Modulation of neural activities during visual processing in the human extrastriate visual cortex.

Magnetoencephalographic responses to single letters (Japanese monosyllabic characters) presented in the left visual field were measured during visual cognition tasks, in which subjects judged matching of characters in two different processes based on phonological and graphical cues. Equivalent current dipoles, which represent focal neural activities, were localized in the extrastriate visual cortex of the occipital to occipitotemporal regions. The main activities were observed in the right lateral area when the subjects detected shape, and also in the medial and inferior areas of both hemispheres when they detected rhyme. These results suggest that the neural activities in the extrastriate cortex, which are related to early processing of familiar visual forms, can be modified by the top-down control.