Functional organization of the human inferior frontal cortex involved in language processing


Yuko Sassa, Hyeonjeong Jeong, Hideyuki Okamot and, Ryuta Kawashima

The Tohoku University 21st Century Center of Excellence Program in Humanities, Graduate School of International Cultural Studies, Graduate School of Medicine, and New Industry Creation Hatchery Center, Tohoku University

The classical model of language in the brain is based on the lesion studies that have reported functional roles of the affected brain region through observation of patient’s symptom(s).　For example, the left inferior frontal gyrus (LIFG) including Broca’s has been believed to be involved only in language production. However, since number of cases in these lesion studies is relatively small, also the localization of lesions is somewhat uncertain, it is difficult to generate comprehensive models for language processing.

Recently, on the other hand, functional neuroimaging techniques enable us to identify non-invasively, the brain areas involved in the cognitive processes of interest of normal subjects. The previous functional neuroimaging studies have reported that a number of the brain areas in addition to Broca’s area and Wernicke’s area, are involved in language functions, and the results of these studies have made it possible to discuss the language systems in the brain precisely. Especially, it has been suggested that the LIFG area plays an important role in the syntactic processing, which forced to change the classical view for the functional role of this area.

In this workshop, we will introduce two functional magnetic resonance imaging (fMRI) studies, which can figure out the functional role of the LIFG in the different aspects of language processing.

In the first study, the brain activations during sentence comprehension in different languages by Korean native speakers were investigated. We used three different languages such as Korean, Japanese, and English, which were divided into two groups from typological differences in the basic word order, that is, English is different from Korean and Japanese. We found that the bilateral superior temporal cortex was activated in during the comprehension of Korean, English and Japanese. It is noteworthy that the LIFG including Broca’s area was significantly activated only when the subjects were listening to English despite the fact that all the Korean subjects had learned English (L2) much longer than Japanese (L3). The results suggest that the brain activation patterns were different according to typological differences in the basic word order.

In the second study, the brain activations during hearing different types of grammatical violation sentences in Japanese by Japanese native speakers were investigated. We found different brain activations depending on each type of grammatical violation sentences. As a result, we found that syntactic processing in Japanese is related to not only the LIFG but also to some specific regions such as the bilateral superior temporal gyrus. The magnitude of activation in the LIFG was different according to processing of different types of grammatical violation sentences. The results suggest that the brain network exists for different syntactic processing in the LIFG, although the pattern of activation is different in relation to the specific syntactic processes.

On the basis of the results of our studies and these of previous investigations, we conclude that the LIFG seems to have a critical role in the processing of specific syntactic aspects.