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 blind spot of the contralateral eye is believed to be monocular. We measured 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 presented around the blind spot of the fellow eye. A large drop in performance was seen because of the flanks, showing clearly the existence of dichoptic contour interaction in this "monocular" region of the visual field. This suggests that the cortical representation of the region of the visual field that corresponds to the contralateral eye's blind spot is not strictly monocular. The absence of direct retinal afferents from one eye to this region of cortex suggests the involvement of horizontal 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 connections in the striate cortex of monkeys. Our results are consistent with the proposition that long-range horizontal connections of the striate cortex may mediate the contour interaction phenomenon.