Disparity sensitivity and binocular integration in mouse visual cortex areas

Binocular disparity, the difference between the two eyes’ images, is a powerful cue to generate the three-dimensional depth percept known as stereopsis. In primates, binocular disparity is processed in multiple areas of the visual cortex, with distinct contributions of higher areas to specific aspects of depth perception. Mice, too, can perceive stereoscopic depth, and neurons in primary visual cortex (V1) and higher-order, lateromedial (LM) and rostrolateral (RL) areas were found to be sensitive to binocular disparity. A detailed characterization of disparity tuning properties across mouse visual areas is lacking, however, and acquiring such data might help clarifying the role of higher areas for disparity processing and establishing putative functional correspondences to primate areas. We used two-photon calcium imaging to characterize the disparity tuning properties of neurons in mouse visual areas V1, LM, and RL in response to dichoptically presented binocular gratings, as well as correlated and anticorrelated random dot stereograms (RDS). In all three areas, many neurons were tuned to disparity, showing strong response facilitation or suppression at optimal or null disparity, respectively. This was even the case in neurons classified as monocular by conventional ocular dominance measurements. Spatial clustering of similarly tuned neurons was observed at a scale of about 10 μm. Finally, we probed neurons’ sensitivity to true stereo correspondence by comparing responses to correlated and anticorrelated RDS. Area LM, akin to primate ventral visual stream areas, showed higher selectivity for correlated stimuli and reduced anticorrelated responses, indicating higher-level disparity processing in LM compared to V1 and RL.

An Unexpected Spontaneous Motion-In-Depth Pulfrich Phenomenon in Amblyopia

The binocular viewing of a fronto-parallel pendulum with a reduced luminance in one eye results in the illusory tridimensional percept of the pendulum following an elliptical orbit in depth, the so-called Pulfrich phenomenon. A small percentage of mild anisometropic amblyopes who have rudimentary stereo are known to experience a spontaneous Pulfrich phenomenon, which posits a delay in the cortical processing of information involving their amblyopic eye. The purpose of this study is to characterize this spontaneous Pulfrich phenomenon in the mild amblyopic population. In order to assess this posited delay, we used a paradigm where a cylinder rotating in depth, defined by moving Gabor patches at different disparities (i.e., at different interocular phases), generates a strong to ambiguous depth percept. This paradigm allows one to accurately measure a spontaneous Pulfrich phenomenon and to determine how it depends on the spatio-temporal properties of stimulus. We observed a spontaneous Pulfrich phenomenon in anisometropic, strabismic, and mixed amblyopia, which is posited to be due to an interocular delay associated with amblyopic processing. Surprisingly, the posited delay was not always observed in the amblyopic eye, was not a consequence of the reduced contrast sensitivity of the amblyopic eye, and displayed a large variability across amblyopic observers. Increasing the density, decreasing the spatial frequency, or increasing the speed of the stimulus tended to reduce the observed delay. The spontaneous Pulfrich phenomenon seen by some amblyopes was variable and depended on the spatio-temporal properties of the stimulus. We suggest it could involve two conflicting components: an amblyopic delay and a blur-based acceleration.

A Multiuser Multiperspective Stereographic QTVR Browser Complemented by Java3D Visualizer and Emulator

To support multiperspective and stereographic image display systems intended for multiuser applications, we have developed two integrated multiuser multiperspective stereographic browsers, respectively featuring IBR-generated egocentric and CG exocentric perspectives. The first one described, “VR4U2C” (‘virtual reality for you to see’), uses Apple's QuickTime VR technology and the Java programming language together with the support of the QuickTime for Java library. This unique QTVR browser allows coordinated display of multiple views of a scene or object, limited only by the size and number of monitors or projectors assembled around or among users (for panoramas or turnoramas) in various viewing locations. The browser also provides a novel solution to limitations associated with display of QTVR imagery: its multinode feature provides interactive stereographic QTVR (dubbed SQTVR) to display dynamically selected pairs of images exhibiting binocular parallax, the stereoscopic depth percept enhanced by motion parallax from displacement of the viewpoint through space coupled with rotation of the view through a 360° horizontal panorama. This navigable approach to SQTVR allows proper occlusion/disocclusion as the virtual standpoint shifts, as well as natural looming of closer objects compared to more distant ones. We have integrated this stereographic panoramic browsing application in a client/server architecture with a sibling client, named “Just Look at Yourself!” which is built with Java3D and allows realtime visualization of the dollying and viewpoint adjustment as well as juxtaposition and combination of stereographic CG and IBR displays. “Just Look at Yourself!” visualizes and emulates VR4U2C, embedding avatars associated with cylinder pairs wrapped around the stereo standpoints texture-mapped with a set of panoramic scenes into a 3D CG model of the same space as that captured by the set of panoramas. The transparency of the 3D CG polygon space and the photorealistic stereographic 360° scenes, as well as the size of the stereo goggles through which the CG space is conceptually viewed and upon which the 360° scenes are texture-mapped, can be adjusted at runtime to understand the relationship of the spaces.

Comparing Perceptual Signals of Single V5/MT Neurons in Two Binocular Depth Tasks

Neurons in the extrastriate visual area V5/MT show perceptually relevant signals in binocular depth tasks, which can be measured as a choice probability (CP) for the neuron. The presence of a CP in a particular paradigm may be an indicator that the neuron is generally part of the substrate for the perception of binocular depth. We compared the responses of those single neurons that show CPs in one stereoscopic depth task with their responses in another stereo task. Each neuron was tested for the presence of 1) CPs during a task in which macaques responded to the sign of binocular depth in a structure-from-motion stimulus, to judge its direction of three-dimensional rotation and 2) a consistent response to the stereo disparity of binocularly anti-correlated stimuli. Previous work, confirmed here, shows that changing the disparity of these binocularly anti-correlated stimuli often fails to yield a coherent change in the depth percept. For each test alone, there are V5/MT neurons that carry signals that are congruent with the perceptual effects. However, on comparing tests, there is no fixed pool of neurons that can account for the binocular depth percept. Excitation of neurons with a measurable CP does not necessarily lead to a change in perception. The cortical circuitry must be able to make dynamic changes in the pools of neurons that underlie perceptual judgments according to the demands of the task.

Experiments on the Role of Painted Cues in Hughes's Reverspectives

The English artist Patrick Hughes has created an extraordinary class of painted artpieces, most commonly referred to as ‘reverspectives’. They consist of truncated pyramids and prisms with their smaller faces closer to the viewer, in such a way as to allow a realistic scene to be painted on them. The works of art contain rich perspective and other painted cues that conspire to elicit an illusory depth percept that is the reverse of the physical depth arrangement. This reverse depth is obtained under a wide range of viewing conditions, and competes with the veridical depth percept in a classical bistable paradigm that was found to exhibit a high degree of hysteresis. Under the illusory depth percept, reverspectives appear to move vividly as the viewer moves in front of them. This paper reports two experiments that were designed to assess the effectiveness of the painted cues in eliciting the illusory depth percept by using three different measures for the strength of the illusion. As expected, the illusion was favored by monocular viewing and large viewing distances. The results from these two experiments are in close agreement with each other, and they indicate that the painted cues are powerful in influencing the ultimate percept.

Learning to See Random-Dot Stereograms

In the present study some specific properties of the learning effects reported for random-dot stereograms are examined. In experiment 1 the retinal position-specific learning effect was reproduced and in a follow-up experiment it was shown that the position specificity of learning can be accounted for by selective visual attention. In experiments 2 and 3 evidence was obtained that suggests that observers can learn, to a certain degree, monocular random-dot patterns and that this learning facilitates the depth percept. This result indicates that the traditional belief that random-dot stereograms are devoid of monocularly recognizable or useful forms should be reconsidered. In the second set of experiments the learning of two binocular surface properties of random-dot stereograms, depth edges and internal depth regions, was investigated. It was shown in experiment 4 that the depth edges of random-dot stereograms are not learned, whereas the results of experiment 5 indicate that the internal depth regions are learned. Finally, in experiment 6 it was shown that depth edges are learned when the internal depth regions of the stereogram are ambiguous. The results are discussed in terms of the importance of the particular type of stimulus used in the learning process and in terms of perceptual learning and attention.

Stereoscopic Illusion Based on the Proximity Principle

A class of ambiguous random-dot stereograms were created that share the following interesting property: Although the binocular disparity forms a periodic ‘sawtooth’ waveform as a function of row number (the disparity is constant for a given row), these stimuli yield a monotonically increasing depth percept along the rows. The random-dot pattern of each row is periodic along the horizontal direction for the purpose of producing an ambiguous depth percept. It is this ambiguity that makes it possible for the periodic stimulus to give rise to a monotonic percept. This monotonic percept is substantially enhanced when the rows are shown in temporal sequence instead of all being displayed together. Experiments are reported which indicate that this illusion is due to the proximity, or pulling, effect in stereopsis.