Brain potentials, perceptual mechanisms and semantic categorisation

Current exposure to a second language modulates bilingual visual word recognition: An EEG study

Bilingual word recognition has been the focus of much empirical work, but research on potential modulating factors, such as individual differences in L2 exposure, are limited. This study represents a first attempt to determine the impact of L2-exposure on bilingual word recognition in both languages. To this end, highly fluent bilinguals were split into two groups according to their L2-exposure, and performed a semantic categorisation task while recording their behavioural responses and electro-cortical (EEG) signal. We predicted that lower L2-exposure should produce less efficient L2 word recognition processing at the behavioural level, alongside neurophysiological changes at the early pre-lexical and lexical levels, but not at a post-lexical level. Results confirmed this hypothesis in accuracy and in the N1 component of the EEG signal. Precisely, bilinguals with lower L2-exposure appeared less accurate in determining semantic relatedness when target words were presented in L2, but this condition posed no such problem for bilinguals with higher L2-exposure. Moreover, L2-exposure modulates early processes of word recognition not only in L2 but also in L1 brain activity, thus challenging a fully non-selective access account (cf. BIA + model, Dijkstra and van Heuven, 2002). We interpret our findings with reference to the frequency-lag hypothesis (Gollan et al., 2011).

Professional Music Training and Novel Word Learning: From Faster Semantic Encoding to Longer-lasting Word Representations

On the basis of previous results showing that music training positively influences different aspects of speech perception and cognition, the aim of this series of experiments was to test the hypothesis that adult professional musicians would learn the meaning of novel words through picture–word associations more efficiently than controls without music training (i.e., fewer errors and faster RTs). We also expected musicians to show faster changes in brain electrical activity than controls, in particular regarding the N400 component that develops with word learning. In line with these hypotheses, musicians outperformed controls in the most difficult semantic task. Moreover, although a frontally distributed N400 component developed in both groups of participants after only a few minutes of novel word learning, in musicians this frontal distribution rapidly shifted to parietal scalp sites, as typically found for the N400 elicited by known words. Finally, musicians showed evidence for better long-term memory for novel words 5 months after the main experimental session. Results are discussed in terms of cascading effects from enhanced perception to memory as well as in terms of multifaceted improvements of cognitive processing due to music training. To our knowledge, this is the first report showing that music training influences semantic aspects of language processing in adults. These results open new perspectives for education in showing that early music training can facilitate later foreign language learning. Moreover, the design used in the present experiment can help to specify the stages of word learning that are impaired in children and adults with word learning difficulties.

Cross-modal symbolic processing can elicit either an N2 or a protracted N2/N400 response

A cross-modal symbolic paradigm was used to elicit EEG activity related to semantic incongruence. Twenty-five undergraduate students viewed pairings of visual lexical cues (e.g., DOG) with congruent (50% of trials) or incongruent (50%) auditory nonlexical stimuli (animal vocalizations; e.g., sound of a dog woofing or a cat meowing). In one condition, many different pairs of congruent/incongruent stimuli were shown, whereas in a second condition only two pairs of stimuli were repeatedly shown. A typical N400-like pattern of incongruence-related activity (including activity in the N2 time window) was evident in the condition using many stimuli, whereas the incongruence-related activity in the two-stimuli condition was confined to differential N2-like activity. A supplementary analysis excluded stimulus characteristics as the source of this differential activity between conditions. We found that a single individual performing a fixed task can demonstrate either a protracted N400-like pattern of activity or a more temporally focused N2-like pattern of activity in response to the same stimulus, which suggests that the N2 may be a precursor to the protracted N400 response.

Distributed patterns of event-related potentials predict subsequent ratings of abstract stimulus attributes

Exposure to pleasant and rewarding visual stimuli can bias people's choices towards either immediate or delayed gratification. We hypothesised that this phenomenon might be based on carry-over effects from a fast, unconscious assessment of the abstract 'time reference' of a stimuli, i.e. how the stimulus relates to one's personal understanding and connotation of time. Here we investigated whether participants' post-experiment ratings of task-irrelevant, positive background visual stimuli for the dimensions 'arousal' (used as a control condition) and 'time reference' were related to differences in single-channel event-related potentials (ERPs) and whether they could be predicted from spatio-temporal patterns of ERPs. Participants performed a demanding foreground choice-reaction task while on each trial one task-irrelevant image (depicting objects, people and scenes) was presented in the background. Conventional ERP analyses as well as multivariate support vector regression (SVR) analyses were conducted to predict participants' subsequent ratings. We found that only SVR allowed both 'arousal' and 'time reference' ratings to be predicted during the first 200 ms post-stimulus. This demonstrates an early, automatic semantic stimulus analysis, which might be related to the high relevance of 'time reference' to everyday decision-making and preference formation.

Reading aloud: a psychophysiological investigation in children

This study investigated the electrophysiological responses to single-letter reading in children (reading-related potentials) and explored the morphological differences between covert and overt reading conditions. Sixty-five healthy children (6-13 years) participated in this study. Reading-related potentials were recorded during visual stimulation with single Italian alphabetic letters. Stimuli were displayed for 5 ms either automatically at a randomly jittered time lag or upon voluntary self-paced button press by children. In the covert conditions, children had to passively look at single letters, while in the overt conditions children were required to read aloud the letters. Electromyographic activity of the forearm and lips was additionally recorded during all tasks. Superimposition of reading-related potentials with the electromyographic activity of forearm and lips during self-paced reading aloud allowed to segregate the reading-related components into four periods: preparatory, pre-lexical, lexical and post-lexical. Reading-related potentials of the preparatory period can be related to preparation/intention to read, those of the pre-lexical period to visual-perceptual processes, those of the lexical period to the external/internal reafferent activity and those of the post-lexical period to the feedback processes following task completion. Analysis of variance showed a significant interaction of reading-related components with electrode locations and task conditions in all periods. The systematic characterization of the neurophysiological correlates of the elementary association between letters and sounds is helpful to highlight the neurobiological and functional basis of reading in healthy as well as impaired readers, for possibly developing neurophysiologically grounded rehabilitation therapies and further improving the explanatory models of dyslexia.

Effects of verbal event structure on online thematic role assignment

Event structure describes the relationships between general semantics (Aktionsart) of the verb and its syntactic properties, separating verbs into two classes: telic verbs, which denote change of state events with an inherent end-point or boundary (catch, rescue), and atelic, which refer to homogenous activities (tease, host). As telic verbs describe events, in which the internal argument (Patient) is affected, we hypothesized that processing of telic verb template would activate syntactic position of the Patient during sentence comprehension. Event-related brain potentials (ERPs) were recorded from 20 English speakers, who read sentences with reduced Object relative clauses, in which the verb was either telic or atelic. ERPs in relative clauses diverged on the definite article preceding the Agent: the atelic condition was characterized by larger amplitude negativity at the N100. Such processing differences are explained by activation of the syntactic position for the Patient by the event structure template of telic verbs.

Sound categorization and conceptual priming for nonlinguistic and linguistic sounds

The aim of these experiments was to compare conceptual priming for linguistic and for a homogeneous class of nonlinguistic sounds, impact sounds, by using both behavioral (percentage errors and RTs) and electrophysiological measures (ERPs). Experiment 1 aimed at studying the neural basis of impact sound categorization by creating typical and ambiguous sounds from different material categories (wood, metal, and glass). Ambiguous sounds were associated with slower RTs and larger N280, smaller P350/P550 components, and larger negative slow wave than typical impact sounds. Thus, ambiguous sounds were more difficult to categorize than typical sounds. A category membership task was used in Experiment 2. Typical sounds were followed by sounds from the same or from a different category or by ambiguous sounds. Words were followed by words, pseudowords, or nonwords. Error rate was highest for ambiguous sounds and for pseudowords and both elicited larger N400-like components than same typical sounds and words. Moreover, both different typical sounds and nonwords elicited P300 components. These results are discussed in terms of similar conceptual priming effects for nonlinguistic and linguistic stimuli.

Meaning and modality: influences of context, semantic memory organization, and perceptual predictability on picture processing

Using event-related potentials (ERPs), the authors investigated the influences of sentence context, semantic memory organization, and perceptual predictability on picture processing. Participants read pairs of highly or weakly constraining sentences that ended with (a) the expected item, (b) an unexpected item from the expected semantic category, or (c) an unexpected item from an unexpected category. Pictures were unfamiliar in Experiment 1 but preexposed in Experiment 2. ERPs to pictures reflected both contextual fit and memory organization, as do ERPs to words in the same contexts (K. D. Federmeier & M. Kutas, 1999). However, different response patterns were observed to pictures than to words. Some of these arose from perceptual predictability differences, whereas others seem to reflect true modality-based differences in semantic feature activation. Although words and pictures may share semantic memory, the authors' results show that semantic processing is not amodal.

Right words and left words: electrophysiological evidence for hemispheric differences in meaning processing

Both cerebral hemispheres are involved in language processing, each playing a unique role that may derive from differences in knowledge organization and on-line meaning integration. Here, we examine lateralized differences in knowledge representation and retrieval using event-related potentials (ERPs) elicited by words in sentences. Volunteers read pairs of sentences ending with three target types: (1) expected words, (2) unexpected words from the expected semantic category, and (3) unexpected words from an unexpected category. Context was presented word by word at fixation while targets were presented two degrees to the right or left of fixation. ERPs to unexpected endings were more negative than those to expected endings in both visual fields. However, when presented to the right visual field (left hemisphere), unexpected items from the expected category elicited smaller N400s than those from an unexpected category. In contrast, when presented to the left visual field (right hemisphere) all unexpected endings elicited N400s of similar amplitude. Thus, while both hemispheres are sensitive to context, only the left hemisphere is sensitive to semantic similarity between an unexpected ending and the expected completion. The results suggest lateralized differences in how new information is integrated into sentences. We propose that right hemisphere processing is best characterized as 'integrative'; new information is compared directly with context information. In contrast, left hemisphere processing is better characterized as 'predictive'; the processing of context leads to an expectation about the semantic features of upcoming items and new information is compared with that expectation rather than directly with the context.

Word, Pseudoword, and Nonword Processing: A Multitask Comparison Using Event-Related Brain Potentials

Event-related brain potentials (ERPs) to words, pseudowords, and nonwords were recorded in three different tasks. A letter search task was used in Experiment 1. Performance was affected by whether the target letter occurred in a word, a pseudoword, or a random nonword. ERP results corroborated the behavioral results, showing small but reliable ERP differences between the three stimulus types. Words and pseudowords differed from nonwords at posterior sites, whereas words differed from pseudowords and nonwords at anterior sites. Since deciding whether the target letter was present or absent co-occurred with stimulus processing in Experiment 1, a delayed letter search task was used in Experiment 2. ERPs to words and pseudowords were similar and differed from ERPs to nonwords, suggesting a primary role of orthographic and phonological processing in the delayed letter search task. To increase semantic processing, a categorization task was used in Experiment 3. Early differences between ERPs to words and pseudowords at left posterior and anterior locations suggested a rapid activation of lexico-semantic information. These findings suggest that the use of ERPs in a multiple task design makes it possible to track the time course and the activation of multiple sources of linguistic information when processing words, pseudowords, and nonwords. The task-dependent nature of the effects suggests that the language system can use multiple sources of linguistic information in flexible and adaptive ways.

Event-related potentials in a two-choice task involving within-form comparisons of pictures and words

Behavioral as well as electrophysiological evidence suggests that words are processed differently than pictures in a variety of tasks. In this study fifteen adult subjects were tested on a speeded "same-different" judgment task between printed names and drawings of common objects. In one condition, subjects decided on the identity between their internal image of the object that was named by S1 (word) and a subsequently presented drawing of an object (Word-Picture condition). In a second condition, comparisons were made on the basis of the name of the depicted object (Picture-Word trials). Event-related potentials (ERPs) were recorded from 12 scalp locations in response to the second item in each pair. No-match waveforms were characterized by larger N350 and P500 deflections compared to Match ERPs. The two responses could be distinguished on the basis of their lateral and anterior-posterior scalp distribution. Within-form comparisons involving pictures produced increased positivity between 150 and 600 ms poststimulus onset at posterior recording sites, whereas the opposite effect was noted at anterior sites during the early portion of the ERP. Decisions on word stimuli were associated with prolonged reaction time and longer N350 peak latency compared to decisions on pictures. These results demonstrate the existence of independent sources of ERP variability, each possibly reflecting a different aspect of cognitive comparisons. Latency and reaction time data provided valuable information regarding differences in the course of the comparison process when linguistic, as opposed to pictorial, stimuli/representations were involved.

Learning disability subtypes and the effects of auditory and visual priming on visual event-related potentials to words

Three learning-disability (LD) subtype groups and a normal control group of children were compared in their visual event-related potentials (ERPs) to primed and unprimed words. The LD subtypes were defined by deficient performance on tests of arithmetic (Group A), reading and spelling (Group RS), or both (Group RSA). The primed words were preceded by pictures or spoken words having a related meaning, while unprimed words were preceded by non-associated pictures or spoken words. For normal controls, N450 amplitude was greater to unprimed words than to words primed by pictures and spoken words. For Group A, N450 amplitude was reduced by spoken-word primes, but not by picture primes, an effect that demonstrates a deficit in processing visual-spatial information. For Group RS and Group RSA, neither picture nor spoken-word primes reduced N450 amplitude. These effects can be understood in terms of deficiencies in processing auditory-verbal information. Normal controls displayed a greater left- than right-hemispheric asymmetry in frontal N450 amplitude to unprimed words, an effect that is consistent with the association of skilled reading with hemispheric specialization. This asymmetry was absent in the ERPs of all the LD subtypes. The distinct ERP effects for the groups endorses the value of defining LD subtypes on the basis of patterns of deficits in arithmetic and reading and spelling.

Perceptual closure and object identification: electrophysiological responses to incomplete pictures

Event-related potentials were recorded during the naming of pictures of concrete objects. The pictures were presented at three levels of completeness: 10, 30, and 60%. The ERP waveforms were evaluated according to the level of picture completeness and the correctness of naming. A negative wave in the latency range of 250-550 ms was significantly more negative when the pictures were more incomplete, regardless of the correctness of response. This N400 wave is proposed as being related to hypotheses about the identity of the object. A late positive wave in the latency range of 550-650 ms followed the negativity, but only when the response was correct. This may reflect the subject's certainty about the perceptual analysis, a verification of the identity of the object. A slow parietal negativity lasting up to 2 sec was largest for the least complete picture. This therefore varied with the perceptual difficulty.

Time of day and semantic category effects on late components of the visual ERP

The N100, P200, N400 and P600 components of the visual event-related potential were recorded from 11 female subjects every 2 h from 09:00 to 21:00 hours using a semantic categorization task. All subjects scored as "Intermediate" or marginal "Evening" types on a Morningness-Eveningness questionnaire. Amplitude and latency of components, tympanic temperature, and performance measures for positive and negative category instances were assessed. Amplitude of P200 increased across the day. Amplitude of N400 was larger, and latency of P600 was longer, for negative category instances, but neither component varied with time of day. N100 was unaffected by time of day. The results suggest that previous reports of diurnal variations in visual N100-P200 were due to variations in P200 alone, and that diurnal variations in P200 may reflect diurnal variations in underlying arousal levels. In addition, overlap with P600 may have obscured time of day effects for N400.

An Event-Related Potential (ERP) Analysis of Semantic Congruity and Repetition Effects in Sentences

In two experiments, event-related brain potentials (ERPs) and cued-recall performance measures were used to examine the consequences of semantic congruity and repetition on the processing of words in sentences. A set of sentences, half of which ended with words that rendered them semantically incongruous, was repeated either once (eg, Experiment 1) or twice (e.g., Experiment 2). After each block of sentences, subjects were given all of the sentences and asked to recall the missing final words.

Repetition benefited the recall of both congruous and incongruous endings and reduced the amplitude and shortened the duration of the N400 component of the ERP more for (1) incongruous than congruous words, (2) open class than closed class words, and (3) low-frequency than high-frequency open class words. For incongruous sentence terminations, repetition increased the amplitude of a broad positive component subsequent to the N400.

Assuming additive factors logic and a traditional view of the lexicon, our N400 results indicate that in addition to their singular effects, semantic congruiry, repetition, and word frequency converge to influence a common stage of lexical processing. Within a parallel distributed processing framework, our results argue for substantial temporal and spatial overlap in the activation of codes subserving word recognition so as to yield the observed interactions of repetition with semantic congruity, lexical class, and word frequency effects.

The effect of picture priming on event-related potentials of normal and disabled readers during a word recognition memory task

Normal and one subtype of disabled readers were compared in their visual event-related potentials (ERPs) that were elicited by primed and unprimed words during a recognition memory task. The primed words were preceded by a picture having the same denotative meaning, while unprimed words were preceded by a picture having a non-associated meaning. Normal readers exhibited consistently greater amplitude than the disabled readers to unprimed words with a negative wave at 455 ms (N400). For the disabled readers, this N400 was evident, though somewhat smaller, than for controls, at fronto-central placements, but absent at the lateral parietal and occipital sites. Priming a word with a picture reduced N400 amplitude for both the normal and disabled readers. There were no remarkable differences between groups in their ERPs to the pictures. The pattern of ERP results obtained seems to reflect a failure of this subtype of disabled readers to engage long-term, semantic memory, while their short-term linguistic processing is intact.

Saccade-related brain potentials during reading correct and incorrect versions of proverbs

Using a reading task, the present study investigated saccade-related brain potentials (SRPs) accompanying the perception of the final words of proverbs, i.e. of sentences where the context allows a strong anticipation of the final word. The sentences were presented one at a time on a TV monitor. The proverbs appeared either in their original form or with their final word changed to be incongruous with the sentence context. SRPs to the two types of final words were recorded from 4 scalp areas. The onset of the saccade leading to the final word was used to trigger the averaging of SRPs. Incongruent and congruent brain responses were also compared by means of difference waveforms. The results showed that a difference between SRPs to congruous vs incongruous final words of proverbs already appeared simultaneously with the SRP component indicating the analysis of the visual pattern of the word. This finding supports an interactive model of word perception.

Event-related potential asymmetries during the reading of sentences

This report is an overview of the lateral distribution of event-related brain potentials (ERPs) recorded during silent reading in 7 different experiments. Both single word and cross-sentence averages revealed the presence of several ERP asymmetries. The P1, P2 and a negativity between 300 and 500 msec were found to be larger over the right than the left hemisphere. It was argued that this asymmetric negativity was due primarily to the contribution of the N400 elicited by all content words. The degree of N400 asymmetry was unaffected either by the rate of sentence presentation or the ratio of congruent to incongruent sentences but was quite sensitive to family history of left-handedness. In contrast, the P1 and P2 asymmetries were uninfluenced by lexical class or familial sinistrality.

Recognition memory for words and event-related potentials: a comparison of normal and disabled readers

Visual event-related potentials elicited during a word-recognition memory task were examined for groups of normal and disabled readers (RD). The strong association of reading ability with recognition memory performance endorsed the appropriateness of this signal detection paradigm as a reading-related task. Enhanced amplitude of the P200 component for the RD group was evident during both the acquisition and recognition series and it is indicative of differences at an early sensory stage of item encoding and retrieval. Normal readers displayed greater N400 amplitude than the RD group during both the acquisition and recognition series, an effect which is consistent with more extensive semantic evaluation or memory search that is attributed to that component. In the absence of any remarkable differences in P300 amplitude between groups, the poorer recognition memory performance for the RD group may not be attributable to attentional deficits.

Context effects on saccade-related brain potentials to words during reading

In two experiments saccade-related brain potentials (SRPs) to sentences were investigated under conditions approximating natural reading. Our aim was to look for electrophysiological (SRP) signs of sentence context on the processing of final words that were either congruent or incongruent with the meaning of the sentence. In Experiment 1 subjects indicated by a button-press whether or not the final word was congruent with the context, while in Experiment 2 they read silently without an overt decision. In Experiment 1, SRPs to incongruent words were more negative than SRPs to congruent words between 80-310 msec (from saccade offset). In Experiment 2, however, the inconcruent SRPs became more negative than the congruent SRPs only between 280-460 msec. These results suggest that in Experiment 1, during the processing of incongruent words the early sign of registering mismatch appears simultaneously with the analysis of the visual features of the word.

Visual and auditory event-related potentials elicited by linguistic and non-linguistic incongruities

To test the hypothesis that violation of overlearned relations between stimuli in a sequence is critical for eliciting the event-related potential (ERP) N400, we recorded scalp ERPs in sequences where interstimulus relations were either overlearned (sentences or famous songs) or not (geometric figures or scale notes): 25% of the sequences ended with an incongruous stimulus. N400 occurred after incongruous words in sentences only. It seems to index the further processing necessary to specify the nature of the mismatch detected between attended and expected stimuli, when complex information is involved. Earlier components also varied as a function of experimental factors.

Stimulus offset P3 and temporal resolution of uncertainty

Stimulus offset evoked potentials were recorded to short (1 250 ms) and long (2 500 ms) tones in the context of the odd-ball paradigm. In one condition, the short tone was presented frequently with a probability of 0.85 and the long tone with a probability of 0.15, and in a second condition, the probabilities of tone presentation were reversed. Offset P3s were obtained only to short and not to long rare tones, indicating that resolution of uncertainty rather than stimulus probability or task relevance is responsible for eliciting this late positive component in the context of this experiment.

Saccade-related brain potentials in semantic categorization tasks

Saccade-related brain potentials (SRPs) were recorded in word categorization tasks in which subjects had to perform a saccade in order to perceive the stimulus. For all three conditions representing different degrees of complexity of semantic categorization, the stimuli belonged to one of two categories which appeared with the respective probabilities of either 0.20 or 0.80. The late positivity (P4) of the SRPs to infrequent stimuli appeared systematically later as the complexity of stimulus evaluation increased: The easiest categorization was accompanied by a P4 at 400 msec, in the more complex condition it peaked at 600 msec, and in the most difficult semantic categorization the P4 peaked even later, at 680 msec. This shift in peak latency with increasing complexity of categorization is in agreement with the results for traditional ERPs (e.g. Kutas and Donchin, 1978). The possible overlap of the late components was investigated by applying Principal Component-Varimax Analysis to the SRPs.

Event-related potentials associated with the discrimination of acoustic and semantic aspects of speech

Analysis of a word's acoustic structure must precede identification of its meaning. Therefore, these aspects of speech processing could be associated with event-related potential (ERP) components that differed in their timing. To identify electrophysiologic indices of the cortical processing of acoustic and semantic features of speech, we recorded ERPs to the random presentation of nonsense or real words in four conditions designed to manipulate the extent to which the speech sounds were processed. In one condition subjects responded to all stimuli; in a second and third, to a designated nonsense or real word; and in the final condition to words within a specified semantic category. To define the cortical activity associated with acoustic processing, ERPs obtained when no discrimination was required were subtracted from those recorded during the identification of a specified speech target. The difference waveforms exhibited a negative potential that began about 50 msec after stimulus onset and lasted about 200 msec. Difference waveforms obtained by subtracting the non-discrimination ERP from those obtained during semantic discrimination exhibited a negative potential with similar onset timing. We concluded that the early negative potential indexed acoustic processes necessary for stimulus identification. To identify potentials associated with determination of a word's meaning, we subtracted the verbal discrimination from the semantic discrimination ERPs. This difference waveform exhibited a later negativity beginning at 150 msec and lasting about 250 msec. This potential may be related to the semantic processing of speech.

The effects of physical and semantic incongruities on auditory event-related potentials

Twenty subjects listened to a series of simple sentences, spoken by a male voice, in which the last word was on occasions either made semantically incongruous or was unexpectedly spoken by a female voice. Averages of ERPs to the last words revealed that a consistent late negative component (N456) was associated with semantic incongruity and a late positive component (P416) with physical (voice) incongruity. The results were consistent with those in the visual modality by Kutas and Hillyard (1980a,b) and are interpreted in terms of the facilitatory and inhibitory effects of contextual priming on the processing of the words concerned. In a subsidiary experiment 6 subjects were required to repeat the last words of the same set of sentences as rapidly as possible. Verbal response latency increased by 62 msec to physically incongruous words and by 185 msec to semantically incongruous words.

Brain potentials during reading reflect word expectancy and semantic association

The neuroelectric activity of the human brain that accompanies linguistic processing can be studied through recordings of event-related potentials (e.r.p. components) from the scalp. The e.r.ps triggered by verbal stimuli have been related to several different aspects of language processing. For example, the N400 component, peaking around 400 ms post-stimulus, appears to be a sensitive indicator of the semantic relationship between a word and the context in which it occurs. Words that complete sentences in a nonsensical fashion elicit much larger N400 waves than do semantically appropriate words or non-semantic irregularities in a text. In the present study, e.r.ps were recorded in response to words that completed meaningful sentences. The amplitude of the N400 component of the e.r.p. was found to be an inverse function of the subject's expectancy for the terminal word as measured by its 'Cloze probability'. In addition, unexpected words that were semantically related to highly expected words elicited lower N400 amplitudes. These findings suggest N400 may reflect processes of semantic priming or activation.

Brain potentials during sentence verification: late negativity and long-term memory strength

Subjects decided whether self-referential statements were true or false. Event-related potentials (ERPs) associated with final words creating false statements displayed a late negativity (N340) relative to ERPs for true completions. The size of this difference between true and false statements was greater for highly familiar statements (e.g. "My name is Ira") than for less familiar ones (e.g. "I go to bed late") even after all the statements had been practised a number of times. The late negativity appears to be associated with a discrepancy between presented and remembered information, and its magnitude reflects the long-term familiarity or strength of the remembered information.

Further study of the electrophysiological correlates of lexical decision

Discriminations were required between words, pseudohomophones, and visually matched nonwords. Two tasks were employed, one which could be accomplished on the basis of a visual code (the REAL task, involving discrimination between words and both types of nonword) and another necessitating the use of a phonological code (the REAL/PSEUD task, words and pseudohomophones vs. non-words). ERPs were recorded from three midline sites and from left and right inferior parietal sites. Two principal results were observed, (i) the peak latency of a late positive component, P637, covaried with RT, with variations in latency of around one half the corresponding RT variations, and (ii) the peak-to-peak amplitude of N100-P187 interacted with stimulus and task, such that it was larger for nonwords in the REAL task and words in the REAL/PSEUD task. No task- or stimulus-dependent asymmetries were observed in any ERP component. The P637 latency data support a model of RT variation based on the interaction of changes in parallel response preparation and stimulus evaluation processes. The observations with respect to N100-P187 suggest that ERPs are sensitive to factors related to the early processing of words and word-like visual material.

Evoked potentials and the dynamics of language processing

It is observed, in a brief review of studies of evoked potentials and language processing, that for the most part only global and rather static aspects have been investigated. It is argued that the important topic of the dynamics of work recognition has been neglected. The evolution of 'spreading activation' models of semantic processing is briefly traced. These postulate pre-conscious word recognition in a semantic network where radiation of excitation between semantically related 'nodes' or 'logogens' account for important context effects on word recognition. It is suggested that evoked potential studies could be used to test and elaborate this model. An experiment is described in which discrimination between positive and negative instances of primed categories by enhancement of the N1-P2 (N130-P212) amplitude measure supports the notion of early and possibly pre-conscious attainment of work meaning and also indicates enhanced activation from word detection units sensitized by spread of activation from a category prime.