A flat surface in the frontal plane appears slanted about a vertical axis when the image in one eye is vertically compressed relative to the image in the other eye: the induced effect (Ogle, 1938). The image appears to slant away from the eye with the smaller image. We have examined the horizontal eye movements of 2 human subjects as they shifted their gaze horizontally across a large random dot pattern that could be identical at the 2 eyes(15 deg, square) or was compressed vertically by 12% at 1 eye. Subjects were required to transfer fixation between two vertically centered vertical lines embedded in the patterns with a horizontal separation of 10 deg. Subjects were first given time to inspect the patterns and to report their percept with a button press, after which they were required to fixate one of the two lines and shift their gaze to the second line when the fixated one was extinguished. If the gaze shift was based solely on the horizontal visual information then the vertical compression should not affect it, but if it was influenced by the perceived slant then we would expect to see changes in the vergence angle (computed from the difference in the horizontal positions of the 2 eyes). Gaze shifts were accompanied by changes in vergence angle that were always in accordance with the perceived slant: compared with the situation in which the images in the two eyes were identical, gaze shifts towards the side of the smaller image were accompanied by divergence and gaze shifts away from the side of the smaller image were accompanied by convergence. The changes in vergence angle were comparable with published estimates of the magnitude of the induced effect.
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