Disparity selective units in the superior colliculus of the opossum.

Binocular visual responses can be recorded in two regions of the superficial layers of the superior colliculus of the opossum. The direct binocular region (DBR) represents the binocular portion of the contralateral hemifield whereas the rostral pole (RP) represents the binocular portion of the ipsilateral hemifield. In the present study single units from both of these regions were tested with binocular and monocular stimulation. Most cells in both regions showed response facilitation when both eyes were simultaneously stimulated and, when tested with different binocular disparities, most cells showed broadly-tuned disparity selectivity. DBR units usually preferred disparities near zero whereas RP units had a wider range of preferred disparities, with a tendency toward positive (crossed) values. This data indicates that the superior colliculus of the opossum could provide a neural substrate for a coarse analysis of depth and also might help control vergence eye movements. The different ranges of disparity selectivity of DBR and RP are consistent with the previously reported monocular receptive-field data and suggest that DBR and RP analyze different depths of the 3-dimensional visual scene.