Valérie Cornilleau-Pérès

Visual perception of surface curvature: Psychophysics of curvature detection induced by motion parallax

The continuous approach to optic-flow processing shows that the curvature of a moving surface is related to a second spatial derivative of the velocity field, the spin variation (Droulez & Cornilleau-Pérès, 1989).With this approach as a theoretical framework, visual sensitivity to the curvature of a cylinder in motion was measured using a task of discrimination between cylindrical and planar patches. The results confirm the predictions suggested by the theory: (1) Sensitivity to curvature was always greater when the cylinder axis and the frontal translation were parallel than when they were orthogonal. The ratio of curvature detection thresholds in the two cases was between 1.3 and 2.5; the value predicted from the spin variation theory is about 2. (2) Sensitivity to curvature increased strongly with the velocity of the motion but was only weakly affected by its amplitude and the duration of viewing for the range of values used in our experiments.

Stereo-motion cooperation and the use of motion disparity in the visual perception of 3-D structure

When an observer views a moving scene binocularly, both motion parallax and binocular disparity provide depth information. In Experiments lA-1C, we measured sensitivity to surface curvature when these depth cues were available either individually or simultaneously. When the depth cues yielded comparable sensitivity to surface curvature, we found that curvature detection was easier with the cues present simultaneously, rather than individually. For 2 of the 6 subjects, this effect was stronger when the component of frontal translation of the surface was vertical, rather than horizontal. No such anisotropy was found for the 4 other subjects. If a moving object is observed binocularly, the patterns of optic flow are different on the left and right retinae. We have suggested elsewhere (Cornilleau-Pérès & Droulez, in press) that this motion disparity might be used as avisual cue for the perception of a 3-D structure. Our model consisted in deriving binocular disparity from the left and right distributions of vertical velocities, rather than from luminous intensities, as has been done in classical studies on stereoscopic vision. The model led to some predictions concerning the detection of surface curvature from motion disparity in the presence or absence of intensity-based disparity (classically termedbinocular disparity). In a second set of experiments, we attempted to test these predictions, and we failed to validate our theoretical scheme from a physiological point of view.

The visual perception of plane tilt from motion in small field and large field: Psychophysics and theory

Subjects indicated the tilt of dotted planes rotating in depth, in monocular viewing, under perspective projection. The responses depended on the FOV (field of view) and on the angle W between the tilt and frontal translation (orthogonal to the rotation axis). Response accuracy increased with the FOV, and decreased with W. Our results support the processing of the second-order optic flow in all cases, but indicate that this flow is quantitatively small in small-field, leading to tilt ambiguities. We examine computational models based on the affine components of the optic flow to interpret our results.

Spatial Constancy and the Visual Perception of Surface Curvature from Optical Flow

Multiple views (successive or simultaneous) of the same object provide the visual system with depth information through motion or stereopsis. The retinal velocity field and binocular disparities are coded in the visual system, and may be used in an intermediate step of the visual processing of 3-D structure. Here we address the problem of recovering an intrinsic characteristic of surfaces — curvature — from moving images. We provide a theory about visual detection of surface curvatures and show how our model can be extended to stereopsis. Experimental validation of this model is brought about by our own experimental work as well as by that of others. We also discuss how spatial constancy can emerge from the visual processing of multiple views.

Visual Encoding of Tilt from Optic Flow: Psychophysics and Computational Modelling

Many computational models indicate ambiguities in the recovery of plane orientation from optic flow. Here we questioned whether psychophysical responses agree with these models. We measured the perceived tilt of a plane rotating in depth with two-view stimuli for 9 human observers. Response accuracy was higher under wide-field perspective projection (60°) than in small field (8°). Also, it decreased when the tilt and frontal translation were orthogonal rather than parallel. This effect was stronger in small field than in large field. Different computational models focusing on the recovery of plane orientation from optic flow can account for our results when associated with a hypothesis of minimal translation in depth. However, the twofold ambiguity predicted by these models is usually not found. Rather, most responses show a shift of the reported tilts toward the spurious solution with concomitant increase in response variability. Such findings point to the need for further simulations of the computational models.