Illusory depth induced by binocular torsional misalignment

Large-scale cortico-cerebellar computations for horizontal and vertical vergence in humans

Horizontal and vertical vergence eye movements play a central role in binocular coordination. Neurophysiological studies suggest that cortical and subcortical regions in animals and humans are involved in horizontal vergence. However, little is known about the extent to which the neural mechanism underlying vertical vergence overlaps with that of horizontal vergence. In this study, to explore neural computation for horizontal and vertical vergence, we simultaneously recorded electrooculography (EOG) and whole-head magnetoencephalography (MEG) while presenting large-field stereograms for 29 healthy human adults. The stereograms were designed to produce vergence responses by manipulating horizontal and vertical binocular disparities. A model-based approach was used to assess neural sensitivity to horizontal and vertical disparities via MEG source estimation and the theta-band (4 Hz) coherence between brain activity and EOG vergence velocity. We found similar time-locked neural responses to horizontal and vertical disparity in cortical and cerebellar areas at around 100–250 ms after stimulus onset. In contrast, the low-frequency oscillatory neural activity associated with the execution of vertical vergence differed from that of horizontal vergence. These findings indicate that horizontal and vertical vergence involve partially shared but distinct computations in large-scale cortico-cerebellar networks.

Vertical size disparity induces enhanced neural responses in good stereo observers

Stereoscopic three-dimensional vision requires cortical processing for horizontal binocular disparity between the two eyes' retinal images. Behavioral and theoretical studies suggest that vertical size disparity is used to recover the viewing geometry and to generate the slant of a large surface. However, unlike horizontal disparity, the relation between stereopsis and neural responses to vertical disparity remains controversial. To determine the role of cortical processing for vertical size disparity in stereopsis, we measured neuromagnetic responses to disparities in people with good and poor stereopsis, using magnetoencephalography (MEG). Healthy adult participants viewed stereograms with a horizontal or vertical size disparity, and judged the perceived slant of the pattern. We assessed neural activity in response to disparities in the visual cortex and the phase locking of oscillatory responses including the alpha frequency range using MEG. For participants with good stereopsis, activity in the visual areas was significantly higher in response to vertical size disparity than to horizontal size disparity. The time-frequency analysis revealed that early neural responses to vertical size disparity were more phase-locked in good stereo participants than in poor stereo participants. These results provide neuromagnetic evidence that vertical-size disparity processing plays a role in good stereo vision.

Effects of Light, Rotation, and Texture on Errors in the Estimated Length of a Three-Dimensional Object in a Two-Dimensional Display

This study investigated the effects of light type (spot, directional, and point), forward-rotated stimulus angle (20°, 45°, and 70°), and surface texture (wood, metal, transparent glass, dark brown leather, and translucent plastic) on errors in the estimated vertical diameter of a three-dimensional object. Twenty female and 12 male Taiwanese college students, ages 18–22 years, participated in the current study. Participants were asked to judge the vertical diameter of an elliptical hole within a square plate that was displayed on an LCD monitor. Analysis indicated that the errors in estimated vertical diameter for rotated angles of 20° and 45° were greater than were those for 70°. Additionally, the error of estimated vertical diameter for a rotated angle of 45° was less than was that for 20°. The surface texture also affected errors in estimated vertical diameter: transparent glass was associated with more errors than were wood and metal textures. However, light type had no effect on errors in estimated vertical diameter. These results have implications for the graphics-based design of interfaces.

Vertical Size Disparity and the Correction of Stereo Correspondence

We examined the stage of vertical-disparity processing that produces a global stereoscopic slant. In two psychophysical experiments, we measured perceived slant about a vertical axis for two-dimensional stereoscopic patterns consisting of random dots, concentric lines, and radial lines. Binocular image differences were introduced into each pattern by vertically magnifying either the entire image for the right eye or that for the left eye. Because the continuous lines were geometrically ambiguous in local stereo correspondence, the three patterns differed from each other in the local horizontal disparity measured in retinal coordinates. The two experiments revealed that, despite the differences in the retinal horizontal disparity, the slant settings were generally similar for the three patterns, in both short and long viewing distances (25 cm and 120 cm, respectively). These results are consistent with the idea that the visual system uses vertical disparity at least when establishing local stereo correspondence. A Bayesian model is proposed to account for the results.

Perception of Relative Depth Interval: Systematic Biases in Perceived Depth

Given an estimate of the binocular disparity between a pair of points and an estimate of the viewing distance, or knowledge of eye position, it should be possible to obtain an estimate of their depth separation. Here we show that, when points are arranged in different vertical geometric configurations across two intervals, many observers find this task difficult. Those who can do the task tend to perceive the depth interval in one configuration as very different from depth in the other configuration. We explore two plausible explanations for this effect. The first is the tilt of the empirical vertical horopter: Points perceived along an apparently vertical line correspond to a physical line of points tilted backwards in space. Second, the eyes can rotate in response to a particular stimulus. Without compensation for this rotation, biases in depth perception would result. We measured cyclovergence indirectly, using a standard psychophysical task, while observers viewed our depth configuration. Biases predicted from error due either to cyclovergence or to the tilted vertical horopter were not consistent with the depth configuration results. Our data suggest that, even for the simplest scenes, we do not have ready access to metric depth from binocular disparity.