Vergence eye movements elicited by stimuli without corresponding features

The role of monocularly visible regions in depth and surface perception

The mainstream of binocular vision research has long been focused on understanding how binocular disparity is used for depth perception. In recent years, researchers have begun to explore how monocular regions in binocularly viewed scenes contribute to our perception of the three-dimensional world. Here we review the field as it currently stands, with a focus on understanding the extent to which the role of monocular regions in depth perception can be understood using extant theories of binocular vision.

Vergence in reverspective: percept-driven versus data-driven eye movement control

'Reverspectives' (by artist Patrick Hughes) consist of truncated pyramids with their small faces closer to the viewer, allowing realistic scenes to be painted on them. Because their pictorial perspective reverses the physical depth arrangement, reverspectives provide a bistable paradigm of two radically different, competing depth percepts, even when viewed binocularly: points that are physically further are perceived to be closer and vice versa. The key question addressed here is whether vergence is governed by the physical and/or the perceived depth of fixated targets. Vergence eye movements were recorded using the EyeLink II system under conditions optimized to obtain both the veridical and illusory depth percepts of a reverspective. Six gaze locations were signaled by LEDs placed at strategically selected depths on the stimulus surface. We obtained strong evidence that stable vergence fixations were governed by the percept: for the same LED position, eyes converged under veridical depth percepts and diverged under illusory percepts, thus rendering pictorial cues to be as effective as physical cues in vergence control. These results, obtained with stable fixations, do not disagree with earlier studies that found rapid fixational eye movements to be governed by physical depth cues. Together, these results allow us to speculate on the existence of at least two eye movement systems: an automatic, data-driven system for rapid successions of fixations; and a deliberate schema-driven vergence system that accounts for stable fixations based on the perceptual state of the observer.

Curvature biases in stereoscopic vision: a nasotemporal asymmetry

The reliability of curvature judgments for linear elements was studied, with stereograms that contained a binocular arc with curvature in depth, and either a binocular frontoparallel arc or a monocular one, on a background representing a hemiellipsoid. The subjects made about 15% errors on binocular arcs with curvature in depth, and 60%-80% of these occurred when both the hemiellipsoid and the arc were convex, the arc being perceived as concave, by transparency through the hemiellipsoid. There were also about 15%-30% errors on frontoparallel arcs, but spread among all situations, with a small prevalence of concave judgments. Curvature in depth was assigned to the monocular stimuli in more than 60% of the cases. There was a curvature bias when the monocular arcs were on the nasal side, and were viewed against a concave background. Assuming parallel viewing, nasal ingoing arcs were usually perceived as concave, and nasal outgoing arcs usually perceived as convex, in agreement with geometrical likelihood. Nasal-side elements captured by one eye are, in general, those with the highest likelihood of having matching elements in the other eye. Then the observed nasal bias effect suggests that the matching process in stereopsis could be driven from the nasal sides of the projections in the two cerebral hemispheres.

Quantitative depth for a phantom surface can be based on cyclopean occlusion cues alone

Liu, L., Stevenson, S.B., and Schor, C.M. (1994, Nature, 367, 66-669) reported quantitative stereoscopic depth in a phantom rectangle which appeared to lack conventional matching elements. Later, Gillam, B.J. (1995, Nature, 373, 202-203) and Liu, L., Stevenson, S.B., and Schor, C.M. (1995, Nature, 373, 203) and Liu, L., Stevenson, S.B., and Schor, C.M. (1997, Vision Research, 37(5), 633-644) indicated that the varying depth of the phantom rectangle could be based on stereoscopic matching. To remove the contaminating effects of conventional stereopsis from the Liu et al. (1994) original example, we presented a pair of parallel vertical lines to each eye where there is a central gap in the right line for the left eye's view and in the left line for the right eye's view. Observers saw a phantom rectangle bounded by subjective contours whose depth increased with the thickness of the lines. We attribute the quantitative variation of depth to a purely cyclopean (binocular) process sensitive to the pattern of contour presence and absence in the two eye's view.