Nasotemporal overlap at the retinal vertical meridian: investigations with a callosotomy patient.

To evaluate nasotemporal overlap at the retinal vertical midline, we asked a callosotomy patient to compare the orientation of square wave gratings that were presented in his opposing visual fields. The gratings were horizontally or vertically oriented and had spatial frequencies ranging from 1 to 8 cycles per degree (cpd). Retinal stabilization ensured the gratings remained properly lateralized during sustained presentations. In accord with previous investigations, when the gratings were presented for only 200 msec, or their medial edges were 2 from the vertical meridian, performance was generally at chance. However, when presentations lasted 2 sec and the medial edges of the gratings were 1 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 significantly better when the gratings were offset vertically from each other than when both were displayed on the horizontal meridian. Since this improvement did not occur when gratings were presented horizontally aligned above or below the horizontal meridian, it must be distributed to the relative offset between the gratings. The data suggest a narrow zone of nasotemporal overlap at the retinal vertical midline where very limited visual information is encoded by crossed temporal and uncrossed nasal retinal ganglion cells. An experiment in which only one grating in a pair was close to the vertical meridian indicates that this zone may be more pronounced in the nasal hemiretina.