Biomedical subjects

Wendy J Adams

Publications and source records attributed to Wendy J Adams.

7 recordsLinked to original sources

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

Experience can change the 'light-from-above' prior.

To interpret complex and ambiguous input, the human visual system uses prior knowledge or assumptions about the world. We show that the 'light-from-above' prior, used to extract information about shape from shading is modified in response to active experience with the scene. The resultant adaptation is not specific to the learned scene but generalizes to a different task, demonstrating that priors are constantly adapted by interactive experience with the environment.

Adaptation, Physiological↗

PubMed2004

Prior depth information can bias motion perception.

Previous studies of the motion aperture problem have shown that the direction of grating motion can be biased by using binocular disparity to designate borders of the aperture as intrinsic (belonging to the grating) or extrinsic (resulting from occlusion of the grating). Observers report motion in the direction of the extrinsic border, as if the grating was extended and moving underneath an occluding surface. Here we investigate whether prior information about depth ordering, given by structure-from-motion, can bias the perceived motion direction of a subsequent moving grating in a similar manner. We presented an aperture stimulus that rotated about its vertical and horizontal axes, revealing the depth relationships (intrinsic and extrinsic) of the aperture borders. The grating then translated within the aperture and observers reported the direction of perceived motion. The test stimulus contained no information about the depth ordering of the scene. We found that observers' reported motion shifted toward the direction of the occluding edges, consistent with the intrinsic-extrinsic border predictions. These results indicate that prior scene information, not just depth information explicitly defined in the test stimulus, is used to help solve the motion aperture problem.

Bias↗

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↗

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

Modulating motion-induced blindness with depth ordering and surface completion.

Motion-induced blindness is a striking phenomenon in which salient static visual stimuli "disappear" for seconds at a time in the presence of specific moving patterns. Here we investigate whether the phenomenon is due to surface completion of the moving patterns. Stereo-depth information was added to the motion stimulus to create depth ordering between the static and moving components of the display. Depth ordering consistent with the perceptual occlusion of the static elements increased motion-induced blindness whereas placing the moving components behind the static elements decreased the static dot disappearance. In a second experiment we used an induced surface stimulus configuration to drive the motion-induced blindness phenomenon as further evidence of the importance of surface completion and interactions during visual processing.

Analysis of Variance↗