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Gunnar Blohm, Aarlenne Z. Khan, Kai M. Schreiber, Lei Ren, J. Douglas Crawford; The role of extraretinal signals in egocentric depth estimation. Journal of Vision 2007;7(9):807. doi: 10.1167/7.9.807.
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To interact with an object in three-dimensional (3 D) space, the brain must construct the object's egocentric spatial location using binocular vision; both retinal images are merged to provide angular eccentricity (horizontal and vertical position) and retinal disparity provides information about distance. We have demonstrated theoretically that the reconstruction of egocentric distance from binocular retinal information requires 3 D eye and head position information in addition to ocular vergence in order to provide a unique depth position. Here, we tested whether subjects interpret identical retinal input as different depth positions depending on changing eye and head positions.
We first asked 5 subjects to fixate targets in different depth planes and under different head roll and head pitch angles. Recording the 3 D eye position from both eyes allowed us to determine each subject's binocular Listing's law and static VOR gains. These parameters were input into our theoretical model which provided us with different eye-head positions predicted to lead to different depth interpretations (with the same retinal stimulation and vergence). We then performed the actual experiment; first, we flashed a pointing target (ultra-bright LED which produced a retinal afterimage). Next we positioned the subjects' eyes and head into the predicted positions and asked subjects to align their finger tip with the recalled position of the target's retinal after-image. When comparing predicted and observed depth positions, we found significant correlations in all subjects, showing that depth was modulated in a way that could not be explained by fixation distance or vergence angle.
Our results demonstrate that extra-retinal eye and head position signals are indeed used by the human brain to compute depth from binocular visual information. Taken together with our theoretical findings, we suggest that extra-retinal signals in early visual areas encoding retinal disparity serve the purpose of depth reconstruction.
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