Temporal Dynamics of Japanese Morphogram and Syllabogram Processing in the Left Basal Temporal Area Studied by Event-Related Potentials

Visual language processing in the left basal temporal area (LBTA) was investigated using subdural electrodes in two patients with medically intractable temporal lobe epilepsy. Reading of Japanese syllabogram (kana) and morphogram (kanji) both provoked two types of event-related potentials, one with the peak latency of similar to 200 milliseconds and the other similar to 300 milliseconds, at the anterior and posterior portion of the fusiform, parahippocampal, and inferior temporal gyri within LBTA. By electrical stimulation using the same electrodes, in contrast, kana reading was selectively impaired in one area in the posterior portion of LBTA where a sharp event-related potential was recorded at similar to 200 milliseconds, and kanji reading was selectively impaired in another area also in the posterior portion of LBTA, where a blunt event-related potential was recorded at similar to 300 milliseconds. Because kana reading is functionally fundamental compared with kanji reading, relatively early signals at 200 milliseconds might be related to postperceptual but still fundamental functions such as letter-by-letter language processing and relatively late signals at 300 milliseconds might be associated with postperceptual, relatively higher-order functions such as word-as-a-whole language processing. The results indicate the existence of two distinct functional areas for written-language processing within LBTA. Both kanji and kana signals reach specific area at the same timing.