Local vs global stereoscopic matching in neurons of cortical area V1

Purpose. To test the binocular responses of V1 neurons with patterns that possess a single global stereoscopic interpretation but have multiple local features for binocular matching. Methods. Single neurons were recorded extracellularly in the alert monkey. Animals were trained to fixate and binocular eye position was monitored with magnetic search coils. The main visual stimuli were stereoscopically-presented sinusoidal gratings delimited by sharp circular or elliptical windows. The binocular disparity of the stimulus was manipulated by shifting both window and grating together by the same amount. The repetitive nature of the grating ensures that a new local feature enters the neuron's binocular receptive field when the disparity has been shifted by an exact multiple of the grating's period. The disparity tuning of neurons was measured for both windowed grating targets and dynamic random dot stereograms. Results. The steresropic depth of the windowed grating (as assessed psychophysically by human observers) is stable and governed by its overall binocular disparity. By contrast, neurons in V1 showed multiple peaks in their tuning functions which correspond to a shift in the disparity by a complete cycle of the grating. With random-dot stereograms, these same neurons showed disparity tuning functions consistent with the responses to gratings. Conclusions. The neurons identified in V1 respond to local features possessing an appropriate binocular disparity. These neurons may be a precursor to sterescopic depth perception rather than being directly responsible for perceptual performance. The tuning of these neurons for random-dot stereograms may reflect the operation of local mechanisms for binocular matching.