LONG-RANGE DICHOPTIC INTERACTIONS IN THE HUMAN VISUAL-CORTEX IN THE REGION CORRESPONDING TO THE BLIND SPOT

The region of the visual field of one eye that corresponds to the blin d spot of the contralateral eye is believed to be monocular. We measur ed dichoptic contour interaction in this region of the visual field in humans by having observers report the orientation of a test letter '' T'' presented to this region, in the presence of flanking T's presente d around the blind spot of the fellow eye. A large drop in performance was seen because of the flanks, showing clearly the existence of dich optic contour interaction in this ''monocular'' region of the visual f ield. This suggests that the cortical representation of the region of the visual field that corresponds to the contralateral eye's blind spo t is not strictly monocular. The absence of direct retinal afferents f rom one eye to this region of cortex suggests the involvement of horiz ontal cortical connections in the contour interaction phenomenon. Our estimates of the extent of contour interaction in mm of striate cortex are comparable to the reported lengths of the long-range horizontal c onnections in the striate cortex of monkeys. Our results are consisten t with the proposition that long-range horizontal connections of the s triate cortex may mediate the contour interaction phenomenon.