The effect of stimulus repetition on cortical magnetic responses evoked by words and nonwords

Multiple functions of the angular gyrus at high temporal resolution

Here, the functions of the angular gyrus (AG) are evaluated in the light of current evidence from transcranial magnetic/electric stimulation (TMS/TES) and EEG/MEG studies. 65 TMS/TES and 52 EEG/MEG studies were examined in this review. TMS/TES literature points to a causal role in semantic processing, word and number processing, attention and visual search, self-guided movement, memory, and self-processing. EEG/MEG studies reported AG effects at latencies varying between 32 and 800 ms in a wide range of domains, with a high probability to detect an effect at 300-350 ms post-stimulus onset. A three-phase unifying model revolving around the process of sensemaking is then suggested: (1) early AG involvement in defining the current context, within the first 200 ms, with a bias toward the right hemisphere; (2) attention re-orientation and retrieval of relevant information within 200-500 ms; and (3) cross-modal integration at late latencies with a bias toward the left hemisphere. This sensemaking process can favour accuracy (e.g. for word and number processing) or plausibility (e.g. for comprehension and social cognition). Such functions of the AG depend on the status of other connected regions. The much-debated semantic role is also discussed as follows: (1) there is a strong TMS/TES evidence for a causal semantic role, (2) current EEG/MEG evidence is however weak, but (3) the existing arguments against a semantic role for the AG are not strong. Some outstanding questions for future research are proposed. This review recognizes that cracking the role(s) of the AG in cognition is possible only when its exact contributions within the default mode network are teased apart.

Rapid acquisition of novel written word-forms: ERP evidence

Background

Novel word acquisition is generally believed to be a rapid process, essential for ensuring a flexible and efficient communication system; at least in spoken language, learners are able to construct memory traces for new linguistic stimuli after just a few exposures. However, such rapid word learning has not been systematically found in visual domain, with different confounding factors obscuring the orthographic learning of novel words. This study explored the changes in human brain activity occurring online, during a brief training with novel written word-forms using a silent reading task

Results

Single-trial, cluster-based random permutation analysis revealed that training caused an extremely fast (after just one repetition) and stable facilitation in novel word processing, reflected in the modulation of P200 and N400 components, possibly indicating rapid dynamics at early and late stages of the lexical processing. Furthermore, neural source estimation of these effects revealed the recruitment of brain areas involved in orthographic and lexico-semantic processing, respectively.

Conclusions

These results suggest the formation of neural memory traces for novel written word-forms after a minimal exposure to them even in the absence of a semantic reference, resembling the rapid learning processes known to occur in spoken language.

Neural Correlates of Semantic Prediction and Resolution in Sentence Processing

Most brain-imaging studies of language comprehension focus on activity following meaningful stimuli. Testing adult human participants with high-density EEG, we show that, already before the presentation of a critical word, context-induced semantic predictions are reflected by a neurophysiological index, which we therefore call the semantic readiness potential (SRP). The SRP precedes critical words if a previous sentence context constrains the upcoming semantic content (high-constraint contexts), but not in unpredictable (low-constraint) contexts. Specific semantic predictions were indexed by SRP sources within the motor system—in dorsolateral hand motor areas for expected hand-related words (e.g., “write”), but in ventral motor cortex for face-related words (“talk”). Compared with affirmative sentences, negated ones led to medial prefrontal and more widespread motor source activation, the latter being consistent with predictive semantic computation of alternatives to the negated expected concept. Predictive processing of semantic alternatives in negated sentences is further supported by a negative-going event-related potential at ∼400 ms (N400), which showed the typical enhancement to semantically incongruent sentence endings only in high-constraint affirmative contexts, but not to high-constraint negated ones. These brain dynamics reveal the interplay between semantic prediction and resolution (match vs error) processing in sentence understanding.

SIGNIFICANCE STATEMENT Most neuroscientists agree on the eminent importance of predictive mechanisms for understanding basic as well as higher brain functions. This contrasts with a sparseness of brain measures that directly reflects specific aspects of prediction, as they are relevant in the processing of language and thought. Here we show that when critical words are strongly expected in their sentence context, a predictive brain response reflects meaning features of these anticipated symbols already before they appear. The granularity of the semantic predictions was so fine grained that the cortical sources in sensorimotor and medial prefrontal cortex even distinguished between predicted face- or hand-related action words (e.g., the words “lick” or “pick”) and between affirmative and negated sentence meanings.

Word-specific repetition effects revealed by MEG and the implications for lexical access

This magnetoencephalography (MEG) study investigated the early stages of lexical access in reading, with the goal of establishing when initial contact with lexical information takes place. We identified two candidate evoked responses that could reflect this processing stage: the occipitotemporal N170/M170 and the frontocentral P2. Using a repetition priming paradigm in which long and variable lags were used to reduce the predictability of each repetition, we found that (i) repetition of words, but not pseudowords, evoked a differential bilateral frontal response in the 150-250ms window, (ii) a differential repetition N400m effect was observed between words and pseudowords. We argue that this frontal response, an MEG correlate of the P2 identified in ERP studies, reflects early access to long-term memory representations, which we tentatively characterize as being modality-specific.

Word repetition priming-induced oscillations in auditory cortex: a magnetoencephalography study

Magnetoencephalography was used in a passive repetition priming paradigm. Words in two frequency bins (high/low) were presented to the participants auditorily. Participants' brain responses to these stimuli were analyzed using synthetic aperture magnetometry. The main finding of this study is that single-word repetition of low-frequency word pairs significantly attenuated the post-second word event-related desynchronization in the θ-α (5-15 Hz) bands, at 200-600 ms of post-second word stimulus onset. Peak significance between repeated high and low frequency words was evident at approximately 365-465 ms of posttarget onset. This finding has implications for: (i) the role of θ-α event-related desynchronization in lexical representation and access, (ii) the study of repetition suppression in the spectral-temporal domain, and (iii) the connection of neuronal repetition suppression with behavioral effects of repetition priming.

Spatiotemporal neural dynamics of word understanding in 12- to 18-month-old-infants

Learning words is central in human development. However, lacking clear evidence for how or where language is processed in the developing brain, it is unknown whether these processes are similar in infants and adults. Here, we use magnetoencephalography in combination with high-resolution structural magnetic resonance imaging to noninvasively estimate the spatiotemporal distribution of word-selective brain activity in 12- to 18-month-old infants. Infants watched pictures of common objects and listened to words that they understood. A subset of these infants also listened to familiar words compared with sensory control sounds. In both experiments, words evoked a characteristic event-related brain response peaking ∼400 ms after word onset, which localized to left frontotemporal cortices. In adults, this activity, termed the N400m, is associated with lexico-semantic encoding. Like adults, we find that the amplitude of the infant N400m is also modulated by semantic priming, being reduced to words preceded by a semantically related picture. These findings suggest that similar left frontotemporal areas are used for encoding lexico-semantic information throughout the life span, from the earliest stages of word learning. Furthermore, this ontogenetic consistency implies that the neurophysiological processes underlying the N400m may be important both for understanding already known words and for learning new words.

Masked immediate-repetition-priming effect on the early lexical process in the bilateral anterior temporal areas assessed by neuromagnetic responses

A masked priming paradigm has been used to measure unconscious and automatic context effects on the processing of words. However, its spatiotemporal neural basis has not yet been clarified. To test the hypothesis that masked repetition priming causes enhancement of neural activation, we conducted a magnetoencephalography experiment in which a prime was visually presented for a short duration (50 ms), preceded by a mask pattern, and followed by a target word that was represented by a Japanese katakana syllabogram. The prime, which was identical to the target, was represented by another hiragana syllabogram in the "Repeated" condition, whereas it was a string of unreadable pseudocharacters in the "Unrepeated" condition. Subjects executed a categorical decision task on the target. Activation was significantly larger for the Repeated condition than for the Unrepeated condition at a time window of 150-250 ms in the right occipital area, 200-250 ms in the bilateral ventral occipitotemporal areas, and 200-250 ms and 200-300 ms in the left and right anterior temporal areas, respectively. These areas have been reported to be related to processing of visual-form/orthography and lexico-semantics, and the enhanced activation supports the hypothesis. However, the absence of the priming effect in the areas related to phonological processing implies that automatic phonological priming effect depends on task requirements.

Multiple forms of learning yield temporally distinct electrophysiological repetition effects

Prior experience with a stimulus leads to multiple forms of learning that facilitate subsequent behavior (repetition priming) and neural processing (repetition suppression). Learning can occur at the level of stimulus-specific features (stimulus learning), associations between stimuli and selected decisions (stimulus-decision learning), and associations between stimuli and selected responses (stimulus-response learning). Although recent functional magnetic resonance imaging results suggest that these distinct forms of learning are associated with repetition suppression (neural priming) in dissociable regions of frontal and temporal cortex, a critical question is how these different forms of learning influence cortical response dynamics. Here, electroencephalography (EEG) measured the temporal structure of neural responses when participants classified novel and repeated stimuli, using a design that isolated the effects of distinct levels of learning. Event-related potential and spectral EEG analyses revealed electrophysiological effects due to stimulus, stimulus-decision, and stimulus-response learning, demonstrating experience-dependent cortical modulation at multiple levels of representation. Stimulus-level learning modulated cortical dynamics earlier in the temporal-processing stream relative to stimulus-decision and stimulus-response learning. These findings indicate that repeated stimulus processing, including the mapping of stimuli to decisions and actions, is influenced by stimulus-level and associative learning mechanisms that yield multiple forms of experience-dependent cortical plasticity.

Masked repetition priming using magnetoencephalography

Masked priming is used in psycholinguistic studies to assess questions about lexical access and representation. We present two masked priming experiments using MEG. If the MEG signal elicited by words reflects specific aspects of lexical retrieval, then one expects to identify specific neural correlates of retrieval that are sensitive to priming. To date, the electrophysiological evidence has been equivocal. We report findings from two experiments. Both employed identity priming, where the prime and target are the same lexical item but differ in case (NEWS-news). The first experiment used only forward masking, while the prime in the second experiment was both preceded and followed by a mask (backward masking). In both studies, we find a significant behavioral effect of priming. Using MEG, we identified a component peaking approximately 225 ms post-onset of the target, whose latency was sensitive to repetition. These findings support the notion that properties of the MEG response index specific lexical processes and demonstrate that masked priming can be effectively combined with MEG to investigate the nature of lexical processing.

The effect of the number of possible words on word completion: a magnetoencephalographic study

We examined the effect of the number of possible words in a word completion task on left superior temporal activity using magnetoencephalography. If this area is involved in access to phonological word representations, as assumed in dual-route models of reading, its activation for word fragments having many possible words (Many condition) would be greater than for fragments with one possible word (Single condition). Left superior temporal activity did not differ in strength between the conditions, while behavioral data suggested multiple accesses in the Many condition. Activation was shorter in the Many than in the Single condition, probably owing to bigram frequency difference. These results are inconsistent with the view that the left superior temporal area is related to lexical-route processing.

Neural correlates of Chinese word-appropriateness judgment: An MEG study

To study the neural correlates of Chinese word-appropriateness judgment, we used 2-word phrases and corresponding meaningless pairs produced by replacing the second words (W2) with homophones. Fourteen right-handed healthy adults viewed word pairs randomly presented one word at a time, and judged the lexical appropriateness of the W2 for combining its preceding first word (W1) into a meaningful phrase. We measured magnetoencephalographic (MEG) responses to W1, appropriate W2, and inappropriate W2 stimuli. For each subject, multi-dipole analyses revealed sequential neuromagnetic activations which involved the bilateral visual cortices at approximately 100 milliseconds (ms), the bilateral occipitotemporal regions at approximately 190 ms, and the left temporal lobe at approximately 350 ms (M350) following stimuli. We found that the word appropriateness had no clear effect on the occipitotemporal activation to W2 stimuli, whereas the M350 activation to inappropriate W2 was greater than that to W1 or appropriate W2. In 8 of our subjects, we found an additional activation in the right temporal region, with a smaller amplitude as compared with the left M350. Our results suggest that the M350 activity reflects both lexical and semantic appropriateness assessment. The lateralized M350 strengths may be used to determine the language dominance hemisphere; and additionally, our 2-word contexture judgment paradigm can be applied in further research on the cortical processing of lexicon-semantic information in Chinese speakers.

Increased inhibition and decreased facilitation effect during a lexical decision task in children

Widespread theories propose that implicit memory is established in the early stages of development, while explicit memory continues to develop until later stages. However, recent studies have argued that implicit memory changes developmentally. In order to elucidate the differences of implicit memory and semantic memory structures between children and adults, a single-word presentation method was developed using lexical decision tasks in which repeated real words, related real words, unrelated real words and pseudowords were presented in a list. Semantic priming and repetition priming using a single-word presentation method were employed in 25 children and 18 adults. Reaction time (RT) and correct rate for real words following pseudowords served as the base. Inhibition effect was defined as an increase in RT between a real word preceded by an unrelated real word and a real word preceded by a pseudoword. Facilitation effect was defined as a decrease in RT between a real word preceded by a related real word and a real word preceded by a pseudoword. Although the effect sizes of semantic priming did not differ between children and adults, the inhibition effect was larger and the facilitation effect was smaller in children. Repetition priming in children did not differ from that in adults. These findings challenge the idea of developmental invariance in implicit memory, suggesting that the strength of links between nodes in the semantic network increases and the range of spreading activation expands developmentally.

Equivalent Current Dipole of Word Repetition Effects in Patients with Obsessive-Compulsive Disorder

We investigated cortical source localization of word repetition effects in patients with obsessive-compulsive disorder (OCD) by employing the equivalent current dipole (ECD) model with high-density 128 channels EEG and individual MRI as a realistic head model. Twelve OCD patients and 13 healthy control subjects performed a word/nonword discrimination task, in which the words and nonwords were visually presented, and some of the words appeared twice with a lag of one or five items. During the 200-500 ms post-stimulus period, the control group showed more positivity in the ERPs elicited by old words than in those elicited by new words, whereas the OCD patients did not. Furthermore, the OCD patients showed prolonged response times to the old words, as compared to the controls. We calculated the location and the power of the ECD sources at approximately 400 ms post-stimulus with the peak mean global field potentials. In both groups, the sources of word repetition effects were determined to be located in the inferior frontal gyrus. The right ECD powers of the ERP generators elicited by the new words were significantly higher in the OCD patients than in the control subjects. The OCD patients also exhibited significant alterations in the hemispheric asymmetry of ECD power during the processing of new words. These results suggest that OCD patients suffer from the encoding deficits in word processing, particularly in the left hemisphere.

Activity of left inferior frontal gyrus related to word repetition effects: LORETA imaging with 128-channel EEG and individual MRI

We investigated the brain substrate of word repetition effects on the implicit memory task using low-resolution electromagnetic tomography (LORETA) with high-density 128-channel EEG and individual MRI as a realistic head model. Thirteen right-handed, healthy subjects performed a word/non-word discrimination task, in which the words and non-words were presented visually, and some of the words appeared twice with a lag of one or five items. All of the subjects exhibited word repetition effects with respect to the behavioral data, in which a faster reaction time was observed to the repeated word (old word) than to the first presentation of the word (new word). The old words elicited more positive-going potentials than the new words, beginning at 200 ms and lasting until 500 ms post-stimulus. We conducted source reconstruction using LORETA at a latency of 400 ms with the peak mean global field potentials and used statistical parametric mapping for the statistical analysis. We found that the source elicited by the old words exhibited a statistically significant current density reduction in the left inferior frontal gyrus. This is the first study to investigate the generators of word repetition effects using voxel-by-voxel statistical mapping of the current density with individual MRI and high-density EEG.

Linking semantic priming effect in functional MRI and event-related potentials

The aim of this study is to examine the neural substrates involved in semantic priming using a combined event-related functional magnetic resonance imaging (fMRI) and event-related potentials (ERP) study. Twelve subjects were instructed to judge whether the presented target word was a real word or a nonword. Under the related condition, target words were preceded by a semantically related prime word. On the other hand, under the unrelated condition, prime words did not have semantic relatedness with the target word. The reaction time for reaching a judgment was longer under the unrelated condition than under the related condition, indicating that the recognition of target words is promoted by semantic priming under the related condition. In the fMRI results, we found reduced activity in the dorsal and ventral left inferior frontal gyrus, the anterior cingulate, and left superior temporal cortex for related versus unrelated conditions (i.e., the repetition suppression effect). ERP analysis revealed that the amplitude of the N400 component was reduced under the related condition compared with the unrelated condition (i.e., the N400 priming effect). Correlation analysis between the BOLD repetition suppression effect and the N400 priming effect decomposed by independent component analysis (ICA) across subjects showed significant correlation in the left superior temporal gyrus. This finding is consistent with the recent MEG data suggesting that the source of N400 is judged to be the bilateral superior temporal lobe. We discussed this finding herein in relation to the modulation of access to the phonological representation caused by semantic priming.

The effect of phonological repetition on cortical magnetic responses evoked by visually presented words

Neuroimaging studies have reported that the left superior temporal cortical area is activated by visually presented words. In the present study, we recorded cortical magnetic responses evoked by visual words and examined the effect of phonological repetition (e.g., hair-hare) on left superior temporal cortical activity, using pairs of homophonic Japanese words as stimuli. Unlike English, Japanese has a large number of homophone pairs with a totally different orthography. By taking advantage of this feature of the Japanese writing system, the effect of phonological repetition can be solely examined without being confounded by the effect of orthographic similarity. Magnetic responses were recorded over the bilateral temporal sites of the brain while subjects silently read words. The words were presented one by one; a quarter of them was immediately followed by a homophonic word. Clear magnetic responses in the latency range of 300-600 msec were observed in the left hemisphere, and the responses to the homophones were smaller than those to the first presented words. In the right hemisphere, clear responses were not consistently recorded in the same latency range, and no effect of phonological repetition was observed. The sources of the responses recorded over the left hemisphere were estimated to be in the left superior temporal cortical area adjacent to the auditory cortex and the source strength as well as the magnetic responses showed a reduction by phonological repetition. This result suggests that the activity in the left superior temporal cortical area is associated with access to the phonological representation of words.

Magnetic brain activity elicited by visually presented symbols and Japanese characters

A standard model of word reading postulates that the posterior inferior temporal cortex is involved in the processing of written words. This processing probably occurs within 200 ms after stimulus presentation. In order to characterize this process more precisely, we conducted a MEG study during a reading task in nine right-handed normal Japanese subjects. The subjects were required to respond to a word pertaining to the human body so that all stimuli would be subject to the same semantic processing. The trials for non-target conditions, such as kanji words, meaningful kana words, kana pseudowords and symbols were analysed to avoid possible P300 effect. The magnetic response peak of around 200 ms for symbols was smaller than any of the other three letter conditions. This result may suggest that M200 reflects the word-specific process such as visual word form recognition.

Spatiotemporal dynamics of modality-specific and supramodal word processing

The ability of written and spoken words to access the same semantic meaning provides a test case for the multimodal convergence of information from sensory to associative areas. Using anatomically constrained magnetoencephalography (aMEG), the present study investigated the stages of word comprehension in real time in the auditory and visual modalities, as subjects participated in a semantic judgment task. Activity spread from the primary sensory areas along the respective ventral processing streams and converged in anterior temporal and inferior prefrontal regions, primarily on the left at around 400 ms. Comparison of response patterns during repetition priming between the two modalities suggest that they are initiated by modality-specific memory systems, but that they are eventually elaborated mainly in supramodal areas.

Cortical evidence of the perceptual backward masking effect on /l/ and /r/ sounds from a following vowel in Japanese speakers

We examined the influence of stimulus duration of foreign consonant vowel stimuli on the MMNm (magnetic counter part of mismatch negativity). In Experiment 1, /ra/ and /la/ stimuli were synthesized and subjects were native Japanese speakers who are known to have difficulty discriminating the stimuli. "Short" duration stimuli were terminated in the middle of the consonant-to-vowel transition (110 ms). They were nevertheless clearly identifiable by English speakers. A clear MMNm was observed only for short-duration stimuli but not for untruncated long-duration (150-ms) stimuli. We suggest that the diminished MMNm for longer duration stimuli result from more effective masking by the longer vowel part. In Experiment 2 we examined this hypothesis by presenting only the third formant (F3) component of the original stimuli, since the acoustic difference between /la/ and /ra/ is most evident in the third formant, whereas F1 and F2 play a major role in vowel perception. If the MMNm effect depends on the acoustic property of F3, a stimulus duration effect comparable to that found with the original /la/ and /ra/ stimuli might be expected. However, if the effect is attributable to the masking effect from the vowel, no influence of stimulus duration would be expected, since neither stimulus contains F1 and F2 components. In fact, the results showed that the "F3 only" stimuli did not show a duration effect; MMNm was always elicited independent of stimulus duration. The MMN stimulus duration effect is thus suggested to come from the backward masking of foreign consonants by subsequent vowels.