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Scott B. Stevenson; Disparity vergence responses to luminance and contrast-defined patterns. Journal of Vision 2002;2(10):75. doi: 10.1167/2.10.75.
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Disparity vergence is a binocular eye alignment response that is driven by binocular disparity in retinal images and has both voluntary and reflexive aspects. The voluntary component is manifest in the attentional selection of targets at different horizontal disparities. The reflex component is manifest in the uncontrollable re-alignment of the eyes in response to horizontal, vertical or torsional disparity. The larger purpose of this study is to compare visual mechanisms that produce reflexive vertical vergence to those that mediate both voluntary vergence control and the perceptual awareness of disparity.
Here we investigated the responses to contrast-defined targets, to determine whether the visual processes driving reflexive, vertical vergence have access to this kind of second-order information. For comparison we also measured voluntary, horizontal responses. Vergence was measured with a binocular, dual-Purkinje image eye tracker. Disparity was controlled through motion of galvo mirrors. Stimuli were contrast-defined gratings, created by modulating contrast of binocularly uncorrelated dynamic visual noise. Grating bars were horizontal for vertical disparity measurements, and vice versa. Luminance gratings of various contrasts were added to the contrast-defined gratings.
Voluntary, horizontal vergence responses were robust for contrast-defined gratings, but showed more delay when the added luminance contrast was near zero. Reflexive, vertical vergence responses were absent for contrast-defined targets, but became robust as contrast was increased in the added luminance grating.
The information in second-order gratings appears to be unavailable to the binocular visual mechanisms that control eye alignment reflexes. Perceptually, this same information produces a sense of depth and a basis for voluntary control of vergence.
Stevenson SB, Lott LA, Yang J. (1997). The influence of subject instruction on horizontal and vertical vergence tracking. Vision Research 37, 2891–2898.
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