Biomedical subjects

Pascal Mamassian

Publications and source records attributed to Pascal Mamassian.

18 recordsLinked to original sources

PubMed2006

Temporal dynamics of stereo correspondence bi-stability.

Periodic stereoscopic stimuli offer multiple viable solutions to the stereo correspondence problem. When viewing such stimuli for prolonged periods of time, observers continually switch their perceptual state between alternative correspondence solutions. We examine the temporal dynamics of this correspondence bi-stability. Participants were presented with an ambiguous stereogram comprised of regularly spaced dots. This stimulus was perceived as a fronto-parallel plane situated either behind or in front of fixation, depending on the achieved correspondence solution. The stimulus was presented continuously for one minute, with participants instructed to report the sign of the perceived depth at the sound of an auditory prompt presented, on average, every 2 s. Inter-ocular contrast and available disparities were varied so as to manipulate preferred correspondence. We find that participants are initially biased to perceive the stimulus as having an uncrossed disparity. Furthermore, we find that following an initial period of change, perceptual preference and perceptual stability (measured as the probability of an observer's percept changing between consecutive responses) remain constant over the presentation period. Finally, we find that manipulations of matching preference affect both the transient preference for, and stability of, one percept over another. Our results suggest two distinct phases of biasing in the correspondence matching process, one early, the other sustained.

Depth Perception↗

PubMed2006

Bayesian inference of form and shape.

The ability to visually perceive two-dimensional (2D) form and three-dimensional (3D) shape is one of our most fundamental faculties. This ability relies on considerable prior knowledge about the way edge elements in an image are likely to be connected together into a contour as well as the way these 2D contours relate to 3D shapes. The interaction of prior knowledge with image information is well modeled within a Bayesian framework. We review here the experimental evidence of shape perception seen as a Bayesian inference problem.

Bayes Theorem↗

PubMed2005

Temporal dynamics in bistable perception.

Bistable perception is fundamentally a dynamic process: Our perceptual experience continuously alternates when an ambiguous or rivalrous stimulus is observed. Here we present a method to analyze instantaneous measures of dominance and transition between percepts. The analysis extracts three time-varying probabilities. First, the transient preference represents the probability of perceiving one interpretation at one instant. Second, the reversal probability is the probability that the current percept will change at the next evaluation. Finally, the survival probabilities are the probability that at one instant the current percept will not switch to the alternative interpretation. We derive the relationships between these probabilities and offer a test of independence between consecutive percepts. We also introduce a simple technique to sample the observer's perception at regular intervals. The analyzing method is illustrated with the example of binocular rivalry. We demonstrate Levelt's second proposition with the survival probability measure and show that the consecutive rivalrous percepts are not independent.

Dominance, Ocular↗

PubMed2005

Selective biasing of stereo correspondence in an ambiguous stereogram.

In spite of numerous studies in stereoscopic perception, it is still not clear how the visual system matches features between the two eyes. One reason is that these previous studies used stimuli that presented little perceptual ambiguity, so the correspondence problem had only one solution. We present here a novel stimulus that presents a more complex correspondence problem. This stimulus is inspired by "wallpaper" stimuli and was specifically designed to put into conflict two possible constraints underlying stereo correspondence matching. These constraints are the nearest neighbour matching rule--that biases surfaces towards the horopter--and the nearest disparity rule--that biases surfaces to be smooth. By varying the contrast of adjacent image features in this stimulus, we were able to reveal and quantify a preference for nearest disparity matching. The magnitude of this preference is dependent upon the magnitude of possible disparities in the scene and is consistent with the idea that the visual system seeks to minimise local differences in disparity. We discuss these results with regard to the use of prior constraints in models of stereo matching.

Contrast Sensitivity↗

PubMed2004

Bayesian combination of ambiguous shape cues.

We 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.

Bayes Theorem↗

PubMed2004

The effects of task and saliency on latencies for colour and motion processing.

In 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.

Attention↗

PubMed2004

No evidence for sequential effects of the interaction of stereo and motion cues in judgements of perceived shape.

The 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 objects.

Cues↗

PubMed2004

The efficiency of depth discrimination for non-transparent and transparent stereoscopic surfaces.

The perception of transparency in binocular vision presents a challenge for any model of stereopsis. We investigate here how well human observers cope with stereo transparency by comparing their efficiency between transparent and opaque depth judgments. In two experiments, the efficiency measure was computed relative to an ideal observer to take into account the larger correspondence ambiguity in the transparent condition. We found that thresholds for human and ideal observers were consistently higher in the transparent condition than in the opaque condition, across a range of dot densities (Experiment 1) and disparity ratios (Experiment 2). Efficiencies (the ratio of human to ideal performance) were approximately equivalent for the opaque and transparent conditions across all stimulus conditions. Therefore, the cost for stereoscopic transparency can be accounted for by the greater correspondence problem in that condition. Indeed, the fact that efficiencies were very low, around 1%, and decreased with increasing dot density demonstrates that human observers use far less information than is available to perform the task. This account contrasts with previous interpretations for the cost in stereoscopic transparency in terms of inhibitory interactions specific to transparent configurations. We relate our findings to a previous and comparable study of motion efficiency, and discuss our findings in terms of a physiologically plausible model.

Depth Perception↗

PubMed2004

Internal surface representations approximated by reverse correlation.

We presented two nai ve observers with 20,000 random-dot stereograms. On each trial, the observers had to indicate the presence or absence of a complex 3D pattern (a large '+' sign in relief). However, unbeknownst to them, the stereograms did not contain any signal, but only disparity noise. Responses and verbal reports indicate that the observers 'saw' the suggested 3D surface configuration in roughly half the trials even though structured local low-level signal was never presented. Using reverse correlation, we derived an approximation of the internal surface-based representations, or templates, that best accounted for the observers' responses. These templates were shown to be spatially well defined and temporally stable. We propose that the 3D surface-based representations that we derived are the first approximations and depictions of the intermediary process that allows the visual system to successfully link degraded, bottom-up signal and high-level, top-down object recognition.

Adult↗

PubMed2004

The influence of object size and surface shape on shape constancy from stereo.

The 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.

Depth Perception↗

PubMed2004

Impossible shadows and the shadow correspondence problem.

Shadows cast by objects contain potentially useful information about the location of these objects in the scene as well as their surface reflectance. However, before the visual system can use this information, it has to solve the shadow correspondence problem, that is to match the objects with their respective shadows. In the first experiment, it is shown that the estimate of the light source position is affected by a gradual luminance ramp added to the image. In the second experiment, it is shown that observers process impossible shadow images as if they ignored the local features of the objects. All together, the results suggest that the visual system solves the shadow correspondence problem by relying on a coarse representation of the scene.

Humans↗

PubMed2004

Object perception as Bayesian inference.

We perceive the shapes and material properties of objects quickly and reliably despite the complexity and objective ambiguities of natural images. Typical images are highly complex because they consist of many objects embedded in background clutter. Moreover, the image features of an object are extremely variable and ambiguous owing to the effects of projection, occlusion, background clutter, and illumination. The very success of everyday vision implies neural mechanisms, yet to be understood, that discount irrelevant information and organize ambiguous or noisy local image features into objects and surfaces. Recent work in Bayesian theories of visual perception has shown how complexity may be managed and ambiguity resolved through the task-dependent, probabilistic integration of prior object knowledge with image features.

Bayes Theorem↗

PubMed2003

Neural correlates of shape from shading.

Psychophysical studies have shown that human observers resolve shape-from-shading ambiguities by assuming that light is coming from above-left. Using event-related potentials (ERPs), we measured the processing time of the perception of an ambiguous shaded pattern. We found that the N2 component followed the change of perceived shape with stimulus orientation. We also found that the P1 component in occipital and temporal areas was correlated with the observers' idiosyncratic bias for light source position. The precocity of the correlated ERP components suggests that the light source is represented early in the visual system. Altogether, our results indicate that shape-from-shading is a mostly bottom-up mechanism.

Adult↗

PubMed2003

Spatial and temporal tuning of motion in depth.

We 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 difference.

Computer Simulation↗

PubMed2003

The efficiency of speed discrimination for coherent and transparent motion.

Transparent motion involves the integration and segmentation of local motion signals. Previous research found a cost for processing transparent random dot motions relative to single coherent motions. However, this cost can be the result of the increased complexity of the transparent stimuli. We investigated this possibility by measuring the efficiency of transparent and coherent motions. Since efficiency normalises human performance to that of an ideal observer in the same task, performance can be compared fairly across tasks. Our task, identical in both transparent and coherent conditions, was to discriminate the fastest speed between two opposite motion directions. In two experiments where we varied dot density and speed, we confirmed the cost in human sensitivity for transparent motion but also found a cost for the ideal observer. The outcome was a consistent residual cost in efficiency for transparent motion. This result points to a processing limitation for transparent motion analogous to previously suggested inhibitory mechanisms between opposite directions of motion. Furthermore, we found that both transparent and coherent motion efficiencies decreased as dot density increased. This latter result stresses the importance of the correspondence problem and suggests that local motion signals are integrated over large areas.

Computer Simulation↗

PubMed2003

Bayesian modeling of cue interaction: bistability in stereoscopic slant perception.

Our 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.

Bayes Theorem↗

PubMed2002

Common mechanisms for 2D tilt and 3D slant after-effects.

By 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.

Adaptation, Ocular↗

PubMed2002

Amodal completion and the perception of depth without binocular correspondence.

Half-occlusions and illusory contours have recently been used to show that depth can be perceived in the absence of binocular correspondence and that there is more to stereopsis than solving the correspondence problem. In the present study we show a new way for depth to be assigned in the absence of binocular correspondence, namely amodal completion. Although an occluder removed all possibility of direct binocular matching, subjects consistently assigned the correct depth (convexity or concavity) to partially occluded 'folded cards' stimuli. Our results highlight the importance of more global, surface-based processes in stereopsis.

Adult↗