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要旨:
The retinal velocity of an object moving in space depends on its distance from us. Thus, to interpret retinal motions the visual system must estimate an object's distance. Which sources of information are used? Here we consider the use of horizontal binocular disparity and vergence cues to distance. Specifically, we investigated whether disparity and vergence cues provide a depth distance estimate required to judge the physical velocity of objects moving at different distances (velocity constancy). Observers (n=6) viewed computer-rendered objects (either wire-frame spheres or small points) translating in the fronto-parallel plane. A trial consisted of two objects presented sequentially; observers judged whether the first or second moved faster. A staircase procedure was used to adjust the velocity of the second object to obtain the point of subjective equality between the two presented motions. Trials for objects moving with different velocities, directions and displacements were randomly interleaved. Velocity judgments were made for objects presented at different distances defined by disparity, vergence angle and changing size cues. Judgments of perceived velocity were systematically affected by the depth distance between the objects, with velocity matches close to those expected for perfect velocity constancy. This was true even for small points, suggesting that, in contrast a previous report (McKee Welch, Vision Research, 29, 553), disparity-defined depth can provide a sufficient distance cue for judgments of object velocity. However, settings made under conditions of different states of eye vergence had little effect on velocity matches. These results support a constancy mechanism for velocity that takes disparity-defined depth as an input, but that is little affected by static vergence posture.
