A flat surface lying in a frontal plane appears slanted in depth about a vertical axis when the image in one eye is horizontally magnified relative to the image in the other eye. The surface appears to slant away from the eye seeing the smaller image. Horizontal magnification disparity also produces shape distortion. Since the vertical angular size of the surface remains the same both with and without horizontal magnification of the image, the side that appears farther away appears larger. A rectangular figure with horizontal magnification disparity is therefore perceived as a horizontally tapered isosceles trapezoid slanted about a vertical axis. It seems that the apparent shape distortion induced by disparity has not been measured systematically although it is well established that the apparent slant approximates to the geometrical prediction. The aim here is to examine the apparent shape distortion induced by disparity. The test stimulus was a random-dot stereogram presented in a mirror stereoscope in a darkroom. The dots were depicted in a rectangular area. The stereoscopic image was a 100-mm-square at 500 mm ahead of the subject. Ten magnitudes of slant were tested: ?±50, 40, 30, 20, and 10?°. Subjects indicated the perceived slant of the test stimulus with an unseen paddle and then adjusted the taper of a trapezoid on a computer monitor to coincide with the apparent shape with buttons. The apparent slant and shape distortion from motion parallax were also investigated. Subjects monoularly viewed a single random-dot pattern displayed on a computer monitor while making side-to-side head movements. Stimulus translation and head movement were synchronized. For both disparity and motion parallax the perceived taper angle was smaller than prediction even though the perceived slant was almost veridical. While the predicted taper increases as slant increases, the perceived taper was immutably about 1?°.
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