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PubMed1986

The Saturn illusion: a new stereokinetic effect.

When a 2-D pattern composed of a solid ellipse with two symmetrical semi-rings (corresponding to the visible parts of a contour ellipse whose major axis is perpendicular to that of the solid ellipse) is slowly rotated about an axis coincident with the line of sight, a compelling 3-dimensional impression occurs. Subjects report seeing an egg-shaped object which is inserted into a circular ring: the two objects move solidly into 3-D space and a moving visual phantom is generated so that the ring appears completed by an illusory curved segment in the region nearer to the observer during rotation. A sequence of nonrigid and rigid percepts (both 2-D and 3-D) precedes this Saturn-like configuration.

Depth Perception↗
PubMed1987

Stereoscopic contours and optokinetic nystagmus in normal and stereoblind subjects.

Moving stereoscopic contours in a dynamic random-dot stereogram have been previously shown to induce optokinetic nystagmus in subjects with normal stereopsis. For this to be validated as an objective test of stereopsis, stereoblind subjects must also be shown not to develop OKN, especially since it has been shown that the optomotor system of stereoblind individuals retains sensitivity to some cyclopean stimuli. In this report we verify that stereoblind subjects do not have an optomotor response to stereoscopic contours--regardless of the alignment angle at which the stereo image pair is presented.

Depth Perception↗
PubMed1995

Disambiguating velocity estimates across image space.

A translating homogeneous edge viewed through an aperture is an ambiguous stimulus, while a translating edge discontinuity is unambiguous. Under what conditions does the visual system use unambiguous velocity estimates to interpret ambiguous velocity estimates? We considered a translating rectangle visible through a set of stationary apertures. One aperture displayed a rectangle edge while the other apertures displayed corners. Observers reported the direction in which the edge appeared to translate. The results suggest that collinearity and terminator proximity determine whether the unambiguous corner velocity was used to interpret the ambiguous edge velocity. These results suggest some of the ways in which the visual system controls the integration of velocity estimates across image space.

Cues↗
PubMed1982

Posterior inferotemporal neuron activities during a visual fixation task and a visual tracking task.

Single neuron activities were recorded from the posterior inferotemporal cortex (PIT) while a monkey was fixating or tracking a target spot. Of 168 recorded neurons, 86 were activated by the extrafoveal slit stimuli; 48 were activated during eye fixation. Twenty were activated only during tracking, and not by fixation or a extrafoveal slit stimuli. It was suggested the PIT participates not only in the perception mechanisms but also in the foveation mechanisms such as fixation or tracking.

Animals↗
PubMed1989

Visual responses of sheep temporal cortex cells to moving and stationary human images.

Single-cell extracellular recordings were made from temporal cortical neurones in the conscious sheep. The visual responses of these cells to stationary or moving images of humans were investigated. Results from 6 animals showed that a small population of cells responded preferentially to the sight of humans as opposed to other objects or food. These cells did not respond to visual images of the human face, or to individual body parts (legs or arms) or to the smell of a human. The majority of cells showed direction selectivity, with the most effective stimulus being a human moving towards the animal. Cells did not respond differentially to the front and back view of a human although the side view was less effective. The posture adopted by the human was important, since responses were diminished or absent if the human adopted a quadrupedal as opposed to the normal bipedal posture. These results provide evidence for integrated neural processing of both visual recognition, movement and posture in the sheep temporal cortex.

Animals↗
PubMed2006

A new response-time measure of object persistence in the tunnel effect.

The recognition of information about an object is facilitated by a preview of the information concerning that object. This facilitation is regarded as evidence for the representational persistence of the object. It is not known, however, if such facilitation is obtained even under the tunnel effect, in which a moving object is temporarily occluded. This facilitation may be a new way to measure the representational persistence of a moving object in the tunnel effect. We addressed this question by a "same-different" judgment task of a target symbol (" composite function" or "+"), drawn within the moving object, before and after encountering the occluder. Response times (RTs) were shorter when the object reappeared with spatial continuity at the proper place than it reappeared at the improper place, as in Experiments 1 and 3. Thus, facilitation was obtained even in the tunnel effect. When the occluder was invisible and deletion/accretion cues along the contour of the occluder were either removed (Experiment 2) or given improperly (Experiment 4), no facilitation was found. These results clearly indicate that the facilitated recognition was caused by amodal integration of the persisting representation from the unoccluded and modal phases. The present study demonstrates that the facilitated recognition (RT measurement) can be used to investigate the representational persistence in the tunnel effect.

Depth Perception↗
PubMed2006

Amodal completion and visual holes (static and moving).

Occlusion is a frequent occurrence in a cluttered world of opaque objects. Often information about the shape of partly occluded objects can be gathered from the visible portion of the object and in particular its contours. Here we address the case where a region of a surface is visible exclusively through an aperture (visual hole). We make several observations about the grouping of surface regions visible through holes, and the appearance of moving objects and holes. These observations support the view that holes are shape properties of the object-with-hole.

Form Perception↗
PubMed2005

Depth perception by the active observer.

The connection between perception and action has classically been studied in one direction only: the effect of perception on subsequent action. Although our actions can modify our perceptions externally, by modifying the world or our view of it, it has recently become clear that even without this external feedback the preparation and execution of a variety of motor actions can have an effect on three-dimensional perceptual processes. Here, we review the ways in which an observer's motor actions--locomotion, head and eye movements, and object manipulation--affect his or her perception and representation of three-dimensional objects and space. Allowing observers to act can drastically change the way they perceive the third dimension, as well as how scientists view depth perception.

Depth Perception↗
PubMed2005

Systematic distortions of perceptual stability investigated using immersive virtual reality.

Using an immersive virtual reality system, we measured the ability of observers to detect the rotation of an object when its movement was yoked to the observer's own translation. Most subjects had a large bias such that a static object appeared to rotate away from them as they moved. Thresholds for detecting target rotation were similar to those for an equivalent speed discrimination task carried out by static observers, suggesting that visual discrimination is the predominant limiting factor in detecting target rotation. Adding a stable visual reference frame almost eliminated the bias. Varying the viewing distance of the target had little effect, consistent with observers underestimating distance walked. However, accuracy of walking to a briefly presented visual target was high and not consistent with an underestimation of distance walked. We discuss implications for theories of a task-independent representation of visual space.

Computer Simulation↗
PubMed2005

Quantitative perceived depth from sequential monocular decamouflage.

We present a novel binocular stimulus without conventional disparity cues whose presence and depth are revealed by sequential monocular stimulation (delay > or = 80 ms). Vertical white lines were occluded as they passed behind an otherwise camouflaged black rectangular target. The location (and instant) of the occlusion event, decamouflaging the target's edges, differed in the two eyes. Probe settings to match the depth of the black rectangular target showed a monotonic increase with simulated depth. Control tests discounted the possibility of subjects integrating retinal disparities over an extended temporal window or using temporal disparity. Sequential monocular decamouflage was found to be as precise and accurate as conventional simultaneous stereopsis with equivalent depths and exposure durations.

Cues↗
PubMed2005

An extension of the transparent-motion detection limit using speed-tuned global-motion systems.

When transparent motion is defined purely by direction differences, no more than two signal directions can be detected simultaneously. This limit appears to occur because higher signal intensities are required to detect transparent motion compared with uni-directional motion (Edwards, M., & Greenwood, J. A. (2005). The perception of motion transparency: A signal-to-noise limit. Vision Research, 45, 1877-1884). Increasing the effective signal intensities should therefore increase the number of signals that can be detected. We achieved this by adding speed differences, dividing transparent-motion signals between two speed-tuned global-motion systems. When some signals moved at appropriate low speeds and others at high speeds, up to three signals were detected. This is consistent, at least in part, with the signal-to-noise processing basis of the transparency limit. Differences in contrast polarity were also used to assess whether the limit could be extended using stimulus features without independent global-motion systems. A modest improvement in performance was obtained, suggesting that there may be multiple routes to extending the transparent-motion limit.

Computer Graphics↗
PubMed2006

Systematic perceptual distortion of 3D slant by disconjugate eye movements.

When an observer pursues an object moving away from him or her, both eyes rotate in the opposite direction, and this type of disconjugate eye movement can generate eye movement-induced disparities in the case of dynamic objects that are present around the pursuit object. Such disparities are not usually generated by conjugate eye movement. The aim of this study was to determine whether eye movement-induced disparities could be calibrated with eye position information. Observers were requested to judge the slant of an object defined by the spatiotemporal pattern of occlusion during disconjugate eye movement. Interestingly, the observers' perception of the slant of the target object was systematically distorted, although the perceptual distortion decreased somewhat in the presence of a salient reference around the target. This suggests that eye movement-induced disparities are not calibrated properly with eye position information.

Eye Movements↗
PubMed2006

Endogenous influences on perceptual bistability depend on exogenous stimulus characteristics.

We investigated the influence of changing physical parameters and task on bistable perception of an ambiguously rotating sphere (SFM). Increasing dot-density and velocity decreased the duration of perceptual phases during both passive viewing and voluntary control exertion. Our main finding is that voluntary control of perception depends on the physical parameters constituting the stimulus. This dependency places important constraints on the mechanisms mediating voluntary control as these mechanisms cannot operate independently of stimulus characteristics. In addition, local asymmetries in dot-densities can trigger alternations towards the most salient direction, which is not necessarily associated with largest number of dots: competition between perceptual interpretations during SFM appears to occur between surface-based representations rather than between individual elements. Finally, we show that voluntary control remains effective, even when attentive tracking of individual stimulus elements is no longer possible.

Attention↗
PubMed2006

Stereo channels with different temporal frequency tunings.

To investigate the spatial and temporal frequency tunings for stereopsis, we measured the contrast sensitivity for depth discrimination with variable spatiotemporal frequencies and disparities using drifting sinusoidal gratings. The results showed that the contrast sensitivity changed with the stimulus disparity and the disparity tuning function varied with the spatial frequency. The disparity in the peak sensitivity decreased proportionally with the spatial frequency (size-disparity correlation). Although the temporal frequency exhibited a limited influence on the peak disparity, the temporal frequency tuning varied with the spatial frequency. The shape of the temporal frequency tuning function was lowpass for higher spatial frequencies, whereas it was bandpass for low spatial frequencies. These results suggest that more than one channel with different temporal as well as spatial frequency tunings contribute to stereopsis.

Contrast Sensitivity↗
PubMed2000

Failure to detect changes in color for lines rotating in depth: the effects of grouping and type of color change.

A new technique for measuring change detection was introduced in which contours rotating in depth around a vertical axis (in a computer display) could be altered in color as they passed through their point of minimum extension (the median plane) where a thin static vertical occluder hid the change. Sets of five or six contours were either strongly grouped (similar in length, orientation and spacing) or weakly grouped (of variable length, orientation and spacing). Changes consisted of one line changing to a new color or else two lines swapping colors. The measure was the proportion of missed changes. When subjects were not instructed to look for change almost no changes were reported although subjects were told beforehand that they would have to describe the configuration after viewing it. When subjects were instructed to look for changes, it was found that detection of color change was significantly better for strongly grouped lines. It is proposed that grouping, by reducing redundancy, also reduces attentional demands with respect to the properties on which it is based, making it easier to attend to and therefore detect changes in other properties. We found that it was much easier to detect the introduction of a new color than to detect a swap between two existing colors. It is hypothesized that swap-type changes were harder to detect because they required attention to a conjunction of position and color.

Adult↗
PubMed2000

Stereopsis with persisting and dynamic textures.

We measured the percept of changing depth from changing disparity in stereograms composed of random-dot textures that were either persistent or dynamically changed on every frame (a dynamic random-dot stereogram). Disparity was changed between frames to depict a surface undergoing smooth temporal changes in simulated slant. Matched depth was greater with dynamic random-dot stereograms than with persistent random-dot stereograms. These results confirm and extend earlier observations at depth threshold. We posit an explanation based on cue conflict between stereopsis and monocular depth cues.

Contrast Sensitivity↗
PubMed2002

Binocular information about time to collision and time to passage.

It is well known that, when an object's horizontal relative disparity is changing appropriately, most observers report a compelling impression that the object is approaching and will collide with the observers at some future instant. Here I derive a new equation, namely TTC approximates (ddelta/dt)/(d(2)delta/dt(2)). This equation relates TTC to retinal image variables without involving a knowledge of the approaching object's distance or speed. In this respect the new equation is the binocular equivalent of the well-known equation for tau.

Automobile Driving↗
PubMed2002

Speed of response initiation in a time-to-contact discrimination task reflects the use of eta.

Avoiding collisions and making interceptions seem to require an organism to estimate the time that will elapse before an object will arrive to the point of observation (time-to-contact). The most outstanding account for precise timing has been the tau hypothesis. However, recent studies demonstrate that tau is not the only source of information in judging time-to-contact. By measuring reaction time in a time-to-contact discrimination task, we show that the eta function, which is a specific combination of optical size and rate of expansion, explains both accuracy and the observed RT pattern. The results conform to the hypothesis that the observers initiate the response when eta reaches a response threshold value.

Depth Perception↗