Aya Ihara

Determination of language dominance with synthetic aperture magnetometry: comparison with the Wada test.

Cerebral dominance for language function was investigated with synthetic aperture magnetometry (SAM). The results were compared with those of the Wada test. SAM is a spatial filtering technique that enables demonstration of the spatiotemporal distribution of oscillatory changes (synchronization and desynchronization) in magnetoencephalography (MEG) signals elicited by specific brain activation. MEG was conducted during a silent reading task in 20 consecutive preoperative neurosurgical patients who also underwent a Wada test. The spatial distribution of oscillatory changes related to silent reading was shown tomographically with SAM as statistical images. Language dominance was estimated by the laterality index, which scales the lateralization of the beta (13-25 Hz) and low gamma (25-50 Hz) band desynchronizations in the inferior frontal gyrus (IFG) or middle frontal gyrus (MFG). Oscillatory changes were distributed multifocally and bilaterally in the occipital cortex, IFG or MFG, and temporo-parieto-occipital border regions. In 19 patients (95%), language lateralization estimated by the laterality index was congruent with the result of the Wada test. In left-handed patients, SAM analysis clearly differentiated language dominance (left, right, or bilateral), and the findings were confirmed by the Wada test. Lateralization of beta or low gamma band desynchronizations in the IFG or MFG is a good indicator of the side of language dominance. Reliability of MEG imaging with SAM is sufficient to evaluate language dominance preoperatively in neurosurgical patients.

New human-specific brain landmark: the depth asymmetry of superior temporal sulcus

Significance In the human brain, from early in development through to adulthood, the superior temporal sulcus is deeper in the right than the left cerebral hemisphere in the area ventral of Heschl’s gyrus. Irrespective of gender, handedness, and language lateralization, and present in several pathologies, this asymmetry is widely shared among the human population. Its appearance early in life suggests strong genetic control over this part of the brain. In contrast, the asymmetry is barely visible in chimpanzees. Thus this asymmetry probably is a key locus to look for variations in gene expression among the primate lineage that have favored the evolution of crucial cognitive abilities sustained by this sulcus in our species, namely communication and social cognition. Identifying potentially unique features of the human cerebral cortex is a first step to understanding how evolution has shaped the brain in our species. By analyzing MR images obtained from 177 humans and 73 chimpanzees, we observed a human-specific asymmetry in the superior temporal sulcus at the heart of the communication regions and which we have named the “superior temporal asymmetrical pit” (STAP). This 45-mm-long segment ventral to Heschl’s gyrus is deeper in the right hemisphere than in the left in 95% of typical human subjects, from infanthood till adulthood, and is present, irrespective of handedness, language lateralization, and sex although it is greater in males than in females. The STAP also is seen in several groups of atypical subjects including persons with situs inversus, autistic spectrum disorder, Turner syndrome, and corpus callosum agenesis. It is explained in part by the larger number of sulcal interruptions in the left than in the right hemisphere. Its early presence in the infants of this study as well as in fetuses and premature infants suggests a strong genetic influence. Because this asymmetry is barely visible in chimpanzees, we recommend the STAP region during midgestation as an important phenotype to investigate asymmetrical variations of gene expression among the primate lineage. This genetic target may provide important insights regarding the evolution of the crucial cognitive abilities sustained by this sulcus in our species, namely communication and social cognition.

Lexical access and selection of contextually appropriate meaning for ambiguous words

To clarify the neural mechanisms of lexical access and selection of contextually appropriate meanings for ambiguous words, we investigated the spatio-temporal characteristics of neural activities during silent reading and semantic judgment of lexically ambiguous or unambiguous target words that were preceded by semantically related or unrelated words by using magnetoencephalography. The left posterior superior temporal/inferior parietal area and the left anterior middle/inferior temporal area consistently showed a clear context effect, regardless of the ambiguity: the activities for related words were weaker than those for unrelated words. The activities in the left inferior frontal cortex, in contrast, were influenced by ambiguities. From approximately 200 to 300 ms, the activities in the left anterior inferior frontal cortex (aIFC) were stronger for ambiguous words than for unambiguous words, regardless of context. The stronger activities in the left aIFC, reflecting an increase in controlled semantic retrieval, indicate that multiple meanings for lexically ambiguous words are accessed irrespective of context. At approximately 400 ms, the left posterior inferior frontal cortex (pIFC) showed a clear context effect for unambiguous words but not for ambiguous ones. In addition, the activation in the left pIFC was stronger for related ambiguous words than for related unambiguous ones. These results suggest that in ambiguous words, not only contextually appropriate meanings but also two or more inappropriate meanings would be semantically integrated with a context. We conclude that the left IFC plays an important role in selecting an appropriate meaning from multiple alternatives after the integration of contextual information.

Early neural activation for lexico-semantic access in the left anterior temporal area analyzed by an fMRI-assisted MEG multidipole method.

To determine the time and location of lexico-semantic access, we measured neural activations by magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) and estimated the neural sources by fMRI-assisted MEG multidipole analysis. Since the activations for phonological processing and lexico-semantic access were reported to overlap in many brain areas, we compared the activations in lexical and phonological decision tasks. The former task required visual form processing, phonological processing, and lexico-semantic access, while the latter task required only visual form and phonological processing, with similar phonological task demands for both tasks. The activation areas observed among 9 or 10 subjects out of 10 were the superior temporal and inferior parietal areas, anterior temporal area, and inferior frontal area of both hemispheres, and the left ventral occipitotemporal area. The activations showed a significant difference between the 2 tasks in the left anterior temporal area in all 50-ms time windows between 200-400 ms from the onset of visual stimulus presentation. Previous studies on semantic dementia and neuroimaging studies on normal subjects have shown that this area plays a key role in accessing semantic knowledge. The difference between the tasks appeared in common to all areas in the time windows of 100-150 ms and 400-450 ms, suggesting early differences in visual form processing and late differences in the decision process, respectively. The present results demonstrate that the activations for lexico-semantic access in the left anterior temporal area start in the time window of 200-250 ms, after early visual form processing.

Gamma-band desynchronization in language areas reflects syntactic process of words

The aim of this study was to verify the relation between gamma-band activity and process of function words. We recorded the neuromagnetic signals in six healthy volunteers during silent reading of verbs (verb task) and forming of the past tenses (past-tense task) and investigated the spatio-temporal distribution of event-related desynchronization (ERD) and synchronization using synthetic aperture magnetometry. In both tasks, ERDs were observed simultaneously at multiple language-related areas. The left junctional area of inferior frontal sulcus and precentral sulcus and the left supramarginal gyrus showed stronger and/or longer-lasting ERDs in past-tense task than in verb task. This result suggests that the gamma-activities reflect the syntactic process of words.

Neuroimaging study on brain asymmetries in situs inversus totalis

Situs inversus totalis (SI) is a rare condition in which all visceral organs are arranged as mirror images of the usual pattern. The objective of this study was to determine whether SI individuals have reversed brain asymmetries. We performed a neuroimaging study on 3 SI subjects and 11 control individuals with normally arranged visceral organs. The language-dominant hemisphere was determined by magnetoencephalography. Left-hemispheric dominance was observed in 1 SI subject and all controls, whereas right-hemispheric dominance was observed in the remaining 2 SI subjects. Statistical analysis revealed that language dominance patterns in SI subjects were different from those in the controls, suggesting that the developmental mechanisms underlying visceral organ asymmetries are related to those underlying functional brain asymmetry. Anatomical brain asymmetries were determined by magnetic resonance imaging. SI subjects had the same planum temporale (PT) asymmetry pattern as the controls, but a reversed petalia asymmetry pattern. The inferior frontal gyrus (IFG) asymmetry pattern varied within both groups, indicating a relationship between the rightward IFG and right-hemispheric language dominance. These results suggest that the developmental mechanisms underlying visceral organ asymmetries are related to those underlying petalia asymmetry but not to those underlying PT and IFG asymmetries, and that brain asymmetries might develop via multiple region-dependent mechanisms.

Selective increase of intention-based economic decisions by noninvasive brain stimulation to the dorsolateral prefrontal cortex.

The intention behind anothers action and the impact of the outcome are major determinants of human economic behavior. It is poorly understood, however, whether the two systems share a core neural computation. Here, we investigated whether the two systems are causally dissociable in the brain by integrating computational modeling, functional magnetic resonance imaging, and transcranial direct current stimulation experiments in a newly developed trust game task. We show not only that right dorsolateral prefrontal cortex (DLPFC) activity is correlated with intention-based economic decisions and that ventral striatum and amygdala activity are correlated with outcome-based decisions, but also that stimulation to the DLPFC selectively enhances intention-based decisions. These findings suggest that the right DLPFC is involved in the implementation of intention-based decisions in the processing of cooperative decisions. This causal dissociation of cortical and subcortical backgrounds may indicate evolutionary and developmental differences in the two decision systems.

Oscillatory activity in the occipitotemporal area related to the visual perception of letters of a first/second language and pseudoletters

The objective of this study was to reveal the oscillatory activity in the occipitotemporal area related to the visual perception of the letters of first (L1) and second languages (L2) and pseudoletters. We recorded neuromagnetic signals while Korean native speakers were exposed to a phonogram of Korean, acquired at school age as their L1 (Hangul), that of Japanese, learned in adulthood as a L2 (Kana) and pseudoletter (Pseudo), and quantified the event-related desynchronization (ERD) and synchronization (ERS). In all conditions, sustained ERDs in the alpha band were observed in both hemispheres. ERD for Pseudo was gradually attenuated after approximately 400-500 ms after stimulus onset, whereas both Hangul and Kana produced stronger and longer-lasting ERD. ERD for Kana showed a broader alpha band than Hangul. Furthermore, transient ERSs in the gamma band around 70 Hz were observed between 100 and 400 ms in the bilateral occipitotemporal areas. In the left hemisphere, gamma band oscillations showed similar enhancement in all conditions, suggesting that gamma band activity in the left occipitotemporal area might be enhanced not only by the bottom-up process as visual perception but also by the top-down process as attention to prelexical visual stimuli. In the right hemisphere, gamma band ERS was stronger for Hangul than Pseudo and no differences were shown between Kana and Pseudo. The differences of oscillatory activity in the alpha and gamma bands suggest that neuronal networks, including the occipitotemporal area, are related to the visual perception of letters differing between L1 and L2.

Transcranial magnetic stimulation over the cerebellum evokes late potential in the soleus muscle

Transcranial magnetic stimulation (TMS) with a double cone coil placed over the left lateral side of the basal occiput was able to elicit late electromyographic (EMG) responses at the bilateral soleus muscles (SOL) averaged over 30 stimulation events, with a mean latency of approximately 100 ms. These EMG responses were detected using a low frequency bandpass filter with 0.05 Hz magnetic stimulation on ten healthy subjects in standing posture. As magnetic stimulation over the left basal occiput with a double cone coil can stimulate cerebellar structure, this late response seems to be conducted from the cerebellar structure to the SOL via an as yet unknown descending pathway. Here, we report new late EMG responses in relation to cerebellum or cerebellum related structures.

Late response evoked by cerebellar stimuli: effect of optokinetic stimulation.

We previously demonstrated that late electromyographic responses with a latency of 100 ms were evoked bilaterally in soleus muscles following transcranial magnetic stimulation over the left cerebellum. Efferent fibers from the left cerebellum modulate vestibulospinal tract influences on the extensor muscles of the left hindlimb. Here, we investigated whether the vestibulospinal tract mediates this late response. We activated the vestibulospinal tract by optokinetic stimulation. Our results show that the latency of the soleus electromyographic response is shortened by optokinetic stimulation, but the latency of the motor response evoked by the corticospinal tract is unchanged. These findings support our hypothesis that vestibulospinal tracts mediate late electromyographic responses, and allow the development of techniques to assess the human vestibulospinal system function.