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PubMed1995

Distortions of three-dimensional space in the perceptual analysis of motion and stereo.

A fundamental issue in the study of human vision is the accuracy with which observers can perceive the three-dimensional structures of objects in the environment. The formal mapping from physical to perceived space is considered as a geometric transformation, and the literature is reviewed to identify which aspects of object structure are systematically distorted by this transformation and which ones remain invariant. In the perceptual analysis of several different sources of optical information, including motion and stereo, both individually and in combination, there is a consistent pattern of results to indicate that perceived depth intervals are scaled differently from comparable intervals in either horizontal or vertical directions. These and other findings provide strong evidence that the relationship between physical and perceived space is noneuclidean, and that the three-dimensional structures of objects can appear systematically distorted even when viewed under full cue conditions.

Cues↗
PubMed1995

Integration of stereo, texture, and outline cues during pinhole viewing of real ridge-shaped objects and stereograms of ridges.

Three experiments are reported in which the possible role of blur cues as a factor needing to be taken into account in cue-integration studies involving conflicts between stereo and texture/outline cues was investigated. The earlier suggestion was tested that uncontrolled blur cues might have caused the quite different patterns of cue integration reported for real ridge-shaped objects oriented vertically and for stereograms depicting similar surfaces. Blur cues were manipulated by pinhole viewing intended to render accommodation open loop. The results for real ridges were as predicted by the blur-cue hypothesis: pinhole viewing strengthened texture/outline cues in vertically oriented ridges, thereby diminishing the pattern of stereo dominance hitherto observed for these stimuli (and as observed here in non-pinhole-viewing control conditions and in horizontally oriented ridges). The results for the stereograms did not conform to predictions: pinhole viewing, assumed to remove blur cues from the cue-integration process, still produced the pattern observed in control conditions in which a texture/outline cue for a shallow ridge overwhelmed stereo cues for a steep ridge. This result is against the hypothesis that perhaps blur cues for the stereogram projection surface differentially favoured the shallow texture/outline cues. A new variant of the blur-cue hypothesis is offered to account for this result. The main conclusion from the study is: beware drawing firm conclusions from stereograms about the pattern of cue integration that can be expected when real objects are being viewed. The two situations can produce very different results as far as cue integration is concerned. This is a conclusion with serious implications for the use of stereograms for studying the integration of stereo with other cues.

Adolescent↗
PubMed1995

Integration by association: combining three-dimensional cues to extrinsic surface shape.

Surface shape is visually derived from multiple sources of three-dimensional (3-D) information. The apparent unity of the percept creates an assumption that there are perceptual processes that resolve and combine the different channels of information within some common 3-D representation. However, integration by the explicit conversion of the information from different sources into a common representation is computationally difficult and unnecessary. An alternative, integration by association, is discussed and related to a variety of experimental results on 3-D-cue conflict.

Association↗
PubMed1995

Amodal completion and vernier acuity: evidence of 'top-but-not-very-far-down' processes?

Vernier thresholds were measured by means of a forced-choice paradigm for physically separated bars that were perceived either as separate or as amodally completed behind an occluder. The finding that thresholds were not reduced (improved) with amodal completion argues against an early neural code for amodal completion that is equivalent to physical completion of the vernier bars (ie neural propagation/interpolation) and limits the stage at which amodal representations could arise via bottom-up processes to neural sites located centrally with respect to those involved in position coding. An alternative explanation is also considered, that this result reflects the activity of top-down processes that mediate amodal completion by imposing grouping/organizational constraints upon earlier representations of position. In support of this proposition it was found that when tested by means of a non-forced-choice paradigm without feedback, observers demonstrated a reduced tendency to report the presence of a vernier offset when the bars were perceived as amodally completed (ie a shift in response bias occurred). This separability of vernier acuity and response bias suggests the existence of 'top-but-not-very-far-down' grouping processes whose influence does not extend to early representations of relative position.

Analysis of Variance↗
PubMed1996

Visual control of steering without course information.

An analysis and experimental evidence are presented that the visual control of steering in following a straight demarcated path is possible even when information about the direction of one's travel (course) is not available. Two likely sources of optical information available to observers are considered, bearing and splay, that might be used for steering under these conditions. In an experiment involving a computer-driven driving simulator, observers attempted to steer a straight path while subjected to lateral perturbing forces. When only bearing and its time derivative, motion parallax, were available, performance fell off as expected with the optical gain of motion parallax as the preview distance of the viewing aperture was varied. When splay angle and its time derivative, splay rate, were added to the display, performance generally improved and remained relatively constant with changing distance of the viewing aperture, as expected because of the constant optical gain of splay rate. Making course information available by adding point features to both displays improved steering performance only in the motion-parallax conditions.

Automobile Driving↗
PubMed2001

The absence of depth constancy in contour stereograms.

Stereoscopic surfaces constructed from Kanizsa-type illusory contours or explicit luminance contours were tested for three-dimensional (3-D) shape constancy. The curvature of the contours and the apparent viewing distance between the surface and the observer were manipulated. Observers judged which of two surfaces appeared more curved. Experiment 1 allowed eye movements and revealed a bias in 3-D shape judgment with changes in apparent viewing distance, such that surfaces presented far from the observer appeared less curved than surfaces presented close to the observer. The lack of depth constancy was approximately the same for illusory-contour surfaces and for explicit-contour surfaces. Experiment 2 showed that depth constancy for explicit-contour surfaces improved slightly when fixation was required and eye movements were restricted. These experiments suggest that curvature in depth is misperceived, and that illusory-contour surfaces are particularly sensitive to this distortion.

Adult↗
PubMed2000

Mapping the zone of eye-height utility for seated and standing observers.

In a series of experiments, we delimited a region within the vertical axis of space in which eye height (EH) information is used maximally to scale object heights, referred to as the "zone of eye height utility" (Wraga, 1999b Journal of Experimental Psychology, Human Perception and Performance 25 518-530). To test the lower limit of the zone, linear perspective (on the floor) was varied via introduction of a false perspective (FP) gradient while all sources of EH information except linear perspective were held constant. For seated (experiment 1a) observers, the FP gradient produced overestimations of height for rectangular objects up to 0.15 EH tall. This value was taken to be just outside the lower limit of the zone. This finding was replicated in a virtual environment, for both seated (experiment 1b) and standing (experiment 2) observers. For the upper limit of the zone, EH information itself was manipulated by lowering observers' center of projection in a virtual scene. Lowering the effective EH of standing (experiment 3) and seated (experiment 4) observers produced corresponding overestimations of height for objects up to about 2.5 EH. This zone of approximately 0.20-2.5 EH suggests that the human visual system weights size information differentially, depending on its efficacy.

Adult↗
PubMed2000

Large systematic deviations in visual parallelism.

The visual environment is distorted with respect to the physical environment. Luneburg [1947, Mathematical Analysis of Binocular Vision (Princeton, NJ: Princeton University Press)] assumed that visual space could be described by a Riemannian space of constant curvature. Such a space is described by a metric which defines the distance between any two points. It is uncertain, however, whether such a metric description is valid. Two experiments are reported in which subjects were asked to set two bars parallel to each other in a horizontal plane. The backdrop consisted of wrinkled black plastic sheeting, and the floor and ceiling were hidden by means of a horizontal aperture restricting the visual field of the subject vertically to 10 deg. We found that large deviations (of up to 40 degrees) occur and that the deviations are proportional to the separation angle: on average, the proportion is 30%. These deviations occur for 30 degrees, 60 degrees, 120 degrees, and 150 degrees reference orientations, but not for 0 degree and 90 degrees reference orientations; there the deviation is approximately 0 degree for most subjects. A Riemannian space of constant curvature, therefore, cannot be an adequate description. If it were, then the deviation between the orientation of the test and the reference bar would be independent of the reference orientation. Furthermore, we found that the results are independent of the distance of the bars from the subject, which suggests either that visual space has a zero mean curvature, or that the parallelity task is essentially a monocular task. The fact that the deviations vanish for a 0 degree and 90 degrees orientation is reminiscent of the oblique effect reported in the literature. However, the 'oblique effect' reported here takes place in a horizontal plane at eye height, not in a frontoparallel plane.

Adult↗
PubMed2000

Curvature biases in stereoscopic vision: a nasotemporal asymmetry.

The reliability of curvature judgments for linear elements was studied, with stereograms that contained a binocular arc with curvature in depth, and either a binocular frontoparallel arc or a monocular one, on a background representing a hemiellipsoid. The subjects made about 15% errors on binocular arcs with curvature in depth, and 60%-80% of these occurred when both the hemiellipsoid and the arc were convex, the arc being perceived as concave, by transparency through the hemiellipsoid. There were also about 15%-30% errors on frontoparallel arcs, but spread among all situations, with a small prevalence of concave judgments. Curvature in depth was assigned to the monocular stimuli in more than 60% of the cases. There was a curvature bias when the monocular arcs were on the nasal side, and were viewed against a concave background. Assuming parallel viewing, nasal ingoing arcs were usually perceived as concave, and nasal outgoing arcs usually perceived as convex, in agreement with geometrical likelihood. Nasal-side elements captured by one eye are, in general, those with the highest likelihood of having matching elements in the other eye. Then the observed nasal bias effect suggests that the matching process in stereopsis could be driven from the nasal sides of the projections in the two cerebral hemispheres.

Depth Perception↗
PubMed2002

How context influences predominance during binocular rivalry.

Variations in the predominance of an object engaged in binocular rivalry may arise from variations in the durations of dominance phases, suppression phases, or both. Earlier work has shown that the predominance of a binocular rival target is enhanced if that target fits well-via common color, orientation, or motion-with its surrounding objects. In the present experiments, the global context outside of the region of rivalry was changed during rivalry, to learn whether contextual information alters the ability to detect changes in a suppressed target itself. Results indicate that context will maintain the dominance of a rival target, but will not encourage a suppressed target to escape from suppression. Evidently, the fate of the suppressed stimulus is determined by neural events distinct from those responsible for global organization during dominance. To reconcile diverse findings concerning rivalry, it may be important to distinguish between processes responsible for selection of one eye's input for dominance from processes responsible for the implementation and maintenance of suppression.

Adult↗
PubMed2003

Distance perception for points at equiconvergence and equidistance loci.

Subjects viewed luminescent targets placed at an equidistance (ED) locus or on a comparable equiconvergence locus, the Vieth-Müller circle (VMC). In darkness, subjects moved the index finger to the apparent distance of the visible target. This was repeated at leftward eccentricities of 0 degrees (straight ahead), 15 degrees, 22.5 degrees, 30 degrees, 37.5 degrees, and 45 degrees. Contrary to theoretical expectations, apparent distance to points on the VMC did not describe an equidistance locus, but diminished significantly with increasing eccentricity, whereas ED settings increased slightly but significantly. Binocular and monocular groups performed equivalently. Veridicality of results appears to be the outcome of an accommodation-convergence conflict with a dominant role for accommodation.

Accommodation, Ocular↗
PubMed2005

Motion parallax driven by head movements: conditions for visual stability, perceived depth, and perceived concomitant motion.

Yoking the movement of the stimulus on the screen to the movement of the head, we examined visual stability and depth perception as a function of head-movement velocity and parallax. In experiment 1, for different head velocities, observers adjusted the parallax to find (a) the depth threshold and (b) the concomitant-motion threshold. Between these thresholds, depth was seen with no perceived motion. In experiment 2, for different head velocities, observers adjusted the parallax to produce the same perceived depth. A slower head movement required a greater parallax to produce the same perceived depth as faster head movements. In experiment 3, observers reported the perceived depth for different parallax magnitudes. Perceived depth covaried with smaller parallax without motion perception, but began to decrease with larger parallax and concomitant motion was seen. Only motion was seen with the larger parallax.

Adult↗
PubMed2005

Depth and motion in historical descriptions of motion parallax.

Motion parallax was described as a cue to depth over 300 years ago and as producing apparent motion over 150 years ago. In recent years, experimental interest in motion parallax has increased, following the rediscovery of the idea that stimulus motion can be yoked to head movement. We compare the historical descriptions with some contemporary research, which indicates how depth and motion perception are dependent on the conditions of stimulation.

Depth Perception↗
PubMed2005

Perceptual alternation induced by visual transients.

When our visual system is confronted with ambiguous stimuli, the perceptual interpretation spontaneously alternates between the competing incompatible interpretations. The timing of such perceptual alternations is highly stochastic and the underlying neural mechanisms are poorly understood. We show that perceptual alternations can be triggered by a transient stimulus presented nearby. The induction was tested for four types of bistable stimuli: structure-from-motion, binocular rivalry, Necker cube, and ambiguous apparent motion. While underlying mechanisms may vary among them, a transient flash induced time-locked perceptual alternations in all cases. The effect showed a dependence on the adaptation to the dominant percept prior to the presentation of a flash. These perceptual alternations show many similarities to perceptual disappearances induced by transient stimuli (Kanai and Kamitani, 2003 Journal of Cognitive Neuroscience 15 664-672; Moradi and Shimojo, 2004 Vision Research 44 449-460). Mechanisms linking these two transient-induced phenomena are discussed.

Adaptation, Psychological↗
PubMed1995

Visual field tunneling in aviators induced by memory demands.

Aviators are required rapidly and accurately to process enormous amounts of visual information located foveally and peripherally. The present study, expanding upon an earlier study (Williams, 1988), required young aviators to process within the framework of a single eye fixation a briefly displayed foveally presented memory load while simultaneously trying to identify common peripheral targets presented on the same display at locations up to 4.5 degrees of visual angle from the fixation point. This task, as well as a character classification task (Williams, 1985, 1988), has been shown to be very difficult for nonaviators: It results in a tendency toward tunnel vision. Limited preliminary measurements of peripheral accuracy suggested that aviators might be less susceptible than nonaviators to this visual tunneling. The present study demonstrated moderate susceptibility to cognitively induced tunneling in aviators when the foveal task was sufficiently difficult and reaction time was the principal dependent measure.

Adult↗
PubMed1999

Effects of consciousness on human brain waves following binocular rivalry.

When the two eyes of an observer are exposed to conflicting stimuli, they enter into binocular rivalry and the two possible percepts will alternate in dominance. We investigated neural activity and its time course following binocular rivalry by measuring human event-related brain potentials to transitions from rivalrous to non-rivalrous stimulation. When these changes did not entail a change in conscious perception they elicited a markedly attenuated N1 component and a delayed and attenuated P3 peak as compared to percept-incompatible changes and non-rivalrous control conditions. These results suggest that in humans binocular rivalry is resolved at latest in extrastriate visual areas.

Adult↗