Human visual processing as revealed by magnetoencephalography

This chapter begins with a brief overview of the human visual system through an introduction of magnetoencophalographic (MEG) studies. Studies using MEG to examine the striate cortex, dorsal pathway, and ventral pathway are described in subsequent. sections. First, the MEG responses from the primary visual cortex are discussed. Emphasis is placed on the fact that even the first response component peaking at around 100 ms could originate from the extrastriate cortex in addition to the striate cortex; inducing a response from the primary visual cortex alone is difficult, unlike the somatosensory and auditory cortices. Second, we show that various visual motion stimuli evoke responses from cortical regions in the dorsal pathway, including MT/V5+. Detailed analysis of response properties reveals: 1) the role of MT/V5+ in the perception of apparent motion; 2) representation of the spatio-temporal gradient detection mechanisms in the human brain; and 3) spatial integration of speed information independent of direction information. Scalar fields theory is introduced as a mechanism for the detection of complex motions, such as the transparency of two incoherent motions with different speeds. Third, the role of the ventral pathway is discussed with reference to our recent MEG studies. Physiological evidence of a restricted locus in the human brain that processes visual shape irrespective of differences in visual cues is shown. We confirmed involvement of the interior and lateral temporal areas for the neural processing of face perception. Comparing responses to inverted and upright images of faces revealed the exclusive role of the right hemisphere in holistic processing of the normal upright face configuration.