Stéphane Rainville

The effects of spatial frequency overlap on face recognition.

The effects of spatial frequency overlap between pairs of low-pass versus high-pass images on face recognition and matching were examined in 6 experiments. Overlap was defined as the range of spatial frequencies shared by a pair of filtered images. This factor was manipulated by processing image pairs with high-pass/low-pass filter pairs whose 50% cutoff points varied in their separation from one another. The effects of the center frequency of filter pairs were also investigated. In general, performance improved with greater overlap and higher center frequency. In control conditions, the image pairs were processed with identical filters and thus had complete overlap. Even severely filtered low-pass or high-pass images in these conditions produced superior performance. These results suggest that face recognition is more strongly affected by spatial frequency overlap than by the frequency content of the images.

The functional role of oriented spatial filters in the perception of mirror symmetry — psychophysics and modeling

We investigated human sensitivity to vertical mirror symmetry in noise patterns filtered for narrow bands of variable orientations. Sensitivity is defined here as the amount of spatial phase randomization corresponding to 75% correct performance in a 2AFC detection task. In Experiment 1, sensitivity was found to be high for tests patterns of all orientations except those parallel to the axis of symmetry. This implies that corresponding mirror-orientations (e.g. -45 and +45 degrees ) are combined prior to symmetry detection. In Experiment 2, observers detected symmetry in tests of variable orientation in the presence of either non-symmetric or symmetric masks filtered for orientations either parallel or perpendicular to the axis. Observers were found to be primarily affected by masks of the same orientation as the test, thus suggesting that symmetry is computed separately in distinct mirror-orientation channels. In Experiment 3, observers detected a symmetric test of variable height and width embedded in random noise. Data revealed that mirror symmetry is computed over a spatial integration region (IR) that remains approximately constant in area but whose height-to-width aspect ratio changes from 20:1 to 2:1 as orientation is varied from parallel to perpendicular to the axis. We compare human data against that of an ideal observer to identify key factors that limit visual performance and discuss the implications for the functional architecture of symmetry perception. We also propose a multi-channel model of symmetry detection that combines the output of oriented spatial filters in a simple and physiologically plausible manner. Particular emphasis is placed on the notion that changes in the shape of the IR with orientation compensate for changes in information density and partially equate performance across orientations.

On the apparent collapse of stereopsis in random-dot-stereograms at isoluminance

We have investigated the apparent collapse of stereopsis obtained with random-dot-stereograms at isoluminance. Contrast thresholds for both depth and form discrimination of targets in random-dot- and figural stereograms were measured at a number of disparitics, using both isoluminant and isochromatic stimuli. All contrast thresholds for stereoscopic tasks were normalised to contrast thresholds for detecting the appropriate stimulus. We found that at isoluminance contrast thresholds for depth judgements were not higher for random-dot compared to figural stereograms, even when normalised to the same thresholds obtained with isochromatic stimuli. On the other hand contrast thresholds for three-dimensional form judgements were much higher than those for depth judgements in isoluminant, compared to isochromatic random-dot-stereograms. This specific impairment of stereoscopic form (as opposed to depth) processing at isoluminance was confirmed in a further experiment in which subjects were required to judge the presence and orientation of depth corrugations in a disparity-modulated random-dot-stereogram.

The role of color and attention-to-color in mirror-symmetry perception

The role of color in the visual perception of mirror-symmetry is controversial. Some reports support the existence of color-selective mirror-symmetry channels, others that mirror-symmetry perception is merely sensitive to color-correlations across the symmetry axis. Here we test between the two ideas. Stimuli consisted of colored Gaussian-blobs arranged either mirror-symmetrically or quasi-randomly. We used four arrangements: (1) ‘segregated’ – symmetric blobs were of one color, random blobs of the other color(s); (2) ‘random-segregated’ – as above but with the symmetric color randomly selected on each trial; (3) ‘non-segregated’ – symmetric blobs were of all colors in equal proportions, as were the random blobs; (4) ‘anti-symmetric’ – symmetric blobs were of opposite-color across the symmetry axis. We found: (a) near-chance levels for the anti-symmetric condition, suggesting that symmetry perception is sensitive to color-correlations across the symmetry axis; (b) similar performance for random-segregated and non-segregated conditions, giving no support to the idea that mirror-symmetry is color selective; (c) highest performance for the color-segregated condition, but only when the observer knew beforehand the symmetry color, suggesting that symmetry detection benefits from color-based attention. We conclude that mirror-symmetry detection mechanisms, while sensitive to color-correlations across the symmetry axis and subject to the benefits of attention-to-color, are not color selective.

The mechanisms for detecting compressively sampled gratings

Contrast thresholds for sine-wave gratings are raised when the gratings are compressively sampled into a set of narrow bright bars on a dark background, even though this method of sampling preserves the mean luminance and contrast of the grating. Burr et al. [(1985). Vision Research, 25, 717-727] suggested threshold elevation was due to localized luminance adaptation to the sample bars, whose average peak luminance necessarily increased when fewer bars per cycle were present. Previously, we reported results using decrement-bar compressively sampled gratings (CSGs), which consist of dark sample bars on a bright background, which favoured the local luminance adaptation hypothesis (Kingdom & Rainville, 1995). Here we report experiments that suggest that this hypothesis is untenable. Using increment-bar CSGs (bright sample bars on a dark background) we found that raising background luminance while holding sample bar luminance constant reduced thresholds by as much as a factor of ten. This suggests that it is the contrast of the bars, rather than their luminance, which determines thresholds. Further experiments showed that CSG detection was facilitated by unsampled grating pedestals, and thresholds were elevated when the fundamental was physically cancelled. This implied that CSGs were detected by the same mechanisms as the unsampled gratings from which they are derived. Finally, we provide evidence for the involvement of a dynamic gain control component for increment-bar CSG detection.

Detection of compressively sampled gratings

Contrast thresholds for sine-wave gratings are increased when the gratings are compressively sampled into a set of narrow bright bars on a dark background, even though the mean luminance and contrast of the grating are unchanged by this sampling method. Burr, Ross and Morrone, who first demonstrated this phenomenon, suggested this was due to local luminance adaptation to the sample bars, whose average peak luminance necessarily increases with the degree of compressive sapling. However, the results could also be explained on the basis of either a luminance compressive non-linearity, or a local contrast-based non-linearity. Previously we reported results with decrement CSGs, which consist of dark sample bars on a bright background, which favored the local luminance adaptation hypothesis. However here we show that this hypothesis is untenable. Using increment CSGs (bright bars on a dark background) we found that raising background luminance while holding average peak sample bar luminance constant reduced thresholds by as much as a factor of ten. This demonstrates that it is the contrast of the bars, rather than their peak luminance, which is the important feature determining thresholds, at least with increment CSGs. We also provide evidence for the involvement of a gain control mechanism which serves to partially reduce the deleterious effects of the contrast-based non-linearity on CSG thresholds. Finally we show that CSG thresholds can be reduced by the presence of a low-contrast unsampled grating mask. This suggests that although local contrast processing is initially involved in CSG detection, the cortical mechanisms which ultimately detect CSGs are the same as those which detect the unsampled gratings from which they are derived.