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PubMed2003

Unconscious adaptation: a new illusion of depth induced by stimulus features without depth.

Here, we show a new illusion of depth induced by psychophysical adaptation to dynamic random-dot stereograms (RDS) that are interocularly anticorrelated (i.e., in which the images for the two eyes have reversed contrast polarity with each other). After prolonged viewing of anticorrelated RDS, the presentation of uncorrelated RDS (i.e., in which two images are mutually independent random-dot patterns) produces the sensation of depth, although both anticorrelated and uncorrelated RDSs are perceptually rivalrous with no consistent depth by themselves. Contrary to other aftereffects demonstrated in a number of visual dimensions, including motion, orientation, and disparity, this illusion results from unconscious adaptation; observers are not aware of what they are being adapted to during the process of adaptation. We further demonstrate that this illusion can be predicted from the simulated responses of disparity-selective neurons based on a local filtering model. Model simulations indicate that the inspection of anticorrelated RDS causes the adaptation of all disparity detectors except one sensitive to its disparity; therefore, those selectively unadapted detectors show relatively strong activation in response to the subsequent presentation of uncorrelated RDS and produce depth perception.

Adaptation, Psychological↗
PubMed1991

Combining binocular and monocular curvature features.

A study is reported of the perception of visual surfaces in wire-frame stimuli generated by combinations of monocular surface contours and binocular disparity that provide differing information about 3-D relief. Observers vary considerably in the relative contribution made by the binocular and monocular cues to the perception of overall 3-D form. Without training, many observers may entirely fail to perceive surface curvature from the binocular disparity patterns, interpreting the form of the surface only according to the monocular information. For other observers, both cues contribute to the end percept, with the monocular interpretation dominating where the disparity information indicates planarity and with disparity dominating where disparity information suggests curvature and the monocular interpretation suggests planarity. Where stereo and monocular interpretations indicate inconsistent surface curvature features at a common location, more complex resolution strategies are suggested.

Cues↗
PubMed1994

Binocular interactions in striate cortical neurons of cats reared with discordant visual inputs.

The postnatal development of cortical binocularity is known to be adversely affected by early abnormal visual experience. However, little information exists on how the signals from the two eyes are combined in individual cortical neurons of animals reared with early discordant binocular visual experience. Since this is a fundamental issue in understanding visual cortical development, we used extracellular single-unit recording methods to study binocular integration in striate cortical neurons of strabismic cats. Specifically, we measured the sensitivity of individual cells to the relative interocular spatial phase of dichoptically presented drifting sinusoidal gratings (i.e., to binocular retinal image disparity). Clear alterations in ocular dominance were observed in all strabismic subjects. Nevertheless, the majority of cortical neurons exhibited some form of binocular interactions when both eyes were stimulated together. The most prominent aspect of cortical physiology in the strabismic animals was the relatively high prevalence of suppressive binocular interactions. Suppression was most frequently found in kittens reared with 2 weeks of early optical dissociation and among adult cats that received 2 weeks of early optical dissociation and a prolonged recovery period. However, substantial excitatory binocular interactions were also maintained in these animals. With an extended period of interocular misalignment (3 or 8 months), all forms of binocular interactions, excitatory and suppressive, were drastically reduced and a greater number of neurons were truly monocular. Although the reduction in the strength of binocular interactions occurred in all units irrespective of their monocular spatial properties, the effect was more pronounced among those units tuned to higher spatial frequencies and this spatial-frequency-dependent effect was larger in the subjects receiving longer periods of binocular dissociation. The results suggest that the "breakdown" of cortical binocular properties in strabismic subjects is not an all-or-none process, and that suppressive binocular interactions may be closely associated with the abnormal binocular interactions exhibited by strabismic humans. Furthermore, our findings are consistent with the notion that cortical disparity-detecting mechanisms are spatial-frequency dependent and, thus, can be selectively altered depending on an animal's early visual experience.

Animals↗
PubMed1996

Receptive field asymmetries and sensitivity to random dot stereograms.

The differences between the two monocular receptive fields of cortical cells were measured and compared to their disparity tuning in the awake behaving monkey. Several receptive field properties (direction selectivity, orientation preference, eye preference and response modulation) were determined for each eye using sweeping bright bars. The disparity sensitivity of these cells was also assessed by plotting their response profile, determined for each cell under strictly binocular cyclopean stimulation (dynamic random dot stereograms, RDS). We have found that large differences between the two monocular receptive fields were infrequent and, apparently, not related with the disparity sensitivity profile. We conclude that the monocular asymmetries tested in this study and the sensitivity to positional binocular disparities present in RDS, might be linked to different mechanisms involved in depth perception in the visual system.

Animals↗
PubMed2004

Asymmetry between horizontal and vertical illusory lines in determining the depth of their embedded surface.

To investigate how the visual system integrates disparity information from horizontal and vertical edges and conveys it to the regions without any depth cues, we introduce a new phenomenon of subjective surface formation in an Ehrenstein-style configuration with inducing elements at different depths, and without explicit monocular occlusion zones. Different sets of experiments by separate groups of subjects suggest that when a subjective (illusory) square forms, it is at the depth of vertical illusory sides rather than horizontal ones. When the vertical side inducers are stereoscopically behind the horizontal ones, subjective surface formation is less likely. In depth assignment, we interpret the dominance of vertical sides over horizontal ones geometrically: vertical orientation can convey the horizontal disparity--a critical factor for Wheatstone (classic) stereopsis--but horizontal orientation per se lacks horizontal disparity information. Therefore, in the disparity integration, vertical illusory sides play a dominant role and their depth information influences the embedded subjective surface as well as the horizontal illusory sides.

Cues↗
PubMed1996

Short-latency disparity vergence responses and their dependence on a prior saccadic eye movement.

1. A dichoptic viewing arrangement was used to study the initial vergence eye movements elicited by brief horizontal disparity steps applied to large textured patterns in three rhesus monkeys. Disconjugate steps (range, 0.2-10.9 degrees) were applied to the patterns at selected times (range, 13-303 ms) after 10 degrees leftward saccades into the center of the pattern. The horizontal and vertical positions of both eyes were recorded with the electromagnetic search coil technique. 2. Without training or reinforcement, disparity steps of suitable amplitude consistently elicited vergence responses at short latencies. For example, with 1.8 degrees crossed-disparity steps applied 26 ms after the centering saccade, the mean latency of onset of convergence for each of the three monkeys was 52.2 +/- 3.8 (SD) ms, 52.3 +/- 5.2 ms, and 53.4 +/- 4.1 ms. 3. Experiments in which the disparity step was confined to only one eye indicated that each eye was not simply tracking the apparent motion that is saw. For example, when crossed-disparity steps were confined to the right eye (which saw leftward steps), the result was (binocular) convergence in which the left eye moved to the right even though that eye had seen only a stationary scene. This movement of the left eye cannot have resulted from independent monocular tracking and indicates that the vergences here derived from the binocular misalignment of the two retinal images. 4. The initial vergence responses to crossed-disparity steps had the following main features. 1) They were always in the correct (i.e., convergent) direction over the full range of stimuli tested, the initial vergence acceleration increasing progressively with increases in disparity until reaching a peak with steps of 1.4-2.4 degrees and declining thereafter to a nonzero asymptote as steps exceeded 5-7 degrees. 2) They showed transient postsaccadic enhancement whereby steps applied in the immediate wake of a saccadic eye movement resulted in much higher initial vergence accelerations than the same steps applied some time later. The response decline in the wake of a saccade was roughly exponential with time constants of 67 +/- 5 (SD) ms, 35 +/- 2 ms, and 54 +/- 4 ms for the three animals. 3) That the postsaccadic enhancement might have resulted in part from the visual stimulation associated with the prior saccade was suggested by the finding that enhancement could also be observed when the disparity steps were applied in the wake of (conjugate) saccadelike shifts of the textured pattern. However, this visual enhancement did not reach a peak unit 17-37 ms after the end of the "simulated" saccade, and the peak enhancement averaged only 45% of that after a "real" saccade. 4) Qualitatively similar transient enhancements in the wake of real and simulated saccades have also been reported for initial ocular following responses elicited by conjugate drifts of the visual scene. We replicated the enhancement effects on ocular following to allow a direct comparison with the enhancement effects on disparity vergence using the same animals and visual stimulus patterns and, despite some clear quantitative differences, we suggest that the enhancement effects share a similar etiology. 5. Initial vergence responses to uncrossed-disparity steps had the following main features. 1) They were in the correct (i.e., divergent) direction only for very small steps (< 1.5-2.5 degrees), and then only when postsaccadic delays were small; when the magnitude of the steps was increased beyond these levels, responses declined to zero and thereafter reversed direction, eventually reaching a nonzero (convergent) asymptote similar to that seen with large crossed-disparity steps; convergent responses were also seen with larger vertical disparity steps, suggesting that they represent default responses to any disparity exceeding a few degrees. 2) As the postsaccadic delay was increased, responses to small steps (1.8 degrees) declined to zero and thereafter re

Animals↗
PubMed2002

Fixation disparity: binocular vergence accuracy for a visual display at different positions relative to the eyes.

Some observers do not fixate accurately at the point of regard: Their vergence angle (between the visual axes of the two eyes) may correspond to points slightly nearer or farther away. This vergence error, or fixation disparity, was measured with nonius (vernier) lines at six positions of a visual display relative to the eyes. At viewing distances of 40, 60, and 100 cm, the display was located either at eye level or at a downward inclination of gaze direction of -25 degrees relative to horizontal. Viewing conditions resembled typical office work. Lowering the screen induced a near shift in mean vergence response of 0.6 min arc, irrespective of viewing distance; the closer the screen, the more distant was the vergence response relative to the target (by 2.5 min arc on average). The slope of this proximity-fixation-disparity curve is an individual parameter of the vergence system. Actual or potential applications of this research include recommendations for the comfortable viewing distance of visual displays.

Accommodation, Ocular↗
PubMed1997

Spatial scale interactions in stereo sensitivity and the neural representation of binocular disparity.

How are binocular disparities encoded and represented in the human visual system? An 'encoding cube' diagram is introduced to visualise differences between competing models. To distinguish the models experimentally, the depth-increment-detection function (discriminating disparity d from d +/- delta d) was measured as a function of standing disparity (d) with spatially filtered random-dot stereograms of different centre spatial frequencies. Stereothresholds degraded more quickly as standing disparity was increased with stimuli defined by high rather than low centre spatial frequency. This is consistent with a close correlation between the spatial scale of detection mechanisms and the disparities they process. It is shown that a simple model, where discrimination is limited by the noisy ratio of outputs of three disparity-selective mechanisms at each spatial scale, can account for the data. It is not necessary to invoke a population code for disparity to model the depth-increment-detection function. This type of encoding scheme implies insensitivity to large interocular phase differences. Might the system have developed a strategy to disambiguate or shift the matches made at fine scales with those made at the coarse scales at large standing disparities? In agreement with Rohaly and Wilson, no evidence was found that this is so. Such a scheme would predict that stereothresholds determined with targets composed of compounds of high and low frequency should be superior to those of either component alone. Although a small stereoacuity benefit was found at small disparities, the more striking result was that stereothresholds for compound-frequency targets were actually degraded at large standing disparities. The results argue against neural shifting of the matching range of fine scales by coarse-scale matches posited by certain stereo models.

Computer Graphics↗
PubMed1993

Interaction of stereo, texture and outline cues in the shape perception of three-dimensional ridges.

We report five psychophysical experiments that employed a cue conflict paradigm to investigate integration by the human visual system of surface shape information from stereo, texture and outline cues. The experiments used convex parabolic and triangular three-dimensional ridge stimuli, with amplitudes (base to peak) in the range 3-9 cm, viewed from 57 cm. The observers' task was to judge ridge amplitude using a scale of two-dimensional drawings of ridge profiles. Cue integration was studied using both vertically and horizontally oriented ridges and both real ridges and stereograms of ridges. The main findings were: (a) stereo strongly dominated all horizontal ridge stereograms; (b) texture and outline cues strongly dominated low (3-6 cm) but not high (9 cm) amplitude vertical ridge stereograms; (c) stereo strongly dominated all real ridge stimuli. These results are evidence against explanations of the vertical/horizontal stereo anisotropy which propose that it derives from stereo mechanisms being tuned only to disparity cues with non-zero second-order spatial derivatives or to disparity discontinuities. They also show that radically different results can be obtained when stereo mechanisms are explored using stereograms and real surfaces and possible reasons for this are discussed.

Adult↗
PubMed2004

Early computation of contour curvature and part structure: evidence from holes.

We used holes to study unilateral border ownership and in particular the information carried by the sign of the curvature along the contour (ie the difference between convex and concave regions). When people perceive a hole, its shape has a reversed curvature polarity (ie a changed sign of curvature) compared to the same region perceived as an object. Bertamini (2001 Perception 30 1295-1310), and Bertamini and Croucher (2003 Cognition 87 33-54) suggested and found evidence to support the hypothesis that, because convex regions are perceived as parts, positional information is more readily available for convex regions. Therefore a change is predicted when a given region is perceived as either a hole or a figure. We confirm that finding in this study, using holes defined by binocular disparity. We conclude that a change from figure to hole always reverses the encoding of curvature polarity. In turn, polarity obligatorily affects perceived part structure and the processing of position.

Analysis of Variance↗
PubMed1993

Effect of induced fixation disparity by negative lenses on the visually evoked potential wave.

The effect of induced fixation disparity produced by negative lenses and base-in prisms on the pattern visually evoked potential (VEP) was investigated. Monocular and binocular pattern reversal VEP were recorded through the negative lenses and base-in prisms using a 9.5 min arc check size. The results suggest that the mean binocular amplitude was 27% larger than the mean monocular amplitude which indicates partial summation in the absence of negative lenses or base-in prisms. A reduction of the binocular VEP amplitude appears to arise in the presence of fixation disparity induced by both the negative lenses and base-in prisms. Both the prisms and the negative lenses had no appreciable effect on the monocular VEP amplitude.

Evoked Potentials, Visual↗
PubMed2006

Effect of age on adult stereoacuity as measured by different types of stereotest.

AIM: To examine how stereoacuity changes with age as measured by a variety of stereotests. METHODS: Stereoacuity has been measured in 60 normal subjects aged 17-83 years by a single observer using TNO, Titmus, Frisby near, and Frisby-Davis distance stereotests. Motor fusion was measured at (1/3) metre and 6 metres. RESULTS: Overall stereoacuity measured by all tests showed a mild decline with age (p<0.001 for all tests; Spearman rank correlation). A marked reduction to screening or absent levels of stereoacuity was seen in five subjects aged over 55, but only with the TNO stereotest. All these subjects were able to achieve a stereoacuity of 200 seconds of arc or better with the Titmus test and 340 seconds of arc or better using the Frisby near stereotest. There was a small decline with age in the base out motor fusion range measured at 6 metres (p<0.05; Spearman rank correlation). No subject described difficulty in judging distances for everyday tasks. CONCLUSIONS: Although subjects showed some decline in stereoacuity with age by all tests, the large drop in stereoacuity seen in some older subjects using the TNO test was probably due to difficulty overcoming the dissociative effect of the test rather than a true reduction in cortical disparity detection. Results of random dot stereotests should be interpreted with caution in older patients, particularly with respect to their ability to perform everyday visual tasks.

Adolescent↗
PubMed2002

Quantitative analysis of the responses of V1 neurons to horizontal disparity in dynamic random-dot stereograms.

Horizontal disparity tuning for dynamic random-dot stereograms was investigated for a large population of neurons (n = 787) in V1 of the awake macaque. Disparity sensitivity was quantified using a measure of the discriminability of the maximum and minimum points on the disparity tuning curve. This measure and others revealed a continuum of selectivity rather than separate populations of disparity- and nondisparity-sensitive neurons. Although disparity sensitivity was correlated with the degree of direction tuning, it was not correlated with other significant neuronal properties, including preferred orientation and ocular dominance. In accordance with the Gabor energy model, tuning curves for horizontal disparity were adequately described by Gabor functions when the neuron's orientation preference was near vertical. For neurons with orientation preferences near to horizontal, a Gaussian function was more frequently sufficient. The spatial frequency of the Gabor function that described the disparity tuning was weakly correlated with measurements of the spatial frequency and orientation preference of the neuron for drifting sinusoidal gratings. Energy models make several predictions about the relationship between the response rates to monocular and binocular dot patterns. Few of the predictions were fulfilled exactly, although the observations can be reconciled with the energy model by simple modifications. These same modifications also provide an account of the observed continuum in strength of disparity selectivity. A weak correlation between the disparity sensitivity of simultaneously recorded single- and multiunit data were revealed as well as a weak tendency to show similar disparity preferences. This is compatible with a degree of local clustering for disparity sensitivity in V1, although this is much weaker than that reported in area MT.

Animals↗
PubMed2005

Ordinal configural cues combine with metric disparity in depth perception.

Prior research on the combination of depth cues generally assumes that different cues must be in the same units for meaningful combination to occur. We investigated whether the geometrically ordinal cues of familiarity and convexity influence depth perception when unambiguous metric information is provided by binocular disparity. We used bipartite, random dot stereograms with a central luminance edge shaped like a face in profile. Disparity specified that the edge and dots on one side were closer than the dots on the other side. Configural cues suggested that the familiar, face-shaped region was closer than the unfamiliar side. Configural cues caused an increase in perceived depth for a given disparity signal when they were consistent with disparity and a decrease in perceived depth when they were inconsistent. Thus, geometrically ordinal configural cues can quantitatively influence a metric depth cue. Implications for the combination of configural and depth cues are discussed.

Cues↗
PubMed2005

A new look at binocular stereopsis.

We report a new phenomenon, which illustrates that the role of binocular disparity in 3D shape perception critically depends on whether the parts are interpreted as belonging to a single object. The nature of this phenomenon was studied in four experiments. In the first two experiments the subjects were shown a sequence of stereoscopic images of a cube, in which binocular disparity indicated that the individual parts move towards or away from one eye. However, when the parts of the cube were perceived as elements of a single object, they appeared to move in a rigid fashion and the direction of motion was orthogonal to that predicted by the binocular disparities. The third experiment generalized these results to more complex polyhedra. The last experiment showed that constraints related to motion, such as rigidity, are important, but not critical for this phenomenon to occur. All these results imply that the interpretation as to what corresponds to a single object affects the importance (weight) of binocular disparity and may even eliminate its contribution altogether; the percept of a 3D shape is dominated by a priori constraints, and depth cues play a secondary role.

Cues↗
PubMed2006

Vergence accommodation and monocular closed loop blur accommodation have similar dynamic characteristics.

Retinal blur and disparity are two different sensory signals known to cause a change in accommodative response. These inputs have differing neurological correlates that feed into a final common pathway. The purpose of this study was to investigate the dynamic properties of monocular blur driven accommodation and binocular disparity driven vergence-accommodation (VA) in human subjects. The results show that when response amplitudes are matched, blur accommodation and VA share similar dynamic properties.

Accommodation, Ocular↗
PubMed1997

Non-Fourier information in bandpass noise patterns.

Random dot patterns and white-noise luminance textures are widely used in psychophysical experiments to study low-level visual processes. Because these noise patterns are broadband, bandpass filtered versions are employed to limit their frequency content. It is not recognized, however, that bandpass noise patterns have amplitude-modulation (AM) components. The AM signal is not present in the Fourier spectrum, nonetheless, it is a valid signal for second-order mechanisms. We characterize the properties of the AM signal in bandpass noise textures: the relevant periodicities of the AM signal are always much lower than the actual passband; the upper frequency limit of the AM signal increases with the linear bandwidth. We present psychophysical data to demonstrate the perceptual significance of the AM signal in bandpass noise. We provide a method for obtaining AM-free bandpass patterns, and compare psychophysical performance in experiments employing AM-present and AM-free bandpass noise patterns as stereoscopic stimuli. The results show that the AM component contributes to stereoscopic discrimination performance at large disparities. We suggest that the low-frequency AM signal is a possible confounding factor in experiments employing bandpass noise textures, and that linear filtering can isolate spatial scales effectively only for linear systems.

Depth Perception↗