Biomedical subjects

Kuniyoshi L Sakai

Publications and source records attributed to Kuniyoshi L Sakai.

15 recordsLinked to original sources

PubMed2005

Cortical hemoglobin-concentration changes under the coil induced by single-pulse TMS in humans: a simultaneous recording with near-infrared spectroscopy.

We measured cortical hemoglobin-concentration changes under the coil induced by single-pulse transcranial magnetic stimulation (TMS) using a technique of simultaneous recording with near-infrared spectroscopy (NIRS). Single-pulse TMS was delivered over the hand area of the left primary motor cortex at an intensity of 100, 120, or 140% of the active motor threshold (AMT). NIRS recordings were also made during sham stimulation. These four different stimulation sessions (TMS at three intensities and sham stimulation) were performed both when the subject slightly contracted the right first dorsal interosseous muscle and when relaxed it (active and resting conditions). Under the active condition with TMS at 100% AMT, we observed a transient increase in oxy-hemoglobin (oxy-Hb), which was significantly larger than sham stimulation. Under the resting conditions with TMS at 120 and 140% AMT, we observed significant decreases in both deoxy-hemoglobin (deoxyHb) and total-hemoglobin (total-Hb) as compared to sham stimulation. We suggest that the increase of oxy-Hb concentration at 100% AMT under the active condition reflects an add-on effect by TMS to the active baseline and that decrease of deoxy-Hb and total-Hb concentrations at 120 and 140% AMT under the resting condition are due to reduced baseline firings of the corticospinal tract neurons induced by a lasting inhibition provoked by a higher intensity TMS.

Adult↗

PubMed2005

Language acquisition and brain development.

Language acquisition is one of the most fundamental human traits, and it is obviously the brain that undergoes the developmental changes. During the years of language acquisition, the brain not only stores linguistic information but also adapts to the grammatical regularities of language. Recent advances in functional neuroimaging have substantially contributed to systems-level analyses of brain development. In this Viewpoint, I review the current understanding of how the "final state" of language acquisition is represented in the mature brain and summarize new findings on cortical plasticity for second language acquisition, focusing particularly on the function of the grammar center.

Brain↗

PubMed2005

Sign and speech: amodal commonality in left hemisphere dominance for comprehension of sentences.

The neural basis of functional lateralization in language processing is a fundamental issue in systems neuroscience. We used functional MRI (fMRI) to examine hemispheric dominance during the processing of signed and spoken sentences. By using tasks involving comprehension of sentences (Sc) and sentential non-word detection (Sn), we compared different groups and stimulus conditions. Under the sign condition with sentence stimuli in Japanese Sign Language (JSL), we tested two groups of subjects: Deaf signers (Deaf) of JSL, and hearing bilinguals (children of Deaf adults, CODA) of JSL and Japanese (JPN). Under the speech condition, we tested hearing monolinguals (Mono) of JPN with auditory JPN stimuli alone (AUD), or with an audiovisual presentation of JPN and JSL stimuli (A&V). We found that the overall bilateral activation patterns under the four experimental conditions of Deaf, CODA, AUD and A&V were almost identical, despite differences in stimuli (JSL and JPN) and groups (Deaf, CODA and Mono). Moreover, consistently left-dominant activations involving frontal and temporo-parietal regions were observed across all four conditions. Furthermore, irrespective of the modalities of sign and speech, the main effects of task (Sc-Sn) were found primarily in the left regions: the ventral part of the inferior frontal gyrus (F3t/F3O), the precentral sulcus, the superior frontal gyrus, the middle temporal gyrus, the angular gyrus and the inferior parietal gyrus. Among these regions, only the left F3t/F3O showed no main effects of modality condition. These results demonstrate amodal commonality in the functional dominance of the left cortical regions for comprehension of sentences, as well as the essential and universal role of the left F3t/F3O in processing linguistic information from both signed and spoken sentences.

Acoustic Stimulation↗

PubMed2005

Language-related activations in the left prefrontal regions are differentially modulated by age, proficiency, and task demands.

It remains to be elucidated how cortical activations are modulated by factors of age, proficiency, and language task demands when mastering first language (L1) and a second language (L2). Using functional magnetic resonance imaging, we tested subjects aged 13 (the age 13 group) and 19 (the age 19 group), thereby comparing the cortical activations involved in past-tense verb identification with those involved in verb matching. We found that the activation in the dorsal triangular part of the left inferior frontal gyrus (IFG) was lower, corresponding to a higher proficiency in English (L2) in the older subjects, suggesting that the proficiency level plays a major role in the activation of this region during L2 acquisition. Moreover, the lower activation in the triangular and orbital parts of the left IFG (F3t/F3O) for the irregular past tense corresponding to a higher proficiency in L2, together with the nonsignificant activation for the regular past tense when its performance almost reached perfection for age 19, suggests that the modulation of the left F3t/F3O activation reflects language task demands for identifying correct past-tense forms. On the other hand, the left F3t/F3O activation in Japanese (L1) for age 13 was significantly greater than that for age 19, despite the matched performances in L1. These results suggest that the left IFG subserves language-specific functions that are critically required when mastering any language.

Adolescent↗

PubMed2004

Correlated functional changes of the prefrontal cortex in twins induced by classroom education of second language.

As the neural substrates of second language (L2) acquisition are largely unknown, they may or may not be similar to those of first language (L1) representation. We used functional imaging to examine whether training in the conjugation of English verbs from present to past tense alters brain activations in 13-year-old twins. A novel experimental design contrasted past tense verb identification and verb matching, which were tested in either English (L2) or Japanese (L1). After 2 month classroom training in the past tense using bingo games, the two individuals in each pair exhibited significantly correlated performances. When pre- and post-scanning sessions were compared with each other for L2, the left dorsal inferior frontal gyrus (IFG) exhibited significantly correlated activation increases within each pair of twins and the increases were positively correlated with individual performance improvements. Moreover, the cortical plasticity for L2 acquisition was guided toward the L1 specialization of the left dorsal IFG, in spite of notable differences between L1 and L2 in the students' linguistic knowledge and in their performance in making past tense forms. These findings suggest a cortical mechanism underlying L2 acquisition, which critically depends on shared genetic and environmental factors for each twin in a surprisingly predictive manner.

Adolescent↗

PubMed2004

Learning letters in adulthood: direct visualization of cortical plasticity for forming a new link between orthography and phonology.

To identify which brain regions in adults show plasticity for learning letters, Hangul letters were experimentally associated with either speech sounds (HS condition) or nonspeech sounds (HN condition) in fMRI sessions over two consecutive days. Selective activations under the HS condition were found in several regions including the left posterior inferior temporal gyrus (PITG) and the parieto-occipital cortex (PO), as compared with activations under a condition for familiar letters and speech sounds, and with those under the HN condition. The left PITG showed a selective activation increase under the HS condition over two days, the degree of which predicted individual performance improvement. Further, functional connectivity between the left PITG and the left PO was selectively enhanced under the HS condition. These results demonstrate that a new link between orthography and phonology is formed by the plasticity of a functional system involving the left PITG in association with the left PO.

Acoustic Stimulation↗

PubMed2003

Brain activations during conscious self-monitoring of speech production with delayed auditory feedback: an fMRI study.

When a speaker's voice returns to one's own ears with a 200-ms delay, the delay causes the speaker to speak less fluently. This phenomenon is called a delayed auditory feedback (DAF) effect. To investigate neural mechanisms of speech processing through the DAF effect, we conducted a functional magnetic resonance imaging (fMRI) experiment, in which we designed a paradigm to explore the conscious overt-speech processing and the automatic overt-speech processing separately, while reducing articulatory motion artifacts. The subjects were instructed to (1) read aloud visually presented sentences under real-time auditory feedback (NORMAL), (2) read aloud rapidly under real-time auditory feedback (FAST), (3) read aloud slowly under real-time auditory feedback (SLOW), and (4) read aloud under DAF (DELAY). In the contrasts of DELAY-NORMAL, DELAY-FAST, and DELAY-SLOW, the bilateral superior temporal gyrus (STG), the supramarginal gyrus (SMG), and the middle temporal gyrus (MTG) showed significant activation. Moreover, we found that the STG activation was correlated with the degree of DAF effect for all subjects. Because the temporo-parietal regions did not show significant activation in the comparisons among NORMAL, FAST, and SLOW conditions, we can exclude the possibility that its activation is due to speech rates or enhanced attention to altered speech sounds. These results suggest that the temporo-parietal regions function as a conscious self-monitoring system to support an automatic speech production system.

Acoustic Stimulation↗

PubMed2003

Sentence processing is uniquely human.

In this article, we will focus on three fundamental issues concerning language processing in the human brain, and update recent advances made by functional neuroimaging and magnetic stimulation studies of language. First, we will provide the first experimental evidence that the neural basis of sentence comprehension is indeed specialized. Specifically, our recent functional magnetic resonance imaging (fMRI) study has clarified that the human left prefrontal cortex (PFC) is more specialized in the syntactic processes of sentence comprehension than other domain-general processes such as short-term memory. Second, the distinction between explicit and implicit syntactic processes will be clarified, based on our fMRI studies that elucidate syntactic specialization in the left PFC. Third, we will advance a hypothesis stating that distinct subregions of the left PFC are recruited for the syntactic integration of lexico-semantic information. The current direction of research in the neuroscience of language is beginning to reveal the uniqueness of the human mind.

Animals↗

PubMed2003

An event-related optical topography study of cortical activation induced by single-pulse transcranial magnetic stimulation.

To visualize cortical activations during transcranial magnetic stimulation (TMS), it is necessary to measure those activations at high spatiotemporal resolution while preventing interference with the magnetic property of a coil. One suitable method that satisfies these demands is optical topography (OT), which has been used in cortical activation studies. In the present study, single-pulse TMS was applied to the left primary motor area, and cortical responses at the stimulation site were measured simultaneously with event-related OT. When TMS was applied at 110% motor threshold (MT), we observed significant oxyhemoglobin increases that were both time-locked and correlated with the hemodynamic basis function. Moreover, when TMS was applied at 90% MT, significant oxyhemoglobin increases were detected even though there were no motor-evoked potentials. These results demonstrate that OT can directly measure cortical responses to subthreshold single-pulse TMS, independent of the afferent feedback from the peripheral neuromuscular activity.

Adult↗

PubMed2003

Selective enhancement of functional connectivity in the left prefrontal cortex during sentence processing.

We present the results of correlation analyses for identifying temporally correlated activations between multiple regions of interest. We focused on functional connectivity for two regions in the prefrontal cortex: the left inferior frontal gyrus (L. F3t/F3O) and the left precentral sulcus (L. PrCS). Temporal correlations of functional magnetic resonance imaging signals were separately examined during a sentence comprehension task and a lexical decision task, thereby averaging data throughout all voxels within a region of interest used as a reference region. We found that the reciprocal connectivity between L. F3t/F3O and L. PrCS was significantly enhanced during sentence processing, but not during lexico-semantic processing, which was confirmed under both auditory and visual conditions. Furthermore, significantly correlated regions were mostly concentrated in the left prefrontal cortex during the sentence task. These results demonstrate that the functional connectivity within the left prefrontal cortex is selectively enhanced for processing sentences, which may subserve the use of syntactic information for integrating lexico-semantic information.

Adult↗

PubMed2003

An event-related fMRI study of explicit syntactic processing of normal/anomalous sentences in contrast to implicit syntactic processing.

Using event-related functional magnetic resonance imaging (fMRI), we examined activation of cortical language areas for explicit syntactic processing. In a syntactic decision (Syn) task, the participants judged whether the presented sentence was syntactically correct, where syntactic knowledge about the distinction between transitive and intransitive verbs was required. In a semantic decision (Sem) task, lexico-semantic knowledge about selectional restrictions was indispensable. In a phonological decision (Pho) task, phonological knowledge about accent patterns was required. The Sem and Pho tasks involved implicit syntactic processing, as well as explicit semantic and phonological processing, respectively. We also tested a voice-pitch comparison (Voi) task in which no explicit linguistic knowledge was required. In the direct comparison of Syn - (Sem + Pho + Voi), we found localized activation in the left inferior frontal gyrus (F3op/F3t), indicating that activation of the left F3op/F3t is more prominently enhanced in explicit syntactic processing than in implicit syntactic processing. Moreover, we determined that its activation is selective to syntactic judgments regarding both normal and anomalous sentences. These results suggest that explicit information processing in the syntactic domain critically involves the left F3op/F3t, which is functionally separable from other regions.

Acoustic Stimulation↗

PubMed2002

Selective priming of syntactic processing by event-related transcranial magnetic stimulation of Broca's area.

It remains controversial whether Broca's aphasia is an articulatory deficit, a lexical-access problem, or agrammatism. In spite of recent neuroimaging studies, the causal link between cortical activity and linguistic subcomponents has not been elucidated. Here we report an experiment with event-related transcranial magnetic stimulation (TMS) to clarify the role of Broca's area, more specifically, the left inferior frontal gyrus (F3op/F3t), in syntactic processing. An experimental paradigm contrasted sentences requiring syntactic decisions with those requiring semantic decisions. We found selective priming effects on syntactic decisions when TMS was administered to the left F3op/F3t at a specific timing, but not to the left middle frontal gyrus (F2). Our results provide direct evidence of the involvement of the left F3op/F3t in syntactic processing.

Adult↗

PubMed2002

Specialization in the left prefrontal cortex for sentence comprehension.

Using functional magnetic resonance imaging (fMRI), we examined cortical activation under syntactic decision tasks and a short-term memory task for sentences, focusing on essential properties of syntactic processing. By comparing activation in these tasks with a short-term memory task for word lists, we found that two regions in the left prefrontal cortex showed selective activation for syntactic processing: the dorsal prefrontal cortex (DPFC) and the inferior frontal gyrus (IFG). Moreover, the left DPFC showed more prominent activation under the short-term memory task for sentences than that for word lists, which cannot be explained by general cognitive factors such as task difficulty and verbal short-term memory. These results support the proposal of specialized systems for sentence comprehension in the left prefrontal cortex.

Adolescent↗

PubMed2002

Lateralized activation in the inferior frontal cortex during syntactic processing: event-related optical topography study.

Functional imaging with near-infrared light has the potential to provide novel information that cannot be obtained with other imaging techniques. An event-related paradigm has not been fully established for studying human cognitive functions with near-infrared optical imaging. We conducted language experiments to develop an event-related paradigm with optical topography (OT). We directly compared cortical activation during syntactic and semantic decision tasks, both of which involved error detection in a sentence stimulus that consisted of a noun phrase and a verb. In the syntactic decision task, subjects judged whether the presented sentence is syntactically correct, where the syntactic knowledge about the distinction between transitive and intransitive verbs was required. In the semantic decision task, subjects judged whether the presented sentence is semantically correct, where the lexico-semantic knowledge about selectional restrictions was indispensable. We found local increases in oxyhemoglobin concentration, which were selectively associated with the syntactic decision task. Activation in the left inferior frontal gyrus was detected when syntactically anomalous sentences were presented, whereas there was no significant activation in this region when semantically anomalous sentences were presented. Moreover, identical stimuli of normal sentences elicited activation in the left inferior frontal gyrus, only when the employment of syntactic knowledge was required. This task-selective activation was not observed in any other measured regions, including the right homologous region. These results demonstrate that OT techniques, when coupled with the event-related paradigm, are useful for studying the higher cognitive functions of the human cerebral cortex.

Adult↗

PubMed2002

From perception to sentence comprehension: the convergence of auditory and visual information of language in the left inferior frontal cortex.

We used functional magnetic resonance imaging (fMRI) to characterize cortical activation associated with sentence processing, thereby elucidating where in the brain auditory and visual inputs of words converge during sentence comprehension. Within one scanning session, subjects performed three types of tasks with different linguistic components from perception to sentence comprehension: nonword (N(AV); auditory and visual), phrase (P; either auditory or visual), and sentence (S; either auditory or visual) tasks. In a comparison of the P and N(AV) tasks, the angular and supramarginal gyri showed bilateral activation, whereas the inferior and middle frontal gyri showed left-lateralized activation. A comparison of the S and P tasks, together with a conjunction analysis, revealed a ventral region of the left inferior frontal gyrus (F3t/F3O), which was sentence-processing selective and modality-independent. These results unequivocally demonstrated that the left F3t/F3O is involved in the selection and integration of semantic information that are separable from lexico-semantic processing.

Adult↗