Visual perception fields in the superficial layers of the opossum's superior colliculus: representation of the ipsi and contralateral hemifields by each eye.

Visual receptive fields of single units in the superior colliculus of the opossums were analysed in regions representing the ipsilateral (rostral pole - RP) and the contralateral hemifields (direct binocular region - DBR) close to and including the representation of the horizontal meridian. Of 242 units half had receptive field centres of the contralateral eye at the ipsilateral hemifield and the other half at the contralateral hemifield. The proportion of units driven by the ipsilateral eye was considerably smaller at the RP than at the DBR and the frequency of unit response to this eye diminished within the ipsilateral hemifield with progressively more rostral penetrations in the RP. The midline was unequally represented by the two eyes in RP units: while 24 out of 26 receptive fields of the ipsilateral eye straddled the vertical meridian, only 50 out of 120 of the contralateral eye did so. Furthermore, at the RP receptive fields of units responsive to the contralateral eye extended up to 46.8 degrees into the ipsilateral visual field while those mapped for the ipsilateral eye extended only up to 26 degrees. As a result of this tendency for the receptive fields of the ipsilateral, but not the contralateral eye, to overlap the vertical meridian, progressively more rostral recordings yielded an increase in the binocular convergent disparity as a function of the eccentricity for the receptive field of the contralateral eye. Since the disparity didn't vary with the eccentricity in the DBR units, we suggest that stimuli at the midline and close to the animal might be the natural ones to drive large disparity units present in the RP.(ABSTRACT TRUNCATED AT 250 WORDS)