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Purpose: To test whether binocular neurons in V1 respond to absolute or relative disparities. Absolute disparity refers to the difference in the anatomical locations of a single feature on the two retinae, and this depends on vergence angle. Relative disparity refers to the differences in absolute disparity between multiple image features, and this does not change with vergence. Methods: Extracellular single unit recordings were performed in the alert monkey. Animals were trained to fixate during stereoscopic stimulus presentation on two CRT monitors viewed via a haploscope. The positions of both eyes were recorded with scleral search coils. The stimuli used were sinusoidal gratings and random dot stereograms (with a surround at the same disparity as the fixation marker). The disparity of the stimulus covering the receptive field was varied by changes on the CRT displays. Additional absolute disparities were superimposed on the entire display, including the fixation marker by rotating the mirrors of the haploscope. The recorded vergence position was used in a feedback loop to ensure that the absolute disparity of the fixation marker was fixed for periods of one second. This allowed stimuli with the same pattern of relative disparities to be presented at different absolute disparities. Results: All cells tested responded consistently to the absolute disparity of the stimulus within the receptive field, regardless of the disparity of the rest of the display Conclusions: This population of neurons in V1 is not selective for relative disparities. They presumably do not therefore support the finest psychophysical stereoacuities, which require the presence of relative disparities.


Journal article


Investigative Ophthalmology and Visual Science

Publication Date