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1. Depth discrimination, using disparity cues alone, was studied with a small fixation point and briefly exposed, vertical slit-shaped targets.

2. The upper limit for reliable qualitative localization of a slit as nearer or further than the fixation point is 4-7 deg of absolute disparity in a convergent direction and 9-12 deg in a divergent direction. Even larger absolute disparities can be recognized in the peripheral visual field.

3. Relative depth discrimination between two slit targets was measured as a function of their spatial position. The horopter (the locus of targets that appear to be fused binocularly) is the region of maximum stereoacuity and this does not necessarily coincide with the Vieth—Müller circle (the locus of zero geometric or absolute disparity). There is a gradual increase in stereo-threshold as the targets are moved out along the horopter, away from the fixation point into the peripheral visual field. The relative disparity threshold also rises, approximately exponentially, as the targets are moved in depth or absolute disparity away from the horopter.

4. Relative depth discrimination is, then, operative over a very wide band of visual space around the horopter (about 3 deg of absolute disparity in the centre of the visual field and even more in the periphery).

5. The findings are discussed in relation to the neurophysiology of binocular neurones of the cat cortex. The dimensions of visual space under observation by the binocular apparatus of cat and man are rather similar. The sharper decline of stereo-acuity with absolute disparity in the centre of the visual field may be related to the limits of bilateral representation of a central strip of retina in the human brain.

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