An event-related potential study of memory for words spoken aloud or heard

Subjects made old/new recognition judgements to visually presented words, half of which had been encountered in a prior study phase. For each word judged old, subjects made a subsequent source judgement, indicating whether they had pronounced the word aloud at study (spoken words), or whether they had heard the word spoken to them (heard words). Event-related potentials (ERPs) were compared for three classes of test item; words correctly judged to be new (correct rejections), and spoken and heard words that were correctly assigned to source (spoken hit/hit and heard hit/hit response categories). Consistent with previous findings (Wilding, E. L. and Rugg, M. D., Brain, 1996, 119, 889-905), two temporally and topographically dissociable components, with parietal and frontal maxima respectively, differentiated the ERPs to the hit/hit and correct rejection response categories. In addition, there was some evidence that the frontally distributed component could be decomposed into two distinct components, only one of which differentiated the two classes of hit/hit ERPs. The findings suggest that at least three functionally and neurologically dissociable processes can contribute to successful recovery of source information.

ERP repetition effects in indirect and direct tasks: effects of age and interitem lag

Event-related potentials (ERPs) were recorded for visually presented words in young and older participants while they performed two tasks. In the indirect task, participants responded to occasional target words. Some of the nontarget words were repeated after a single intervening trial, and others were repeated after a mean of 10 trials. In the direct task participants responded to every item, discriminating between words presented for the first and the second times. Compared with ERPs to unrepeated words, those to words repeated in the indirect task after either lag were more positive in both participant groups. For the short lag, this effect was larger among the older participants. In the direct task, words repeated after either lag elicited a positive shift in the ERPs of the young participants. In the older participants, short lag repeats elicited a repetition effect, but smaller than the equivalent effect among the young participants. Long lag repeats failed to elicit a repetition effect in the direct task in the older participants. The findings show that word repetition in these tasks reflects the modulation of two ERP components, which differ in their sensitivity to age-related changes in memory function.

The functional neuroanatomy of episodic memory

Functional neuroimaging studies have revealed that effective encoding in episodic memory is associated with enhanced activity in left prefrontal cortex, whereas retrieval is accompanied by the enhancement of predominantly right-sided prefrontal activity. The extent of the contribution of prefrontal cortex to episodic memory, and the fact that the encoding and retrieval operations it supports are differentially lateralized, were unexpected on the basis of evidence from lesion studies. Such studies have highlighted the crucial role in episodic memory played by the hippocampus and related medial temporal lobe structures. Neuroimaging studies, however, have had only limited success in elucidating the role of the hippocampus in episodic memory. Refinements in experimental design and improved spatial resolution should promote rapid future progress with respect to this issue.

An event-related potential study of explicit memory on tests of cued recall and recognition

The event-related potential (ERP) correlates of performance on test of word-stem cued recall and recognition memory were contrasted. ERPs elicited by stems attracting successful recall exhibited a sustained positive-going shift relative to ERPs elicited by stems completed with unstudied items. This positive shift was maximal at electrode sites on and adjacent to the midline. An equally sustained positive-going ERP modulation was observed for the recognition memory task in ERPs elicited by recognised 'old' items relative to ERPs elicited by correctly rejected 'new' items. The scalp topography of this effect shifted from a parietally distributed asymmetry favouring left hemisphere sites, to a frontally distributed effect maximal over midline and right hemisphere sites. The findings indicate that ERP correlates of explicit memory are task-dependent. The disparate ERP effects are interpreted as reflecting a common explicit retrieval mechanism which is sensitive to the nature of retrieval cues provided at test.

Brain regions supporting intentional and incidental memory: a PET study

Regional brain activity associated with intentional and incidental memory retrieval was studied with PET. Previously studied and new words were presented in either an intentional or an incidental memory task. Type of task was crossed with an encoding manipulation ('deep' vs 'shallow') which varied the probability that studied items would be remembered. In both tasks, deeply encoded items were associated with greater activation in the left hippocampus than were items that had received shallow encoding, suggesting that the involvement of the hippocampus in memory retrieval is independent of whether remembering is intentional or incidental. Right prefrontal and bilateral parietal cortex were more activated during the international task than during the incidental task, irrespective of encoding condition. Thus, these regions play a more extensive role in memory retrieval when remembering is intentional.

Event-related potentials and the recognition memory exclusion task

Subjects heard words that were presented in either a male or a female voice, and were required to perform one of two encoding tasks according to the gender of the voice. At test studied words were presented visually, along with a set of words new to the experiment. Subjects were required to respond on one key to words belonging to one of the two classes of studied word (targets), and to respond on a different key both to words belonging to the other study class (non-targets), and to words new to the experiment. In comparison to the event-related potentials (ERPs) elicited by new words, the ERPs elicited by correctly detected targets exhibited two temporally and topographically distinct positive going effects: one of these was phasic, showed a parietal maximum, and was larger over the left than the right hemisphere. The second effect was more sustained in time, frontally distributed, and was larger over the right hemisphere. The ERPs elicited by correctly classified non-targets contained the parietal effect only. These findings confirm that retrieval of contextual information in tests of recognition memory (recollection) is associated with two distinct ERP modulations. While one of these may be closely tied to process necessary for recollection, the other may reflect less obligatory processes which operate on the products of successful retrieval.

An event-related potential study of retroactive interference in memory

Neural activity associated with overcoming retroactive interference in episodic memory was investigated with event-related brain potentials (ERPs). ERPs were recorded during a combined recognition/associative recall memory test in which subjects were required to identify previous studied words and recall their study associates. A retroactive interference condition (A-B, A-C, A-?) was compared with two control conditions A-B, C-D, A-? and A-B, A-B, A-?). No ERP effect specific to the interference condition was identified. ERPs to all three classes of old word differed however from those to new words. These differences involved two effects, one with a left parietal maximum that has been described before, and a left frontal effect of earlier onset which has not been reported previously. This latter effect may indicate a role for the left prefrontal cortex in associative retrieval.

Differential activation of the prefrontal cortex in successful and unsuccessful memory retrieval

Six subjects underwent PET scans while they performed three versions of a recognition memory test for words and three versions of a control task. In each memory condition, the subjects discriminated between words presented in a prescan study list and words new to the experiment. During the 30 s scanning interval, the ratio of old and new words was 0:20, 4:16 or 16:4, depending on the experimental condition. Outside this interval, the ratio was 50:50 in all three conditions. The requirement in the control task was to discriminate between two character strings, the ratios of which were also manipulated during the 30 s scanning interval. Employing the control task as a covariate, analysis with statistical parametric mapping revealed that regional cerebral blood flow (rCBF) covaried with increasing density of old items in three regions of prefrontal cortex: right dorsolateral [Brodmann area (BA) 9/46], right medial (BA 32/8) and bilateral frontopolar cortex (BA 10). It is concluded that the prefrontal cortex, especially in the right hemisphere, is more active when a retrieval attempt succeeds than when it fails. This finding is consistent with the idea that the prefrontal cortex supports processes that operate selectively on the products of memory retrieval.

An event-related potential study of word-stem cued recall

Event-related potentials (ERPs) were recorded from 13 scalp sites while subjects attempted to recall studied words using word-stems. If recall failed, stems were to be completed with the first suitable word to come to mind. To distinguish between correct completions accompanied and unaccompanied by explicit memory, subjects were required to make an overt recognition ("old/new") judgement for each completion. Semantically studied words were associated with higher levels of recall and recognition than were words subjected to non-semantic study. The sole ERP effect was a sustained positive shift in ERPs evoked by stems attracting correct completions that were correctly judged to be old. The shift was anteriorly distributed, and onset was around 300 ms post stimulus. It is interpreted as a reflection of processes either contributing to, or contingent upon, explicit memory retrieval.

Event-related potentials and the recollection of associative information

Event-related potentials (ERPs) were recorded from 13 scalp sites during the performance of an associative recall task. At study, subjects were presented with a series of word pairs and were required to incorporate the two members of each pair into a sentence. At test, the first members of each pair were presented intermixed with an equal number of unstudied items. Subjects were required to discriminate between new and studied (old) words and, for each word judged old, to attempt to recall the word with which it had been associated at study. Compared to the ERPs elicited by new words, the ERPs elicited by words correctly judged to be old and for which the associate was correctly recalled showed a sustained, positive-going shift (the "parietal old/new effect"). This effect was strongly lateralised to the left hemisphere and was maximal at temporo-parietal electrodes. The effect was absent in ERPs elicited by words that were correctly judged to be old, but for which the studied associate could not be recalled. The findings are taken as support for the idea that the parietal old/new effect reflects neural activity associated with the recollection of specific past episodes, and hence that the effect may index retrieval operations supported by the medial temporal lobe memory system.

An event-related potential study of recognition memory with and without retrieval of source

Event-related potentials (ERPs) were recorded during the test phase of a recognition memory task in two experiments. In both experiments subjects made initial old/new judgements to visually presented words, and for words judged old, indicated in which of two voices (male/female) the words had been heard at study. In the second experiment only subjects had the option to signal that they were uncertain about the status of a test word. Two positive-going ERP effects differentiated the ERPs evoked by correctly recognized old words from those evoked by words correctly judged new. The two effects differed in their scalp topography and time course, and were both of greater magnitude in the ERPs evoked by recognized words for which a correct voice judgement was made. The findings are consistent with the view that multiple neural systems underlie the ability to recognize an item and to recall its study context. However, the findings offer little support for the view, articulated in certain 'dual-process' models of recognition memory, that recognition judgements with and without retrieval of study context depend upon qualitatively different memory processes or systems.

A multivariate analysis of evoked responses in EEG and MEG data

This paper presents a multivariate analysis of evoked responses and their spatiotemporal dynamics as measured with electro- or magnetoencephalography. This analysis uses standard techniques (ManCova) to make possible statistical inference about differential responses, after the data have been transformed using singular value decomposition. The generality of this approach is limited only by the assumptions implicit in the general linear model and can range from simple analyses like Hotelling's T2 test (in comparing evoked responses among different conditions) to complex analyses of a multivariate regression type (e.g., characterizing the response components associated with a behavioral or psychophysical parameter). To illustrate the technique we have characterized time-dependent changes (both within and between trials) in magnetic fields, evoked by self-paced movements. Our illustrative analysis showed that movement-evoked components were less prone to adaptation than premovement components, suggesting that functionally distinct (preparatory and early executive) biomagnetic signals show differential adaptation.

A comparison of the electrophysiological effects of formal and repetition priming

Three experiments investigated the electrophysiological consequences of repetition (e.g., scandal-scandal, buple-buple) and formal (e.g., scan-scandal, bup-buple) priming using words and nonwords. In each experiment, repetition and formal priming resulted in positive-going shifts in the event-related potential (ERP) that onset at approximately 200 ms poststimulus and were initially of similar magnitude. Subsequently, the repetition effect became larger than the formal priming effect. Although the word and nonword formal priming effects and the nonword repetition effects were greatest over midline and right hemisphere sites, the word repetition effects were greatest over the midline. It is suggested that the positive-going shift seen in the repetition and formal priming conditions was a modulation of the well-documented N400 component of the ERP. The topographic differences between the priming effects may have reflected differences in the nature of the representations to which cognitive operations are applied rather than differences in the nature of the operations themselves.

Modulation of event-related potentials by the repetition of drawings of novel objects

In experiment 1, event-related potentials (ERPs) were recorded from 13 scalp sites while subjects viewed a series of line drawings of novel objects. Most of the drawings were of objects that were structurally possible, while the remainder were of structurally impossible objects; the task was to respond to each presentation of an impossible object. Approximately one third of the possible objects were repetitions of the immediately preceding drawing. Compared to the ERPs elicited by first presentations, the ERPs elicited by repetitions were more negative-going. This negative repetition effect comprised two temporally and topographically distinct components. In experiment 2, subjects monitored drawings of unstructured patterns, so as to detect occasional 'targets' containing a pair of parallel lines. Repetitions of 'non-target' patterns elicited ERPs which were largely indistinguishable from those elicited by first presentations. Thus, the negative repetition effects found in experiment 1 are not merely a consequence of repeating visual patterns in a demanding discrimination task. Possible reasons why novel objects should elicit ERP repetition effects opposite in polarity to those observed in tasks employing verbal or meaningful pictorial stimuli are discussed.

Cognitive brain potentials and regional cerebral blood flow equivalents during two- and three-sound auditory "oddball tasks"

Ten healthy volunteers were examined with single photon emission tomography and 99mTc-exametazime. They were studied on 2 occasions, during a 2- and a 3-sound auditory discrimination (oddball) task. Twenty healthy volunteers were used as controls, studied once at rest. During the 2-tone task there was a bilateral posterior (occipito-) temporal and medial frontal activation, a left pericentral increase, and posterior cingulate suppression. During the 3-sound task activation was again found in posterior (occipito-) temporal, medial frontal cortex, left pericentral, with a small non-significant reduction in posterior cingulate uptake. Compared with the 2-tone task, there was a trend towards higher activity in left medial frontal, right posterior temporal and posterior cingulate cortex in the 3-sound task. P3b amplitudes were negatively correlated with posterior cingulate tracer uptake during both tasks. Positive correlations with P3b amplitudes were found in various frontal and temporal regions. These results are consistent with more invasive localisation studies of P3b. Posterior cingulate cortex appears to be inhibited during the oddball tasks, the more so, the more restricted the range of stimuli, and the greater the task-related recruitment of neurones (P3b amplitude). As expected from its more frontal distribution, P3a amplitude was positively correlated with anterior cingulate tracer uptake, and negatively correlated with temporal cortical activity.

Memory and consciousness: a selective review of issues and data

Methods for dissociating and independently studying conscious (explicit) and unconscious (implicit) memory are discussed. Recent work in the field of amnesia is then briefly reviewed, focusing on the question of how clearly the disorder fractionates according to the implicit-explicit distinction. Finally, evidence supporting dual-process models of recognition memory is reviewed, and data suggesting that amnesic patients have relatively spared familiarity-based recognition memory are critically assessed.

Recognition memory with and without retrieval of context: an event-related potential study

In two recognition memory tests subjects made initial old/new judgements and subsequently judged whether 'old' words had been presented auditorily or visually at study. Test words were presented visually in Experiment 1 and auditorily in Experiment 2. ERPs evoked at test by words correctly judged 'old' were separated according to whether they were correctly or incorrectly assigned to their study modality. In both experiments, ERPs to correctly assigned words were more positive than those evoked by words correctly judged to be new. This ERP old/new effect was absent for words incorrectly assigned to study modality in Experiment 1, and was observed over a restricted latency range in Experiment 2. The findings are consistent with the hypothesis that the old/new effect is associated with recognition memory based on recollection of the study episode, rather than familiarity.

Event-related potentials and the recollection of low and high frequency words

A recognition memory test was conducted in which low and high frequency words were initially presented in one of two different study tasks. A word was defined as recollected if, at test, it was both confidently judged 'old', and confidently assigned to its correct study context. Low frequency words were more accurately recognised than high frequency items, and were also more likely to be assigned to their correct study context. The results are consistent with the view that low frequency words are better recognised because they are more likely to be recollected, rather than because they engender higher levels of relative familiarity. Event-related potentials (ERPs) evoked at test by correctly classified new words were contrasted with those evoked by old, recollected words. The ERPs to low frequency words exhibited large and reliable 'old/new' effects, in that from approx. 300 msec post-stimulus, waveforms were more positive-going for old than for new items. These effects were markedly smaller, and indeed non-significant, in the ERPs evoked by high frequency items. The results show that the interaction between word frequency and old/new differences in ERPs does not arise because of a confound between frequency and the probability of recollection. Together with other findings, they suggest that recollection is better conceived of as a graded, rather than as an all-or-none phenomenon.

Word and Nonword Repetition Within- and Across-Modality: An Event-Related Potential Study

The effects on event-related potentials (ERPs) of within- and across-modality repetition of words and nonwords were investigated. In Experiment 1, subjects detected occasional animal names embedded in a series of words. AU items were equally likely to be presented auditorily or visually. Some words were repetitions, either within- or across-modality, of words presented six items previously. Visual-visual repetition evoked a sustained positive shift, which onset around 250 msec and comprised two topographically and temporally distinct components. Auditory-visual repetition modulated only the later of these two components. For auditory EMS, within- and across-modality repetition evoked effects with similar onset latencies. The within-modality effect was initially the larger, but only at posterior sites. In Experiment 2, critical items were auditory and visual nonwords, and target items were auditory words and visual pseudohomophones. Visual-visual nonword repetition effects onset around 450 msec, and demonstrated a more anterior scalp distribution than those evoked by auditory-visual repetition. Visual-auditory repetition evoked only a small, late-onsetting effect, whereas auditory-auditory repetition evoked an effect that, at parietal sites only, was almost equivalent to that from the analogous condition of Experiment 1. These findings indicate that, as indexed by ERF's, repetition effects both within- and across-modality are influenced by lexical status. Possible parallels with the effects of word and nonword repetition on behavioral variables are discussed.

Modulation of event-related brain potentials by word repetition: effects of local context

In four experiments, event-related brain potentials (ERPs) were evoked by visually presented word pairs in a task requiring responses to occasional target pairs. In Experiments 1 and 2, some pairs comprised items that had been presented previously. These repeated pairs consisted of words that had been paired together when first presented (same context condition) or words that had first been presented on consecutive trials (different context condition). ERP repetition effects were equivalent in the two conditions. In Experiment 3, same-context repeats were contrasted with a condition in which a repeated word was paired with a new word. Only the same-context pairs evoked a repetition effect. Experiment 4 showed that repetition effects to different- and same-context repeats remained equivalent when first presentations of the members of different-context pairs were separated by six intervening trials. We conclude that the ERP repetition effect shows little sensitivity to local context.

Word repetition effects on event-related potentials in healthy young and old subjects, and in patients with Alzheimer-type dementia

Event-related potentials (ERPs) were recorded from 16 healthy young (mean age 21 years) and 16 healthy old subjects (mean age 64 years), and from 11 subjects with a diagnosis of Dementia of Alzheimer Type (DAT). The task requirement was to attend to a series of visually presented words so as to respond to occasional animal names. Non-animal names repeated after either a single or six intervening items. In the young subjects ERPs evoked by repeated words displayed a widespread, sustained positive-going shift relative to ERPs evoked by first presentations (the ERP repetition effect). This effect onset around 220 msec and did not differ as a function of inter-item lag. Other than for a delay in onset of approximately 80 msec, the ERP repetition effect in the healthy old group was in all respects equivalent to that of the young subjects. The ERP repetition effects in the DAT patients were statistically indistinguishable from those of an appropriately matched sub-set of the healthy old subjects. These results indicate that the ERP repetition effect remains robust in subjects in whom explicit memory has declined as a result of normal aging or DAT. Thus they suggest that the effect reflects processes independent of those underlying explicit memory, and that it may index a form of memory relatively unaffected by the pathology underlying DAT.

Modulation of event-related potentials by word repetition: the role of visual selective attention

Event-related potentials (ERPs) were recorded while subjects viewed visually presented words, some of which occurred twice. Each trial consisted of two colored letter strings, the requirement being to attend to and make a word/nonword discrimination for one of the strings. Attention was manipulated by color in Experiment 1, and color and a precue were used in Experiment 2. As in previous ERP studies of word repetition, a positive offset to repeated words developed when both first and second presentations were the focus of attention. In Experiment 2, ERPs showed evidence of positive-going repetition effects in all conditions in which at least one of the two presentations of the repeated word was attended. In the visual modality, the positive-going ERP repetition effect occurs only when at least one of the two presentations of a repeated item is the object of attention, which suggests that one or more of the processes reflected by the effect is capacity limited.

Cognitive brain potentials in a three-stimulus auditory "oddball" task after closed head injury

Event-related potentials (ERPs) were recorded in a three stimulus oddball task from 16 patients who had sustained a severe closed head injury at least 6 months before testing, and from 16 control subjects. The stimuli comprised a random sequence of frequent non-target tones (P = 0.70), rare target tones (P = 0.15), and rare novel sounds (P = 0.15). The task requirement was to respond promptly to each target tone. From a latency of 200 msec onwards, the ERPs evoked by frequent non-targets were substantially more negative-going in the head-injured than in the control group. When this difference in the ERPs to the frequent tones was taken into account, there was no evidence to suggest that either the latency or the amplitude of the target-evoked N2 and P3b components differed between the groups. The novel stimuli evoked a prominent P3a component. The amplitude and scalp distribution of this component differed little between the groups, but its peak latency was reliably longer in the head-injured subjects. The findings in respect of the N2 and P3b components suggest that impairments in early processing of task-relevant stimuli are not an invariant feature of closed head injury. The findings regarding P3a suggest that, in the majority of patients, head injury has only a limited effect on the neural systems underlying involuntary shifts of attention.

Word frequency and multiple repetition as determinants of the modulation of event-related potentials in a semantic classification task

We recorded event-related potentials while subjects performed a category membership decision task. The stimuli were words of high and low frequency of occurrence in written English, and each was presented four times. The experiment was intended to explore the interaction of word frequency and multiple repetition on the event-related potential, and thence to investigate the possible loci in time of the effects of these variables. First presentations of low and high frequency words evoked event-related potentials which differed in the presence, in the low word frequency waveforms, of a right-hemisphere dominant negativity peaking at 400 ms. This negativity was very similar to the N400 which may be sensitive to the semantic relations among words in a sequence. Initial repetition diminished this midlatency difference, but gave rise to both earlier and later frequency-related differences. Subsequent multiple repetition abolished the early frequency-related repetition effect, but did not affect amplitudes in the region of N400, nor did it abolish a late positivity, present only for repeated low frequency words.

Modulation of event-related potentials (ERPs) by word repetition in a task with inconsistent mapping between repetition and response

ERPs were recorded during 5 blocks of a continuous recognition memory task. In each block words were shown twice, separated by an average of 6 intervening items, and the task was to respond discriminatively on the basis of whether each word was appearing in that block for the first (first presentations) or the second (second presentations) time. In blocks 2-5, half of the words had also been shown in the immediately preceding block, and half were new to the experiment. Subjects were slower and less accurate at identifying first presentations of words that had appeared in the preceding block, as compared to first presentations of entirely new words. In the latency ranges of the N400 and late positive components, ERPs to words that had previously been presented were more positive-going than were ERPs to new words. These 'across-block' ERP repetition effects were qualitatively very similar to, and additive with, those evoked by words repeated within a block. These findings suggest that the effects of word repetition on ERPs are not dependent on an invariant mapping between repetition and response and are not a consequence of facilitated decision/response processing for repeated items.

The effects of scopolamine on event-related potentials in a continuous recognition memory task

Event-related potentials (ERPs) were recorded during a task requiring continuous recognition memory for visually-presented words. Twelve subjects each performed the task twice, once following the administration of scopolamine, and once after receiving a saline placebo. In the placebo condition, correctly detected "old" words (i.e., words that had been presented once before during the task) evoked more positive-going ERPs than did "new" words. Scopolamine caused a substantial impairment in task performance, but did not reduce the size of these old-word/new-word ERP differences. It is concluded that old/new ERP effects are unlikely to reflect cholinergically-mediated neural activity underlying normal recognition memory.

Event-related potentials related to recognition memory. Effects of unilateral temporal lobectomy and temporal lobe epilepsy

Event-related potentials (ERPs) were recorded during a continuous recognition memory task for visually presented words, with a 6 item lag between the first and second presentation of each word. The subjects consisted of: (i) a control group of patients with primary generalized epilepsy; (ii) patients who had undergone either a left or a right anterior temporal lobectomy; (iii) unoperated patients with either left or right unilateral temporal lobe epilepsy. In the controls, ERPs to detected 'old' words were reliably more positive-going in the interval 300 to 600 ms post-stimulus than were ERPs to 'new' items. In both left and right lobectomy patients, 'old/new' ERP differences in the same latency range were significantly smaller than in the controls, and did not differ significantly from zero. At midline electrodes, old/new effects in the temporal lobe epilepsy patients were of similar magnitude to those of the controls. In contrast to the control data, the old/new effects in the temporal lobe epilepsy patients were asymmetric, in that they were smaller over the hemisphere ipsilateral to the seizure focus than over the contralateral hemisphere. No relationship was found across subjects between the magnitude of old/new ERP effects and verbal memory performance. In a second task, occasional non-words had to be discriminated against a background of sequentially presented words, some of which were repetitions of the immediately preceding item. ERPs evoked by repeated words were more positive-going than were those to first presentations; this effect was reliable, and of equivalent size, in all patient groups. It is concluded that in the recognition task, old/new ERP effects are dependent on temporal lobe functioning, but that the anterior temporal lobe is not the principal locus of the generators of these effects. The cognitive processes reflected by these effects do not appear to be strongly lateralized to one hemisphere, and neither do they seem to be necessary for normal verbal memory.

Normal P300 following extensive damage to the left medial temporal lobe

Event-related potentials (ERPs) were recorded during auditory and visual "oddball" tasks from a patient with a severe verbal memory deficit due to a low grade infiltrating glioma which involved the full extent of the left medial temporal lobe. In both sensory modalities, the patient's oddball-evoked P300s were symmetrical and of normal amplitude. These findings are difficult to reconcile with the hypothesis that the hippocampus, or any other medial temporal structure, makes a substantial contribution to the scalp P300.

Event-related potentials and the semantic matching of pictures

Event-related potentials (ERPs) were recorded from one midline and three pairs of lateral electrodes while subjects determined whether pairs of sequentially presented pictures were semantically associated. The ERPs evoked by the second picture of each pair differed as a consequence of whether it was associated with its predecessor, such that ERPs to nonassociated pictures were more negative-going than those to associated items. These differences resulted from the modulation of two ERP components, one frontally distributed and centered on an N300 deflection, the other distributed more widely over the scalp and encompassing an N450 deflection. The modulation of N450 is interpreted as further evidence that the "N400" ERP component is sensitive to semantic relationships between nonverbal stimuli. The earlier N300 effects, which do not appear to occur when ERPs are evoked by semantically primed and unprimed words, could suggest that the semantic processing of pictorial stimuli involves neural systems different from those associated with the semantic processing of words.

Endogenous event-related potentials from sphenoidal electrodes

Event-related potentials (ERPs) were recorded from left and right sphenoidal electrodes during 2 cognitive tasks (visual oddball and a word repetition procedure) known to evoke endogenous ERP activity in the medial temporal lobe. Both tasks gave rise to large, reliable modulations of scalp-recorded ERPs. In the oddball task, no consistent task-related ERP activity could be recorded from the sphenoidal electrodes concurrently with the scalp-recorded P3 component, although in the latency region following the peak of P3 these electrodes exhibited an enhanced late negative wave to target stimuli. In the word repetition task, no consistent repetition-related ERP effects of any kind were observed from the sphenoidal electrodes. Sphenoidal electrodes do not appear to detect the endogenous ERP components that are generated in the medial temporal lobe concurrently with scalp-recorded components.

Event-related brain potentials dissociate repetition effects of high- and low-frequency words

Event-related brain potentials (ERPs) were recorded while subjects detected nonwords interspersed among sequences of words of high or low frequency of occurrence. In Phase 1, a proportion of the words were repeated after six intervening items. In Phase 2, which followed after a break of approximately 15 min, the words were either repeats of items presented in the previous phase or new. Unrepeated low-frequency words evoked larger N400 components than did high-frequency items. In Phase 1, this effect interacted with repetition, such that no frequency effects were observed on N400s evoked by repeated words. In addition, the post-500-msec latency region of the ERPs exhibited a substantial repetition effect for low-frequency words, but did not differentiate unrepeated and repeated high-frequency words. In Phase 2, ERPs evoked by "old" and "new" high-frequency words did not differ in any latency region, while those evoked by old and new low-frequency words differed only after 500 msec. The interactive effects of frequency and repetition suggest that these variables act jointly at multiple loci during the processing of a word. The specificity of the post-500-msec repetition effect for low-frequency words may reflect a process responsive to a discrepancy between words' intra and extraexperimental familiarity.

Modulation of event-related potentials by word repetition: the effects of inter-item lag

The modulation of event-related potentials by word repetition was investigated in two experiments. In both experiments, subjects responded to occasional nonwords interspersed among a series of words. A proportion of the words were repetitions of previously presented items. Words were repeated after 0 or 6 intervening items in Experiment 1 and after 6 or 19 items in Experiment 2. Event-related potentials to repeated words were characterised by a sustained, widespread positive-going shift with an onset of approximately 300 ms. This effect did not vary significantly as a function of lag in either experiment. When words were repeated immediately, this repetition-evoked positive shift was preceded by a transient negative deflection (onset ca. 200 ms) which was absent in event-related potentials to words repeated at longer lags. These results suggest that the modulation of event-related potentials by word repetition is influenced by at least two processes. One of these processes acts relatively early during the processing of a repeated word, but subsides rapidly as inter-item lag between first and second presentations increases. The second process occurs later in time, but is considerably more robust over variations in inter-item lag.

Event-related potentials and recognition memory for words

Event-related potentials (ERPs) were recorded while subjects performed 2 recognition memory tasks. In the first task, a series of words was presented in which items were repeated after either 6 or 19 intervening words. Subjects were required to make an 'old/new' discrimination to every word. In the second task, which was performed approximately 45 min after the first, subjects had to discriminate between words employed in the first task and previously unpresented items. In the first task, ERPs to 'old' words were more positive-going, from around 250 msec post stimulus, than those to 'new' items. Old/new ERP differences in the second task were smaller than in the first, less widespread over the scalp, and had an onset latency of around 500 msec, although subjects' ability to make 'old/new' discriminations was at a high level. It is suggested that 'old/new' ERP differences may not reflect processes necessary for discrimination between old and new items in recognition memory tasks.

CNV abnormalities following closed head injury

Event-related potentials (ERPs) were recorded in a GO/NO-GO reaction time task from 20 closed head injury patients, at least 6 months postinjury, and from 20 controls. In this task the pitch of an initial tone (Sl) indicated whether or not a response was required to a second tone (S2) occurring 1.5 s later. In the control group both the early, frontal-maximum, and the later vertex-maximum, components of the contingent negative variation (CNV) were larger on GO than NO-GO trials. In the patients, the early frontal CNV wave did not differentiate GO and NO-GO trials, and the late CNV showed a smaller separation between these trial types than did the late CNV of the control group. These CNV abnormalities may reflect impairments in selectively orienting to salient stimuli, and in differential response preparation. Such impairments might in turn reflect the damage to the frontal lobes and/or their connections that commonly occurs as a result of closed head injury.

Asymmetries in event-related potentials during rhyme-matching: confirmation of the null effects of handedness

Event-related potentials (ERPs) were recorded from 16 right- and 16 left-handed subjects during two variants of a sequential rhyme-judgement task. These variants manipulated the demands placed on short-term memory, by requiring subjects to match a word either with one or with three previously presented words. In both tasks, ERPs exhibited two lateral asymmetries: (i) during the interval prior to the final word, the CNV was more negative over the left hemisphere, and (ii) rhyme/non-rhyme differences in the amplitude of the N450 component of the ERPs to the final word were larger over the right than the left hemisphere. Neither of these asymmetries was smaller or more variable in left- than in right-handed subjects. It is suggested that these data may indicate that some aspects of language processing are, irrespective of handedness, relatively invariant in the direction and degree of their cerebral lateralization.

Event-related potentials and the semantic matching of faces

Event-related brain potentials (ERPs) were recorded while subjects determined whether two sequentially presented famous faces depicted individuals belonging to the same or to different occupational categories. During the 1.56 sec interval between the onset of the faces, ERPs recorded from right hemisphere electrodes were more negative-going than those from electrodes over the left hemisphere. The ERPs evoked by the second face on each trial differed as a consequence of whether or not the person depicted belonged to the occupational category specified by the first face. This difference took the form of a bilaterally-distributed negative-going shift in the ERPs evoked by non-matching as opposed to matching faces. This negativity was maximal around 450 msec post-stimulus. The ERP asymmetries during the inter-stimulus interval are interpreted as evidence for the engagement of cognitive processes lateralized to the right hemisphere. The match/non-match differences are considered to reflect the modulation of an "N400" component similar to that evoked by words, and thus suggest that such components can be modulated by associative priming between non-linguistic stimuli.

Latencies of visually responsive neurons in various regions of the rhesus monkey brain and their relation to human visual responses

The temporal characteristics of visually responsive neurons in a variety of areas of the monkey brain are presented. These data allow a comparison to be made between the latencies of components of the human visual ERP, and the onset latencies of neurons in regions which are candidate sources of these ERP phenomena.

Event related potentials from closed head injury patients in an auditory "oddball" task: evidence of dysfunction in stimulus categorisation

Event-related potentials (ERPs) were recorded from 19 closed head injury (CHI) patients, at least 6 months after injury, and an equal number of control subjects, during a task requiring the covert counting of rare auditory "target" stimuli against a background of frequent "non-targets". In both groups, ERPs to targets contained enhanced frontal N2 and parietal P3 components. N2 was larger in amplitude in the CHI patients than in the controls, and its peak latency was delayed. P3 amplitude was smaller in the patients, but its latency was not significantly different from that of the control group. The delay in N2 latency is interpreted as evidence of an increase in the time needed to achieve stimulus categorisation in CHI patients. The larger N2s in this group are thought to reflect the additional cognitive effort required after CHI to cope with the task. The negative findings with respect to P3 latency suggest that this may be a less sensitive measure of information-processing efficiency in this task than the latency of N2.

Event-related potentials and the matching of familiar and unfamiliar faces

Event-related potentials (ERPs) were recorded from one midline and three pairs of lateral sites while subjects made same/different judgements on sequentially presented pairs of familiar or unfamiliar faces. During the interval between the first and second face, a slow wave was more negative-going over the right than the left hemisphere, particularly when the faces were familiar. Following the second face, two regions of the waveforms were more negative-going when this face did not match the identity of its predecessor. In the early region (less than 160 msec), this effect was confined to posterior electrode sites and familiar faces. In the later region (greater than 250 msec), the match/non-match effect was widespread across the scalp and was evident for both familiar and non-familiar faces, although in the latency range 350-450 msec (encompassing the "N400" component), it was greater in magnitude in the case of familiar stimuli. It is suggested that the slow wave asymmetries reflect the engagement of short-term memory mechanisms lateralized to the right hemisphere. The match/non-match differences are thought to reflect multiple processes, including the modulation of the "N400" component. The sensitivity of this component to the familiarity manipulation is consistent with the hypothesis that the amplitude of N400 reflects an item's compatibility with currently activated memory representations.

Phonological similarity effects, memory span and developmental reading disorders: the nature of the relationship

Eight-and 11-year-old poor readers of average and below average intelligence carried out a visually presented immediate memory task, recalling strings of similar- and dissimilar-sounding letters. Task difficulty was adjusted for each child by determining memory span for the dissimilar items prior to the memory task. All poor reader groups recalled significantly more dissimilar- than similar-sounding items, and the magnitude of these effects was comparable to those shown by their chronological and reading age controls. A close association was found between memory span and reading age, such that average intelligence poor readers had memory spans indistinguishable from those of their reading age controls. Furthermore, a highly significant correlation was found between memory span and reading age. However, it was concluded that this relationship could not be primarily due to a substantial immediate memory component in word recognition as the below average intelligence poor readers had poorer memory spans than the brighter poor readers, yet they had very similar reading ages. Additionally, a weaker association was found between memory span and non-word reading than between memory span and word reading. It was hypothesized that recognition difficulties may be the primary problem underlying both poor word recognition and immediate memory impairments in children with reading disorders.

Modulation of visual event-related potentials by spatial and non-spatial visual selective attention

Event-related potentials (ERPs) were recorded in two experiments involving selective visual processing. In Experiment 1, subjects attended to light flashes emanating from one visual field, in order to detect occasional slightly deviant "targets", while ignoring equiprobable stimuli from the opposite field. ERPs elicited by stimuli in an attended field were characterised by larger posteriorly distributed P120 and N170 components, and a larger anteriorly distributed N145 component. In addition, these ERPs were, in comparison to those elicited by unattended stimuli, more negative-going in the latency region of approx. 200-400 msec. This late effect had a marked fronto-central distribution. In Experiment 2 subjects attended to either horizontal or vertical bars, displayed equiprobably in the same spatial location. No enhancement of early components was observed as a function of attention but, as in Experiment 1, a late, sustained, fronto-centrally distributed negative shift was observed in ERPs elicited by "attended" compared to "unattended" stimuli. It was concluded that the enhancement of P120 (P1) observed in Experiment 1 reflects the engagement of attentional mechanisms specific to the selection of stimuli on the basis of spatial cues. The later sustained negative shift seen in both experiments was considered to reflect a feature of within-channel processing common to both spatial and non-spatial selective tasks.

Visual evoked potentials to lateralised stimuli in two cases of callosal agenesis

Visual evoked potentials (VEPs) to lateralised light flashes were recorded from two acallosal patients. In one patient, these recordings were made while he performed a choice-reaction time task, and in the other patient the VEPs were obtained during a simple reaction time task. In both cases the patient's VEPs from electrode sites contralateral to the visual field of stimulus delivery resembled those of controls. Their VEPs from ipsilateral sites were aberrant, however, in that while control subjects showed a smaller and slightly delayed ipsilateral N160 component, this was not discernible in the patients' data. It is concluded that the ipsilateral N160 relies for its generation on the transcallosal transfer of information processed initially by the contralateral hemisphere.

Further investigation of visual evoked potentials elicited by lateralized stimuli: effects of stimulus eccentricity and reference site

Visual evoked potentials (VEPs) to small lateralized flashes were recorded from the parietal midline, homotopic lateral central and occipital electrodes, and from left and right mastoid processes, all referred to a balanced non-cephalic reference. Two stimulus eccentricities, 4 degrees and 10 degrees, were employed, and a comparison made between the non-cephalic and linked mastoid references. P120 (measured at lateral occipital sites only) peaked earlier and was of smaller amplitude at the electrode contralateral to the visual field of stimulus exposure. N160 peaked earlier at central than occipital sites, was larger from electrodes over the contralateral hemisphere, and at the occipital sites only, peaked earlier in the electrode contralateral to the visual field of stimulus exposure. These effects were virtually unaffected by the eccentricity manipulation and it is concluded that light scatter across the visual midline is unlikely to be responsible for the observed pattern of ipsilateral-contralateral VEP asymmetries. The mastoids were found to detect significant stimulus-locked activity in the same latency range as the occipital N160 component. However, comparison of the non-cephalic and linked mastoid references revealed only non-specific effects, and no sign of any change in the pattern of ipsilateral-contralateral VEP asymmetries, or the magnitude of the associated latency differences. It is concluded that these effects may be of value in the study of callosal transfer.

The effects of handedness on event-related potentials in a rhyme-matching task

Event-related potentials (ERPs) were recorded from one midline and three pairs of homotopic lateral sites while subjects determined whether or not sequentially presented words rhymed. ERPs from 11 left-handed males with a family history of sinistrality were compared with those from 12 right-handers, none of whom had such a family history. As in previous work (e.g. Rugg, M. D. Neuropsychologia 22, 435-443, 1984) the slow negative wave developing during the interval in which subjects waited for the final word was more negative from left-hemisphere electrodes, and the rhyme/non-rhyme differences in the ERPs following this word were greater over the right hemisphere. Neither of these asymmetries differed between the left and right-handers. Possible reasons for the occurrence of these ERP asymmetries during rhyme-matching are discussed, and it is suggested that the aspect(s) of linguistic processing tapped by this task, and influencing concurrently recorded ERPs, may be similarly lateralised in the brains of left- and right-handers.

Event-related potentials in phonological matching tasks

Two experiments investigated Event-Related Potentials (ERPs) in matching tasks. In Experiment 1 subjects judged, in separate conditions, whether two words rhymed or were written in the same case. The CNV developing between the two words was larger in the latter task compared to the former at the right temporal site. In the rhyme judgment task, an increased late negativity differentiated the ERPs to nonrhyming words from those that rhymed with the previously presented word. This difference was maximal at the midline and over the right hemisphere. Experiment 2 further investigated ERPs in the rhyme judgment task, increasing memory demands with an extended interstimulus interval (ISI) and varying the number of items subjects had to hold in memory during this period (one vs. three). Irrespective of memory load, CNVs during the ISI were more negative from the left hemisphere, and the ERPs to the rhyming and nonrhyming words showed the same differences as in Experiment 1. The CNV asymmetries are interpreted as being associated with the engagement of lateralized short-term memory processes. The rhyme/nonrhyme differences are possibly related to the "N400" component elicited by semantically incongruous words. Possible reasons for their scalp distribution are discussed.

The effect of stimulus intensity on visual evoked potential estimates of interhemispheric transmission time

Visual evoked potentials (VEPs) to bright or dim lateralised light flashes were recorded from homologous occipital and central sites. In a GO/NOGO reaction time task (Experiment 1) the latency of the N160 component of the VEP was found to be shorter from the contralateral hemisphere by approximately 16 ms at occipital sites, but only 3 ms centrally. In addition, there was a trend for the occipital contralateral latency advantage to increase with decreasing stimulus brightness. In Experiment 2 a wider intensity range and a simple visual reaction time task were employed. Contralateral N160 latency advantages were again found to be larger occipitally (approx 13 ms) than centrally (3 ms). Furthermore the occipital contralateral latency advantage was significantly increased at the lower stimulus intensity, while that from central sites remained constant. These data suggest that two types of interhemispheric relay can be distinguished-a sensory one recorded occipitally and a non-sensory one recorded from central sites.