[Binocular fusion and spatial localization of vertical and horizontal gratings of different spatial frequencies]

End-stopped cells and binocular depth discrimination in the striate cortex of cats

Proposals concerning neural mechanisms for binocular depth discrimination have been criticized on the grounds that only striate cells with a preferred stimulus orientation not too far from the vertical can make significant horizontal disparity discriminations. We investigated this claim by preparing a two-dimensional array of position-disparity response profiles to moving light and dark bars from each of 18 cells in the simple family. From these arrays, it was possible to reconstruct disparity response profiles along any axis across the receptive field, irrespective of the cell's optimal stimulus orientation. This analysis showed that cells with a predominantly excitatory binocular response (N = 10) can make precise horizontal disparity discriminations, independent of their optimal stimulus orientation, provided that they are sufficiently end stopped. End-free cells, on the other hand, are effective for horizontal disparity discriminations only if their preferred orientation are near the vertical. Nearly all striate cells we examined were end-stopped to some degree and nearly half had an end inhibition sufficient to reduce the monocular response from the dominant eye to half its maximal amplitude. Cells having a predominantly inhibitory disparity response profile of the symmetric type (N = 8) have an inhibitory profile along every axis across the receptive field. An outline is given of a neural mechanism for the determination of absolute viewing distance based on the sensitivities of striate cells to vertical retinal-image disparities.

Sensitivity for vertical retinal image differences

The detection of horizontal disparities between the right and left retinal images is accepted as an important component in the visual perception of depth. It has occasionally been pointed out that a disparity will also occur in the vertical when a target is nearer to one eye than the other and could be used as a second component. Here it is shown that while it is possible to detect size differences between the retinal images in the vertical direction, the sensitivity is at least one order of magnitude less than the horizontal differences. It had been generally accepted that vertical disparities cannot be detected (literature summarized by Tschermak), but there are clear reports of the ability of subjects to respond to relative differences in the vertical magnification of the images of the two eyes, the so-called induced size effect. It manifests itself in the appearance of a visual scene as if there had been an opposite horizontal magnification and is, therefore, in the correct direction if it is to be utilized in asymmetrical convergence.