1,721,091 research outputs found
Métamères perceptifs et perception bistable
No full textPerceptual Metamers and Bistable Perception. Perceptual metamers and bistable perception are two key phenomena of perceptual awareness. In the first case, two physically distinct stimuli are perceived the same way, whereas in the second case, the very same stimulus is perceived in two distinct ways. What can externalist and internalist approaches contribute to explain these phenomena ?Les métamères perceptifs et la perception bistable sont deux phénomènes clés de la perception consciente. Dans le premier cas, deux stimuli physiquement distincts sont perçus de la même manière, alors que dans le second cas, le même stimulus est perçu de deux manières distinctes. Que peuvent apporter les approches externalistes et internalistes pour expliquer ces phénomènes ?Mamassian Pascal. Métamères perceptifs et perception bistable. In: Intellectica. Revue de l'Association pour la Recherche Cognitive, n°43, 2006/1. Internalisme / externalisme. pp. 73-77
The effects of task and saliency on latencies for colour and motion processing
No full textIn human visual perception, there is evidence that different visual attributes, such as colour, form and motion, have different neural-processing latencies. Specifically, recent studies have suggested that colour changes are processed faster than motion changes. We propose that the processing latencies should not be considered as fixed quantities for different attributes, but instead depend upon attribute salience and the observer's task.We asked observers to respond to high- and low-salience colour and motion changes in three different tasks. The tasks varied from having a strong motor component to having a strong perceptual component. Increasing salience led to shorter processing times in all three tasks. We also found an interaction between task and attribute: motion was processed more quickly in reaction-time tasks, whereas colour was processed more quickly in more perceptual tasks.Our results caution against making direct comparisons between latencies for processing different visual attributes without equating salience or considering task effects. More-salient attributes are processed faster than less-salient ones, and attributes that are critical for the task are also processed more quickly
Bayesian modelling of cue interaction: bi-stability in stereoscopic slant perception
No full textOur two eyes receive different views of a visual scene, and the resulting binocular disparities enable us to reconstruct its three-dimensional layout. However, the visual environment is also rich in monocular depth cues. We examined the resulting percept when observers view a scene in which there are large conflicts between the surface slant signaled by binocular disparities and the slant signaled by monocular perspective. For a range of disparity-perspective cue conflicts, many observers experience bistability: They are able to perceive two distinct slants and to flip between the two percepts in a controlled way. We present a Bayesian model that describes the quantitative aspects of perceived slant on the basis of the likelihoods of both perspective and disparity slant information combined with prior assumptions about the shape and orientation of objects in the scene. Our Bayesian approach can be regarded as an overarching framework that allows researchers to study all cue integration aspects - including perceptual decisions - in a unified manner
Common mechanisms for 2D tilt and 3D slant after-effects
No full textBy presenting oriented Gabor patches either monocularly or binocularly, we dissociated retinal orientation from perceived tilt and perceived slant. After adapting to binocular patches, with zero apparent tilt and non-zero slant, small tilt after-effects (TAEs) and large slant after-effects (SAE) were measured. Adapting to monocular patches with non-zero tilt and zero slant produced large TAEs and smaller SAEs. This pattern of results suggests that a common, low-level adaptation to monocular orientation is involved in slant and tilt after-effects. However, the incomplete transfer between slant and tilt makes it clear that higher-level adaptation is also involved, perhaps at the level of surface representation
Spatial and temporal tuning of motion-in-depth
No full textWe used the Pulfrich effect to investigate perception of motion in depth. Independent manipulation of spatial and temporal frequency content in stereoscopic motion stimuli revealed the tuning characteristics of motion-in-depth perception. Sensitivity to interocular phase difference between sinusoidally oscillating sine-wave gratings was measured in four observers who judged direction of motion in depth. Discrimination thresholds in terms of interocular phase difference were determined to investigate spatial and temporal tuning characteristics of a system that is based on interocular phase difference, interocular delay, binocular disparity and velocity difference. Temporal frequency tuning of interocular phase difference thresholds was band pass and relatively dependent on spatial frequency variation. These results together with evidence from two control experiments support the idea that sensitivity to direction of motion in depth is limited by a stereo-motion system that monitors binocular horizontal disparity and motion rather than interocular phase difference, interocular delay, or interocular velocity differenc
The influence of object size and surface shape on shape constancy from stereo.
No full textThe failure of shape constancy from stereoscopic information is widely reported in the literature. In this study we investigate how shape constancy is influenced by the size of the object and by the shape of the object's surface. Participants performed a shape-judgment task on objects of five sizes with three different surface shapes. The shapes used were: a frontoparallel rectangle, a triangular ridge surface, and a cylindrical surface, all of which contained the same maximum depth information, but different variations in depth across the surface. The results showed that, generally, small objects appear stretched and large objects appear squashed along the depth dimension. We also found a larger variance in shape judgments for rectangular stimuli than for cylindrical and ridge-shaped stimuli, suggesting that, when performing shape judgments with cylindrical and ridge-shaped stimuli, observers rely on a higher-order shape representation
Bayesian combination of ambiguous shape cues
No full textWe investigate how different depth cues are combined when one cue is ambiguous. Convex and concave surfaces produce similar texture projections at large viewing distances. Our study considered unambiguous disparity information and its combination with ambiguous texture information. Specifically, we asked whether disparity and texture were processed separately, before linear combination of shape estimates, or jointly, such that disparity disambiguated the texture information. Vertical ridges of various depths were presented stereoscopically. Their texture was consistent (in terms of maximum likelihood) with both a convex and a concave ridge. Disparity was consistent with either a convex or concave ridge. In a separate experiment the stimuli were defined solely by texture (monocular viewing). Under monocular viewing observers consistently reported the convex interpretation of the texture cue. However, in stereoscopic stimuli, texture information modulated shape from disparity in a way inconsistent with simple linear combination. When disparity indicated a concave surface, a texture pattern perceived as highly convex when viewed monocularly caused the stimulus to appear more concave than a “flat” texture pattern. Our data confirm that different cues can disambiguate each other. Data from both experiments are well modeled by a Bayesian approach incorporating a prior for convexity
No evidence for sequential effects of the interaction of stereo and motion cues in judgements of perceived shape
No full textThe interaction of the depth cues of binocular disparity and motion parallax could potentially be used by the visual system to recover an estimate of the viewing distance. The present study investigated whether an interaction of stereo and motion has effects that persist over time to influence the perception of shape from stereo when the motion information is removed. Static stereoscopic ellipsoids were presented following the presentation of rotating stereoscopic ellipsoids, which were located either at the same or a different viewing distance. It was predicted that shape judgements for static stimuli would be better after presentation of a rotating stimulus at the same viewing distance, than after presentation of one at a different viewing distance. No such difference was found. It was concluded that an interaction between stereo and motion depth cues does not influence the perception of subsequently presented static object
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