Ross Goutcher

Prior knowledge on the illumination position

Visual perception is fundamentally ambiguous because an infinite number of three-dimensional scenes are consistent with our retinal images. To circumvent these ambiguities, the visual system uses prior knowledge such as the assumption that light is coming from above our head. The use of such assumptions is rational when these assumptions are related to statistical regularities of our environment. In confirmation of previous visual search experiments, we demonstrate here that the assumption on the illumination position is in fact biased to the above-left rather than directly above. This bias to the left reaches 26 degrees on average in a more direct shape discrimination task. Both right-handed and left-handed observers have a similar leftward bias. We discuss the possible origins of this singular bias on the illumination position.

Temporal dynamics in bistable perception

Bistable perception is fundamentally a dynamic process: Our perceptual experience continuously alternates when an ambiguous or rivalrous stimulus is observed. Here we present a method to analyze instantaneous measures of dominance and transition between percepts. The analysis extracts three time-varying probabilities. First, the transient preference represents the probability of perceiving one interpretation at one instant. Second, the reversal probability is the probability that the current percept will change at the next evaluation. Finally, the survival probabilities are the probability that at one instant the current percept will not switch to the alternative interpretation. We derive the relationships between these probabilities and offer a test of independence between consecutive percepts. We also introduce a simple technique to sample the observers perception at regular intervals. The analyzing method is illustrated with the example of binocular rivalry. We demonstrate Levelts second proposition with the survival probability measure and show that the consecutive rivalrous percepts are not independent.

Mechanisms for similarity matching in disparity measurement

Early neural mechanisms for the measurement of binocular disparity appear to operate in a manner consistent with cross-correlation-like processes. Consequently, cross-correlation, or cross-correlation-like procedures have been used in a range of models of disparity measurement. Using such procedures as the basis for disparity measurement creates a preference for correspondence solutions that maximize the similarity between local left and right eye image regions. Here, we examine how observers’ perception of depth in an ambiguous stereogram is affected by manipulations of luminance and orientation-based image similarity. Results show a strong effect of coarse-scale luminance similarity manipulations, but a relatively weak effect of finer-scale manipulations of orientation similarity. This is in contrast to the measurements of depth obtained from a standard cross-correlation model. This model shows strong effects of orientation similarity manipulations and weaker effects of luminance similarity. In order to account for these discrepancies, the standard cross-correlation approach may be modified to include an initial spatial frequency filtering stage. The performance of this adjusted model most closely matches human psychophysical data when spatial frequency filtering favors coarser scales. This is consistent with the operation of disparity measurement processes where spatial frequency and disparity tuning are correlated, or where disparity measurement operates in a coarse-to-fine manner.

Motion transparency from opposing luminance modulated and contrast modulated gratings.

Two luminance gratings of identical orientation and opposite directions of motion are seen as moving across one another (i.e. moving transparently) only if they differ in spatial frequency (SF) by a factor of four or more. Identical SF gratings produce counter-phase flicker. This suggests that opposite motions cancel each other at the level of motion detection. Here we show that motion transparency is perceived with two gratings of the same SF and orientation moving in opposite directions, when one grating is a first-order, luminance modulated (LM) stimulus and the other is a second-order, contrast modulated (CM) stimulus. Participants were presented with various combinations of LM and CM gratings. In experiment 1, the test stimulus contained the summation of oppositely moving LM and CM gratings. In order to assess the simultaneous perception of both motions, we used a paradigm where observers were required to discriminate the direction of motion of each component from counter-phase flicker. Results show that observers can accurately discriminate both LM and CM directions of motion in a transparent configuration. We next measured the effect of varying the contrast/modulation depth of LM and CM gratings on the perception of transparency. The perception of motion transparency depends upon the relative contrast/modulation depth of the component gratings: raising the contrast of the LM component necessitates a greater modulation depth for the CM component if motion transparency is to be perceived. Our results are consistent with a motion system comprised of two separate, but not wholly independent, pathways for the encoding of LM and CM signals. We hypothesise that the observed contrast dependence is the result of contrast gain control mechanisms that receive inputs from separate motion systems.

Temporal dynamics of stereo correspondence bi-stability

Periodic stereoscopic stimuli offer multiple viable solutions to the stereo correspondence problem. When viewing such stimuli for prolonged periods of time, observers continually switch their perceptual state between alternative correspondence solutions. We examine the temporal dynamics of this correspondence bi-stability. Participants were presented with an ambiguous stereogram comprised of regularly spaced dots. This stimulus was perceived as a fronto-parallel plane situated either behind or in front of fixation, depending on the achieved correspondence solution. The stimulus was presented continuously for one minute, with participants instructed to report the sign of the perceived depth at the sound of an auditory prompt presented, on average, every 2 s. Inter-ocular contrast and available disparities were varied so as to manipulate preferred correspondence. We find that participants are initially biased to perceive the stimulus as having an uncrossed disparity. Furthermore, we find that following an initial period of change, perceptual preference and perceptual stability (measured as the probability of an observers percept changing between consecutive responses) remain constant over the presentation period. Finally, we find that manipulations of matching preference affect both the transient preference for, and stability of, one percept over another. Our results suggest two distinct phases of biasing in the correspondence matching process, one early, the other sustained.

Encoding and estimation of first- and second-order binocular disparity in natural images

Highlights • First- and second-order responses to natural binocular images are correlated.• Second-order mechanisms can improve the accuracy of disparity estimation.• Second-order mechanisms can extend the depth range of binocular stereopsis.

Evidence for relative disparity matching in the perception of an ambiguous stereogram

To compute depth from binocular disparity, the visual system must correctly link corresponding points between two images, given multiple possible correspondences. Typically, model solutions to this problem use some form of local spatial smoothing, with many physiologically inspired models doing so implicitly, through the use of local cross-correlation-like procedures. In this paper we show that implicit smoothing, without the explicit consideration of relative disparity, cannot account for biases in the perception of a novel ambiguous stereo stimulus. Observers viewed a stereogram consisting of multiple strips of periodic random-dot patterns, perceived as either a slanted surface, or a triangular wedge in depth, and reported their perception in a 4AFC task. Biases in the perception of this stimulus are shown to depend upon the stimulus configuration in its entirety, and cannot be accounted for by low-level preferences for disparity sign. Such results are not consistent with local smoothing effects arising solely at the level of cross-correlation-like absolute disparity detectors. Instead, our results suggest the presence of smoothing constraints that consider the differences in disparity between neighboring image regions. These results further suggest that such smoothing generally biases matching toward solutions that minimize relative disparity, regardless of the presence of changes in disparity sign.

Representation and Measurement of Stereoscopic Volumes

Binocular disparity information provides the human visual system with a basis for the compelling perception of both three-dimensional (3-D) object shape, and of the 3-D space between objects. However, while an extensive body of research exists into the perception of disparity-defined surface shape, relatively little research has been conducted on the associated perception of disparity-defined volume. In this paper, we report three experiments that examine this aspect of binocular vision. Participants were asked to make judgments about the 3-D spread, location-in-depth, and 3-D shape of stereoscopic volumes. Volumes were comprised of random dots with disparities drawn from a uniform distribution, a Gaussian distribution, or a combination of both. These results were compared to two models: One of these made judgments about stereoscopic volumes using information about the distributions of disparities in each stimulus, while the other was limited to only maximum and minimum disparity information. Psychophysical results were best accounted for by the maximum-minimum decision rule model. This suggests that, although binocular vision affords a compelling phenomenal sense of 3-D volume, when required to make judgments about such volumes, the visual systems default strategies make only limited use of available binocular disparity signals.