McGill Vision Research

Montréal, Canada

McGill Vision Research

Montréal, Canada
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Kingdom F.A.A.,McGill Vision Research | Bell J.,University of Western Australia | Haddad C.,McGill Vision Research | Bartsch A.,McGill Vision Research
Journal of Vision | Year: 2015

We have measured the relative perceptual scales for chromatic and luminance blur in dense textures comprised of color and luminance Gabors, using a modification of the method of paired comparisons. We find that the rate at which perceived blur grows with physical blur, when normalized to 1.0 for luminance, is 0.2 for red-green and 0.06 for blue-yellow blur. It is argued that the relatively severely compressed perceptual scales for red-green and blue-yellow blur are a contributary factor to the observation that when the color but not luminance layer of an image of a natural scene is blurred, there is little or no impression of blur (Wandell, 1995). © 2015 ARVO.

Kingdom F.A.A.,McGill Vision Research | Baldwin A.S.,McGill Vision Research | Schmidtmann G.,McGill Vision Research
Journal of Vision | Year: 2015

Many studies have investigated how multiple stimuli combine to reach threshold. There are broadly speaking two ways this can occur: additive summation (AS) where inputs from the different stimuli add together in a single mechanism, or probability summation (PS) where different stimuli are detected independently by separate mechanisms. PS is traditionally modeled under high threshold theory (HTT); however, tests have shown that HTT is incorrect and that signal detection theory (SDT) is the better framework for modeling summation. Modeling the equivalent of PS under SDT is, however, relatively complicated, leading many investigators to use Monte Carlo simulations for the predictions. We derive formulas that employ numerical integration to predict the proportion correct for detecting multiple stimuli assuming PS under SDT, for the situations in which stimuli are either equal or unequal in strength. Both formulas are general purpose, calculating performance for forced-choice tasks with M alternatives, n stimuli, in Q monitored mechanisms, each subject to a non-linear transducer with exponent τ. We show how the probability (and additive) summation formulas can be used to simulate psychometric functions, which when fitted with Weibull functions make signature predictions for how thresholds and psychometric function slopes vary as a function of τ, n, and Q. We also show how one can fit the formulas directly to real psychometric functions using data from a binocular summation experiment, and show how one can obtain estimates of τ and test whether binocular summation conforms more to PS or AS. The methods described here can be readily applied using software functions newly added to the Palamedes toolbox. © 2015 ARVO.

Kingdom F.A.A.,McGill Vision Research | Bell J.,McGill Vision Research | Gheorghiu E.,Catholic University of Leuven | Malkoc G.,Dogus University
Journal of Vision | Year: 2010

Most objects in natural scenes are suprathreshold in both color (chromatic) and luminance contrast. How salient is each dimension? We have developed a novel method employing a stimulus similar to that used by B. C. Regan and J. D. Mollon (1997) who studied the relative saliencies of the two chromatic cardinal directions. Our stimuli consist of left- and right- oblique modulations of color and/or luminance dened within a lattice of circles. In the "separated" condition, the two modulations were presented separately as forced-choice pairs, and the task was to indicate which was more salient. In the "combined" condition, the two orthogonal-in-orientation modulations were added, and the task was to indicate the more salient orientation. The ratio of color to luminance contrast at the PSE was calculated for both conditions. Across color directions, 48% more luminance contrast relative to color contrast was required to achieve a PSE in the "combined" compared to the "separated" condition. A second experiment showed that the PSE difference was due to the luminance being masked by the color, rather than due to superior color grouping. We conclude that suprathreshold brightness variations are masked by suprathreshold color variations. © ARVO.

Huang P.C.,McGill Vision Research
Journal of vision | Year: 2012

To assess the effects of spatial frequency and phase alignment of mask components in pattern masking, target threshold vs. mask contrast (TvC) functions for a sine-wave grating (S) target were measured for five types of mask: a sine-wave grating (S), a square-wave grating (Q), a missing fundamental square-wave grating (M), harmonic complexes consisting of phase-scrambled harmonics of a square wave (Qp), and harmonic complexes consisting of phase-scrambled harmonics of a missing fundamental square wave (Mp). Target and masks had the same fundamental frequency (0.46 cpd) and the target was added in phase with the fundamental frequency component of the mask. Under monocular viewing conditions, the strength of masking depends on phase relationships among mask spatial frequencies far removed from that of the target, at least 3 times the target frequency, only when there are common target and mask spatial frequencies. Under dichoptic viewing conditions, S and Q masks produced similar masking to each other and the phase-scrambled masks (Qp and Mp) produced less masking. The results suggest that pattern masking is spatial frequency broadband in nature and sensitive to the phase alignments of spatial components.

Gheiratmand M.,McGill Vision Research | Mullen K.T.,McGill Vision Research
Scientific Reports | Year: 2014

We measure the orientation tuning of red-green colour and luminance vision at low (0.375 c/deg) and mid (1.5 c/deg) spatial frequencies using the low-contrast psychophysical method of subthreshold summation. Orientation bandwidths of the underlying neural detectors are found using a model involving Minkowski summation of the rectified outputs of a bank of oriented filters. At 1.5 c/deg, we find orientation-tuned detectors with similar bandwidths for chromatic and achromatic contrast. At 0.375 c/deg, orientation tuning is preserved with no change in bandwidth for achromatic stimuli, however, for chromatic stimuli orientation tuning becomes extremely broad, compatible with detection by non-oriented colour detectors. A non-oriented colour detector, previously reported in single cells in primate V1 but not psychophysically in humans, can transmit crucial information about the color of larger areas or surfaces whereas orientation-tuned detectors are required to detect the colour or luminance edges that delineate an object's shape.

Kingdom F.A.A.,McGill Vision Research | Libenson L.,McGill Vision Research
Journal of Vision | Year: 2015

We demonstrate a new type of interaction between suprathreshold color (chromatic) and luminance contrast in the context of binocular vision. When two isoluminant colored disks of identical hue but different saturations are presented to different eyes, the apparent saturation of the resulting "dichoptic" mix is close to that of the more saturated patch if presented binocularly. This result is commensurate with previous findings using luminance contrast and is close to the scenario termed "winner-takeall." However, when binocularly matched luminance contrast is added to the dichoptic saturation mixture, the apparent saturation of the mixture shifts away from winner-take-all towards the average of the two dichoptic saturations. The likely cause of this effect is that the matched luminance contrasts reduce the interocular suppression between the unmatched color saturations. We suggest that the presence of binocularly matched luminance contrast promotes the interpretation that the dichoptic color saturations, even though unmatched, nevertheless originate from the same object. We term this idea the "object commonality" hypothesis. © 2015 ARVO.

Schmidtmann G.,McGill Vision Research | Jennings B.J.,McGill Vision Research | Bell J.,University of Western Australia | Kingdom F.A.A.,McGill Vision Research
Journal of Vision | Year: 2015

Previous studies investigating signal integration in circular Glass patterns have concluded that the information in these patterns is linearly summed across the entire display for detection. Here we test whether an alternative form of summation, probability summation (PS), modeled under the assumptions of Signal Detection Theory (SDT), can be rejected as a model of Glass pattern detection. PS under SDT alone predicts that the exponent β of the Quick- (or Weibull-) fitted psychometric function should decrease with increasing signal area. We measured spatial integration in circular, radial, spiral, and parallel Glass patterns, as well as comparable patterns composed of Gabors instead of dot pairs.We measured the signal-to-noise ratio required for detection as a function of the size of the area containing signal, with the remaining area containing dot-pair or Gabor-orientation noise. Contrary to some previous studies, we found that the strength of summation never reached values close to linear summation for any stimuli. More importantly, the exponent β systematically decreased with signal area, as predicted by PS under SDT. We applied a model for PS under SDT and found that it gave a good account of the data. We conclude that probability summation is the most likely basis for the detection of circular, radial, spiral, and parallel orientation-defined textures. © 2015 ARVO.

Ouhnana M.,McGill Vision Research | Kingdom F.A.A.,McGill Vision Research
Vision Research | Year: 2016

Previous studies have shown that spatial context influences the perceptual interpretation of ambiguous figures such as the Necker cube; however, the properties that mediate the influences of an unambiguous spatial context have yet to be investigated. Here we consider the effect of the motion and position of an unambiguous rotating skeleton cube on the perceived motion direction of an ambiguous rotating Necker cube. We aimed to determine whether the motion of the two figures could be perceptually bound, and if it could, to determine the properties of the binding. We employed a novel procedure analogous to reverse correlation to establish the correlation between the rotation directions of the context and the perceived rotation directions of the target, across 32. s trial presentations. Our results showed that changes in the rotation direction of the context triggered above-chance changes in the perceived rotation direction of the target. However, the relative speeds of rotation of the context and target had little effect on the correlations. Position on the other hand had a significant effect: correlations were higher when the context was below compared to when above the target. Our results reveal that change-synchrony not common fate is the factor mediating perceptual motion binding between the context and Necker cube. We also suggest that prior knowledge of friction forces could underlie the position dependency of the context and Necker-cube correlation. © 2016.

Kingdom F.A.A.,McGill Vision Research
Current Biology | Year: 2012

Our two eyes' views of the outside world are slightly different, providing the basis for stereopsis. A new study has found evidence that the human visual system has separately adaptable channels for adding and subtracting the neural signals from the two eyes, supporting an unconventional view of the initial stages of stereopsis. © 2012 Elsevier Ltd.

The past quarter century has witnessed considerable advances in our understanding of Lightness (perceived reflectance), Brightness (perceived luminance) and perceived Transparency (LBT). This review poses eight major conceptual questions that have engaged researchers during this period, and considers to what extent they have been answered. The questions concern 1. the relationship between lightness, brightness and perceived non-uniform illumination, 2. the brain site for lightness and brightness perception, 3 the effects of context on lightness and brightness, 4. the relationship between brightness and contrast for simple patch-background stimuli, 5. brightness " filling-in", 6. lightness anchoring, 7. the conditions for perceptual transparency, and 8. the perceptual representation of transparency. The discussion of progress on major conceptual questions inevitably requires an evaluation of which approaches to LBT are likely and which are unlikely to bear fruit in the long term, and which issues remain unresolved. It is concluded that the most promising developments in LBT are (a) models of brightness coding based on multi-scale filtering combined with contrast normalization, (b) the idea that the visual system decomposes the image into " layers" of reflectance, illumination and transparency, (c) that an understanding of image statistics is important to an understanding of lightness errors, (d) Whittle's log. W metric for contrast-brightness, (e) the idea that " filling-in" is mediated by low spatial frequencies rather than neural spreading, and (f) that there exist multiple cues for identifying non-uniform illumination and transparency. Unresolved issues include how relative lightness values are anchored to produce absolute lightness values, and the perceptual representation of transparency. Bridging the gap between multi-scale filtering and layer decomposition approaches to LBT is a major task for future research. © 2010 Elsevier Ltd.

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