Auditory event-related potentials dissociate early and late memory processes

Investigating the Effect of Contextual Cueing with Face Stimuli on Electrophysiological Measures in Younger and Older Adults

Extracting repeated patterns from our surroundings plays a crucial role in contextualizing information, making predictions, and guiding our behavior implicitly. Previous research showed that contextual cueing enhances visual search performance in younger adults. In this study, we investigated whether contextual cueing could also improve older adults' performance and whether age-related differences in the neural processes underlying implicit contextual learning could be detected. Twenty-four younger and 25 older participants performed a visual search task with contextual cueing. Contextual information was generated using repeated face configurations alongside random new configurations. We measured RT difference between new and repeated configurations; ERPs to uncover the neural processes underlying contextual cueing for early (N2pc), intermediate (P3b), and late (r-LRP) processes; and multiscale entropy and spectral power density analyses to examine neural dynamics. Both younger and older adults showed similar contextual cueing benefits in their visual search efficiency at the behavioral level. In addition, they showed similar patterns regarding contextual information processing: Repeated face configurations evoked decreased finer timescale entropy (1-20 msec) and higher frequency band power (13-30 Hz) compared with new configurations. However, we detected age-related differences in ERPs: Younger, but not older adults, had larger N2pc and P3b components for repeated compared with new configurations. These results suggest that contextual cueing remains intact with aging. Although attention- and target-evaluation-related ERPs differed between the age groups, the neural dynamics of contextual learning were preserved with aging, as both age groups increasingly utilized more globally grouped representations for repeated face configurations during the learning process.

Contextual Processing and the Impacts of Aging and Neurodegeneration: A Scoping Review

Contextual processing (or context processing; CP) is an integral component of cognition. CP allows people to manage their thoughts and actions by adjusting to surroundings. CP involves the formation of an internal representation of context in relation to the environment, maintenance of this information over a period of time, and the updating of mental representations to reflect changes in the environment. Each of these functions can be affected by aging and associated conditions. Here, we introduced contextual processing research and summarized the literature studying the impact of normal aging and neurodegeneration-related cognitive decline on CP. Through searching the PubMed, PsycINFO, and Google Scholar databases, 23 studies were retrieved that focused on the impact of aging, mild cogniitve impairment (MCI), Alzheimer's disease (AD), and Parkinson's disease (PD) on CP. Results indicated that CP is particularly vulnerable to aging and neurodegeneration. Older adults had a delayed onset and reduced amplitude of electrophysiological response to information detection, comparison, and execution. MCI patients demonstrated clear signs of impaired CP compared to normal aging. The only study on AD suggested a decreased proactive control in AD participants in maintaining contextual information, but seemingly intact reactive control. Studies on PD restricted to non-demented older participants, who showed limited ability to use contextual information in cognitive and motor processes, exhibiting impaired reactive control but more or less intact proactive control. These data suggest that the decline in CP with age is further impacted by accelerated aging and neurodegeneration, providing insights for improving intervention strategies. This review highlights the need for increased attention to research this important but understudied field.

The phonological loop: is speech special?

It has been proposed that the maintenance of phonological information in verbal working memory (vWM) is carried by a domain-specific short-term storage center-the phonological loop-which is composed of a phonological store and an articulatory rehearsal system. Several brain regions including the left posterior inferior frontal gyrus (pIFG) and anterior supramarginal gyri (aSMG) are thought to support these processes. However, recent behavioral evidence suggests that verbal and non-verbal auditory information may be processed as part of a unique domain general short-term storage center instead of through specialized subsystems such as the phonological loop. In the current study, we used a single-pulse transcranial magnetic stimulation (TMS)-delayed priming paradigm with speech (syllables) and acoustically complex non-speech sounds (bird songs) to examine whether the pIFG and aSMG are involved in the processing of verbal information or, alternatively, in the processing of any complex auditory information. Our results demonstrate that TMS delivered to both regions had an effect on performance for speech and non-speech stimuli, but the nature of the effect was different. That is, priming was reduced for the speech sounds because TMS facilitated the detection of different but not identical stimuli, and accuracy was decreased for non-speech sounds. Since TMS interfered with both speech and non-speech sounds, these findings support the existence of an auditory short-term storage center located within the dorsal auditory stream.

Abnormal auditory N400 in a case of zolpidem dependence, during a working memory test

Zolpidem is a GABA (A) agonist, which is indicated for the short-term management of insomnia. Recent research provide evidence suggesting that zolpidem produces spatial working memory (WM) deficits and dependence; however, the underlying mechanisms of these effects are unknown. Since the auditory N400 component of event-related potentials (ERPS) is considered as an index of memory use of context processing, the present study focused on N400 waveform of ERPs elicited during a WM task in a case suffering from zolpidem dependence. The patterns of N400 waveform of this case were compared to the patterns obtained from healthy controls. This comparison revealed that zolpidem dependence is accompanied by reduced amplitudes located at posterior brain areas and diffuse prolongation of N400. These findings may indicate that zolpidem dependence manifests alterations with regard to the memory use of context processing, involving or affecting a wide-ranging network of the brain's structures.

Early and late cortical responses to directly gazing faces are task dependent

Gender categorisation of human faces is facilitated when gaze is directed toward the observer (i.e., a direct gaze), compared with situations where gaze is averted or the eyes are closed (Macrae, Hood, Milne, Rowe, & Mason, Psychological Science, 13(5), 460-464, 2002). However, the temporal dynamics underlying this phenomenon remain to some extent unknown. Here, we used electroencephalography (EEG) to assess the neural correlates of this effect, focusing on the event-related potential (ERP) components known to be sensitive to gaze perception (i.e., P1, N170, and P3b). We first replicated the seminal findings of Macrae et al. (2002, Experiment 1) regarding facilitated gender discrimination, and subsequently measured the underlying neural responses. Our data revealed an early preferential processing of direct gaze as compared with averted gaze and closed eyes at the P1, which reverberated at the P3b (Experiment 2). Critically, using the same material, we failed to reproduce these effects when gender categorisation was not required (Experiment 3). Taken together, our data confirm that direct gaze enhances the early P1, as well as later cortical responses to face processing, although the effect appears to be task dependent.

Representation and processing of multi-word expressions in the brain

Language comprehension is sensitive to the predictability of the upcoming information. Prediction allows for smooth, expedient and successful communication. While general discourse-based constraints have been investigated in detail, more specific phrase-level prediction has received little attention. We address this gap by exploring the ERPs elicited during the comprehension of English binomials - familiar and predictable multi-word expressions. In Experiment 1a, participants read binomial expressions (knife and fork), infrequent strongly associated phrases (spoon and fork), and semantic violations (theme and fork). In Experiment 1b, participants read the same stimuli without "and". Experiment 1a revealed that binomials elicited larger P300s and smaller N400s compared to the other conditions, reflecting the activation of a 'template' that matches the upcoming information (P300) and pointing to easier semantic integration (N400). In contrast, no differences were observed between binomials and associates in Experiment 1b. We conclude that distinct mechanisms underlie the processing of predicable and novel sequences.

Neural correlates of local contextual processing across stimulus modalities and patient populations

The objective of the current review is to integrate information from a series of studies, employing a paradigm that evaluates local contextual processing using electrophysiological measures. Collectively these studies provide an overview of how utilization of predictive context changes as a function of stimulus modality and across different patient populations, as well as the networks that may be critical for this function. The following aspects of local contextual processing will be discussed and reviewed: (i) the correlates associated with contextual processing that have been identified in healthy adults, (ii) stimulus modality effects, (iii) specific alterations and deficits of local contextual processing in aging and across different neurological and psychiatric patient populations, including patients with prefrontal cortex lesions, Parkinson's disease, schizophrenia, and major depressive disorder, (iv) the potential for utilizing the correlates of local context as biomarkers for frontal cognitive dysfunction and (v) the role of frontal networks in the processing of contextual information. Overall findings show that behavioral and neural correlates associated with processing of local context are comparable across stimulus modalities, but show specific alterations in aging and across different neurological and psychiatric disorders.

The neural bases of distracter-resistant working memory

A major difference between humans and other animals is our capacity to maintain information in working memory (WM) while performing secondary tasks, which enables sustained, complex cognition. A common assumption is that the lateral prefrontal cortex (PFC) is critical for WM performance in the presence of distracters, but direct evidence is scarce. We assessed the relationship between fMRI activity and WM performance within subjects, with performance matched across distracter and no-distracter conditions. Activity in the ventrolateral PFC during WM encoding and maintenance positively predicted performance in both conditions, whereas activity in the presupplementary motor area (pre-SMA) predicted performance only under distraction. Other parts of the dorsolateral and ventrolateral PFCs predicted performance only in the no-distracter condition. These findings challenge a lateral-PFC-centered view of distracter resistance, and suggest that the lateral PFC supports a type of WM representation that is efficient for dealing with task-irrelevant input but is, nonetheless, easily disrupted by dual-task demands.

Distributed cortical network for visual attention

The contribution of prefrontal and posterior association cortex to voluntary and involuntary visual attention was as sessed using electrophysiological techniques in patients with focal lesions in prefrontal (n = 11), temporal-parietal (n = 10), or lateral parietal cortex (n = 7). Subjects participated in a task requiring detection of designated target stimuli embedded in trains of repetitive stimuli. Infrequent and irrelevant novel visual stimuli were randomly interspersed with the target and background stimuli. Controls generated attention dependent N1 (170 msec) and N2 (243 msec) potentials maximal over extrastriate cortex. Anterior and posterior association cortex lesions reduced the amplitude of both the N1 and N2 potentials recorded over extrastriate cortex of the lesioned hemisphere. The pattern of results obtained reveals that an intrahemispheric network involving prefrontal and posterior association cortex modulates early visual processing in extrastriate regions. Voluntary target detection generated a parietal maximal P300 response (P3b) and irrelevant novel stimuli generated a more frontocentrally distributed P300 (P3a). Cortical lesions had differential effects on P3a and P3b potentials. The P3b was not significantly reduced in any cortical lesioned group. Conversely, the P3a was reduced by both prefrontal and posterior lesions with decrements most severe throughout the lesioned hemisphere. These data provide evidence that an association cortex network involving prefrontal and posterior regions is activated during orientation to novel events. The lack of a significant effect on the visual target P3b in patients with novelty P3a reductions supports the notion that different neural systems are engaged during voluntary vs involuntary atten- tion to visual stimuli.

Dissociating linguistic and non-linguistic gesture processing: electrophysiological evidence from American Sign Language

A fundamental advance in our understanding of human language would come from a detailed account of how non-linguistic and linguistic manual actions are differentiated in real time by language users. To explore this issue, we targeted the N400, an ERP component known to be sensitive to semantic context. Deaf signers saw 120 American Sign Language sentences, each consisting of a "frame" (a sentence without the last word; e.g. BOY SLEEP IN HIS) followed by a "last item" belonging to one of four categories: a high-close-probability sign (a "semantically reasonable" completion to the sentence; e.g. BED), a low-close-probability sign (a real sign that is nonetheless a "semantically odd" completion to the sentence; e.g. LEMON), a pseudo-sign (phonologically legal but non-lexical form), or a non-linguistic grooming gesture (e.g. the performer scratching her face). We found significant N400-like responses in the incongruent and pseudo-sign contexts, while the gestures elicited a large positivity.

Memory for sound, with an ear toward hearing in complex auditory scenes

An area of research that has experienced recent growth is the study of memory during perception of simple and complex auditory scenes. These studies have provided important information about how well auditory objects are encoded in memory and how well listeners can notice changes in auditory scenes. These are significant developments because they present an opportunity to better understand how we hear in realistic situations, how higher-level aspects of hearing such as semantics and prior exposure affect perception, and the similarities and differences between auditory perception and perception in other modalities, such as vision and touch. The research also poses exciting challenges for behavioral and neural models of how auditory perception and memory work.

Neural correlates of local contextual processing deficits in schizophrenic patients

Deficits in processing contextual information are one of the main features of cognitive dysfunction in schizophrenia, but the neurophysiologic substrate underlying this dysfunction is poorly understood. We used ERPs to investigate local contextual processing in schizophrenic patients. Local context was defined as the occurrence of a short predictive series of stimuli occurring before delivery of a target event. Response times of predicted targets were faster in controls compared to patients. Schizophrenia patients failed to generate the P3b latency shift between predicted and random targets that was observed in controls and demonstrated a prominent reduction of the peak of an early latency context dependent positivity. The current study provides evidence of contextual processing deficits in schizophrenia patients by demonstrating alteration in the behavioral and neural correlates of local contextual processing.

Surprise? Early visual novelty processing is not modulated by attention

This study investigated the influence of direction of attention on the early detection of visual novelty, as indexed by the anterior N2. The anterior N2 was measured in young subjects (n=32) under an attend and an ignore condition. Subjects were presented standard, target/rare, and perceptually novel visual stimuli under both conditions, but under the ignore condition, attention was directed toward an auditory n-back task. The size of the anterior N2 to novel stimuli did not differ between conditions and was significantly larger than the anterior N2 to all other stimulus types. Furthermore, under the ignore condition, the anterior N2 to visual novel stimuli was not affected by the level of difficulty of the auditory n-back task (3-back vs. 2-back). Our findings suggest that the early processing of visual novelty, as measured by the size of the anterior N2, is not strongly modulated by direction of attention.

Role of frontal and parietal cortices in the control of bottom-up and top-down attention in humans

We investigated the contribution of frontal and parietal cortices to bottom-up and top-down visual attention using electrophysiological measures in humans. Stimuli consisted of triangles, each with a different color and orientation. Subjects were presented with a sample triangle which served as the target for that trial. An array was subsequently presented with the target and three additional distractor stimuli, which were constructed to induce either automatic "pop-out" (50%) or effortful "search" (50%) behavior. For pop-out, both the color and orientation of the distractors differed from the target, which attracted attention automatically. For search, only the orientation of the distractors differed from the target, so effortful attention was required. Pop-out target detection generated a P300 event-related potential (ERP) with a peak amplitude over parietal sites whereas the search condition generated a fronto-centrally distributed P300. Reaction times and associated P300 latency in frontal areas were shorter for pop-out targets than for search targets. We used time-frequency analysis to compare pop-out and search conditions, within a 200-650 ms time-window and a 4-55 Hz frequency band. There was a double dissociation, with significantly increased power from 4 to 24 Hz in parietal areas for pop-out targets and increased power from 4 to 24 Hz in frontal regions for search targets. Taken together the ERP and time-frequency results provide evidence that the control of bottom-up and top-down attention depend on differential contributions from parietal and frontal cortices.

Multimodal effects of local context on target detection: evidence from P3b

We used the P300 component to investigate how changes in local context influenced the ability to detect target stimuli. Local context was defined as the occurrence of a short predictive series of stimuli before delivery of a target event. EEG was recorded in 12 subjects during auditory and visual sessions. Stimuli were presented in the center of the auditory and visual field and consisted of 15% targets (1000 Hz tone or downward facing triangle) and 85% of equal amounts of three types of standards (1500, 2000, and 2500 Hz tones or triangles facing left, upward, and right). Recording blocks consisted of targets preceded by either randomized sequences of standards or by sequences including a three-standard predictive sequence signaling the occurrence of a subsequent target event. Subjects pressed a button in response to targets. Peak target P300 (P3b) amplitude and latency were evaluated for targets after predictive and nonpredictive sequences using conventional averaging and a novel single-trial analysis procedure. Reaction times were shorter for predictable targets than for nonpredicted targets. P3b latency was shorter for predicted targets than for nonpredictive targets, and there were no significant P3b amplitude differences between predicted and random targets, as determined by both conventional averaging and single-trial analysis. Comparable effects on amplitude and latency were observed in both the auditory and visual modalities. The results indicate that local context has differential effects on P3b amplitude and latency, and exerts modality-independent effects on cognitive processing.

Prefrontal cortex is critical for contextual processing: evidence from brain lesions

We investigated the role of prefrontal cortex (PFC) in local contextual processing using a combined event-related potentials and lesion approach. Local context was defined as the occurrence of a short predictive series of visual stimuli occurring before delivery of a target event. Targets were preceded by either randomized sequences of standards or by sequences including a three-stimulus predictive sequence signalling the occurrence of a subsequent target event. PFC lesioned patients were impaired in their ability to use local contextual information. The response time for controls revealed a larger benefit for predictable targets than for random targets relative to PFC patients. PFC patients had reduced amplitude of a context-dependent positivity and failed to generate the expected P3b latency shift between predictive and non-predictive targets. These findings show that PFC patients are unable to utilize predictive local context to guide behaviour, providing evidence for a critical role of PFC in local contextual processing.

Remembered but unused: the accessory items in working memory that do not guide attention

If we search for an item, a representation of this item in our working memory guides attention to matching items in the visual scene. We can hold multiple items in working memory. Do all these items guide attention in parallel? We asked participants to detect a target object in a stream of objects while they maintained a second item in memory for a subsequent task. On some trials, we presented this memory item as a distractor in the stream. Subjects did not confuse these memory items with the search target, as the false alarm rate on trials where the memory item was presented in the stream was comparable to that on trials with only regular distractors. However, a comparable performance does not exclude that the memory items are processed differently from normal distractors. We therefore recorded event-related potentials (ERPs) evoked by search targets, memory items, and regular distractors. As expected, ERPs evoked by search targets differed from those evoked by distractors. Search targets elicited an occipital selection negativity and a frontal selection positivity indexing selective attention, whereas the P3b component, which reflects the matching of sensory events to memory representations, was enhanced for targets compared to distractors. Remarkably, the ERPs evoked by memory items were indistinguishable from the ERPs evoked by normal distractors. This implies that the search target has a special status in working memory that is not shared by the other items. These other, "accessory" items do not guide attention and are excluded from the matching process.

New insights into name category-related effects: is the Age of Acquisition a possible factor?

BACKGROUND

Electrophysiological, hemodynamic and neuropsychological studies have provided evidence of dissociation in the way words belonging to different semantic categories (e.g., animals, tools, actions) are represented in the brain. The aim of the present study was to investigate whether a word's semantic domain may affect the amplitude and latency of ERP components, independently of any other factor.

METHODS

EEGs were recorded from 16 volunteers engaged in a lexical decision task (word/non-word discrimination) involving 100 words (flora and fauna names). This task allowed us to evaluate differences in processing between words belonging to different categories (fauna vs. flora) independently of task demands. All stimuli were balanced in terms of length, frequency of occurrence, familiarity and imageability. Low Resolution Electromagnetic Tomography (LORETA) was performed on ERP difference waves of interest.

RESULTS

Our findings showed that the two categories were discriminated as early as 200 ms post-stimulus, with larger responses to flora names over the left occipito-temporal areas, namely BA37 and BA20. Category-related ERP differences were also observed in the amplitudes of the later centro-parietal N400, posterior P300 and anterior LP components. Behavioral responses to words denoting fauna were more accurate than to words denoting flora.

CONCLUSION

Overall, it seems that it was easier to access the lexical properties of fauna, probably because of their biologically relevant status. The results are discussed in the light of the possible role played by different factors.

Electrophysiological evidence for aging effects on local contextual processing

We used event-related potentials to investigate how aging affects local contextual processing. Local context was defined as the occurrence of a short predictive series of visual stimuli before delivery of a target event. Stimuli were presented to either the left or right visual field and consisted of 15% targets (downward facing triangle) and 85% of equal numbers of three types of standards (triangles facing left, upwards and right). Recording blocks consisted of targets preceded by either randomized sequences of standards or by sequences including a three-standard predictive sequence signaling the occurrence of a subsequent target event. Subjects pressed a button in response to targets. Predictive local context affected target detection by reducing the duration of stimulus evaluation compared to detection of non-predictive random targets comparably for both young and older adults, as shown by a P3b latency shift. The peak of an earlier latency context positivity, which was seen only in the predicted compared to the random target condition, was prolonged in the older population compared to young adults. Finally, older subjects elicited a late sustained positivity in the predictive condition, not seen in the younger subjects. Taken together, these findings suggest that local contextual effects on target detection processes are altered with age.

To Ignore or Explore: Top–Down Modulation of Novelty Processing

Attending to novelty is a critical element of human behavior and learning. Novel events can serve as task-irrelevant distracters or as potential sources of engagement by interesting or important aspects of one's environment. An optimally functioning brain should have the capacity to respond differentially to novel events depending on the circumstances in which they occur. In the present study, a subject-controlled variant of the visual novelty oddball paradigm was employed under two different conditions in which novel stimuli were characterized either as distracters from a main task or as potentially meaningful “invitations” to explore the environment. Differences in context, derived from varying the emphasis of task instructions, strongly modulated both the behavioral and electrophysiological response to novelty. This modulation was not observed for processing earlier than the P3 component. Subjects who encountered novel events that served as distracters limited the amount of attention and processing resources they appropriated. Remarkably, under this condition, there were no differences in overall P3 amplitude, late positive slow-wave activity, or viewing duration between rare novel and frequent standard events. In contrast, subjects who encountered novel events as potential opportunities to explore augmented the attention and processing resources directed toward these events (as reflected by a larger P3 amplitude, late positive slow-wave activity, and longer viewing durations). Our results suggest that the processing of novelty within the visual modality involves several stages, including: (1) the relatively automatic detection of unfamiliar, novel stimuli (indexed by the N2); (2) the voluntary allocation of resources determined by the broader context in which a novel event occurs (indexed by the P3); and (3) the sustained processing of novelty (indexed by late positive slow-wave activity). This study provides evidence of the brain's ability to generate differential responses to novel events according to the circumstances under which they are encountered. It also points to a greater degree of top–down modulation of the processing of novelty than has been previously emphasized. We suggest that less commonly studied variables, such as subject control, may provide additional insight into the different ways in which novelty is processed.

Behavioral and electrophysiological effects of distractor variation on auditory selective attention

Two experiments investigated the effects on auditory selection of varying distractor values in memory. Participants performed a set of control (single distractor) and distractor-variation (multiple distractors) tasks, classifying targets by pitch (Experiments 1A and 2) or loudness (Experiment 1B) while ignoring previously presented (and spatially separate) distractors. When both targets and distractors varied in pitch, the degree of variation among the distractors increasingly disrupted classification accuracy and reaction time to the targets. Physiologically, the degree of distractor variation boosted the N1 response to distractors, the P2 response to both targets and distractors, and the slow-wave response to targets (400-600 ms after stimulus onset). The results suggest that target representations are diminished in distinctiveness as distractors activate a wider range of the task-relevant continuum in working memory.

Modulation of late alpha band oscillations by feedback in a hypothesis testing paradigm

We used the electroencephalogram (EEG) to investigate whether positive and negative performance feedbacks differentially modulate late time-locked oscillatory brain activity in hypothesis testing. Ten college students serially tested hypotheses concerning a hidden rule by judging its presence or absence in triplets of digits, and revised them on the basis of an exogenous performance feedback. The EEG signal was convolved with a family of complex wavelets and induced brain potentials were extracted in the alpha range (8-13 Hz). The time-varying modulation of alpha activity time-locked to positive and negative feedback was analyzed in the 350-700 ms time-window. The results showed differential feedback-induced modulations of upper-alpha rhythms (> or =10 Hz) between 450 and 700 ms in parieto-occipital and central regions, and of lower-alpha rhythms (<10 Hz) between 350 and 450 ms in central regions. These results were interpreted in terms of differential functional roles of feedback in short-term memory and active inhibition/disinhibition of resources for subsequent hypothesis testing. Some implications for cognitive models of feedback are discussed.

Event-related potential correlates of the word length effect in working memory

It has been proposed that a frontally distributed ERP negativity reflects rehearsal within the phonological loop component of working memory. This study investigated the relationship between phonological rehearsal and frontal negativity, by examining the effects of word length and articulatory suppression (continuously uttering an irrelevant word) on memory for auditorily presented words while ERPs were recorded. P2 amplitude, thought to reflect word identification, was increased for long compared to short words. However, this difference did not remain under conditions of suppression. A centrally maximum early negativity was larger in the short than long word silent conditions and this word length effect was reduced under suppression. The early negativity was interpreted as reflecting the transfer of the information from input to output buffers in the silent conditions that was prevented by the suppression. There was only a word length effect for the late frontally distributed negativity in the suppression conditions, suggesting that this component was not associated with phonological loop rehearsal but rather other working memory processes that operate under high load conditions.

EEG theta and gamma responses to semantic violations in online sentence processing

We explore the nature of the oscillatory dynamics in the EEG of subjects reading sentences that contain a semantic violation. More specifically, we examine whether increases in theta ( approximately 3-7 Hz) and gamma (around 40 Hz) band power occur in response to sentences that were either semantically correct or contained a semantically incongruent word (semantic violation). ERP results indicated a classical N400 effect. A wavelet-based time-frequency analysis revealed a theta band power increase during an interval of 300-800 ms after critical word onset, at temporal electrodes bilaterally for both sentence conditions, and over midfrontal areas for the semantic violations only. In the gamma frequency band, a predominantly frontal power increase was observed during the processing of correct sentences. This effect was absent following semantic violations. These results provide a characterization of the oscillatory brain dynamics, and notably of both theta and gamma oscillations, that occur during language comprehension.

Inhibiting change: Effects of memory on auditory selective attention

Two experiments investigated the behavioral and electrophysiological effects on human auditory selection of the psychophysical discriminability of a distractor channel in memory. Participants performed a set of baseline (single distractor) and filtering (multiple distractors) tasks, classifying the pitch of targets, while ignoring pitch variation in temporally distinct distractors, which differed from targets in timbre (Experiment 1) or loudness (Experiment 2). Increased distractor change progressively disrupted target accuracy and reaction time, and fostered confusion in distinguishing target from distractor channels. Physiologically, relative discriminability only affected distractor waveforms, whether or not distractor values physically differed across tasks, enhancing the N1 response while reducing an inhibitory slow-wave component. The results suggest that inhibition fails as distractors activate a wider range of the task-relevant continuum in working memory.

Oscillatory EEG correlates of episodic trace decay

Recent studies suggest that human theta oscillations appear to be functionally associated with memory processes. It is less clear, however, to what type of memory sub-processes theta is related. Using a continuous word recognition task with different repetition lags, we investigate whether theta reflects the strength of an episodic memory trace or general processing demands, such as task difficulty. The results favor the episodic trace decay hypothesis and show that during the access of an episodic trace in a time window of approximately 200-400 ms, theta power decreases with increasing lag (between the first and second presentation of an item). LORETA source localization of this early theta lag effect indicates that parietal regions are involved in episodic trace processing, whereas right frontal regions may guide the process of retrieval. We conclude that episodic encoding can be characterized by two different stages: traces are first processed at parietal sites at approximately 300 ms, then further processing takes place in regions of the medial temporal lobe at approximately 500 ms. Only the first stage is related to theta, whereas the second is reflected by a slow wave with a frequency of approximately 2.5 Hz.

Expectancy effects on omission evoked potentials in musicians and non-musicians

Abstract An expanded omitted stimulus paradigm was investigated to determine whether expectancy would modulate the amplitude of the omission evoked potentials (OEPs). In addition, we examined the effects of musical expertise on OEPs. Trials started with 3-7 beats randomly and contained 5 omitted beats. Three types of trials (n = 90) were presented with 1, 2, or 3 beats occurring between omissions. A tap response at the end of each trial was used to determine timing accuracy. Clear OEPs were observed over midline sites. We found main omission effects with respect to an N150 and a P400 OEPs component, such that peak amplitudes diminished whenever the occurrence of an omitted stimulus could be expected. In addition, an N600 OEPs component emerged in response to expectedly omitted stimuli toward the end of each trial within the group of musicians. Thus, musical training seems to lead to more efficient and more refined processing of auditory temporal patterns.

Impairment of recognition memory in schizophrenia: event-related potential study using a continuous recognition task

Recognition memory and the dissociation of immediate and delayed repetition in schizophrenic patients were investigated using event-related potential (ERP) and a continuous word recognition task. In total, 14 schizophrenic patients and 14 age and gender-matched control subjects, were recruited. Among 240 stimulus words used, 40 words were not repeated, 100 were repeated immediately, and 100 were repeated after five intervening words. Both schizophrenic and control groups responded faster to words repeated immediately than to words repeated after a delay and to new words. However, schizophrenic patients responded less accurately to words repeated immediately and to words repeated after a delay than the controls. In terms of ERP, schizophrenic patients showed significantly reduced N200, late positive component (LPC), and N400 amplitudes, and a more frontally-distributed N200 topography than the controls. For controls, immediate repetition was associated with a large LPC amplitude and the absence of N400, while delayed repetition was associated with a small LPC amplitude and the presence of N400. However, this dissociation between immediate and delayed repetition was not observed in schizophrenic patients. All of these results suggest that schizophrenic patients have recognition memory impairment, the cause of which may range from early encoding, and memory search to late retrieval.

Differences between auditory evoked responses recorded during spatial and nonspatial working memory tasks

Results from several recent studies suggest that neuronal processing of sound content and its spatial location may be dissociated. The use of modern neuroimaging techniques has allowed for the determination that different brain structures may be specifically activated during working memory processing of pitch and location of sound. The time course of these task-related differences, however, remains uncertain. In the present study, we performed simultaneous whole-head electroencephalogram and magnetoencephalogram recordings, using a new behavioral paradigm, to investigate the dynamics of differences between "what" and "where" evoked responses in the auditory system as a function of memory load. In the location task the latency of the N1m was shorter and its generator was situated more inferiorly than in the pitch task. Working memory processing of the tonal frequency enhanced the amplitude of the N2 component, as well as the negative-going deflection at a latency around 400 ms. A memory-load-dependent task-related difference was found in the positive slow wave which was higher during the location than pitch task at the low load. Late slow waves were affected by memory load but not type of task. These results suggest that separate neuronal networks are involved in the attribute-specific analysis of auditory stimuli and their encoding into working memory, whereas the maintenance of auditory information is accomplished by a common, nonspecific neuronal network.

Event-related brain potentials during auditory and visual word recognition memory tasks

Event-related brain potentials (ERPs) recorded during presentation of a series of words or pictures show enhanced positivity between 300 and 800 ms after presentation of repeated items. However, little attention has been directed to the characterization of this ERP recognition memory effect using auditory stimuli. The present study directly compared the ERP 'old/new effect' for words presented in the visual and auditory modalities. Nose-referenced ERPs were recorded from 30 electrode sites while participants (N=16) were engaged in visual and auditory continuous word recognition memory tasks. Spatially and temporally overlapping ERP components were identified and measured by covariance-based principal components analysis. The expected old/new effect was observed in both modalities, with a comparable time course peaking at 560 ms, but having a more anterior scalp topography for visual items. This suggests a common cognitive process (i.e. successful retrieval of information from memory) associated with separable neural generators in each modality. Despite this temporal synchronization, the old/new effect overlapped ERP components having distinct scalp topographies (N2) or peak latencies (P3) for each modality. The positive-going old/new effect was preceded by an earlier negativity peaking at 370 ms that was greater across modalities for old than new words, likely reflecting semantic processing aspects of word recognition memory. A late (beyond 900 ms), broadly-distributed negativity was also greater for old than new words, prolonged for auditory items, and may represent activity of a post-retrieval process.

Frontal and parietal components of a cerebral network mediating voluntary attention to novel events

Despite the important role that attending to novel events plays in human behavior, there is limited information about the neuroanatomical underpinnings of this vital activity. This study investigated the relative contributions of the frontal and posterior parietal lobes to the differential processing of novel and target stimuli under an experimental condition in which subjects actively directed attention to novel events. Event-related potentials were recorded from well-matched frontal patients, parietal patients, and non-brain-injured subjects who controlled their viewing duration (by button press) of line drawings that included a frequent, repetitive background stimulus, an infrequent target stimulus, and infrequent, novel visual stimuli. Subjects also responded to target stimuli by pressing a foot pedal. Damage to the frontal cortex resulted in a much greater disruption of response to novel stimuli than to designated targets. Frontal patients exhibited a widely distributed, profound reduction of the novelty P3 response and a marked diminution of the viewing duration of novel events. In contrast, damage to posterior parietal lobes was associated with a substantial reduction of both target P3 and novelty P3 amplitude; however, there was less disruption of the processing of novel than of target stimuli. We conclude that two nodes of the neuroanatomical network for responding to and processing novelty are the prefrontal and posterior parietal regions, which participate in the voluntary allocation of attention to novel events. Injury to this network is indexed by reduced novelty P3 amplitude, which is tightly associated with diminished attention to novel stimuli. The prefrontal cortex may serve as the central node in determining the allocation of attentional resources to novel events, whereas the posterior parietal lobe may provide the neural substrate for the dynamic process of updating one's internal model of the environment to take into account a novel event.

The effect of immediate and delayed word repetition on event-related potential in a continuous recognition task

The dissociation of immediate and delayed word repetition was studied using a continuous recognition memory task and event-related potential (ERP). Among 240 stimulus words, 40 words were not repeated, 100 were immediately repeated and 100 were repeated after 5 intervening words. Words presented only once during the experiment were referred to as new words. Subjects responded faster and more accurately to words repeated immediately than to new words and to words repeated after a delay. In terms of ERP results, immediate repetition was associated with large P300 amplitude, early P300 latency and the absence of N400, while delayed repetition was associated with small P300 amplitude, late P300 latency and the presence of N400. N400 was elicited only to new words and to those repeated after intervening words. The general morphology of the waveforms was similar for two repetition conditions until around 310 ms after the onset of stimulus. These results indicate that distinct neural systems subserve the immediate and delayed repetition effect, and that the difference between the two emerges around 310 ms poststimulus. Immediate and delayed word repetition are considered in terms of template matching and memory searching, and are possibly mediated by the parietal and left medial temporal lobes, respectively.

Auditory memory in congenitally blind adults: a behavioral-electrophysiological investigation

Blind people must rely more than sighted people on auditory input in order to acquire information about the world. The present study was designed to test the hypothesis that blind people have better memory than sighted individuals for auditory verbal material and specifically to determine whether memory encoding and/or retrieval are improved in blind adults. An incidental memory paradigm was employed in which 11 congenitally blind people and 11 matched sighted controls first listened to 80 sentences which ended either with a semantically appropriate or inappropriate word. Immediately following, the recognition phase occurred, in which all sentence terminal words were presented again randomly intermixed with the same number of new words. Participants indicated whether or not they had heard the word in the initial study phase. Event-related brain potentials (ERPs) were recorded from 28 electrode positions during both the encoding and the retrieval phase. Blind participants' memory performance was superior to that of sighted controls. In addition, during the recognition phase, previously presented words elicited ERPs with larger positive amplitudes than new words, particularly over the right hemisphere. During the study phase, words that would subsequently be recognized elicited a more pronounced late positive potential than words that were not subsequently recognized. These effects were reliable in the congenitally blind participants but could only be obtained in the subgroup of sighted participants who had the highest memory performance. These results imply that blind people encode auditory verbal material more efficiently than matched sighted controls and that this in turn allows them to recognize these items with a higher probability.

Repetition related ERP effects in a visual object target detection task

ERP responses to initial and repeated presentations of possible and impossible objects were recorded from 61 recording sites in a simple target detection task. In Experiment 1, the non-target objects were line drawings of possible and impossible 3-D geometric figures and the targets were line drawings of familiar everyday objects or combinations of parts of everyday objects. In Experiment 2, the non-target objects were everyday objects and the targets were possible and impossible 3-D geometric figures. In both experiments, at frontal sites, the repeated possible and impossible non-target items elicited less negative ERP waveforms relative to first presentations between 250 and 350-400 ms. At parieto-occipital sites, in both experiments, the repeated possible and impossible non-target items elicited less positive ERP waveforms than did first presentations beginning at about 300 ms. The briefly reduced frontal negativity to repeated items is consistent with familiarity arising from a facilitation of access to conceptual, semantic and visuo-spatial representations during object categorization. The polarity of the parieto-occipital effect was the reverse of what is usually found in stimulus repetition tasks, although it is consistent with earlier work using similar visual stimuli. It is interpreted as reflecting the availability of a newly formed representation (i.e., token) of the object just experienced.

Electrophysiology reveals semantic memory use in language comprehension

The physical energy that we refer to as a word, whether in isolation or embedded in sentences, takes its meaning from the knowledge stored in our brains through a lifetime of experience. Much empirical evidence indicates that, although this knowledge can be used fairly flexibly, it is functionally organized in 'semantic memory' along a number of dimensions, including similarity and association. Here, we review recent findings using an electrophysiological brain component, the N400, that reveal the nature and timing of semantic memory use during language comprehension. These findings show that the organization of semantic memory has an inherent impact on sentence processing. The left hemisphere, in particular, seems to capitalize on the organization of semantic memory to pre-activate the meaning of forthcoming words, even if this strategy fails at times. In addition, these electrophysiological results support a view of memory in which world knowledge is distributed across multiple, plastic-yet-structured, largely modality-specific processing areas, and in which meaning is an emergent, temporally extended process, influenced by experience, context, and the nature of the brain itself.

Event-related potentials during conscious and automatic memory retrieval

The effects of study-test lags of between 0 and 32 items on conscious (C) and automatic (A) memory processes in a running word-completion task were investigated with event-related potentials (ERPs). The process dissociation procedure (PDP) can distinguish between C and A contributions to memory by comparing performance when subjects respond with either an old item (inclusion) or a new item (exclusion). C can be estimated by subtracting the probability of an intrusion of an old item during the exclusion task (due to A without C) from the probability of correctly producing an old item during the inclusion task (due to C and/or A). The behavioral results showed that C was stronger when the test item followed the studied word in the next trial or after a lag of one stimulus. The strength of A did not vary with lag. The ERP waveforms contained a broad parietal positive wave between 300 and 800 ms. This parietal wave distinguished between correctly recalled old and new words. The early portion of this old-new effect was significantly affected by lag. Subtracting waveforms to obtain a measure of C revealed an effect in the later portion of this wave, lateralized over the left hemisphere. A sustained frontal negativity occurred during all recordings and was larger during conscious retrieval. There was no consistent ERP effect related to automatic memory retrieval.

Prefrontal cortical involvement in visual working memory

Studies of human amnesia provide evidence for a short-term memory store with information transfer to long term memory occurring within 60 s of sensory encoding. Human and nonhuman primate research has shown that maintenance of this short-term or working memory store is dependent upon frontal cortical activation, although the critical temporal parameters of frontal involvement throughout this 60-s window are undetermined. We examined prefrontal contributions to rapid (under 2 s) and sustained (over 4 s) visual working memory by recording behavioral performance and event-related potentials (ERPs) in patients with lesions in dorsolateral frontal cortex and age-matched control subjects. Prefrontal lesioned patients generated a reduced sustained frontal positivity at all delays. At short delays, patients generated reduced performance to stimuli presented in the contralesional field. Patients generated a negative potential (N400), greatest to contralesionally presented stimuli, that was observed in the control subjects only at long delays. The results indicate that prefrontal lesions impair the frontal component of an anterior-posterior working memory network activated during rapid and sustained visual memory processing. Frontal patients may require activation of limbic cortex, indexed by N400, for maintenance of both rapid and sustained working memory.

Short- and long-delay intracranial ERP repetition effects dissociate memory systems in the human brain

Prior exposure to a stimulus can facilitate the performance to subsequent presentations of that stimulus. ERP studies have shown that this facilitation is associated with the modulation of two components (N400 and P600). Investigation of the time course of both behavioral and ERP repetition effects have led to the assumption that it is subserved by the combination of at least two mechanisms operating at different time-points: a short-delay semantic activation and a long-lasting episodic mechanism. The present experiment recorded intracranial ERPs from various structures during a continuous recognition memory task to investigate the respective contribution of the different brain regions to short- and long-delay ERP repetition effects. The results are in good agreement with both the classical neuropsychological literature and the more recent data obtained with functional imagery techniques. They provide electrophysiological evidence of multiple anatomo-functional memory systems in the human brain: a short-term semantic activation system and a long-term episodic memory system, with interface structures that coordinate the functioning of these two systems.

Cortico-limbic circuits and novelty: a review of EEG and blood flow data

Novelty detection is a fundamental capacity of all mammalian nervous systems /64/. The ability to orient to unexpected events is critical for both survival and normal memory function /82/. The mechanisms whereby the brain detects and responds to novelty have become of increasing interest to neuroscientists. A review is provided of human electrophysiological and blood flow data focused on delineating the neural systems engaged by novelty. Electrophysiological recording of event-related potentials (ERPs) has shown that novel stimuli activate a distributed network involving prefrontal and posterior association cortex as well as the hippocampus /4,23,24,32,33,36,86,88/. Activation of this network facilitates subsequent memory for novel events /27/. Neural modeling provides additional support for a prominent role of novelty in normal memory function /43/. Blood flow studies employing PET and fMRI have also begun to define the neural regions activated by novelty. The blood flow data provide converging evidence on the role of the hippocampus and cortical association regions in the processing of novelty /30,66,75,76/. The results of the behavioral, ERP and blood flow research confirm that a distributed neocortical-limbic circuit is activated by stimulus novelty. These distributed circuits maintain a template of the recent past /74/. Deviations from the template activate a neocortical-limbic network facilitating behavioral response to and memory storage of novel events.