Electrophysiological correlates of exemplar-specific processes in implicit and explicit memory

Electrophysiological Correlates of Visual Memory Search

In everyday life, we frequently engage in 'hybrid' visual and memory search, where we look for multiple items stored in memory (e.g., a mental shopping list) in our visual environment. Across three experiments, we used event-related potentials to better understand the contributions of visual working memory (VWM) and long-term memory (LTM) during the memory search component of hybrid search. Experiments 1 and 2 demonstrated that the FN400 (an index of LTM recognition) and the CDA (an index of VWM load) increased with memory set size (target load), suggesting that both VWM and LTM are involved in memory search, even when target load exceeds capacity limitations of VWM. In Experiment 3, we used these electrophysiological indices to test how categorical similarity of targets and distractors affects memory search. The CDA and FN400 were modulated by memory set size only if items resembled targets. This suggests that dissimilar distractor items can be rejected before eliciting a memory search. Together, our findings demonstrate the interplay of VWM and LTM processes during memory search for multiple targets.

Using temporo-spatial principal component analysis as tool to dissociate latent ERP components of episodic memory retrieval: Objectifying time-window selection for overlapping ERP components

This report details how principal component analysis (PCA) can be used as a valuable tool to dissociate latent ERP components, even when considerable temporal and spatial overlap makes it difficult to discern ERP effects in standard time windows. We illustrate our methodological approach in a data set from a recognition memory paradigm, in which event-related potential (ERP) correlates of familiarity, recollection and the late parietal negativity (LPN) were partially overlapping. By adapting standard time windows based on the results of a temporo-spatial PCA, small yet reliable ERP correlates reflecting familiarity and recollection for identical items and late recollection for changed items were identified, complementing the result pattern observed in behavioral performance. Due to similar temporo-spatial characteristics and opposing polarities in late parietal ERP correlates associated with memory retrieval, component overlap is often observed in this field of research. Hence, the complex interplay of several processes underlying higher cognitive functions such as memory retrieval may interfere with standard ERP assessment. In such instances, PCA can provide promising ways to objectively assess time window selection for subsequent ERP analyses.

Usefulness of familiarity signals during recognition depends on test format: Neurocognitive evidence for a core assumption of the CLS framework

Familiarity-based discrimination between studied items and similar foils in yes/no recognition memory tests is relatively poor. The complementary learning systems (CLS) framework explains this with the small difference in familiarity strength between targets and foils. The framework, however, also predicts that familiarity values of targets and corresponding similar foils are directly comparable - as long as they are presented side by side in a forced-choice corresponding (FCC) test. This is because in each trial, targets tend to be more familiar than their corresponding foils. In contrast, when forced-choice displays contain non-corresponding foils (FCNC) which are similar to other studied items, familiarity values are not directly comparable (as in yes/no-tasks). In a recognition memory task with pictures of objects, we found that the putative ERP correlate of familiarity, the mid-frontal old/new effect for targets vs. foils, was significantly larger in FCC compared to FCNC displays. Moreover, single-trial target-foil amplitude differences predicted the accuracy of the recognition judgment. This study supports the assumption of the CLS framework that the test format can influence the diagnostic reliability of familiarity. Moreover, it implies that the mid-frontal old/new effect does not reflect the difference in the familiarity signal between studied and non-studied items but the task-adequate assessment of this signal.

Briefly Flashed Scenes Can Be Stored in Long-Term Memory

The capacity of human memory is impressive. Previous reports have shown that when asked to memorize images, participants can recognize several thousands of visual objects in great details even with a single viewing of a few seconds per image. In this experiment, we tested recognition performance for natural scenes that participants saw for 20 ms only once (untrained group) or 22 times over many days (trained group) in an unrelated task. 400 images (200 previously viewed and 200 novel images) were flashed one at a time and participants were asked to lift their finger from a pad whenever they thought they had already seen the image (go/no-go paradigm). Compared to previous reports of excellent recognition performance with only single presentations of a few seconds, untrained participants were able to recognize only 64% of the 200 images they had seen few minutes before. On the other hand, trained participants, who had processed the flashed images (20 ms) several times, could correctly recognize 89% of them. EEG recordings confirmed these behavioral results. As early as 230 ms after stimulus onset, a significant event-related-potential (ERP) difference between familiar and new images was observed for the trained but not for the untrained group. These results show that briefly flashed unmasked scenes can be incidentally stored in long-term memory when repeated.

The impact of perceptual changes to studied items on ERP correlates of familiarity and recollection is subject to hemispheric asymmetries

It is still unclear which role the right hemisphere (RH) preference for perceptually specific and the left hemisphere (LH) bias towards abstract memory representations play at the level of episodic memory retrieval. When stimulus characteristics hampered the retrieval of abstract memory representations, these hemispheric asymmetries have previously only modulated event-related potential (ERP) correlates of recollection (late positive complex, LPC), but not of familiarity (FN400). In the present experiment, we used stimuli which facilitated the retrieval of abstract memory representations. With the divided visual field technique, new items, identical repetitions and color-modified versions of incidentally studied object pictures were presented in either the right (RVF) or the left visual field (LVF). Participants performed a memory inclusion task, in which they had to categorize both identically repeated and color-modified study items as 'old'. Only ERP, but not behavioral data showed hemispheric asymmetries: Compared to identical repetitions, FN400 and LPC old/new effects for color-modified items were equivalent with RVF/LH presentation, but reduced with LVF/RH presentation. By promoting the use of abstract stimulus information for memory retrieval, we were thus able to show that hemispheric asymmetries in accessing abstract or specific memory representations can modulate ERP correlates of familiarity as well as recollection processes.

On the functional significance of retrieval mode: Task switching disrupts the recollection of conceptual stimulus information from episodic memory

Episodic memory retrieval is assumed to be associated with the tonic cognitive state of retrieval mode. Despite extensive research into the neurophysiological correlates of retrieval mode, as of yet, relatively little is known about its functional significance. The present event-related potential (ERP) study was aimed at examining the impact of retrieval mode on the specificity of memory content retrieved in the course of familiarity and recollection processes. In two experiments, participants performed a recognition memory inclusion task in which they had to distinguish identically repeated and re-colored versions of study items from new items. In Experiment 1, participants had to alternate between the episodic memory task and a semantic task requiring a natural/artificial decision. In Experiment 2, the two tasks were instead performed in separate blocks. ERPs locked to the preparatory cues in the test phases indicated that participants did not establish retrieval mode on switch trials in Experiment 1. In the absence of retrieval mode, neither type of studied item elicited ERP correlates of familiarity-based retrieval (FN400). Recollection-related late positive complex (LPC) old/new effects emerged only for identically repeated but not for conceptually identical but perceptually changed versions of study items. With blocked retrieval in Experiment 2, both types of old items instead elicited equivalent FN400 and LPC old/new effects. The LPC data indicate that retrieval mode may play an important role in the successful recollection of conceptual stimulus information. The FN400 results additionally suggest that task switching may have a detrimental effect on familiarity-based memory retrieval.

An ERP study of recognition memory for concrete and abstract pictures in school-aged children

Recognition memory for concrete, nameable pictures is typically faster and more accurate than for abstract pictures. A dual-coding account for these findings suggests that concrete pictures are processed into verbal and image codes, whereas abstract pictures are encoded in image codes only. Recognition memory relies on two successive and distinct processes, namely familiarity and recollection. Whether these two processes are similarly or differently affected by stimulus concreteness remains unknown. This study examined the effect of picture concreteness on visual recognition memory processes using event-related potentials (ERPs). In a sample of children involved in a longitudinal study, participants (N=96; mean age=11.3years) were assessed on a continuous visual recognition memory task in which half the pictures were easily nameable, everyday concrete objects, and the other half were three-dimensional abstract, sculpture-like objects. Behavioral performance and ERP correlates of familiarity and recollection (respectively, the FN400 and P600 repetition effects) were measured. Behavioral results indicated faster and more accurate identification of concrete pictures as "new" or "old" (i.e., previously displayed) compared to abstract pictures. ERPs were characterized by a larger repetition effect, on the P600 amplitude, for concrete than for abstract images, suggesting a graded recollection process dependent on the type of material to be recollected. Topographic differences were observed within the FN400 latency interval, especially over anterior-inferior electrodes, with the repetition effect more pronounced and localized over the left hemisphere for concrete stimuli, potentially reflecting different neural processes underlying early processing of verbal/semantic and visual material in memory.

ERP evidence for hemispheric asymmetries in abstract but not exemplar-specific repetition priming

Implicit memory retrieval is thought to be exemplar-specific in the right hemisphere (RH) but abstract in the left hemisphere (LH). Yet, conflicting behavioral priming results illustrate that the level at which asymmetries take effect is difficult to pinpoint. In the present divided visual field experiment, we tried to address this issue by analyzing ERPs in addition to behavioral measures. Participants made a natural/artificial decision on lateralized visual objects that were either new, identical repetitions, or different exemplars of studied items. Hemispheric asymmetries did not emerge in either behavioral or late positive complex (LPC) priming effects, but did affect the process of implicit memory retrieval proper as indexed by an early frontal negativity (N350/(F)N400). Whereas exemplar-specific N350/(F)N400 priming effects emerged irrespective of presentation side, abstract implicit memory retrieval of different exemplars was contingent on right visual field presentation and the ensuing initial stimulus processing by the LH.

ERP evidence for hemispheric asymmetries in exemplar-specific explicit memory access

The right cerebral hemisphere (RH) appears to be more effective in representing visual objects as distinct exemplars than the left hemisphere (LH) which is presumably biased towards coding objects at the level of abstract prototypes. As of yet, relatively little is known about the role that asymmetries in exemplar-specificity play at the level of explicit memory retrieval. In the present study, we addressed this issue by examining hemispheric asymmetries in the putative event-related potential (ERP) correlates of familiarity (FN400) and recollection (LPC). In an incidental study phase, pictures of familiar objects were presented centrally. At test, participants performed a memory inclusion task on identical repetitions and different exemplars of study items as well as new items which were presented in only one visual hemifield using the divided visual field technique. With respect to familiarity, we observed exemplar-specific FN400 old/new effects that were more pronounced for identical repetitions than different exemplars, irrespective of the hemisphere governing initial stimulus processing. In contrast, LPC old/new effects were subject to some hemispheric asymmetries indicating that exemplar-specific recollection was more extensive in the RH than in the LH. This further corroborates the idea that hemispheric asymmetries should not be generalized but need to be distinguished not only in different domains but also at different levels of processing.

Cerebral correlates of faking: evidence from a brief implicit association test on doping attitudes

Direct assessment of attitudes toward socially sensitive topics can be affected by deception attempts. Reaction-time based indirect measures, such as the Implicit Association Test (IAT), are less susceptible to such biases. Neuroscientific evidence shows that deception can evoke characteristic ERP differences. However, the cerebral processes involved in faking an IAT are still unknown. We randomly assigned 20 university students (15 females, 24.65 ± 3.50 years of age) to a counterbalanced repeated-measurements design, requesting them to complete a Brief-IAT (BIAT) on attitudes toward doping without deception instruction, and with the instruction to fake positive and negative doping attitudes. Cerebral activity during BIAT completion was assessed using high-density EEG. Event-related potentials during faking revealed enhanced frontal and reduced occipital negativity, starting around 150 ms after stimulus presentation. Further, a decrease in the P300 and LPP components was observed. Source analyses showed enhanced activity in the right inferior frontal gyrus between 150 and 200 ms during faking, thought to reflect the suppression of automatic responses. Further, more activity was found for faking in the bilateral middle occipital gyri and the bilateral temporoparietal junction. Results indicate that faking reaction-time based tests alter brain processes from early stages of processing and reveal the cortical sources of the effects. Analyzing the EEG helps to uncover response patterns in indirect attitude tests and broadens our understanding of the neural processes involved in such faking. This knowledge might be useful for uncovering faking in socially sensitive contexts, where attitudes are likely to be concealed.

Sometimes we have to intentionally focus on the details: Incidental encoding and perceptual change decrease recognition memory performance and the ERP correlate of recollection

Prior studies suggest that memory retrieval is based on two independent processes: Recollection and familiarity. Here, we investigated the role of incidental and intentional encoding, and specifically whether perceptual changes between study and test affects behavioral and electrophysiological correlates of both retrieval processes. During retrieval, participants distinguished between identical and changed exemplars as well as novel distractors. Following incidental encoding, participants had difficulty identifying changed exemplars; item and feature recognition increased after intentional encoding, in particular for changed exemplars. Reflecting this increase in memory performance, the ERP correlate of recollection was larger after intentional encoding and for identical item repetitions, whereas the ERP correlate for familiarity was largely unaffected. Pre-response old/new effects corresponding to later aspects of recollection (700-1000 ms relative to stimulus onset) were larger in response-compared to stimulus-locked averages, but also of similar magnitude for identical and changed exemplars. These results corroborate previous findings suggesting that the electrophysiological signature of recollection is modulated as a function of memory performance. The role of task characteristics and material retrieved from memory for modulations in familiarity-based retrieval processes is discussed.

The temporal dynamics of visual object priming

Priming reflects an important means of learning that is mediated by implicit memory. Importantly, priming occurs for previously viewed objects (item-specific priming) and their category relatives (category-wide priming). Two distinct neural mechanisms are known to mediate priming, including the sharpening of a neural object representation and the retrieval of stimulus-response mappings. Here, we investigated whether the relationship between these neural mechanisms could help explain why item-specific priming generates faster responses than category-wide priming. Participants studied pictures of everyday objects, and then performed a difficult picture identification task while we recorded event-related potentials (ERP). The identification task gradually revealed random line segments of previously viewed items (Studied), category exemplars of previously viewed items (Exemplar), and items that were not previously viewed (Unstudied). Studied items were identified sooner than Unstudied items, showing evidence of item-specific priming, and importantly Exemplar items were also identified sooner than Unstudied items, showing evidence of category-wide priming. Early activity showed sustained neural suppression of parietal activity for both types of priming. However, these neural suppression effects may have stemmed from distinct processes because while category-wide neural suppression was correlated with priming behavior, item-specific neural suppression was not. Late activity, examined with response-locked ERPs, showed additional processes related to item-specific priming including neural suppression in occipital areas and parietal activity that was correlated with behavior. Together, we conclude that item-specific and category-wide priming are mediated by separate, parallel neural mechanisms in the context of the current paradigm. Temporal differences in behavior are determined by the timecourses of these distinct processes.

Speaker's voice as a memory cue

Speaker's voice occupies a central role as the cornerstone of auditory social interaction. Here, we review the evidence suggesting that speaker's voice constitutes an integral context cue in auditory memory. Investigation into the nature of voice representation as a memory cue is essential to understanding auditory memory and the neural correlates which underlie it. Evidence from behavioral and electrophysiological studies suggest that while specific voice reinstatement (i.e., same speaker) often appears to facilitate word memory even without attention to voice at study, the presence of a partial benefit of similar voices between study and test is less clear. In terms of explicit memory experiments utilizing unfamiliar voices, encoding methods appear to play a pivotal role. Voice congruency effects have been found when voice is specifically attended at study (i.e., when relatively shallow, perceptual encoding takes place). These behavioral findings coincide with neural indices of memory performance such as the parietal old/new recollection effect and the late right frontal effect. The former distinguishes between correctly identified old words and correctly identified new words, and reflects voice congruency only when voice is attended at study. Characterization of the latter likely depends upon voice memory, rather than word memory. There is also evidence to suggest that voice effects can be found in implicit memory paradigms. However, the presence of voice effects appears to depend greatly on the task employed. Using a word identification task, perceptual similarity between study and test conditions is, like for explicit memory tests, crucial. In addition, the type of noise employed appears to have a differential effect. While voice effects have been observed when white noise is used at both study and test, using multi-talker babble does not confer the same results. In terms of neuroimaging research modulations, characterization of an implicit memory effect reflective of voice congruency is currently lacking.

Identical versus conceptual repetition FN400 and parietal old/new ERP components occur during encoding and predict subsequent memory

This Event-Related Potential (ERP) study investigated whether components commonly measured at test, such as the FN400 and the parietal old/new components, could be observed during encoding and, if so, whether they would predict different levels of accuracy on a subsequent memory test. ERPs were recorded while subjects classified pictures of objects as man-made or natural. Some objects were only classified once, while others were classified twice during encoding, sometimes with an identical picture, and other times with a different exemplar from the same category. A subsequent surprise recognition test required subjects to judge whether each probe word corresponded to a picture shown earlier, and if so whether there were two identical pictures that corresponded to the word probe, two different pictures, or just one picture. When the second presentation showed a duplicate of an earlier picture, the FN400 effect (a significantly less negative deflection on the second presentation) was observed regardless of subsequent memory response; however, when the second presentation showed a different exemplar of the same concept, the FN400 effect was only marginally significant. In contrast, the parietal old/new effect was robust for the second presentation of conceptual repetitions when the test probe was subsequently recognized, but not for identical repetitions. These findings suggest that ERP components that are typically observed during an episodic memory test can be observed during an incidental encoding task, and that they are predictive of the degree of subsequent memory performance.