Referential processing in the human brain: An Event-Related Potential (ERP) study.

A substantial body of ERP research investigating the processing of syntactic long-distance dependencies has shown that, across languages and construction types, the second element in such configurations typically elicits phasic left anterior negativity (LAN). We hypothesized that these effects are not specific to syntactic dependencies, but rather index a more general cognitive operation in which the second (dependent) element in sentence-level linguistic long-distance relationships triggers a process of association with the first element. We tested this hypothesis with straightforward referential dependencies, comparing pronouns with proper name antecedents to those without, and proper names with and without preceding co-referring pronouns. We predicted phasic LAN effects in response to the second referential element in both comparisons, but observed them only in response to pronouns with antecedents; no differences were observed between responses to proper names with and without preceding co-referring pronouns. We argue that LAN effects observed at the pronoun index the cognitive operations necessary for the association of a pronoun with its antecedent, on which it depends for its reference. Similar but non-identical responses were elicited by the main clause verb following the gap position in object relative clause constructions compared to coordinate clause controls in an orthogonal manipulation. LAN effects were thus elicited by the second dependent element in both construction types, suggesting that long-distance syntactic and referential dependencies pose similar processing challenges. These findings help to clarify the cognitive processes indexed by anterior negative responses to associated dependent elements in a variety of language contexts.

Patients with MCI and N400 or P600 abnormalities are at very high risk for conversion to dementia.

OBJECTIVE: We sought cognitive event-related potential (ERP) biomarkers of disease progression and subsequent conversion to dementia in mild cognitive impairment (MCI). BACKGROUND: Two ERP components, the P600 and N400, are sensitive to abnormal episodic/declarative memory and semantic processing. When congruous category-exemplars are repeated, smaller P600s (relative to initial presentation) are normally elicited. Repetitions of semantically incongruous words yield smaller N400 amplitude. In mild Alzheimer disease (AD), abnormalities of both the N400 and P600 repetition effects are present, suggesting a widespread failure of synaptic plasticity. METHODS: Patients with amnestic MCI (n = 32) were longitudinally studied annually with an ERP paradigm in which semantically congruous (50%) and incongruous target words are repeated 10 to 140 seconds after initial presentation. ERP data were analyzed to contrast MCI-to-AD converters (within 3 years) vs nonconverters, using split-plot analyses of variance. RESULTS: A statistically significant P600 congruous word repetition effect was found only in the nonconverter group (F = 9.9, p = 0.005 vs MCI converters). This effect correlated with verbal memory measures. Repetition of incongruous words produced a significant N400 amplitude attenuation (across right-hemisphere sites) in nonconverters, but not in converters. Patients with MCI with abnormal/reduced N400 or P600 word repetition effects had an 87 to 88% likelihood of dementia within 3 years while those with normal/spared N400 and P600 repetition effects had only an 11 to 27% likelihood. CONCLUSIONS: Abnormalities of the P600 or N400 in mild cognitive impairment are associated with an increased risk of subsequent conversion to Alzheimer disease (AD). These event-related potential components may offer useful biomarkers for the detection and staging of very early AD.

Semantic and repetition priming within the attentional blink: an event-related brain potential (ERP) investigation study.

An attentional blink (AB) paradigm was used to directly compare semantic and repetition priming for reported words versus missed words. Three target words (T1, T2, T3) were embedded in a rapidly presented stream of non-word distractors for report at the end of each trial. Whereas T1 was not related to either T2 or T3, T2 and T3 could be unrelated words, semantically related words, or identical. Semantic and repetition priming effects were evident in both behavioral and electrophysiological measures on T3, whether T2 was accurately reported or 'blinked'. These results suggest that semantic and repetition priming effects, under rapid serial visual presentation conditions, are modulated by at least partially overlapping neural mechanisms.

Dissecting out conscious and unconscious memory (sub)processes within the human medial temporal lobe.

The human medial temporal lobe (MTL) system mediates memories that can be consciously recollected. However, the specific natures of the individual contributions of its various subregions to conscious memory processes remain equivocal. Here we show a functional dissociation between the hippocampus proper and the parahippocampal region in conscious and unconscious memory as revealed by invasive recordings of limbic event-related brain potentials recorded during explicit and implicit word recognition: Only hippocampal and not parahippocampal neural activity exhibits a sensitivity to the implicit versus explicit nature of the recognition memory task. Moreover, only within the hippocampus proper do the neural responses to repeated words differ not only from those to new words but also from each other as a function of recognition success. By contrast parahippocampal (rhinal) responses are sensitive to repetition independent of conscious recognition. These findings thus demonstrate that it is the hippocampus proper among the MTL structures that is specifically engaged during conscious memory processes.

Abnormal verbal event related potentials in mild cognitive impairment and incipient Alzheimer's disease.

BACKGROUND: It has been reported that patients with amnesia have a reduced effect of word repetition upon the late positive component of the event related potential (ERP), which peaks at around 600 ms after word onset. OBJECTIVE: To study a word repetition ERP paradigm in subjects with mild cognitive impairment. SUBJECTS: 14 patients with mild cognitive impairment (mean mini-mental state examination score = 27); 14 normal elderly controls. METHODS: Auditory category statements were each followed by a single visual target word (50% "congruous" category exemplars, 50% "incongruous") while ERPs were recorded. N400 (an ERP component elicited by semantically "incongruous" words) and LPC amplitude data were submitted to analysis of variance. RESULTS: The latency of the N400 was slower in mild cognitive impairment. In normal controls, the ERPs to "congruous" targets showed a late positive component to new words, which was greatly diminished with repetition. This repetition effect in normal subjects started before 300 ms at right frontal sites, and peaked at approximately 600 ms post-stimulus over posterior sites. In contrast, the group with mild cognitive impairment had a reduced repetition effect (p < 0.02), which started around 500 ms, with a more central distribution. Further comparisons within the cognitive impairment group showed no appreciable congruous word repetition effect among seven individuals who subsequently converted to probable Alzheimer's disease. The congruous word repetition effect in the group with mild cognitive impairment was almost entirely accounted for by the non-converters. The amplitude of the congruous late positive component word repetition effect was significantly correlated (0.38 < or = r < or = 0.73) with several verbal memory measures. CONCLUSIONS: The congruous word repetition ERP effect appears sensitive to the memory impairment in mild cognitive impairment and could have value in predicting incipient Alzheimer's disease.

Variability in AC amplifier distortions: estimation and correction.

AC amplifiers can introduce significant distortions into the low frequency and DC components of recorded electrophysiological data such as event-related potentials (ERPs). Methods for correcting such distortions (i.e., estimating the waveform of the original data) after the data have been amplified and recorded rely on an accurate estimate of the amplifier's time constant (TC). We show that the filter characteristics of AC amplifiers in at least some commercially available ERP recording instruments may vary considerably across individual channels, even when each houses an identical AC amplifier circuit. Clearly, distortion correction methods must take this variability into account. We propose an empirical means of estimating the correct TC value. This approach yields more accurate correction than those based on TCs calculated analytically.

Getting it: human event-related brain response to jokes in good and poor comprehenders.

Joke comprehension has been decomposed into surprise registration followed by a coherence stage, involving frame-shifting (retrieving a new frame from long-term memory to reinterpret information in working memory). We examined this view by recording event-related brain potentials (ERPs) from adults reading one-line jokes or non-joke controls with equally unexpected endings. Joke and non-joke ERPs differed in several respects depending on participants' ability to get the joke and contextual constraint. In good joke comprehenders, all jokes elicited a left-lateralized sustained negativity (500-900 ms), indexing frame-shifting, low constraint jokes elicited a frontal positivity (500-900 ms), and high constraint jokes elicited an N400 and later posterior positivity. By contrast, poor joke comprehenders showed only a right frontal negativity (300-700 ms) to jokes. This pattern of effects does not map readily onto a two-stage model of joke comprehension.

Electrophysiological estimates of semantic and syntactic information access during tacit picture naming and listening to words.

We investigated the relative time courses of the accessibility of semantic and syntactic information in speaking and comprehension via event-related brain potentials (ERPs). Native German speakers either viewed a series of pictures (tacit picture naming experiment) or heard a series of nouns (listening experiment) and made dual choice go/nogo decisions based on each item's semantic and syntactic features. N200 peak latency results indicate that access to meaning has temporal precedence over access to syntactic information in both speaking (approximately 80 ms) and comprehension (approximately 70 ms), and are discussed in the context of current psycholinguistic theories.

Electrophysiological correlates of emotion-induced recognition bias.

The question of how emotions influence recognition memory is of interest not only within basic cognitive neuroscience but from clinical and forensic perspectives as well. Emotional stimuli can induce a "recognition bias" such that individuals are more likely to respond "old" to a negative item than to an emotionally neutral item, whether the item is actually old or new. We investigated this bias using event-related brain potential (ERP) measures by comparing the processing of words given "old" responses with accurate recognition of old/new differences. For correctly recognized items, the ERP difference between old items (hits) and new items (correct rejections, CR) was largely unaffected by emotional valence. That is, regardless of emotional valence, the ERP associated with hits was characterized by a widespread positivity between 300 and 700 msec relative to that for CRs. By contrast, the analysis of ERPs to old and new items that were judged "old" (hits and false alarms [FAs], respectively) revealed a differential effect of valence by 300 msec: Neutral items showed a large old/new difference over prefrontal sites, whereas negative items did not. These results are the first clear demonstration of response bias effects on ERPs linked to recognition memory. They are consistent with the idea that frontal cortex areas may be responsible for relaxing the retrieval criterion for negative stimuli so as to ensure that emotional events are not as easily "missed" or forgotten as neutral events.

An electrophysiological analysis of the time course of conceptual and syntactic encoding during tacit picture naming.

A central question in psycholinguistic research is when various types of information involved in speaking (conceptual/semantic, syntactic, and phonological information) become available during the speech planning process. Competing theories attempt to distinguish between parallel and serial models. Here, we investigated the relative time courses of conceptual and syntactic encoding in a tacit picture-naming task via event-related brain potential (ERP) recordings. Participants viewed pictures and made dual-choice go/no-go decisions based on conceptual features (whether the depicted item was heavier or lighter than 500 g) and syntactic features (whether the picture's German name had feminine or masculine syntactic gender). In support of serial models of speech production, both the lateralized readiness potential, or LRP (related to response preparation), and the N200 (related to response inhibition) measures indicated that conceptual processing began approximately 80 msec earlier than syntactic processing.

Effects of transient, mild mood states on semantic memory organization and use: an event-related potential investigation in humans.

The effects of transient mood states on semantic memory organization and use were investigated using event-related potentials. Participants read sentence pairs ending with (1) the most expected word, (2) an unexpected word from the expected semantic category, or (3) an unexpected word from a different (related) category; half the pairs were read under neutral mood and half under positive mood. Under neutral mood, N400 amplitudes were smallest for expected items and smaller for unexpected items when these came from the expected category. In contrast, under positive mood, N400 amplitudes to the two types of unexpected items did not differ. Positive mood seemed to specifically facilitate the processing of distantly-related, unexpected items. The results suggest that transient mood states are associated with dynamic changes in how semantic memory is used on-line.

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.

Electrophysiology reveals semantic memory use in language comprehension.

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

Brain responses to nouns, verbs and class-ambiguous words in context.

Recent neuropsychological and imaging data have implicated different brain networks in the processing of different word classes, nouns being linked primarily to posterior, visual object-processing regions and verbs to frontal, motor-processing areas. However, as most of these studies have examined words in isolation, the consequences of such anatomically based representational differences, if any, for the processing of these items in sentences remains unclear. Additionally, in some languages many words (e.g. 'drink') are class-ambiguous, i.e. they can play either role depending on context, and it is not yet known how the brain stores and uses information associated with such lexical items in context. We examined these issues by recording event-related potentials (ERPs) in response to unambiguous nouns (e.g. 'beer'), unambiguous verbs (e. g. 'eat'), class-ambiguous words and pseudowords used as nouns or verbs within two types of minimally contrastive sentence contexts: noun-predicting (e.g. 'John wanted THE [target] but.') and verb-predicting ('John wanted TO [target] but.'). Our results indicate that the nature of neural processing for nouns and verbs is a function of both the type of stimulus and the role it is playing. Even when the context completely specifies their role, word class-ambiguous items differ from unambiguous ones over frontal regions by approximately 150 ms. Moreover, whereas pseudowords elicit larger N400s when used as verbs than when used as nouns, unambiguous nouns and ambiguous words used as nouns elicit more frontocentral negativity than unambiguous verbs and ambiguous words used as verbs, respectively. Additionally, unambiguous verbs elicit a left-lateralized, anterior positivity (approximately 200 ms) not observed for any other stimulus type, though only when these items are used appropriately as verbs (i.e. in verb-predicting contexts). In summary, the pattern of neural activity observed in response to lexical items depends on their general probability of being a verb or a noun and on the particular role they are playing in any given sentence. This implicates more than a simple two-way distinction of the brain networks involved in their storage and processing. Experience, as well as context during on-line language processing, clearly shapes the neural representations of nouns and verbs, such that there is no single neural marker of word class. Our results further suggest that the presence and nature of the word class-based dissociations observed after brain damage are similarly likely to be a function of both the type of stimulus and the context in which it occurs, and thus must be assessed accordingly.

Word repetition in amnesia. Electrophysiological measures of impaired and spared memory.

Amnesic patients often show improved performance when stimuli are repeated, even in the absence of conscious memory for those stimuli. Although these performance changes are typically attributed to perceptual or motor systems, in some cases they may be related to basic language processing. We examined two neurophysiological measures that vary with word repetition in 12 amnesic patients and 12 control subjects: (i) a late positive component of the event-related potential (ERP) linked to conscious memory and (ii) the N400 component that varies with language comprehension. In each trial, the subject heard a category name, then viewed a word, and then decided whether the word was semantically congruous or incongruous (e.g. 'yes' for 'baby animal: cub'; 'no' for 'water sport: kitchen'). Recall and recognition testing at the end of the experiment showed that control subjects had better memory for congruous than for incongruous words, as did the amnesic patients, who performed less well overall. In contrast, amnesic patients were unimpaired on the category decisions required in each trial and, like the control subjects, showed a large N400 for incongruous relative to congruous words. Similarly, when incongruous trials were repeated after 0-13 intervening trials, N400s were reduced in both groups. When congruous trials were repeated, a late positive repetition effect was observed, but only in the control group. Furthermore, the amplitude of the late positive repetition effect was highly correlated with later word recall in both patients and controls. In the patients, the correlation was also observed with memory scores from standardized neuropsychological tests. These data are consistent with a proposed link between the late positive repetition effect and conscious memory. On the other hand, the N400 repetition effect was not correlated with episodic memory abilities, but instead indexed an aspect of memory that was intact in the amnesic patients. The preserved N400 repetition effect is an example of preserved memory in amnesia that does not easily fit into the categories of low-level perceptual processing or of motor learning. Instead, the sensitivity of the N400 to both semantic context and repetition may reflect a short-term memory process that serves language comprehension in realtime.

Electrophysiological estimates of the time course of semantic and phonological encoding during implicit picture naming.

Two different event-related potential (ERP) components were used to investigate the temporal processing of semantic and phonological encoding during implicit picture naming. Participants were shown pictures and carried out a dual choice go/nogo decision based on semantic information (i.e., whether the picture was of an object or an animal) and phonological information (i.e., whether the picture's name starts with a vowel or a consonant). In addition to the already established lateralized readiness potential (LRP; related to response preparation), we introduce the N200 (presumably related to response inhibition) as a tool for measuring online language processing. Both, the LRP and the N200 data indicated that semantic processing began earlier than phonological processing. The data are discussed in the context of language production models. Therein, the LRP and N200 results, taken together, favor a serial or cascaded processing model of language production in contrast to a parallel processing account.

Overt and covert identification of fragmented objects inferred from performance and electrophysiological measures.

The authors investigated visual processing leading to object identification by manipulating the number of fragments and nature of the study. During the study, participants either named or drew objects in Experiment 1 and drew them all in Experiment 2. During the test, participants made an identification judgment at each of 6 different fragmentation levels for studied and new objects. Fewer fragments were needed to identify studied than unstudied objects. Reaction times were faster for studied than unstudied objects both at identification and at the preceding level. Event-related brain potentials (ERPs) to unidentified objects were characterized by a late negativity in contrast to a positivity to identified objects. ERPs to studied but not to new objects contained a smaller and later version of the identification positivity at level just prior to identification, which was not due to differential response confidence. Much covert visual analysis and even object identification may precede overt identification, depending on the nature of prior experience.

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.