Neural correlates of visual form and visual spatial processing

Cortico-cortical projections for visual processing that originate from the striate cortex are organized into two streams. The dorsal stream projects t o the parietal region and the central stream to the inferior temporal regio n. One hypothesis is that the dorsal stream processes visual spatial inform ation, and the ventral stream processes visual object information. Although recognition of human faces or common objects has been shown preferentially to activate the ventral stream, the issue of when such processing starts t o engage the ventral or the dorsal stream is not clear. The question explor ed in this study is whether processing of visual form per se without evokin g the brain mechanisms that are associated with recognition of human faces or common objects is sufficient to activate the ventral stream more signifi cantly relative to the condition when only visual spatial processing is inv olved. Functional magnetic resonance images were acquired while subjects pe rformed a delayed comparison task in which either visual spatial or visual form information was processed. Cortical areas that were preferentially act ivated in visual spatial or visual form processing showed not only ventral- dorsal segregation, but also hemispheric laterality. The results extended p revious findings by showing that preferential activation in the ventral pat hway is not contingent upon such powerful stimuli as faces and common objec ts. Processing of simple visual form information is cause enough for such a ctivation to be observed. A strong left hemisphere dominance in visual form recognition was also revealed. The observed laterality may be a reflection that the left hemisphere is more important in symbolic and/or semantic cod ing of visual form information. a 1999 Wiley-Liss, Inc.