NASOTEMPORAL OVERLAP AT THE RETINAL VERTICAL MERIDIAN - INVESTIGATIONS WITH A CALLOSOTOMY PATIENT

To evaluate nasotemporal overlap at the retinal vertical midline, we a sked a callosotomy patient to compare the orientation of square wave g ratings that were presented in his opposing visual fields. The grating s were horizontally or vertically oriented and had spatial frequencies ranging from 1 to 8 cycles per degree (cpd). Retinal stabilization en sured the gratings remained properly lateralized during sustained pres entations. In accord with previous investigations, when the gratings w ere presented for only 200 msec, or their medial edges were 2 degrees from the vertical meridian, performance was generally at chance. Howev er, when presentations lasted 2 sec and the medial edges of the gratin gs were 1 degrees from the vertical meridian, above chance performance was obtained. Accuracy rates were highest with 2 and 4 cpd gratings, and dropped. at 1 and 8 cpd. Unexpectedly, the subject performed signi ficantly better when the gratings were offset vertically from each oth er than when both were displayed on the horizontal meridian. Since thi s improvement did not occur when gratings were presented horizontally aligned above or below the horizontal meridian, it must be attributed to the relative offset between the gratings. The data suggest a narrow zone of nasotemporal overlap at the retinal vertical midline where ve ry limited visual information is encoded by crossed temporal and uncro ssed nasal retinal ganglion cells. An experiment in which only one gra ting in a hair was close to the vertical meridian indicates that this zone may be more pronounced in the nasal hemiretina. Copyright (C) 199 6 Elsevier Science Ltd.