Kimihiro Nakamura

Task-specific change of unconscious neural priming in the cerebral language network

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 task-specific 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.

Universal brain systems for recognizing word shapes and handwriting gestures during reading

Do the neural circuits for reading vary across culture? Reading of visually complex writing systems such as Chinese has been proposed to rely on areas outside the classical left-hemisphere network for alphabetic reading. Here, however, we show that, once potential confounds in cross-cultural comparisons are controlled for by presenting handwritten stimuli to both Chinese and French readers, the underlying network for visual word recognition may be more universal than previously suspected. Using functional magnetic resonance imaging in a semantic task with words written in cursive font, we demonstrate that two universal circuits, a shape recognition system (reading by eye) and a gesture recognition system (reading by hand), are similarly activated and show identical patterns of activation and repetition priming in the two language groups. These activations cover most of the brain regions previously associated with culture-specific tuning. Our results point to an extended reading network that invariably comprises the occipitotemporal visual word-form system, which is sensitive to well-formed static letter strings, and a distinct left premotor region, Exner’s area, which is sensitive to the forward or backward direction with which cursive letters are dynamically presented. These findings suggest that cultural effects in reading merely modulate a fixed set of invariant macroscopic brain circuits, depending on surface features of orthographies.

On the perceptual origin of loanword adaptations: experimental evidence from Japanese

Japanese shows an asymmetry in the treatment of word-final [n] in loanwords from English and French: while it is adapted as a moraic nasal consonant in loanwords from English, it is adapted with a following epenthetic vowel in loanwords from French. We provide experimental evidence that this asymmetry is due to phonetic differences in the realisation of word-final [n] in English and French, and, consequently, to the way in which English and French word-final [n] are perceived by native speakers of Japanese. Specifically, French but not English word-final [n] has a strong vocalic release that Japanese listeners perceive as their native vowel [ɯ]. We propose a psycholinguistic model in which most loanword adaptations originate in perceptual assimilation, a process which takes place during perception and which maps non-native sounds and sound structures onto the phonetically closest native ones. We compare our model to alternatives couched within phonological theory.

Task-Guided Selection of the Dual Neural Pathways for Reading

The visual perception of words is known to activate the auditory representation of their spoken forms automatically. We examined the neural mechanism for this phonological activation using transcranial magnetic stimulation (TMS) with a masked priming paradigm. The stimulation sites (left superior temporal gyrus [L-STG] and inferior parietal lobe [L-IPL]), modality of targets (visual and auditory), and task (pronunciation and lexical decision) were manipulated independently. For both within- and cross-modal conditions, the repetition priming during pronunciation was eliminated when TMS was applied to the L-IPL, but not when applied to the L-STG, whereas the priming during lexical decision was eliminated when the L-STG, but not the L-IPL, was stimulated. The observed double dissociation suggests that the conscious task instruction modulates the stimulus-driven activation of the lateral temporal cortex for lexico-phonological activation and the inferior parietal cortex for spoken word production, and thereby engages a different neural network for generating the appropriate behavioral response.

Hemispheric asymmetry emerges at distinct parts of the occipitotemporal cortex for objects, logograms and phonograms: a functional MRI study.

Behavioral and neuropsychological studies have suggested that the right hemisphere has a special advantage in the visual recognition of logograms. While this long-standing right hemisphere hypothesis has never been investigated systematically by previous neuroimaging studies, a candidate neural substrate of such asymmetry might be found within the occipitotemporal cortex that is known to exhibit lateralized response to a certain class of stimuli, such as letters and faces. The present study examined the hemispheric specialization of brain activation during naming of objects, logograms and phonograms using functional magnetic resonance imaging. The three types of stimuli overall produced left-predominant activation of the perisylvian and inferior parietal regions relative to the resting baseline. This inter-hemispheric difference was significant irrespective of the stimuli type. In the occipitotemporal cortex, six subregions showing lateralized response were identified. That is, the three stimuli commonly produced left-lateralized response in the posterior fusiform and superior temporal gyri and right-lateralized response in the extrastriate cortex. Only logograms and objects produced a distinct cluster showing right-lateralized activation in the medial anterior fusiform gyrus associated with semantic knowledge, whereas only phonograms produced a left-lateralized activation in the posterior middle temporal cortex close to the site associated with visual perception of alphabetical letters. These findings suggest that while these stimuli similarly recruit the left perisylvian language area as a common neural component for naming, processing of objects and logograms becomes left-lateralized only in the downstream of the occipitotemporal cortex. By contrast, visual processing of phonograms is specialized to the left hemisphere in earlier stages of the area. The present data provide further evidence suggesting that both the left-right and anterior-posterior axes of the occipitotemporal cortex are differentially tuned according to the specific features of visual stimuli.

Selective Impairment of the Auditory-Verbal Short-Term Memory due to a Lesion of the Superior Temporal Gyrus

115 tion: an increase in its volume and slowing down of its circulation that can lead to subdural effusions or hematomas due to extravasation from the dilated venous system [1, 2]. The presence of subdural hematomas in sinus thrombosis, although possible, is rare. The diagnosis of IH requires a CSF pressure measured by lumbar tapping of less than 60 mm H2O. In our case, it could not be carried out due to the risk of cerebral herniation because of the VST. So, we cannot confirm the presence of IH, but the clinical presentation, evolution and MRI findings strongly suggest this diagnosis. The cause of IH could be a CSF leakage due to the strong sustained exercise the patient had practiced the previous month. Other etiologies of IH, like dehydration, diabetic coma, hyperventilation, meningoencephalitis, uremia or systemic infections, were rejected [4, 8]. Chisholm and Campbell [9] reported a superior sagittal sinus thrombosis initially mistaken for an IH syndrome due to the postural headache that the patient suffered. This happened in a young woman in the puerperal period (a risk factor for thrombosis) and without reference to image findings suggesting IH. So, it was understood as a sinus thrombosis that clinically evoked an IH syndrome. However, we interpret our case differently. Based on the physiopathology of IH, we think it could be possible that IH may cause an extreme ingurgitation and stasis of the dural venous system leading to thrombosis. To our knowledge, there is no other reported case prompting the idea of an association between IH and VST. Thus, we consider that VST could complicate an initially trivial IH syndrome. The description of similar cases would increase the knowledge of this association and the physiopathology of both entities.

Transient Functional Suppression and Facilitation of Japanese Ideogram Writing Induced by Repetitive Transcranial Magnetic Stimulation of Posterior Inferior Temporal Cortex

The Japanese writing system is unique in that it is composed of two different orthographies: kanji (morphograms) and kana (syllabograms). The retrieval of the visual orthographic representations of Japanese kanji is crucial to the process of writing in Japanese. We used low-frequency repetitive transcranial magnetic stimulation (rTMS) to clarify the functional relevance of the left and right posterior inferior temporal cortex (PITC) to this process in native Japanese speakers. The experimental paradigms included the mental recall of kanji, kana-to-kanji transcription, semantic judgment, oral reading, and copying of kana and kanji. The first two tasks require the visualization of the kanji image of the word. We applied 0.9 Hz rTMS (600 total pulses) over individually determined left or right PITC to suppress cortical activity and measured subsequent task performance. In the mental recall of kanji and kana-to-kanji transcription, rTMS over the left PITC prolonged reaction times (RTs), whereas rTMS over the right PITC reduced RTs. In the other tasks, which do not involve the mental visualization of kanji, rTMS over the left or right PITC had no effect on performance. These results suggest that the left PITC is crucial for the retrieval of the visual graphic representation of kanji. Furthermore, the right PITC may work to suppress the dominant left PITC in the neural network for kanji writing, which involves visual word recognition.

Functional neuroanatomy of Japanese writing systems

Background: Most of the brain imaging studies to date have addressed the skill of writing as a paradigm to examine the neural correlates for complex limb movements. Although these data have enriched our view of the functional organization of the brain for motor aspects of writing, there have been few attempts to discuss their relevance to aphasiology research. Aims: The aim of the present article is two-fold. First, we examined neuroimaging data on writing to provide an overview of the neurophysiological basis subserving the motor aspects of the skill. Second, while taking into consideration the framework provided by these data, we reviewed lesion and functional imaging studies of Japanese to discuss the possible difference in neural correlates that has been assumed for its two orthographic systems, kanji (logogram) and kana (syllabogram). Main Contributions: The fronto-parietal cortical circuit linking the premotor with posterior parietal areas in the left hemisphere seems to constitute a basic neural substrate for the motor act of writing. Lesion and imaging data with Japanese suggest that writing of kana utilises these structures in conjunction with the left perisylvian area for spoken language. In contrast, writing of kanji shares this network for the later phase of motor execution, but recruits the left basal temporal area as an additional device for the generation of motor output. Conclusions: The execution of writing, irrespective of the script type, is achieved by common brain structures used for a wide range of limb movements. Writing of kanji seems to differ from that of kana only in that the former needs the retrieval of visuospatial information of characters as an additional cognitive operation. This difference does not necessarily imply that the two kinds of script have distinct neural substrates.

Functional neuroanatomy of speech processing within the temporal cortex.

The phonemic structure of the maternal language determines the way of perceiving speech signals. A typical example is that native Japanese listeners map two English phonemes, /r/ and /l/, onto the same /R/. This perceptual assimilation of speech sounds has been associated with the left and/or right posterior perisylvian region, but the precise functional anatomy is unknown. Using functional magnetic resonance imaging and a repetition priming paradigm, we identified three subregions in the left temporal cortex: an anterior division sensitive to language-specific phonological knowledge, and a midlateral and a posterior division related to other vocal stimuli features. Dynamic causal modeling supports the scheme by which the anterior pathway processes perceptual assimilation; the posterior pathway processes lexico-semantic information.

A compensatory mechanism in unilateral akinetic-rigid syndrome: an fMRI study

The motor mechanisms of a patient with unilateral hand clumsiness in the early stages of akinetic-rigid syndrome were assessed by functional magnetic resonance imaging (fMRI). Movements of the unaffected hand produced activation in the contralateral sensorimotor cortex (SMC) and ipsilateral SMC and superior parietal lobule (SPL). The affected hand activated the bilateral SMCs, supplementary motor areas, and SPLs. We speculated that the bilateral activation indicated recruitment of a pre-existing bilaterally organized large-scale neural network to perform the motor task.