Binocular phase interactions in area 21a of the cat.

1. Binocular interactions related to retinal disparity were investigated in single neurons in area 21a of extrastriate cortex in the anaesthetized cat using sinusoidal luminance gratings. 2. The responses of approximately two-thirds of neurons were profoundly modulated by a relative phase difference between identical drifting gratings presented to each eye. This modulation included both facilitatory and inhibitory interocular interactions. The selectivity for binocular disparity was about twice as sharp as the selectivity for monocular spatial position. 3. Significant phase modulation was retained in many neurons at interocular orientation differences exceeding 45 deg. The response suppression associated with stimulation at a phase shift 180 deg from the optimum was stronger than the response suppression to an interocular orientation difference of 90 deg. 4. The proportion of phase modulated neurons and the potency of modulation in area 21a neurons exceed that reported for phase-selective complex cells in area 17. Neurons in area 21a show sharp disparity tuning that is relatively insensitive to changes in orientation and monocular position, which suggests that this extrastriate region has a role in stereoscopic depth perception.