Behavioral evidence for visual perception of 3-dimensional surface structures in monkeys.
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Human subjects perceive two crossing bars, one in front of the other, when shown a cross with disparity added to its horizontal limbs, and they also perceive neon-color spreading when shown a stereoscopic Redies-Spillmann figure. It has thus been hypothesized that the human visual system follows the principle of generic image sampling in reconstructing 3-dimensional (3-D) surface structures. Here we examine whether monkeys also perceive these surface structures. The results indicate that monkeys, like humans, perceive two crossing bars and neon-color spreading and suggest that the principle of generic image sampling may also be applied to visual perception in monkeys. (+info)
The interaction of first- and second-order cues to orientation.
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The visual system is sensitive to orientation information defined both by first-order (luminance) and by second-order (texture) cues. We consider how these orientation cues are computed and how they affect one another. We measured the perceived orientation of the first and second-order components of Gabor patches (the carrier and envelope, respectively) and report a dependence of the perceived orientation of each on the orientation of the other, and on the spatial frequency of the carrier. Fixing the carrier orientation near that of the envelope interferes with envelope orientation judgements. This interference is reduced by adding a small (subthreshold) rotation to the carrier indicating that the site of interference is early. When the gross relative orientation of carrier and envelope is varied, the carrier appears systematically tilted towards the envelope. However, provided envelope and carrier are separated by more than approximately 10 degrees, the perceived envelope orientation appears tilted away from the carrier. The size of these effects increases with decreasing carrier spatial frequency, and with increasing exposure duration. When the envelope and carrier are both non parallel and non-perpendicular Fourier energy is distributed asymmetrically across orientation. We demonstrate that, for a channel-based orientation code, this asymmetry induces a shift in mean orientation that is sufficient to explain illusory tilting of carriers. The illusory tilting of the envelope, as a function of carrier orientation and spatial frequency, demonstrates that human ability to demodulate contrast information is far from ideal and cannot be explained by existing two-stage filter-rectify-filter models. We propose that illusory tilting of the envelope is due to selective connectivity between first- and second-stage filters whose purpose is to dissociate the type of image structure producing each class of cue. (+info)
Path perception and Filehne illusion compared: model and data.
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Pursuit eye movements introduce retinal motion that complicates the recovery of self-motion from retinal flow. An extra-retinal, eye-velocity signal could be used to aid estimation of the observer's path, perhaps by converting retino-centric into head-centric motion. This conversion is apparently not precise because we often misperceive head-centric object velocity: in the Filehne illusion, for example, a stationary object appears to move in the opposite direction to the eye movement. Similar errors should be expected when extra-retinal, eye-velocity signals are used in self-motion tasks. However, most self-motion studies conclude that path direction is recovered quite accurately. Path perception and the Filehne illusion were therefore compared directly in order to examine the apparent discrepancy. A nulling technique determined the velocity of simulated eye rotation that cancelled the perceived curvature of the path or, in a Filehne condition, the perceived rotation of the ground-plane stimulus. In either case, observers typically set the simulated eye rotation to be a fixed proportion of the actual eye pursuit made. No differences were found between path perception and Filehne illusion. The apparent inaccuracy of path perception during a real eye movement was confirmed in a second experiment, using a standard 'mouse-pointing' technique. The experiments provide support for a model of head-centric motion perception based on extra-retinal and retinal signals that are linearly related to pursuit and retinal speed, respectively. (+info)
Temporal recruitment along the trajectory of moving objects and the perception of position.
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The trajectory of a moving object provides information about its velocity, direction and position. This information can be used to enhance the visual system's ability to detect changes in these parameters. We show that the visibility of the trajectory of a moving object influences the perception of its position. This form of temporal recruitment builds up on a long timescale of approximately 500 ms. Temporary occlusion of the trajectory during this time period reduces recruitment, but does not abolish it. Moreover, we found no spatial restrictions on recruitment on the scale of 10 degrees of arc. When the position of objects on trajectories with different degrees of visibility are compared, this recruitment effect causes spatial offsets. This leads to a visual illusion in which the position of moving objects is misperceived. (+info)
Two processes in stereoscopic apparent motion.
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This study investigated the human ability to discriminate the motion direction of sequentially presented depth patterns produced by random-dot stereograms. The stereoscopic (cyclopean) patterns used here consisted of 256 rectangle patches, each of which had an alternative depth position (near or far). Two successive frames of correlated depth patterns made impressions of lateral motion when the pattern position in the second frame shifted laterally. The density of the patches that were near was varied. The Dmax that was measured using the 2AFC method was short when the density was high. The effect of depth reversing in the second frame was also tested. Under low density conditions, the performance was still good against reversing 3-D polarity. However, when the density was high, with depth reversal, motion in the reversed direction was perceived. Reversed motion was observed more often when SOA was small and when the density of near patches was near 1/2. Two strategies seem to exist in stereoscopic motion detecting: a polarity-independent process which matches figures, ignoring their depth polarity, and a polarity-dependent process which operates locally, ignoring 2-D shapes. The latter suggests the existence of a passive process in stereoscopic motion. (+info)
An empirical explanation of the cornsweet effect.
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A long-standing puzzle in vision is the assignment of illusory brightness values to visual territories based on the characteristics of their edges (the Craik-O'Brien-Cornsweet effect). Here we show that the perception of the equiluminant territories flanking the Cornsweet edge varies according to whether these regions are more likely to be similarly illuminated surfaces having the same material properties or unequally illuminated surfaces with different properties. Thus, if the likelihood is increased that these territories are surfaces with similar reflectance properties under the same illuminant, the Craik-O'Brien-Cornsweet effect is diminished; conversely, if the likelihood is increased that the adjoining territories are differently reflective surfaces receiving different amounts of illumination, the effect is enhanced. These findings indicate that the Craik-O'Brien-Cornsweet effect is determined by the relative probabilities of the possible sources of the luminance profiles in the stimulus. (+info)
The representation of illusory and real contours in human cortical visual areas revealed by functional magnetic resonance imaging.
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Illusory contours (perceived edges that exist in the absence of local stimulus borders) demonstrate that perception is an active process, creating features not present in the light patterns striking the retina. Illusory contours are thought to be processed using mechanisms that partially overlap with those of "real" contours, but questions about the neural substrate of these percepts remain. Here, we employed functional magnetic resonance imaging to obtain physiological signals from human visual cortex while subjects viewed different types of contours, both real and illusory. We sampled these signals independently from nine visual areas, each defined by retinotopic or other independent criteria. Using both within- and across-subject analysis, we found evidence for overlapping sites of processing; most areas responded to most types of contours. However, there were distinctive differences in the strength of activity across areas and contour types. Two types of illusory contours differed in the strength of activation of the retinotopic areas, but both types activated crudely retinotopic visual areas, including V3A, V4v, V7, and V8, bilaterally. The extent of activation was largely invariant across a range of stimulus sizes that produce illusory contours perceptually, but it was related to the spatial frequency of displaced-grating stimuli. Finally, there was a striking similarity in the pattern of results for the illusory contour-defined shape and a similar shape defined by stereoscopic depth. These and other results suggest a role in surface perception for this lateral occipital region that includes V3A, V4v, V7, and V8. (+info)
Optical images of visible and invisible percepts in the primary visual cortex of primates.
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We optically imaged a visual masking illusion in primary visual cortex (area V-1) of rhesus monkeys to ask whether activity in the early visual system more closely reflects the physical stimulus or the generated percept. Visual illusions can be a powerful way to address this question because they have the benefit of dissociating the stimulus from perception. We used an illusion in which a flickering target (a bar oriented in visual space) is rendered invisible by two counter-phase flickering bars, called masks, which flank and abut the target. The target and masks, when shown separately, each generated correlated activity on the surface of the cortex. During the illusory condition, however, optical signals generated in the cortex by the target disappeared although the image of the masks persisted. The optical image thus was correlated with perception but not with the physical stimulus. (+info)