Cortical conditions for fused binocular vision

1. The behaviour of twenty-six single neurones has been studied in the visual cerebral cortex of the cat's neurologically isolated and unanaesthetized forebrain. In a separate series of experiments binocular vision was investigated in five human subjects.2. Units in the cat's brain were excited by two straight, light-dark edges, projected independently, one upon each retina; these edges were given identical, artificial saccadic movements. The average response of neurones was measured from the post-stimulus-histogram (P.S.H.), which provided the probability of unit-discharge at various times after the pattern movement.3. A district within either eye-field could usually be found, such that the saccadic movements of edges passing through this part of the field, caused maximal responses; this part of each field is termed the ;representative district'. The most exciting orientations of the stimulating patterns were similar for both eyes. A cell excited by similarly oriented patterns, ;aligned' through the representative districts for each eye, exhibited dramatic spatial summation. Mutual inhibition between retinal inputs was never seen.4. The response of cortical neurones to such aligned patterns was always greater than that when one pattern was misaligned by displacement in either direction from its representative district.5. When the pattern in one eye was inverted, the cell gave less response to aligned than to misaligned patterns.6. Human subjects, viewing similar inverted patterns through a mirror stereoscope, could only obtain stable fusion with a 5 min arc misalignment of the optic axes.7. In other experiments, one eye of each subject was presented with a white rectangular bar upon a black background, while the other eye was excited by a similar black bar upon a white background. Stable fusion was reported, providing percepts of either one single low contrast bar or two neighbouring bars of high contrast. There was a range of relative pattern positions over which alignment of the eyes sought the former percept in preference to the high contrast pattern.8. These results are consistent with the hypothesis that the mammalian visual system maintains binocular fusion by continuously hunting those eye directions that provide the greatest local cortical responses to saccadic movements, restricted to the smallest possible areas of the visual cortex.