Alpha‐band desynchronization reflects memory‐specific processes during visual change detection

Recent work investigating physiological mechanisms of working memory (WM) has revealed that modulation of alpha and beta frequency bands within the EEG plays a key role in WM storage. However, the nature of that role is unclear. In the present study, we examined event‐related desynchronization of alpha and beta (α/β‐ERD) elicited by visual tasks with and without a memory component to measure the impact of a WM demand on this electrophysiological marker. We recorded EEG from 60 healthy participants while they completed three variants on a typical change detection task: one in which participants passively viewed the sample array, passive (WM?); one in which participants viewed and attended the sample array in search of a target color but did not memorize the colors, active (WM?); and one in which participants encoded, attended to, and memorized the sample array, active (WM+). Replicating previous findings, we found that active (WM+) elicited robust α/β‐ERD in frontal and posterior electrode clusters and that α‐ERD was significantly associated with WM capacity. By contrast, α/β‐ERD was significantly smaller in the passive (WM?) and active (WM?) tasks, which did not consistently differ from one another. Furthermore, no such relationship was observed between WM capacity and desynchronization in the passive (WM?) or active (WM?) tasks. Taken together, these results suggest that α‐ERD during memory formation reflects a memory‐specific process such as consolidation or maintenance, rather than serving a generalized role in perceptual gating or engagement of attention.     

Reward‐related distracters and working memory filtering

Reward‐related stimuli capture attention, even when they are task irrelevant. A consequence of attentional prioritization of reward‐related stimuli is that they may also have preferential access to working memory like other forms of emotional information. However, whether reward‐related distracters leak into working memory remains unknown. Here, using a well‐validated change detection task of visual working memory capacity and filtering, we conducted two studies to directly assess the impact of reward‐related distracters on working memory. In both studies, the distracters consisted of colored bars or circles that were previously associated with monetary reward. In Experiment 1, results indicated that previously rewarded distracters did not impact behavioral measures of working memory filtering efficiency compared to neutral distracters. In Experiment 2, using ERPs, we measured the contralateral delay activity (CDA), a psychophysiological index of the number of items retained in working memory, to further assess filtering efficiency. We observed that the CDA for high reward distracters was similar to low reward and neutral distracters. However, in early trials, behavioral measures revealed that previously rewarded stimuli negatively impacted working memory capacity, an effect not observed with neutral distracters. This effect, though, was not found for the CDA in early trials. In summary, our findings across two studies suggest that attentional capture by task‐irrelevant reward may have minimal impact on visual working memory—findings that have important implications for delineating the boundaries of reward‐cognition interactions.     

Socioaffective factors modulate working memory in schizophrenia patients

Working memory deficit in schizophrenia is a core cognitive feature of the disorder and is reliably associated with abnormalities of the prefrontal circuitry. Working memory deficits are also associated with impaired social functioning and present a major obstacle toward successful rehabilitation in schizophrenia. Although the role of prefrontal cortex in working memory has been extensively investigated, the intricate relations among the prefrontal circuitry, working memory and social behaviors are not clearly understood. In this study, we manipulated social context and observed its effects on spatial working memory. In experiment 1, the effects of social and asocial reinforcements on spatial working memory were examined in schizophrenic patients and healthy controls. The results show that social but not asocial reinforcements facilitated spatial working memory in schizophrenic patients. In experiment 2, the effects of human voice reinforcements (with or without affect) on working memory was investigated. Voice reinforcements did not facilitate working memory relative to the no-reinforcement condition. There was no difference between high-affect vs flat-affect voice conditions. In experiment 3, the effects of direct and indirect social interactions on spatial working memory were studied. Direct but not indirect social interaction facilitated working memory in schizophrenic patients. These results suggest that social context might facilitate working memory in schizophrenic patients perhaps by activating frontal lobe systems. In addition, the possibility of improving cognitive functions such as working memory using seemingly non-cognitive methods might lead to potential remediation strategies.     

Theta‐ and delta‐band EEG network dynamics during a novelty oddball task

While the P3 component during target detection and novelty processing has been widely studied, less is known about its underlying network dynamics. A recent cognitive model suggests that frontal‐parietal and frontal‐temporal interregional connectivity are related to attention/action selection and target‐related memory updating during the P3, respectively, but empirical work testing this model is lacking. Other work suggests the importance of theta‐ and delta‐band connectivity between the medial frontal cortex and distributed cortical regions during attention, stimulus detection, and response selection processes, and similar dynamics may underlie P3‐related network connectivity. The present study evaluated the functional connectivity elicited during a visual task, which combined oddball target and novelty stimuli, in a sample of 231 same‐sex twins. It was hypothesized that both target and novel conditions would involve theta frontoparietal connectivity and medial frontal theta power, but that target stimuli would elicit the strongest frontotemporal connectivity. EEG time‐frequency analysis revealed greater theta‐band frontoparietal connectivity and medial frontal power during both target and novel conditions compared to standards, which may index conflict/uncertainty resolution processes. Theta‐band frontotemporal connectivity was maximal during the target condition, potentially reflecting context updating or stimulus‐response activation. Delta‐band frontocentral‐parietal connectivity was also strongest following targets, which may be sensitive to response‐related demands. These results suggest the existence of functional networks related to P3 that are differentially engaged by target oddballs and novel distractors. Compared to simple P3 amplitude, network measures may provide a more nuanced view of the neural dynamics during target detection/novelty processing in normative and pathological populations.     

Functional connectivity reveals load dependent neural systems underlying encoding and maintenance in verbal working memory

One of the main challenges in working memory research has been to understand the degree of separation and overlap between the neural systems involved in encoding and maintenance. In the current study we used a variable load version of the Sternberg item recognition test (two, four, six, or eight letters) and a functional connectivity method based on constrained principal component analysis to extract load-dependent neural systems underlying encoding and maintenance, and to characterize their anatomical overlap and functional interaction. Based on the pattern of functional connectivity, constrained principal component analysis identified a load-dependent encoding system comprising bilateral occipital (Brodmann's area (BA) 17, 18), bilateral superior parietal (BA 7), bilateral dorsolateral prefrontal (BA 46), and dorsal anterior cingulate (BA 24, 32) regions. For maintenance, in contrast, constrained principal component analysis identified a system that was characterized by both load-dependent increases and decreases in activation. The structures in this system jointly activated by maintenance load involved left posterior parietal (BA 40), left inferior prefrontal (BA 44), left premotor and supplementary motor areas (BA 6), and dorsal cingulate regions (BA 24, 32), while the regions displaying maintenance-load-dependent activity decreases involved bilateral occipital (BA 17, 18), posterior cingulate (BA 23) and rostral anterior cingulate/orbitofrontal (BA 10, 11, 32) regions. The correlation between the encoding and maintenance systems was strong and negative (Pearson's r = -.55), indicting that some regions important for visual processing during encoding displayed reduced activity during maintenance, while subvocal rehearsal and phonological storage regions important for maintenance showed a reduction in activity during encoding. In summary, our analyses suggest that separable and complementary subsystems underlie encoding and maintenance in verbal working memory, and they demonstrate how constrained principal component analysis can be employed to characterize neuronal systems and their functional contributions to higher-level cognition.     

Time‐frequency approaches to investigating changes in feedback processing during childhood and adolescence

Processing feedback from the environment is an essential function during development to adapt behavior in advantageous ways. One measure of feedback processing, the feedback negativity (FN), is an ERP observed following the presentation of feedback. Findings detailing developmental changes in the FN have been mixed, possibly due to limitations in traditional ERP measurement methods. Recent work shows that both theta and delta frequency activity contribute to the FN; utilizing time‐frequency methods to measure change in power and phase in these frequency bands may provide more accurate representation of feedback processing development in childhood and adolescence. We employ time‐frequency power and intertrial phase synchrony measures, in addition to conventional time‐domain ERP methods, to examine the development of feedback processing in the theta (4–7 Hz) and delta (.1–3 Hz) bands throughout adolescence. A sample of 54 female participants (8–17 years old) completed a gambling task while EEG was recorded. As expected, time‐domain ERP amplitudes showed no association with age. In contrast, significant effects were observed for the time‐frequency measures, with theta power decreasing with age and delta power increasing with age. For intertrial phase synchrony, delta synchrony increased with age, while age‐related changes in theta synchrony differed for gains and losses. Collectively, these findings highlight the importance of considering time‐frequency dynamics when exploring how the processing of feedback develops through late childhood and adolescence. In particular, the role of delta band activity and theta synchrony appear central to understanding age‐related changes in the neural response to feedback.     

Spatial working memory deficits in schizophrenia patients and their first degree relatives from Palau,Micronesia

Spatial working memory deficits associated with dorsolateral prefrontal dysfunction have been found in Caucasian samples of schizophrenia patients and their first‐degree relatives. This study evaluated spatial working memory function in affected and unaffected members of multiplex schizophrenia families from the Republic of Palau to determine whether the spatial working memory deficits associated with schizophrenia extend to this non‐Caucasian population. Palau is an isolated island nation in Micronesia with an elevated prevalence of schizophrenia and an aggregation of cases in large multigenerational families. Our objective was to evaluate the potential for spatial working memory function to serve as one of multiple endophenotypes in a genetic linkage study of these Palauan schizophrenia families. A spatial delayed response task requiring resistance to distraction and a sensorimotor control task were used to assess spatial working memory in 32 schizophrenia patients, 28 of their healthy first‐degree relatives, and 19 normal control subjects. Schizophrenia patients and their relatives were significantly less accurate than normal control subjects on the spatial delayed response task but not on the sensorimotor control task. On both tasks, patients and relatives were slower to respond than the normal controls. There were no age or gender effects on accuracy, and working memory performance in schizophrenia patients was not significantly correlated with medication dosage. In summary, spatial working memory deficits that have been found in Caucasian schizophrenia patients and relatives were confirmed in this isolated Pacific Island family sample. These results suggest that spatial working memory deficits may be a potentially useful addition to the endophenotypic characterization of family members to be used in a comprehensive genome wide linkage analysis of these Palauan families. © 2002 Wiley‐Liss, Inc.     

Event‐related EEG time‐frequency PCA and the orienting reflex to auditory stimuli

We recently reported an auditory habituation series with counterbalanced indifferent and significant (counting) instructions. Time‐frequency (t‐f) analysis of electrooculogram‐corrected EEG was used to explore event‐related synchronization (ERS)/desynchronization (ERD) in four EEG bands using arbitrarily selected time epochs and traditional frequency ranges. ERS in delta, theta, and alpha, and subsequent ERD in theta, alpha, and beta, showed substantial decrement over trials, yet effects of stimulus significance (count vs. no‐task) were minimal. Here, we used principal components analysis (PCA) of the t‐f data to investigate the natural frequency and time combinations involved in such stimulus processing. We identified four ERS and four ERD t‐f components: six showed decrement over trials, four showed count > no‐task effects, and six showed Significance × Trial interactions. This increased sensitivity argues for the wider use of our data‐driven t‐f PCA approach.     

A high‐density EEG study of differentiation between two speeds and directions of simulated optic flow in adults and infants

A high‐density EEG study was carried out to investigate cortical activity in response to forward and backward visual motion at two different driving speeds, simulated through optic flow. Participants were prelocomotor infants at the age of 4–5 months and infants with at least 3 weeks of crawling experience at the age of 8–11 months, and adults. Adults displayed shorter N2 latencies in response to forward as opposed to backward visual motion and differentiated significantly between low and high speeds, with shorter latencies for low speeds. Only infants at 8–11 months displayed similar latency differences between motion directions, and exclusively in response to low speed. The developmental differences in latency between infant groups are interpreted in terms of a combination of increased experience with self‐produced locomotion and neurobiological development. Analyses of temporal spectral evolution (TSE, time‐dependent amplitude changes) were also performed to investigate nonphase‐locked changes at lower frequencies in underlying neuronal networks. TSE showed event‐related desynchronization activity in response to visual motion for infants compared to adults. The poorer responses in infants are probably related to immaturity of the dorsal visual stream specialized in the processing of visual motion and could explain the observed problems in infants with differentiating high speeds of up to 50 km/h.     

The role of inhibition for working memory processes: ERP evidence from a short‐term storage task

Human working memory is the central unit for short‐term storage of information. In addition to the selection and adequate storage of relevant information, the suppression of irrelevant stimuli from the environment seems to be of importance for working memory processes. To learn more about the interplay of information uptake and inhibition of irrelevant information, the present study used ERP measures and a short‐term storage and retrieval task, in which pairs of either numbers or letters had to be compared. Random sequences of four stimuli (two numbers and two letters) were presented, with either the numbers or the letters being relevant for comparison. The analysis of ERPs to each of the four stimuli indicated more pronounced P2 and P3b amplitudes for relevant than irrelevant stimuli. In contrast, the N2 (reflecting inhibitory control) was only elicited by irrelevant stimuli. Moreover, the N2 amplitude of the second irrelevant stimulus was associated with behavioral performance, indicating the importance of inhibition of task‐irrelevant stimuli for working memory processes. In sum, the findings demonstrate the role of cognitive control mechanisms for protecting relevant contents in working memory against irrelevant information.     

Applying computational modeling to assess age‐, sex‐, and strategy‐related differences in Spin the Pots,a working memory task for 2‐ to 4‐year‐olds

Working memory (WM) develops rapidly during early childhood. In the present study, visual WM (VSM) was measured using the well‐established Spin the Pots task (Hughes & Ensor, 2005), a complex non‐verbal eight‐location object occlusion task. A self‐ordered hiding procedure was adopted to allow for an examination of children's strategy use during a VWM task. Participants (N = 640) between the ages of 2 and 4 years were tested under semi‐naturalistic conditions, in the home or in a museum. Computational modeling was used to estimate an expected value for the total trials to complete Spin the Pots via a random search and child performance was compared to expected values. Based on this approach, we determined that children who found six stickers retrieved them in significantly fewer trials than the expected value, excluding chance performance and implicating VWM. Results also showed age‐related and sex‐related changes in VWM. Between 2 and 4 years of age, 4‐year‐olds performed significantly better than younger children and girls out‐performed the boys. Spontaneous use of a color matching hiding strategy was associated with a higher success rate on the task. Implications of these findings for early development of VWM are discussed.     

The neural fate of neutral information in emotion‐enhanced memory

In this study, we report evidence that neural activity reflecting the encoding of emotionally neutral information in memory is reduced when neutral and emotional stimuli are intermixed during encoding. Specifically, participants studied emotional and neutral pictures organized in mixed lists (in which emotional and neutral pictures were intermixed) or in pure lists (only‐neutral or only‐emotional pictures) and performed a recall test. To estimate encoding efficiency, we used the Dm effect, measured with event‐related potentials. Recall for neutral items was lower in mixed compared to pure lists and posterior Dm activity for neutral items was reduced in mixed lists, whereas it remained robust in pure lists. These findings might be caused by an asymmetrical competition for attentional and working memory resources between emotional and neutral information, which could be a major determinant of emotional memory effects.     

Theta‐ and alpha‐power enhancements in the electroencephalogram as an auditory delayed match‐to‐sample task becomes impossibly difficult

Recent studies have related enhancements of theta‐ (∼4–8 Hz) and alpha‐power (∼8–13 Hz) to listening effort based on parallels between enhancement and task difficulty. In contrast, nonauditory works demonstrate that, although increases in difficulty are initially accompanied by increases in effort, effort decreases when a task becomes so difficult as to exceed one's ability. Given the latter, we examined whether theta‐ and alpha‐power enhancements thought to reflect effortful listening show a quadratic trend across levels of listening difficulty from impossible to easy. Listeners (n = 14) performed an auditory delayed match‐to‐sample task with frequency‐modulated tonal sweeps under impossible, difficult (at ∼70.7% correct threshold), and easy (well above threshold) conditions. Frontal midline theta‐power and posterior alpha‐power enhancements were observed during the retention interval, with greatest enhancement in the difficult condition. Independent component‐based analyses of data suggest that theta‐power enhancements stemmed from medial frontal sources at or near the anterior cingulate cortex, whereas alpha‐power effects stemmed from occipital cortices. Results support the notion that theta‐ and alpha‐power enhancements reflect effortful cognitive processes during listening, related to auditory working memory and the inhibition of task‐irrelevant cortical processing regions, respectively. Theta‐ and alpha‐power dynamics can be used to characterize the cognitive processes that make up effortful listening, including qualitatively different types of listening effort.     

Manipulating the focus of attention in working memory: Evidence for a protection of multiple items against perceptual interference

Visual working memory representations can be shielded from interference by selective attentional focusing using retroactive cues (retro‐cues). However, it is not clear how many representations can be effectively cued and which neural mechanisms provide the protection from distractors. To address these questions, we manipulated the number of attended items by means of a retro‐cue (one, two, or three items) and presented a distractor display during the retention of information in working memory. Analyses of the raw error and a mixture model revealed that a general performance benefit was only present when one item was retro‐cued. Nevertheless, a protection of the item representations against subsequent interference occurred also after a two‐item cue. ERPs revealed a modulation of the posterior negative slow wave following the retro‐cues, reflecting the applied working memory resources dependent on the number of attended representations. Further, the distractor information was encoded into working memory only when the number of attended items was not changed by the retro‐cues (neutral and three‐item conditions), reflected by a P3b following the distractor display. These results suggest that more than one item can be effectively protected from perceptual interference. We propose that the protective effect of selective attention within working memory is based on both a reduction of the number of focused representations and the attentional refreshing of item context and features.     

Stimulus sequence context differentially modulates inhibition‐related theta and delta band activity in a go/no‐go task

Recent work suggests that dissociable activity in theta and delta frequency bands underlies several common ERP components, including the no‐go N2/P3 complex, which can better index separable functional processes than traditional time‐domain measures. Reports have also demonstrated that neural activity can be affected by stimulus sequence context information (i.e., the number and type of preceding stimuli). Stemming from prior work demonstrating that theta and delta index separable processes during response inhibition, the current study assessed sequence context in a go/no‐go paradigm in which the number of go stimuli preceding each no‐go was selectively manipulated. Principal component analysis of time‐frequency representations revealed differential modulation of evoked theta and delta related to sequence context, where delta increased robustly with additional preceding go stimuli, while theta did not. Findings are consistent with the view that theta indexes simpler initial salience‐related processes, while delta indexes more varied and complex processes related to a variety of task parameters.     

Differentiation of neuronal operations in latent components of event‐related potentials in delayed match‐to‐sample tasks

The goal of this study was to decompose ERPs into latent components associated with hypothetical processes of category discrimination, comparison to working memory and action‐related operations. In five variants of the delayed match‐to‐sample s1‐s2 task, instructions were varied for manipulation of the processes. The blind source separation was applied to the collection of ERPs. The category discrimination operation is attributed to three latent components with peak latencies of 130 to 170 ms, which are generated in different parts of the prestriate cortex. The comparison to working memory operation is attributed to a latent component that is generated in the temporal cortex and manifested in a positive deflection with a peak latency of 250 ms after s2. The category discrimination and comparison to working memory effects were dissociated spatially and temporally from attention and action selection effects.     

Voxel‐wise brain‐wide functional connectivity abnormalities in first‐episode,drug‐naive patients with major depressive disorder

Due to different foci and single sample across studies, abnormal functional connectivity (FC) has been implicated in the pathophysiology of major depressive disorder (MDD) with inconsistent results. The inconsistency may reflect a combination of clinical and methodological variability, which leads to limited reproducibility of these findings. The samples included 59 patients with MDD and 31 controls from Sample 1, 29 patients with MDD and 24 controls from Sample 2, and 31 patients with schizophrenia and 37 controls from Sample 3. Global‐brain FC (GFC) and an overlapping technique were applied to analyze the imaging data. Compared with healthy controls, patients with MDD in Samples 1 and 2 showed increased GFC in the overlapped brain areas, including the bilateral insula, right inferior parietal lobule (IPL), and right supramarginal gyrus/IPL. By contrast, decreased GFC in the overlapped brain areas, including the bilateral posterior cingulate cortex/presuneus and left calcarine cortex, was found in patients with MDD. In addition, patients with schizophrenia in Sample 3 did not show any GFC abnormalities in the overlapped areas from the results of Samples 1 and 2. The present study is the first to examine voxel‐wise brain‐wide FC in MDD with two independent samples by using an overlapping technique. The results indicate that aberrant FC patterns of insula‐centered sensorimotor circuit may account for the pathophysiology of MDD.     

Covarying structural alterations in laterality of the temporal lobe in schizophrenia: A case for source‐based laterality

The human brain is asymmetrically lateralized for certain functions (such as language processing) to regions in one hemisphere relative to the other. Asymmetries are measured with a laterality index (LI). However, traditional LI measures are limited by a lack of consensus on metrics used for its calculation. To address this limitation, source‐based laterality (SBL) leverages an independent component analysis for the identification of laterality‐specific alterations, identifying covarying components between hemispheres across subjects. SBL is successfully implemented with simulated data with inherent differences in laterality. SBL is then compared with a voxel‐wise analysis utilizing structural data from a sample of patients with schizophrenia and controls without schizophrenia. SBL group comparisons identified three distinct temporal regions and one cerebellar region with significantly altered laterality in patients with schizophrenia relative to controls. Previous work highlights reductions in laterality (ie, reduced left gray matter volume) in patients with schizophrenia compared with controls without schizophrenia. Results from this pilot SBL project are the first, to our knowledge, to identify covarying laterality differences within discrete temporal brain regions. The authors argue SBL provides a unique focus to detect covarying laterality differences in patients with schizophrenia, facilitating the discovery of laterality aspects undetected in previous work.