The binocular computation of visual direction

How is a single visual direction assigned to a binocular feature for which the left and right eyes are signaling different directions? According to geometrical principles, binocular visual direction is the average of the visual directions measured from the left and right eyes. Contrary to this prediction, we have found that the relative visual direction between two Gabor targets presented at different stereoscopic depths could be manipulated by varying the contrast ratio between the left and right images. This finding is consistent with a new model in which the relative alignment of depth features is determined from a maximum-likelihood combination of the direction signals from the left and right eyes. In a second experiment we provide support for this model, showing that the magnitude of the contrast-dependent bias in visual direction is predicted by the uncertainty for spatial localization in the left and right images. Lastly we show that visual direction and stereopsis have different dependencies on interocular contrast differences, suggesting that the computation of stereo depth and visual-direction are mediated via different mechanisms.