| Background. Fixation disparity (FD) is a constant small deviation of the eyes from the intended vergence position so that the images of a binocularly fixated target are not imaged precisely on corresponding retinal points. However, the images remain within Panum's fusional areas (PFA), and observers continue to maintain binocular single vision. The ability of the human visual system to perceive depth in the visual surroundings, i.e., stereopsis, has been shown to worsen with the distance of the target from the horopter, i.e., with horizontal disparity pedestals. However, previous studies provide contradictory ranges for tolerance of stereopsis to vertical and horizontal disparity pedestals, such as those that result from FD. The purpose of this study is to address the relationship between VFD and HFD and stereothresholds by measuring both FD and the stereothreshold concurrently in the same experiment, under highly similar stimulus conditions.;Conclusions. The results agree with previous findings that FD is a better predictor of an individual's stereothresholds than the magnitude of the vergence demand. The elevation of stereothresholds that occurs in the presence of VFD or HFD may be attributable to the difficulty of binocular neurons to match vertically or horizontally displaced images within their receptive fields, respectively. The greater elevation of stereothresholds with VFD than with an equivalent amount of HFD suggests that VFD and HFD elevate stereothresholds by different mechanisms. |
