Vergence provides veridical depth perception from horizontal retinal imagedisparities

One useful source of depth information available to the human nervous syste m is present in the horizontal disparities that exist between the two retin al images (stereoscopic depth). The relationship between horizontal dispari ty and depth varies with viewing distance so that an interpreting signal is required if disparities are to yield useful information. One potentially u seful interpreting signal is available from ocular vergence. A number of st udies have concluded. however, that a vergence signal does not provide veri dical stereoscopic depth, All of these studies required observers to make a range of judgements under conditions of uncertainty (often using random do t stimuli) and we suggest that the lack of veridicality arose because of a contraction bias: a general tendency to bias judgements towards the centre of the range of possible responses. We re-examined the role of ocular verge nce in the maintenance of stereoscopic depth constancy for teal three-dimen sional objects. Our results question the conclusions reached by previous st udies and suggest that vergence can provide a veridical interpretation of s tereoscopic depth. Our results indicate that horizontal retinal image dispa rities are not interpreted by a 'higher order' signal (i.e. the 'perceived distance' of the fixation point). The results of the experiment have signif icant implications for models of depth processing from disparity.