Direct electrophysiological evidence for the maintenance of retrieval orientations and the role of cognitive control

Retrieval orientations are memory states that bias retrieval towards specific memory contents. Many neuroimaging studies have examined the influence of retrieval orientations on stimulus processing, but very little direct evidence exists regarding the ongoing maintenance of orientations themselves. Participants completed two memory tasks with different retrieval goals. ERPs were time-locked to a pre-stimulus fixation asterisk and contrasted according to retrieval goals. Prestimulus ERPs elicited during the two retrieval tasks diverged at frontal electrode sites. These differences onset early and were sustained throughout the fixation-stimulus interval. The functional and spatiotemporal characteristics of this ERP effect comprise the first direct electrophysiological evidence of the ongoing maintenance of retrieval orientations throughout a task. Moreover, this effect was eliminated in participants who performed a stroop task prior to the memory tests, indicating that reserves of cognitive control play an important role in the maintenance of retrieval orientations throughout memory tasks

Preparation breeds success: Brain activity predicts remembering

Successful retrieval of episodic information is thought to involve the adoption of memory states that ensure that stimulus events are treated as episodic memory cues (retrieval mode) and which can bias retrieval toward specific memory contents (retrieval orientation). The neural correlates of these memory states have been identified in many neuroimaging studies, yet critically there is no direct evidence that they facilitate retrieval success. We cued participants before each test item to prepare to complete an episodic (retrieve the encoding task performed on the item at study) or a non-episodic task. Our design allowed us to separate event-related potentials (ERPs) elicited by the preparatory episodic cue according to the accuracy of the subsequent memory judgment. We predicted that a correlate of retrieval orientation should be larger in magnitude preceding correct source judgments than that preceding source errors. This hypothesis was confirmed. Preparatory ERPs at bilateral frontal sites were significantly more positive-going when preceding correct source judgments than when preceding source errors or correct responses in a non-episodic baseline task. Furthermore this effect was not evident prior to recognized items associated with incorrect source judgments. This pattern of results indicates a direct contribution of retrieval orientation to the recovery of task-relevant information and highlights the value of separating preparatory neural activity at retrieval according to subsequent memory accuracy. Moreover, at a more general level this work demonstrates the important role of pre-stimulus processing in ecphory, which has remained largely neglected to date

Electrophysiological evidence for flexible goal-directed cue processing during episodic retrieval

A widely held assumption is that memory retrieval is aided by cognitive control processes that are engaged flexibly in service of memory retrieval and memory decisions. While there is some empirical support for this view, a notable exception is the absence of evidence for the flexible use of retrieval control in functional neuroimaging experiments requiring frequent switches between tasks with different cognitive demands. This absence is troublesome in so far as frequent switches between tasks mimic some of the challenges that are typically placed on memory outside the laboratory. In this experiment we instructed participants to alternate frequently between three episodic memory tasks requiring item recognition or retrieval of one of two different kinds of contextual information encoded in a prior study phase (screen location or encoding task). Event-related potentials (ERPs) elicited by unstudied items in the two tasks requiring retrieval of study context were reliably different, demonstrating for the first time that ERPs index task-specific processing of retrieval cues when retrieval goals change frequently. The inclusion of the item recognition task was a novel and important addition in this study, because only the ERPs elicited by unstudied items in one of the two context conditions diverged from those in the item recognition condition. This outcome constrains functional interpretations of the differences that emerged between the two context conditions and emphasises the utility of this baseline in functional imaging studies of retrieval processing operations

Magnetoencephalography as a tool in psychiatric research: current status and perspective

The application of neuroimaging to provide mechanistic insights into circuit dysfunctions in major psychiatric conditions and the development of biomarkers are core challenges in current psychiatric research. In this review, we propose that recent technological and analytic advances in Magnetoencephalography (MEG), a technique which allows the measurement of neuronal events directly and non-invasively with millisecond resolution, provides novel opportunities to address these fundamental questions. Because of its potential in delineating normal and abnormal brain dynamics, we propose that MEG provides a crucial tool to advance our understanding of pathophysiological mechanisms of major neuropsychiatric conditions, such as Schizophrenia, Autism Spectrum Disorders, and the dementias. In our paper, we summarize the mechanisms underlying the generation of MEG signals and the tools available to reconstruct generators and underlying networks using advanced source-reconstruction techniques. We then survey recent studies that have utilized MEG to examine aberrant rhythmic activity in neuropsychiatric disorders. This is followed by links with preclinical research, which have highlighted possible neurobiological mechanisms, such as disturbances in excitation/inhibition parameters, which could account for measured changes in neural oscillations. In the final section of the paper, challenges as well as novel methodological developments are discussed which could pave the way for a widespread application of MEG in translational research with the aim of developing biomarkers for early detection and diagnosis

Pre-retrieval event-related potentials predict source memory during task switching

Neural activity preceding memory probes differs according to retrieval goals. These divergences have been linked to retrieval orientations, which are content-specific memory states that bias retrieval towards specific contents. Here, participants were cued to retrieve either spatial location or encoding operations. On the first trial of each memory task (‘switch’ trials), preparatory ERPs preceding correct source memory judgments differed according to retrieval goal, but this effect was absent preceding memory errors. Initiating appropriate retrieval orientations therefore predicted criterial recollection. Preparatory ERPs on the second trial of each memory task (i.e. ‘stay’ trials) also differed according to retrieval goal, but the polarity of this effect was reversed from that observed on switch trials and the effect did not predict memory accuracy. This was interpreted as a correlate of retrieval orientation maintenance, with initiation and maintenance forming dissociable components of these goal-directed memory states. More generally, these findings highlight the importance of pre-retrieval processes in episodic memory

Cognitive control depletion reduces pre-stimulus and recollection-related measures of strategic retrieval

Background: The ability to strategically retrieve task-relevant information from episodic memory is thought to rely on goal-directed executive processes, and there is evidence that neural correlates of strategic retrieval are sensitive to reserves of cognitive control. The present study extended this work, exploring the role of cognitive control in the flexible orienting of strategic retrieval processes across alternating retrieval goals. Method: Pre-stimulus cues directed participants to endorse memory targets from one of two encoding contexts, with the target encoding context alternating every two trials. Items from the nontarget encoding context were rejected alongside new items. One group of participants completed a Stroop task prior to the memory test in order to deplete their reserves of cognitive control, while a second group performed a control task. Event-related potentials (ERPs) were recorded throughout the memory task, and time-locked to both pre-stimulus cues and memory probes. Results: Control participants’ pre-stimulus ERPs showed sustained divergences at frontal electrode sites according to retrieval goal. This effect was evident on the first trial of each memory task, and linked with the initiation of goal-specific retrieval orientations. Control participants also showed enhanced ERP correlates of recollection (the ‘left parietal effect’) for correctly classified targets relative to nontargets on the second trial of each memory task, indexing strategic retrieval of task-relevant information. Both the pre-stimulus index of retrieval orientation and the target/nontarget left parietal effect were significantly attenuated in participants that completed the Stroop task. Conclusions: The reduction of pre-stimulus and stimulus-locked ERP effects following the Stroop task indicates that available reserves of cognitive control play an important role in both proactive and recollection-related aspects of strategic retrieval

Retrieval and monitoring processes during visual working memory: An ERP study of the benefit of visual semantics

In this study we examined electrophysiological indices of episodic remembering whilst participants recalled novel shapes, with and without semantic content, within a visual working memory paradigm. The components of interest were the parietal episodic (PE; 400-800ms) and late posterior negativity (LPN; 500-900ms), as these have previously been identified as reliable markers of recollection and post retrieval monitoring, respectively. Fifteen young adults completed a visual matrix patterns task, assessing memory for low and high semantic visual representations. Matrices with either low semantic or high semantic content (containing familiar visual forms) were briefly presented to participants for study (1500ms), followed by a retention interval (6000ms) and finally a same/different recognition phase. The event-related potentials of interest were tracked from the onset of the recognition test stimuli. Analyses revealed equivalent amplitude for the earlier PE effect for the processing of both low and high semantic stimulus types. However, the LPN was more negative-going for the processing of the low semantic stimuli. These data are discussed in terms of relatively ‘pure’ retrieval of high semantic items, where support can readily be recruited from semantic memory. However, for the low semantic items additional executive resources, as indexed by the LPN, are recruited when memory monitoring and uncertainty exist in order to recall previously studied items more effectively

Flexible updating of visual working memory - The joint roles of attention and action

Visual working memory allows us to retain information over short periods of time, thereby enabling the comparison of objects separated in time or space. This ability is critical for various tasks, but it is highly limited in capacity (e.g., Luck & Vogel, 2013). As visual information constantly gains or loses relevance as we interact with our environment, there is a need to update the contents of visual working memory in a flexible manner to ensure that its limited capacity is used efficiently. In five studies, this dissertation examined how this updating is accomplished. The first part of this dissertation (Studies I-III) investigated updating following so-called retrocues presented during visual working memory maintenance, indicating some memorized items as more task-relevant than others and thereby inducing a strategic internal orienting of attention (e.g., Griffin & Nobre, 2003). Results showed that this attentional selection of representations yields a benefit (i.e., better memory) for task-relevant information (Studies I – III), with the magnitude of this benefit being related to the attentional efficiency of an individual (Study I). The consequence of this attentional selection for the other, unselected representations is sensitive to task context: When there are graded differences in the relevance of maintained information, the contents of visual working memory can be weighted to reflect these differences. While the most important information is robustly maintained inside the focus of attention, less important information can be maintained in a more vulnerable state outside the focus of attention, from where it can be accessed to be refocused and retrieved if need be (Study I). Studies II and III established that different visual properties (e.g., location or colour) can be used to guide the selection of relevant representations. A basic distinction can be drawn between mechanisms of spatial and feature-based attentional selection, which can be dissociated in terms of behavioural signatures (Study II) and involved cortical areas (Study III). The second part of the dissertation looked at the effects of more natural indicators of the relevance of specific aspects of our visual surroundings: actions and action-intentions. Instead of presenting retrocues during visual working memory maintenance, Studies IV and V used dual-task paradigms, in which an action was to be executed or prepared. This action rendered some items in a concurrently performed memory task more potentially relevant than others. Results revealed that selective action-related processing continues to influence visual processing beyond the perceptual stage, inducing an updating of visual working memory that reflects differences in the action-relevance of representations. Representations that hold potential action-relevance because they spatially correspond to the location of an action goal (Study IV) or because they contain information that is coded on a feature-dimension that is critical for a particular type of action being prepared (Study V) are preferentially maintained and recalled with higher accuracy than information that is action-irrelevant. The effect of actions on maintenance was found to be particularly pronounced when memory load corresponded to the average visual working memory capacity (Study IV), suggesting that information holding potential relevance for an action is prioritized when demand on the system is high. Furthermore, Study IV provided evidence that action-related enhancement at the mnemonic level is spatially not as precise as it has been shown to be for perception (e.g., Baldauf et al., 2006). Instead, results indicated a graded enhancement spreading out from the representation corresponding to the action goal location. In sum, the present dissertation demonstrates that updating of visual working memory is remarkably flexible. Maintained information can be weighted to reflect graded differences in relevance (Studies I-V), irrespective of whether this relevance is explicitly indicated by external cues (Studies I-III) or more implicitly indicated by action intentions (Studies IV and V). Different representational characteristics can guide the selection of relevant memory contents: Updating is induced when some representations are more important than others because they correspond to relevant locations (Studies I-IV) or because they contain a feature, which is more relevant than other features of the same dimension (Studies II and III) or coded on a feature dimension that is more relevant than other dimensions (Study V). This flexibility highlights the versatile nature of visual working memory, which allows for an efficient use of its highly limited capacity in any given situation

The cognitive neuroscience of visual working memory

Visual working memory allows us to temporarily maintain and manipulate visual information in order to solve a task. The study of the brain mechanisms underlying this function began more than half a century ago, with Scoville and Milner’s (1957) seminal discoveries with amnesic patients. This timely collection of papers brings together diverse perspectives on the cognitive neuroscience of visual working memory from multiple fields that have traditionally been fairly disjointed: human neuroimaging, electrophysiological, behavioural and animal lesion studies, investigating both the developing and the adult brain