials). Only correct trials preceded by two correct trials were included in this analysis (see Figure 7). A within-subject three-way ANOVA was conducted on the RT data with matching (the mismatch vs. match decision, compared with the reference), alternation versus repetition of the stimulus in Trial N, and alternation versus repetition of the stimulus in Trial N – 1. The alternation/repetition conditions in Trials N and N – 1 reflect the sequential history, which is orthogonal to the outcome of the comparison process, being a match (“same”) or mismatch (“different”) decision. Significant main effects were observed for matching, F(1, 20) ⫽ 34.29, MSE ⫽ 4,225.18, p ⬍ .001, ␩p2 ⫽ .63, Trial N, F(1, 20) ⫽ 21.40, MSE ⫽ 2,315.84, p ⬍ .001, ␩p2 ⫽ .52, and Trial N – 1, F(1, 20) ⫽ 18.13, MSE ⫽ 2,230.11, p ⬍ .001, ␩p2 ⫽ .48. Two-way interactions were observed between matching and Trial N, F(1, 20) ⫽ 26.81, MSE ⫽ 1,152.09, p ⬍ .001, ␩p2 ⫽ .57, matching and Trial N – 1, F(1, 20) ⫽ 20.81, MSE ⫽ 789.06, p ⬍ .001, ␩p2 ⫽ .51, and Trial N and Trial N – 1, F(1, 20) ⫽ 23.28, MSE ⫽ 1,014.41, p ⬍ .001, ␩p2 ⫽ .54. The three-way interaction was also significant, F(1, 20) ⫽ 6.27, MSE ⫽ 959.44, p ⫽ .021, ␩p2 ⫽ .24. To explore this interaction, we examined the simple two-way interaction between Trial N and Trial N – 1, separately under match and mismatch conditions. In match trials, alternations in Trial N were slower than repetitions, F(1, 20) ⫽ 51.54, MSE ⫽ 1,539.02, p ⬍ .001, ␩p2 ⫽ .72. However, the effect of Trial N – 1 was nonsignificant, F(1, 20) ⫽ 1.63. Rather, the effect of Trial N – 1 was modulated through a two-way interaction, F(1, 20) ⫽ 17.14, MSE ⫽ 1,560.07, p ⬍ .001, ␩p2 ⫽ .46. Specifically, alternations in Trial N – 1 were not different from repetitions when Trial N was alternation, F(1, 20) ⫽ 2.90, but were slower than repetitions when Trial N was repeated, F(1, 20) ⫽ 23.00, MSE ⫽ 1,002.46, p ⬍ .001, ␩p2 ⫽ .53. Thus, the effect for alternation in Trial N – 1 was only detected when it was followed by repetition in Trial N. In mismatch trials, the main effect for Trial N – 1 was observed, F(1, 20) ⫽ 38.69, MSE ⫽ 1,404.02, p ⬍ .001, ␩p2 ⫽ .66, but not for Trial N, F(1, 20) ⫽ 0.57. The two-way interaction was also significant, F(1, 20) ⫽ 7.00, MSE ⫽ 413.79, p ⫽ .015, ␩p2 ⫽ .26. Alternations in Trial N – 1 were slower than repetitions when the condition in Trial N was alternation, F(1, 20) ⫽ 14.40, but were even slower when Trial N was repetition, F(1, 20) ⫽ 58.61.



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Figure 7. The three-way interaction in comparison trials between matching (match or mismatch to reference), Trial N (alternation, repetition), and Trial N – 1 (alternation, repetition). RT ⫽ reaction time; PE ⫽ error proportion.

Similar to the results of 2AFC, comparison trials also showed an interaction between Trial N and Trial N – 1, as these trials did not involve WM updating. However, the analysis of comparison trials also revealed a more complicated effect of trial history. The results showed a significant interaction in both matches and mismatches, but different main effects. A main effect for alternation/repetition in Trials N – 1 was significant only in mismatches and not for matches, while a significant effect for alternation/repetition in Trials N was observed only in matches and not for mismatches. We suggest that this difference might reflect an additional refreshing process that occurs during conflict (Camos, Mora, & Oberauer, 2011; Raye, Johnson, Mitchell, Greene, & Johnson, 2007). Specifically, alternations in Trial N in matches elicited interference because each process generated a different response. Mismatch trials also elicit interference because the stimulus changed, but unlike in reference trials, here mismatches must not lead to WM updating. Although this interference can be resolved through gating, it is conceivable that refreshing the reference would minimize this interference and ensure that the reference, rather than the comparison stimulus, will be remembered. This refresh might minimize the effects of history in the trials in which it is prompted. The PE results were consistent with the RT analysis. The threeway interaction between matching, alternation/repetition in Trial N, and alternation/repetition in Trial N – 1, was significant, F(1, 20) ⫽ 8.99, MSE ⫽ .0008, p ⫽ .01, ␩p2 ⫽ .31. To explore this interaction, we examined the simple two-way interaction between

Trial N and Trial N – 1, separately under match and mismatch conditions. In match trials, the same pattern of results is observed as in RT analysis. A significant main effect was observed for Trial N, F(1, 20) ⫽ 7.10, MSE ⫽ .003, p ⫽ .015, ␩p2 ⫽ .26, but not for Trial N – 1, F(1, 20) ⫽ 1.52, and a significant interaction between Trials N and N – 1, F(1, 20) ⫽ 12.11, MSE ⫽ .008, p ⫽ .002, ␩p2 ⫽ .38. In mismatch trials, no effect reached significance, F(1, 20) ⫽ .14 for Trial N, F(1, 20) ⫽ .18 for Trial N – 1, and F(1, 20) ⫽ .91 for the interaction. Reference trials. Unlike comparison trials, matching in reference trials is confounded with alternation/repetition. Hence, twoway ANOVA was conducted with stimulus alternation/repetition in Trial N and Trial N – 1 as independent variables. As explained earlier, Trial N – 1 refers to the previous reference trial. Both main effects were significant, F(1, 20) ⫽ 38.39, MSE ⫽ 8,796.23, p ⬍ .001, ␩p2 ⫽ .65, for Trial N, and F(1, 20) ⫽ 14.82, MSE ⫽ 2,120.58, p ⬍ .001, ␩p2 ⫽ .43, for Trial N – 1. The two-way interaction was nonsignificant, F(1, 20) ⫽ 1.22, MSE ⫽ 960.97, p ⫽ .12, ␩p2 ⫽ .12 (see Figure 8). Notably, the additivity of the alternation versus repetition effects in Trials N and N – 1 resembles the findings observed in the 1-back paradigm in Experiment 1. This pattern suggests that the alternation/repetition condition in Trial N reflects a different process than that of Trial N – 1. Replicating the findings of Experiment 1 in reference trials strongly supports our view that the effect in Trial N reflects WM updating, while sequential effects in earlier trials stem from a different system, presumably LTM. In Experiment 1,

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The limitation of the current design of the reference back was that each reference trial also involved a switching effect because the preceding trial was a comparison trial. Switching from maintenance of the reference letter (in comparison trials) to updating of the reference letter (in reference trials) involves a behavioral cost, termed update switching (Kessler & Oberauer, 2014, in press). Kessler and Oberauer (2014) demonstrated that switching between updating to maintenance, and vice versa, is associated with a large RT cost. Update switching is confounded with the reference trial type (this was not the case in comparison trials, in which the first two trials in a row were excluded from the analysis).

Experiment 3

Figure 8. The effects of alternation/repetition in Trials N and N – 1 were additive in reference trials. Note that in these trials mismatches are also always alternations, matches are also always repetitions. RT ⫽ reaction time; PE ⫽ error proportion.

The paradigm used in Experiment 2 consisted of a sequence of one reference trial followed by three comparison trials, and so on. This design raises two difficulties in comparing reference and comparison trials. First, the effects of trial history were measured differently in the two trial types. Comparison trials enabled examining the immediate trial history, while the history of reference trials was measured across the intervening comparison trials. Accordingly, the different effects of trial history could reflect, at least partially, a differential recency effect. Second, in Experiment 2, each reference trial also involved update switching (Kessler & Oberauer, 2014, in press). The update-switch cost and its interaction with the other processes cannot be estimated in our paradigm. Experiment 3 was designed to address these shortcomings. Three types of blocks were administered, with different trial sequences. One block included mostly comparison trials, a second block included mostly reference trials, and a third block included both trial types with equal frequencies. These blocks allowed us to examine the sequential effects of each trial type in comparable conditions, as well as to assess the switch cost of switching between the two trial types and its potential interaction with the other subprocesses.

Method the additivity of Trials N and N – 1 could be explained by either WM updating or the matching decision because neither was involved in 2AFC. The results of Experiment 2 rule out the latter possibility. The conditions in Trials N and N – 1 are additive in reference trials and interactive in comparison trials, despite the fact that both trial types involve a matching decision. Furthermore, the effect of Trial N – 1 in reference trials demonstrates that the effects of automatic updating is not limited solely to immediate history but can rather persist even across a 3-trial lag (in which comparison trials were presented). This is consistent with our extension of the gating model to include a gate between LTM and WM. It is suggested that automatic updating is a function of a memory system that can maintain the trial history of many events, even when these occur at nonsuccessive time points (cf. Logan, 1988). The PE results were consistent with the RT analysis. A parallel two-way ANOVA was conducted with Trials N and N – 1 as independent variables. Both main effects were significant, F(1, 20) ⫽ 7.62, MSE ⫽ .003, p ⫽ .012, ␩p2 ⫽ .28, for Trial N, and F(1, 20) ⫽ 16.46, MSE ⫽ .0008, p ⬍ .001, ␩p2 ⫽ .45, for Trial N – 1. The two-way interaction only approached significance, F(1, 20) ⫽ 3.97, MSE ⫽ .001, p ⫽ .06, ␩p2 ⫽ .17.

Participants. Thirty undergraduate students from Ben-Gurion University of the Negev participated in the experiment (11 males; age M ⫽ 24.83 years, SD ⫽ 1.73). All of the participants were right-handed. Informed consent was obtained from all participants and they were paid for their participation. Procedure. The reference-back task was used with one major modification: The sequence length of reference letters and comparison letters was varied between blocks. In Block A (“mostly comparison”) each reference trial was followed by a sequence of comparison trials, which varied randomly (1, 3, or 5 trials). The purpose of this block was to enable analyzing the sequential effects within a series of comparison trials, independently of update switching. In Block B (“mostly reference”) each comparison trial was followed by a sequence of 1, 3, or 5 reference trials. The aim of this block was to enable analyzing the immediate trial history among reference trials, independently of update switching. Finally, Block C (“equal proportions”) used an alternating runs design (cf. Rogers & Monsell, 1995), in which three reference trials were followed by three comparison trials, and so on. This block enabled to measure the cost of switching between the two trial types. In all blocks, reference letters were always colored in red and comparison letters in blue.


Each trial started with a fixation screen that was presented for 800 ms, followed by a blank display with fix duration of 250 ms. Unlike the previous experiments, the stimulus in each trial was presented until response with no time limitations. The order of block was counterbalanced between subjects. The experiment comprised six blocks (two consecutive repetitions of each block type; e.g., AA BB CC), Block A and Block B contained 20 sequences with varying length of two to six letters. Block C contained 20 sequences of six letters, resulting in 240 trials in each block.


Results and Discussion For the analysis of Block A and Block B, we examined performance starting from the third trial, to exclude the effect of update switching from the data. Correct trials that were preceded by two correct trials were included in the analysis. RTs that were shorter or longer than 3 SD from the mean RT of each condition for each participant were considered as outliers. In addition, RTs faster than 200 ms were also excluded from the analysis. We started by examining sequential effects in comparison trials in Block A (“mostly comparison”) compared with reference trials in Block B (“mostly reference”). Block A (mostly comparison.). A three-way ANOVA was conducted on comparison trials in Block A, with matching in Trial N (mismatch, match), the condition in Trial N (alternation, repetition) and the condition in Trial N – 1 (alternation, repetition) as within-subject independent variables. All main effects were significant, F(1, 29) ⫽ 17.23, MSE ⫽ 19,956.46, p ⬍ .001, ␩p2 ⫽ .37, for matching, F(1, 29) ⫽ 59.79, MSE ⫽ 19,501.21, p ⬍ .001, ␩p2 ⫽ .67, for Trial N, and F(1, 29) ⫽ 14.15, MSE ⫽ 12,730.45, p ⬍ .001, ␩p2 ⫽ .33, for Trial N – 1. The only significant

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interaction was the two-way interaction between Trial N and Trial N – 1, F(1, 29) ⫽ 9.65, MSE ⫽ 12,116.82, p ⫽ .004, ␩p2 ⫽ .25 (Figure 9a). The results present an underadditive interaction between Trial N and Trial N – 1, replicating the sequential effects attributed to LTM that were also observed in Experiment 1 in 2AFC and in Experiment 2 in comparison trials. The PE data did not compromise the RT data. A parallel three-way ANOVA was conducted as in RT analysis. Only the effect of Trial N reached significance, F(1, 29) ⫽ 6.41, MSE ⫽ .01, p ⫽ .02, ␩p2 ⫽ .18. The two-way interaction between Trial N and Trial N – 1 was not significant, F(1, 29) ⫽ 1.27. Block B (mostly reference). In reference trials, matching in Trial N is confounded with alternation/repetition in Trial N and thus a three-way ANOVA is not possible. A two-way ANOVA was conducted on reference trials in Block B, with condition (alternation vs. repetition) in Trials N and Trial N – 1 as withinsubject independent variables. Both main effects were significant, F(1, 29) ⫽ 66.72, MSE ⫽ 27,931.72, p ⬍ .001, ␩p2 ⫽ .70, for Trial N, and, F(1, 29) ⫽ 98.42, MSE ⫽ 17,795.31, p ⬍ .001, ␩p2 ⫽ .77, for Trial N – 1. The two-way interaction was nonsignificant, F(1, 29) ⫽ 2.36, MSE ⫽ 27,569.62, p ⫽ .13, ␩p2 ⫽ .07 (Figure 9b). This additive pattern replicates the findings obtained in the 1-back task in Experiment 1 and in reference trials in Experiment 2. The PE data did not compromise the RT data. The same two-way ANOVA was conducted as in the RT analysis. Only a significant effect for Trial N – 1 was observed, F(1, 29) ⫽ 36.43, MSE ⫽ 0.01, p ⬍ .001, ␩p2 ⫽ .56. We compared the effects of Trial N in the two trial types, as performed in Experiment 2. A two-way ANOVA was conducted with trial type (comparison trials from Block A, reference trials from Block B) and matching in Trial N as independent variables.

Figure 9. (a) Comparison trials in Block A (“mostly comparison). The two-way interaction between alternation/repetition in Trial N and Trial N – 1. The under-additive interaction replicates the sequential effects seen in Experiment 1 in two-alternative forced choice and in Experiment 2 in comparison trials. (b) Reference trials in lock B (“mostly reference”). The two-way interaction between alternation/repetition in Trials N and Trial N – 1. The additive interaction in reference trials replicates the additive interaction observed in 1-back in Experiment 1. RT ⫽ reaction time; PE ⫽ error proportion.


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Both main effects were significant, F(1, 29) ⫽ 32.33, MSE ⫽ 21,299.17, p ⬍ .001, ␩p2 ⫽ .53, for trial type, and F(1, 29) ⫽ 62.72, MSE ⫽ 13,427.68, p ⬍ .001, ␩p2 ⫽ .68, for Trial N. The two-way interaction was also significant, F(1, 29) ⫽ 46.79, MSE ⫽ 5,257.74, p ⬍ .001, ␩p2 ⫽ .62 (see Figure 10). A larger matching effect (mismatch minus match) was observed in reference trials (258 ms) than in comparison trials (79 ms). As suggested earlier, this difference reflects the additional substitution process that only occurs in reference trials. Similar to Experiments 2, match trials produced longer RT in the reference condition compared with the comparison condition, F(1, 29) ⫽ 9.42, MSE ⫽ 5,912.92, p⫽ .005, ␩p2 ⫽ .24, further suggesting that substitution occurs in all reference trials. The PE data did not compromise the RT data. The same twoway ANOVA was conducted as in RT analysis. Only a significant effect for trial type was observed, F(1, 29) ⫽ 1,247.65, MSE ⫽ 0.02, p ⬍ .001, ␩p2 ⫽ .98. The differential pattern of sequential effects for reference and comparison trials was replicated, consistent with the distinction between WM updating that occurs in Trial N, and the effects of LTM-based automatic updating, which stems from previous trials. However, these results are inconsistent with the three-way interaction observed between matching and previous alternations and repetitions in Experiment 2. The results of Experiment 3 suggest that the comparison process is independent of the sequential effects. Furthermore, the interaction between Trial-Type and match-

Figure 10. The two-way interaction between trial type (comparison from Block A, reference from Block B) and matching in trial. Larger matching effect (mismatch minus match) was observed in reference trials compare to comparison trials. RT ⫽ reaction time; PE ⫽ error proportion.

ing in Trial N was also replicated. This replication suggests that the critical difference between 2AFC and 1-back is WM updating and not a comparison process. Importantly, the reference trials in Block B did not involve an update-switch, in contrast to Experiment 2. Replicating the findings here suggested that update-switch could not account for the differential sequential effects observed between reference and comparison trials. Finally, we examined the effect of switching from one trial type to the other through an analysis of the data in Block C. We first measured the recovery time from switching (also known as asymptotic recovery in task switching literature; Monsell, Sumner, & Waters, 2003) by testing the effect of position-in-run on performance. Because this block comprised alternating sequences of the three trial types in a row, the first trial in the run was a trial-typeswitch trial, and the second and third trials were repetition trials. A three-way ANOVA was conducted with trial type (comparison and reference), position (first, second, and third), and matching (mismatch and match) as independent variables (see Figure 11). Significant main effects were observed for trial type, F(1, 29) ⫽ 34.32, MSE ⫽ 32,226.86, p ⬍ .001, ␩p2 ⫽ .54, position, F(2, 58) ⫽ 69.91, MSE ⫽ 12,498.48, p ⬍ .001, ␩p2 ⫽ .71, and matching, F(1, 29) ⫽ 85.49, MSE ⫽ 69,246.76, p ⬍ .001, ␩p2 ⫽ .75. All two-way interactions were significant: trial type and position, F(2, 58) ⫽ 5.24, MSE ⫽ 13,117.08, p ⫽ .008, ␩p2 ⫽ .15, trial type and matching, F(1, 29) ⫽ 21.25, MSE ⫽ 17,973.67, p ⬍ .001, ␩p2 ⫽ .42, and matching and position, F(2, 58) ⫽ 17.99, MSE ⫽ 8,721.01, p ⬍ .001, ␩p2 ⫽ .38. The three-way interaction was nonsignificant, F(2, 58) ⫽ .92, MSE ⫽ 14,094.92, p ⫽ .40, ␩p2 ⫽ .03. We continued to decompose the two-way interaction between trial type and position to simple effects. The contrast between the first position and positions two and three (the switch cost) was significant in both comparison, F(1, 29) ⫽ 64.72, MSE ⫽ 22,122.60, p ⬍ .001, ␩p2 ⫽ .69, and reference trials, F(1, 29) ⫽ 27.07, MSE ⫽ 16,704.46, p ⬍ .001, ␩p2 ⫽ .48. The contrast between the second and third position was nonsignificant in both trial types: F(1, 29) ⫽ .15, MSE ⫽ 3,603.21, p ⫽ .69, ␩p2 ⫽ .005, for comparison trials and F(1, 29) ⫽ .03, MSE ⫽ 8,800.85, p ⫽ .86, ␩p2 ⫽ .001, for reference trials. Thus, the switch cost lasted for only one trial, demonstrating a clear one-shot effect for switching. Experiment 3 examined the effect of switching between the two trial types. A switching cost of 106 ms was observed in reference trials, and a cost of 189 ms was observed in comparison trials. In both trial types, the entire effect of switching was limited to the first trial in the sequence, with no further reduction in RT along the following repetition trials. Such a “one-shot switching” pattern is typical in the alternating-runs task switching paradigm, where the sequence of tasks is fixed and thus predictable (Monsell et al., 2003). The finding of a switch cost between the two trial types provides further support to the gating model. As suggested above, in the reference-back paradigm, successful performance in comparison trials depends on closing the gate to WM: Robust maintenance of the reference representation is required for protection from frequent alternation of stimuli in comparison trials. However, in reference trials, the gate to WM should be opened, facilitating the updating of the reference representation. This switching between the two states of the gate leads to a marked cost. Subsequently, we tested the difference between the two switch costs in the two trial types. The difference was significant, F(1,



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Figure 11. The two-way interaction between trial type (comparison trials, reference trials) and position (first, second, third) in Block C is presented. A switch cost is observed in both trial types. This cost is larger in comparison trials than in reference trials. RT ⫽ reaction time; PE ⫽ error proportion.

29) ⫽ 6.73, MSE ⫽ 20,424.14, p ⫽ .01, ␩p2 ⫽ .19, presenting a switch-cost asymmetry, where switching to comparison trials is more demanding than switching to reference trials (PE data below). For PE, a parallel three-way ANOVA was conducted with trial type (comparison, reference), position (first, second, third), and matching (mismatch, match). Significant main effects were observed for trial type, F(1, 29) ⫽ 9.09, MSE ⫽ .003, p ⫽ .005, ␩p2 ⫽ .24, and for position, F(2, 58) ⫽ 16.38, MSE ⫽ 0.003, p ⬍ .001, ␩p2 ⫽ .36. Matching did not reach significance, F(1, 29) ⫽ 1.37, MSE ⫽ .005, p ⫽ .25, ␩p2 ⫽ .05. The two-way interaction between trial type and matching, F(1, 29) ⫽ 4.97, MSE ⫽ .008, p ⫽ .034, ␩p2 ⫽ .15, and matching and position, F(2, 58) ⫽ 5.21, MSE ⫽ .004, p ⫽ .008, ␩p2 ⫽ .15, were significant. The two-way interaction between trial type and position, only approached significance, F(2, 58) ⫽ 2.94, MSE ⫽ .004, p ⫽ .06, ␩p2 ⫽ .09. Considered with caution, this finding suggests that switching was more costly, in terms of PE, when switching to reference trials. This finding is inconsistent with the RT data, indicating a switch-cost asymmetry in the other direction. More research is required to investigate this point. The three-way interaction was nonsignificant, F(2, 58) ⫽ 1.22, MSE ⫽ .003, p ⫽ .30, ␩p2 ⫽ .04. The results of Experiment 3 also bare implications on understanding the default mode of the gate. Biological models assume that the gate is closed by default and only opens when required (O’Reilly & Frank, 2006). The results obtained here, however, suggest that the default state of the gate might depend on the task. Update switching was detected only when robust maintenance of the reference trial was necessary. Therefore, when no robust maintenance is required, the gate to WM remains open (cf. the full

display paradigm in Kessler & Oberauer, 2014) and stimuli are updated in every reference trial.

General Discussion The first goal of the present study was to distinguish between controlled and automatic updating at both the theoretical and empirical levels. Building on the gating model of WM, we suggested that WM updating depends on control over the gate to WM. In contrast, automatic updating reflects obligatory encoding and retrieval in LTM. This distinction is analogous to the difference between early and late attentional selection. Automatic updating is triggered through early selection and only depends on attending to the relevant stream of information (perceptual or LTM-based). WM updating also depends on gating, presumably based on the task-relevance of the input for the ongoing activity. We compared the sequential effects observed in the 2AFC and 1-back tasks, as 1-back, as opposed to 2AFC, requires updating WM with task-relevant information in each trial (or, to the very least, in mismatch trials). Previous studies demonstrated the prevalence of sequential effects in 2AFC tasks, at least five trials prior. Apart from observing a similar pattern, Experiment 1 demonstrated that the alternation/repetition effects in all 4 previous trials affect RT through interactions with the alternation/repetition condition in Trial N. Basing on Additive Factors logic, this finding is compatible with the view that the effects of all trials influence the same processing stage, presumably (but not necessarily) perceptual and/or response-related stages. In 1-back however, the effect of alternation/repetition in Trial N was additive to the sequential effects of earlier trials, consistent with the idea that these effects


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are two sources of updating. Trial N reflects WM updating, while earlier trials reflect automatic updating, which affects independently performance. Performance in 1-back and 2AFC tasks can also be described using the procedural and declarative WM framework (Oberauer et al., 2013). According to this model, WM is comprises two modules, a declarative system (WMd) for maintaining and processing declarative information (letters, colors), and a procedural system (WMp) for the maintenance of goals and task-rules. Performance on the 2AFC task is affected through the S-R rule, which is the task rule (e.g., arbitrary or based on LTM; Shahar, Teodorescu, Usher, Pereg, & Meiran, 2014). The association between the stimulus and response representations is initially generated in WMp (Meiran, Pereg, Kessler, Cole, & Braver, 2015). Because S-R mapping is consistent, the stimuli were quickly learned, and consequently performance becomes less dependent on WM and more dependent on LTM (Shahar et al., 2014). In addition to the storage of S-R mappings, LTM also maintains traces of active information from previous trials, such as the previous instances of the stimuli, responses and conditions. This information is stored along with the current perceptual stimulus and the other characteristics of the present trial. In contrast, in the n-back task WMd is needed in addition to LTM to maintain the reference to which the present stimulus should be compared. While LTM generates familiarity, this parameter is not sufficient for correct performance in n-back, particularly for n ⬎ 1, where irrelevant stimuli within the maintained set compete with the relevant reference during retrieval, leading to an intrusion effect (Oberauer, 2005; Szmalec et al., 2011). Although this problem does not occur in 1-back the familiarity of irrelevant items remains high, and WMd is still needed to facilitate recollection of the exact stimulus presented in position n – 1. Relying solely on familiarity is particularly difficult when the overall number of stimuli is small, as in the present study (in which only two stimuli were used, “X” and “O”). In Experiments 2 and 3, we continued after separating between the subprocesses of n-back using the reference-back paradigm. Namely, the comparison process, automatic updating and update switching were measured directly and independently of other subprocesses, thus facilitating the assessment of the unique contribution of WM updating to performance. The results replicated the additive interaction between trials N and N – 1 in reference trials, even when these trials were not immediately followed by each other (Experiment 2). These results reflect a separate memory trace in LTM for each trial type, strengthening the idea that automatic updating must rely on LTM. Moreover, Experiment 3 also provides additional behavioral support for the gating model and suggests that the opening and closing of the gate might require different effort. A potential limitation of the present version of reference-back is the use of only two stimuli. In Experiment 1, the 1-back also comprised only two stimuli, whereas more “standard” versions of the 1-back use a larger stimulus set. However, this two-stimuli design did not limit the implications of the findings (neither for 1-back nor for the reference-back), but rather reinforces these results. This design was advantageous for two reasons: The first is that the design facilitated the direct measurement of the sequential effects, similar to the 2AFC task. Second, this design facilitated the balanced manipulation of stimuli (equal proportion of “X” and

“O”) and the conditions (match/no-update, mismatch/update). This unbiased design maximized interference of familiarity from previous trials, and thus the observed dissociation between WM updating and automatic updating was even more difficult under these conditions. Moreover, because the results of Experiment 1 showed that the local-inconsistency effect (between stimulus and response) was limited to Trial N – 1 it can be safely assumed that the processing of Trial N in the current designs of both 1-back and reference-back is not different from a design that uses more stimuli. However, future studies should test these effects again in a design that uses a larger set of stimuli. Another limitation of the current version of the reference-back is that the conclusions primarily address the major subprocesses of 1-back but not n-back. The achieved understanding of n-back is admittedly limited at present. The obtained findings from the reference-back could serve as a starting point for a broader understanding of performance in the n-back task. The ERP results of Experiments 1 failed to find a direct link between the P3b component and WM updating. Although many studied regard P3b as an index for WM updating (e.g., Dehaene, Changeux, Naccache, Sackur, & Sergent, 2006; George & Coch, 2011; King, Gramfort, Schurger, Naccache, & Dehaene, 2014) a direct empirical support for this finding is still missing. Most studies have demonstrated that the P3b component is sensitive to expectations (i.e., novelty; Donchin, 1981; Fjell, Rosquist, & Walhovd, 2009; Polich, 2007; Polich & Criado, 2006; Schröger, 1996) and sequential history (Jentzsch & Sommer, 2001; Sommer, Leuthold, & Soetens, 1999; Squires et al., 1976). Although these factors are likely associated with memory updating, a direct manipulation of WM was missing in these studies. The present study was designed to address this gap in the literature. As explained earlier, Experiment 1 did not provide support for the WM updating account of P3b (Polich, 2007). Rather, our ERP results are more in line with the event categorization model (Kok, 2001), explaining that P3b is a product of the target categorization mechanism. To conclude, the results of the present study provide further behavioral support in favor of the gating model and extend this model by demonstrating that the gate separates WM from LTM. Importantly, this separation facilitates independent updating at each memory system. Finally, the newly designed reference-back paradigm assessed the main processes involved in updating tasks such as n-back, which are typically confounded within the task.

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Received March 18, 2015 Revision received October 9, 2015 Accepted October 10, 2015 䡲