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PubMed1996

The time course of 2-D shape discrimination in random dot stereograms.

Predictions of a coarse-to-fine and co-operative stereo matching algorithm were compared using a 2-D shape discrimination task for disparity-defined targets in 50% random dot stereograms. Uniform disparity targets, square wave modulated targets with a different mean disparity to the background, or uncorrelated dots could be seen at much briefer exposures (down to 33 msec) than square wave modulated targets with the same mean disparity as the background. In the latter case, performance at brief exposures was improved by using coarser disparity corrugations. The results are readily explained by a coarse-to-fine matching scheme such as that proposed by Marr & Poggio (1979) [Proceedings of the Royal Society of London (B), 204, 301-328] and suggest that the correspondence problem is not the limiting step in the perception of simple cyclopean forms.

Adult↗

PubMed1991

Pre-attentive detection of a target defined by stereoscopic slant.

Does the visual system represent stereoscopic depth purely as a map of local disparities, or does it explicitly represent local relationships of disparity, such as disparity gradients? Experiments are reported in which visual search for a target containing the same disparity range as other elements in the display, but differing in the relationship of the disparities (stereo slant), was used to determine whether the target showed 'pop-out' like a unitary feature, or the serial search characteristic of feature conjunctions. Each stereo pair of elements was selected randomly from a range of outline parallelograms leaning to the right or to the left, so that the target could not be identified using any monocular shape cue. Response times for detection of the target (present on 50% of the trials) were independent of the number of elements in the display. This result was confirmed by varying element size and spacing, and by using oblique crosses rather than parallelograms as stimuli. It is concluded that stereoscopically defined slant, or disparity gradient, can be processed and compared in parallel across the display, and acts in this respect as an explicit unitary visual property. This contrasts with findings in analogous experiments on movement, which show that targets defined by divergence or deformation of optic flow can only be identified by serial search.

Attention↗

PubMed2004

Combining cues while avoiding perceptual conflicts.

A common assumption in cue combination models is that small discrepancies between cues are due to the limited resolution of the individual cues. Whenever this assumption holds, information from the separate cues can best be combined to give a single, more accurate estimate of the property of interest. We examined whether information about the discrepancy itself is lost when this is done. In our experiments, subjects were required to combine cues to match certain properties while avoiding perceptual conflicts. In part 1, they combined expansion and change in disparity to estimate motion in depth; and in part 2, they combined perspective and binocular disparities to estimate slant. We compared the pattern in the way that subjects set the two cues with the patterns predicted by models of cue combination with and without a loss of information about the discrepancy. From this comparison we conclude that little information about the discrepancies between cues is lost when the cues are combined.

Cues↗

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↗

PubMed1995

The visual discrimination of relative surface orientation.

In a series of three experiments, observers' discrimination thresholds for relative surface orientation were measured under full cue conditions, in which surfaces were specified by multiple sources of optical information including shading, texture, motion, and binocular disparity. The results revealed that observers' sensitivity to relative surface orientation varies for different types of structural configuration. Weber fractions were approximately 8% for planar patches connected to form a dihedral angle. They increased to 11% for planar patches that were spatially separated, and jumped to over 26% for patches that were part of a smoothly curved surface.

Depth Perception↗

PubMed2004

Disruption of eye movements by ethanol intoxication affects perception of depth from motion parallax.

Motion parallax, the ability to recover depth from retinal motion generated by observer translation, is important for visual depth perception. Recent work indicates that the perception of depth from motion parallax relies on the slow eye movement system. It is well known that ethanol intoxication reduces the gain of this system, and this produces the horizontal gaze nystagmus that law enforcement's field sobriety test is intended to reveal. The current study demonstrates that because of its influence on the slow eye movement system, ethanol intoxication impairs the perception of depth from motion parallax. Thresholds in a motion parallax task were significantly increased by acute ethanol intoxication, whereas thresholds for an identical test relying on binocular disparity were unaffected. Perhaps a failure of motion parallax plays a role in alcohol-related driving accidents; because of the effects of alcohol on eye movements, intoxicated drivers may have inaccurate or inadequate information for judging the relative depth of obstacles from motion parallax.

Adult↗

PubMed2006

[Study on crossed and uncrossed disparity evoked potential of dynamic random dot stereogram].

OBJECTIVE: To analyze the disparity evoked potentials (DEP) of fine disparities, coarse disparities, crossed disparities and uncrossed disparities and provide parameters of impersonal examining stereopsis. METHODS: A software package for generating dynamic random dot stereogram (DRDS) was developed as a visual stimulus to elicit DEP in 30 normal subjects. The DEP of every subject was recorded in different crossed and uncrossed disparity stimuli (4', 8', 15', 23', 30', 45', 53', 60', 72', 87', 102', 124', 150' of arc). RESULTS: (1) the constant negative-positive complex wave was observed in different disparity stimuli. (2) The N wave's latency of -45' was the longest, the N wave's latency of -150' was the shortest among crossed disparities. The N wave's latency of 4', 23' was the longest, the N wave's latency of 124' was the shortest among uncrossed disparities. (3) The amplitude peak of P wave occurred at -23', -60', -150' among crossed disparities. The amplitude peak of P wave occurred at 15', 45', 72' among uncrossed disparities. (4) All characteristic crossed disparities were smaller than the uncrossed disparities. The change orderliness of small crossed disparities was different from that of small uncrossed disparities. CONCLUSIONS: The N wave's latency and the P wave's amplitude can be used as parameters to impersonally examine stereopsis. These results suggest that stereopsis can be divided into fine crossed stereopsis, fine uncrossed stereopsis, coarse crossed stereopsis, and coarse uncrossed stereopsis.

Adolescent↗

PubMed1993

A model for neural representation of binocular disparity in striate cortex: distributed representation and veto mechanism.

A model in striate cortex is proposed for a distributed neural representation of binocular disparity with a simple cell. In the model, disparity is represented by "far", "near" and "tuned inhibitory" simple cells. However, the representation will be vetoed by model cells where disparity is excessively large. The veto mechanism consists of a neural network of the model cell which received output from simple cells and which interacts with neighbors. The mechanism is necessary, the model cell responds like a simple cell, and the network is physiologically plausible in the brain. Computer simulation on the neural network model with random dot stereography indicates reasonable performance.

Computer Simulation↗

PubMed1990

Suppression of sensitivity to change in target disparity during vergence eye movements.

It has been demonstrated recently in human psychophysical experiments that sensitivity to surround displacement is suppressed during convergence eye movements. To determine whether sensitivity to changes in target disparity is also reduced, responses to test disparities that were superimposed on standard 4 degrees step disparities were investigated. The test disparities consisted of brief (20 ms) positive and negative pulses as well as steps (in the range of +/- 0.6 degrees). A two-alternative forced-choice procedure was used in which the test disparity was presented in either the first or the second portion of a trial. The results showed that suppression of both test pulse and step disparities began before the start of the convergence movement and continued during the movement. Maximum suppression was about 0.50 to 0.85 log units and occurred between 150 ms before to 50 ms after convergence onset. The differences in sensitivity curves for pulse and step stimuli suggest the presence of different central and peripheral neural factors during vergence eye movements.

Eye Movements↗

PubMed2000

Inter-attribute tilt effects and orientation analysis in the visual brain.

A test grating appears to be tilted away from an inducing grating for small angular separations (repulsion), but towards the inducing grating for larger angular separations (attraction). Previous research on luminance gratings suggests that repulsion is caused by local inhibition in cortical areas V1 and/or V2, and that the attraction involves global interactions beyond V1, in extrastriate areas. Experiments reported here demonstrate attribute invariant attraction and repulsion effects for gratings specified by luminance, motion, and disparity contrasts. A frame surrounding the inducing grating abolishes only the attraction effect, but a spatial frequency difference, or a small gap between the inducer and test gratings, abolishes only the repulsion effect, irrespective of the attributes that specify the gratings. It is proposed that detectors selectively sensitive to attribute invariant orientation and size exist in early cortical sites such as V1 and/or V2.

Form Perception↗

PubMed1998

Disparity detection in anticorrelated stereograms.

Recent physiological observations in which stimuli with opposite contrast signs in the two eyes have been used (anticorrelated stereograms) show that these stimuli evoke responses in primary visual cortex which are the reverse of responses to correlated stimuli. Psychophysical investigations reveal no such reversals: reversed-contrast bars with crossed disparities are seen in front of those with uncrossed disparities. For anticorrelated random-dot stereograms human subjects perceive no depth at all, except at low dot densities. However, these human studies were carried out with stimuli that differed in several ways from those used in physiological studies. We therefore reexamined psychophysical responses using stimuli similar to those used for physiological recordings. Our results confirm the previous findings: there is no evidence of a reversed depth sensation for bar stereograms (crossed disparities are never seen behind uncrossed disparities), and subjects are unable to detect depth in anticorrelated random-dot stereograms at the densities used for the physiological recordings. The discrepancy between the psychophysical data and the responses of single neurons in primary visual cortex suggests that further processing outside area V1 is necessary to provide the signals that produce the sensation of stereoscopic depth.

Computer Graphics↗

PubMed1990

Vergence movements in comitant strabismus.

Vergence movements induced by base-out prisms were recorded, with an infrared eye tracker, for 6 patients with comitant esotropia (i.e., convergent strabismus) and anomalous retinal correspondence and for 6 normal subjects. Vergence movements of strabismic subjects were much slower and showed characteristics different from those of controls. It may be argued that vergence movements are induced by disparity and represent the motor fusion component left over in strabismus. In fact, in this study accommodative as well as proximal vergence had been ruled out. Therefore, the movements recorded can be considered the objective representation of the well-known phenomenon of prism compensation or adaptation, found in many strabismic patients.

Accommodation, Ocular↗

PubMed1996

Gain changes in the accommodative convergence cross-link.

The output of the accommodative vergence cross-link, the AC/A ratio, was measured before and after subjects viewed through a telestereoscope which increased their interocular separation (pd) approximately fourfold. The initial paradigm was designed to force an increase in the AC/A ratio in that subjects alternately fixated targets set at differing distances. In this paradigm, the varying vergence demands could not be met by a constant increase in phasic or tonic vergence responses. In a second paradigm, only one target was viewed at a fixed position from the eye. Now the constant vergence demand could be met by a set increase in phasic and/or tonic vergence. A significant increase in the AC/A ratio was found in the two-target study but not in the single-target study. Increases in the accommodative vergence gain did show considerable individual differences. Tonic vergence changes were interpolated from the data. This parameter increased significantly in both cases but more so when two targets were alternately fixated. This finding is consistent with current near triad models which predict that accommodative convergence input acts along with disparity vergence input to increase tonic levels of convergence. When AC/A ratios were calculated using clinical measures of the phoria at 6 and 0.4 m, increases were found in the two-target study and to a lesser degree in the single-target study. This clinical method of AC/A measure appears to have been confounded by changes in tonic vergence.

Accommodation, Ocular↗

PubMed2000

Representation of stereoscopic edges in monkey visual cortex.

Form perception in random-dot stereograms is based on information that resides in the correlation between the two images, but is not present in either image alone. We have studied the coding of stereoscopic figures in the neural activity of areas V1 and V2 of alert behaving monkeys. While cells in V1 generally responded according to the disparity of the surface at the receptive field, we found cells in area V2 that responded selectively to the figure edges. These cells signaled the location and orientation of contrast borders as well as stereoscopic edges, and were often selective for the direction of the step in depth. We concluded that stereoscopic edges are explicitly represented in area V2.

Animals↗

PubMed1992

Mental rotation, physical rotation, and surface media.

Subjects made mirror-normal discriminations on alphanumeric characters shown in different orientations in the picture plane. Either the characters or the background rotated during stimulus presentation in Experiments 1-3. Character rotation in the direction of mental rotation facilitated mental rotation, whereas rotation in the opposite direction inhibited it. In Experiment 4, characters were presented in different surface media so as to stimulate only one representation at a time. Mental rotation performance was similar whether the stimuli were defined by luminance, color, texture, relative motion, or binocular disparity, suggesting that mental rotation occurs at a level beyond that of the independent analyses of these different media. These results support those of Experiments 1-3 in excluding the participation of low-level motion analysis centers in the mental rotation processes.

Adult↗

PubMed1999

The characteristics of near prism induced fixation disparity curve in Hong Kong Chinese.

The characteristics of prism induced fixation disparity curve at near were studied in 56 asymptomatic young adult Hong Kong Chinese (average age 21 years). The percentage distribution of types of fixation disparity curves was found to be similar to some previous studies, with a higher prevalence of Type I curve (64.3%), followed by Type II (28.6%) and Type III (7.1%) curves. No Type IV curve was revealed from the sample. Two modified measurement approaches were applied to study the effect of measurement techniques on the parameters of fixation disparity curve obtained. A thorough description of the techniques was given. Although a significant difference in the x- and y-intercepts was obtained between different methods, the curve type and slope were not affected. The average y-intercepts (fixation disparity) were 2.72 min of arc and 3.54 min of arc from two methods respectively. Forty-one subjects had x-intercept (associated phoria) found from the curve. The average values were 2.42 delta base-out and 3.54 delta base-out from two methods respectively.

Adult↗

PubMed2001

The stereoscopic (cyclopean) motion aftereffect is dependent upon the temporal frequency of adapting motion.

This study investigated whether the stereoscopic (cyclopean) motion aftereffect (induced by adaptation to moving binocular disparity information) is dependent upon the temporal frequency or speed of adapting motion. The stereoscopic stimuli were gratings created from disparity embedded in a dynamic random-dot stereogram. Across different combinations of stereoscopic spatial frequency, temporal frequency and speed of adapting motion, the duration of the aftereffect was dependent upon temporal frequency (maximal aftereffect=1-2 cyc s(-1)). These results support the idea that stereoscopic motion is processed by a cortical mechanism that computes cyclopean motion energy.

Adaptation, Physiological↗

PubMed2001

Visual summation of luminance lines and illusory contours induced by pictorial, motion, and disparity cues.

Illusory contours where no contrast exists in the image can be seen between pairs of spatially separate but aligned inducing real contours defined either by pictorial cues (luminance contrasts or offset gratings), kinetic contrast, or binocular disparity contrast. In previous studies it has been shown that the detection of a thin luminous line is facilitated when the line is superimposed on illusory contours and the inducing flanking elements are defined by luminance contrast. By using a spatial forced-choice technique I show that luminous lines summate with illusory contours induced by luminance contrast, offset gratings, motion contrast, and disparity contrast when the line is superimposed on the illusory contour. Control experiments show that the positional cues, offered by the inducing contours, are unable to account for these results. It is suggested that real luminous lines or edges and illusory contours activate common neural mechanisms in the brain irrespectively of the stimulus attributes that induce the illusory contour.

Contrast Sensitivity↗