Semantic and phonological task-set priming and stimulus processing investigated using magnetoencephalography (MEG (original) (raw)
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Neuroscience Research, 2010
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.
NeuroImage, 2010
Repetition priming is a core feature of memory processing whose anatomical correlates remain poorly understood. In this study, we use advanced multimodal imaging (functional magnetic resonance imaging (fMRI) and magnetoencephalography; MEG) to investigate the spatiotemporal profile of repetition priming. We use intracranial electroencephalography (iEEG) to validate our fMRI/MEG measurements. Twelve controls completed a semantic judgment task with fMRI and MEG that included words presented once (new, 'N') and words that repeated (old, 'O'). Six patients with epilepsy completed the same task during iEEG recordings. Blood-oxygen level dependent (BOLD) responses for N vs. O words were examined across the cortical surface and within regions of interest. MEG waveforms for N vs. O words were estimated using a noise-normalized minimum norm solution, and used to interpret the timecourse of fMRI. Spatial concordance was observed between fMRI and MEG repetition effects from 350 to 450 ms within bilateral occipitotemporal and medial temporal, left prefrontal, and left posterior temporal cortex. Additionally, MEG revealed widespread sources within left temporoparietal regions, whereas fMRI revealed bilateral reductions in occipitotemporal and left superior frontal, and increases in inferior parietal, precuneus, and dorsolateral prefrontal activity. BOLD suppression in left posterior temporal, left inferior prefrontal, and right occipitotemporal cortex correlated with MEG repetition-related reductions. IEEG responses from all three regions supported the timecourse of MEG and localization of fMRI. Furthermore, iEEG decreases to repeated words were associated with decreased gamma power in several regions, providing evidence that gamma oscillations are tightly coupled to cognitive phenomena and reflect regional activations seen in the BOLD signal.
Neural Interactions at the Core of Phonological and Semantic Priming of Written Words
Word processing is often probed with experiments where a target word is primed by preceding semantically or phonologically related words. Behaviorally, priming results in faster reaction times, interpreted as increased efficiency of cognitive processing. At the neural level, priming reduces the level of neural activation, but the actual neural mechanisms that could account for the increased efficiency have remained unclear. We examined whether enhanced information transfer among functionally relevant brain areas could provide such a mechanism. Neural activity was tracked with magnetoencephalography while subjects read lists of semantically or phonologically related words. Increased priming resulted in reduced cortical activation. In contrast, coherence between brain regions was simultaneously enhanced. Furthermore, while the reduced level of activation was detected in the same area and time window (superior temporal cortex [STC] at 250--650 ms) for both phonological and semantic priming, the spatiospectral connectivity patterns appeared distinct for the 2 processes. Causal interactions further indicated a driving role for the left STC in phonological processing. Our results highlight coherence as a neural mechanism of priming and dissociate semantic and phonological processing via their distinct connectivity profiles.
Task-specific change of unconscious neural priming in the cerebral language network
Proceedings of the National Academy of Sciences, 2007
We explored the impact of task context on subliminal neural priming using functional magnetic resonance imaging. The repetition of words during semantic categorization produced activation reduction in the left middle temporal gyrus previously associated with semantic-level representation and dorsal premotor cortex. By contrast, reading aloud produced repetition enhancement in the left inferior parietal lobe associated with print-to-sound conversion and ventral premotor cortex. Analyses of effective connectivity revealed that the task set for reading generated reciprocal excitatory connections between the left inferior parietal and superior temporal regions, reflecting the audiovisual integration required for vocalization, whereas categorization did not produce such backward projection to posterior regions. Thus, masked repetition priming involves two distinct components in the taskspecific neural streams, one in the parietotemporal cortex for task-specific word processing and the other in the premotor cortex for behavioral response preparation. The top-down influence of task sets further changes the directions of the unconscious priming in the entire cerebral circuitry for reading. effective connectivity ͉ masked priming ͉ reading ͉ repetition suppression and enhancement ͉ task set
Brain and Language, 2002
Evidence from electrophysiological measures such as ERPs (event-related potentials) and MEG (magnetoencephalography) suggest that the first evoked brain response component sensitive to stimulus properties affecting reaction times in word recognition tasks occurs at 300-400 ms. The present study used the stimulus manipulation of Vitevich and Luce (1999) to investigate whether the M350, an MEG response component peaking at 300-400 ms, reflects lexical or post-lexical processing. Stimuli were simultaneously varied in phonotactic probability, which facilitates lexical activation, and in phonological neighborhood density, which inhibits the lexical decision process. The present results indicate that the M350 shows facilitation by phonotactic probability rather than inhibition by neighborhood density. Thus the M350 cannot be a post-lexical component.
The Journal of neuroscience : the official journal of the Society for Neuroscience, 2010
Debate surrounds the precise cortical location and timing of access to phonological information during visual word recognition. Therefore, using whole-head magnetoencephalography (MEG), we investigated the spatiotemporal pattern of brain responses induced by a masked pseudohomophone priming task. Twenty healthy adults read target words that were preceded by one of three kinds of nonword prime: pseudohomophones (e.g., brein-BRAIN), where four of five letters are shared between prime and target, and the pronunciation is the same; matched orthographic controls (e.g., broin-BRAIN), where the same four of five letters are shared between prime and target but pronunciation differs; and unrelated controls (e.g., lopus-BRAIN), where neither letters nor pronunciation are shared between prime and target. All three priming conditions induced activation in the pars opercularis of the left inferior frontal gyrus (IFGpo) and the left precentral gyrus (PCG) within 100 ms of target word onset. However, for the critical comparison that reveals a processing difference specific to phonology, we found that the induced pseudohomophone priming response was significantly stronger than the orthographic priming response in left IFG/PCG at ϳ100 ms. This spatiotemporal concurrence demonstrates early phonological influences during visual word recognition and is consistent with phonological access being mediated by a speech production code.
Journal of Clinical Neurophysiology, 2008
Eight right-handed subjects were asked to silently generate a verb to a visual stimulus while the magnetic flux normal to the scalp surface was recorded with a whole-head neuromagnetometer. The spatiotemporal patterns of activation in lateral occipital, inferior parietal, superior temporal, basal temporal, and inferior frontal cortices were estimated using minimum estimation, a distributed source analysis methodology. Although there was significant variability among subjects, averaged data indicated that latencies of peak activation in these regions of interest progressed from posterior to anterior. Peak latencies were earliest in lateral occipital cortex and latest in pars opercularis and pars triangularis in the inferior frontal gyrus. Lateralization of activation was strongest in pars opercularis, which is part of classical Broca's area, with activation being stronger in this area within the left hemisphere in every subject. Results provide support for the use of magnetoencephalography in conjunction with MNE analysis for the purpose of lateralizing and localizing language-specific activation in frontal areas as well as the study of the spatiotemporal parameters of brain activation associated with cognitive function.
Linking semantic priming effect in functional MRI and event-related potentials
NeuroImage, 2005
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. D