Relative Contributions of Eye-Position and Retinal Information to Perceived Egocentric Visual Direction
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Purpose: Visual directions of object(s) in space are judged with reference either to the self (egocentric), or to the eye (oculocentric). Perceived egocentric direction (EVD) is based on the combination of sensed eye-position in the orbits, provided by proprioception and efference copies, and retinal information provided by local sign mechanisms. This dissertation examined whether (1) the eye-position and the retinal information covary within the same subjects, (2) the eye-position information from the non-viewing eye contributes to perceived EVD during complete absence of retinal information from one eye, (3) the contribution of eye-position information varies similarly in the presence and absence of foveal suppression, (4) vergence eye movements contribute to perceived EVD, and (5) differences in eye velocities can account for differences in perceived EVD for different viewing conditions. Methods: The relative contributions of eye-position information from the two eyes during different viewing conditions were determined by open-loop pointing responses to an isolated target presented at one of several horizontal locations on a completely black background. These targets were presented binocularly with either asymmetric or symmetric vergence demands, and monocularly during heterophoria. The targets were either static, or were ramped with a constant asymmetric vergence velocity of 0.75 or 1.5 deg/s. The effect of foveal suppression on the eye-position information was determined using targets with 1.5D of anisometropic blur. The relative contributions of retinal information from each eye were estimated by an alignment task, for contrast-varying targets with horizontal or vertical disparity, and for luminance-varying targets with horizontal disparity. Comparisons of relative contributions of eye-position information during asymmetric vergence and retinal information for targets of unequal contrast or luminance in the two eyes indicated if the two sources of information covary within the same subjects. Comparisons of the contributions of eye-position information during binocular viewing and asymmetric vergence with monocular viewing, suppression of foveal information, and different eye-velocities indicated whether the contribution of eye-position to perceived EVD remains similar during different viewing conditions. Results: Between-eye differences exist in the contributions of eye-position and retinal information in some subjects. Across subjects, the inferred weighting of eye-position and retinal information covaries quantitatively (p < 0.05). During monocular viewing, the contribution of eye-position information from the non-viewing eye varies idiosyncratically. The contribution of eye-position information from a foveally suppressed eye is less than when no suppression occurs. In subjects who weight the position information from the two eyes unequally, changes in perceived direction were noted during symmetric vergence. No statistically significant differences in the contributions of eye-position information occur for different eye velocities. Conclusions: The contributions of eye-position and retinal information to perceived EVD covary similarly. However, the contribution of eye-position information depends to some extent on the retinal information, and/or suppression. Vergence eye movements contribute to perceived EVD. Differences in eye velocity cannot account for differences in the contributions of eye-position information observed under the different viewing conditions. The combination of eye-position and retinal information could occur at a common neural locus.