Elena Gheorghiu

The spatial feature underlying the shape-frequency and shape-amplitude after-effects

The shape-frequency and shape-amplitude after-effects, or SFAE and SAAE, refer respectively to the shifts observed in the perceived shape-frequency and shape-amplitude of a sinusoidal test contour following adaptation to a similar-shaped contour. As with other shape after-effects the shifts are in a direction away from that of the adapting stimulus. Using a variety of procedures we tested whether the spatial feature that was adapted in the SFAE and SAAE was (a) local orientation, (b) average unsigned curvature, (c) periodicity/density, (d) shape-amplitude and (e) local curvature. Our results suggest that the last of these, local curvature, underlies both the SFAE and SAAE. The evidence in favour of local curvature was that the after-effect reached its maximum value when just half-a-cycle of the test contour, in +/-cosine phase, was present. We suggest that the SFAE and SAAE are mediated by intermediate-level mechanisms that encode the shapes of contour fragments with constant sign of curvature. Given the neurophysiological evidence that neurons in area V4 encode parts of shapes with constant sign of curvature, we suggest V4 is the likely neural substrate for both the SFAE and SAAE.

Spatial properties of curvature-encoding mechanisms revealed through the shape-frequency and shape-amplitude after-effects

The shape-frequency and shape-amplitude after-effects, or SFAE and SAAE, are phenomena in which adaptation to a sinusoidal-shaped contour results in a shift in, respectively, the perceived shape-frequency and perceived shape-amplitude of a test contour in a direction away from that of the adapting stimulus. Recent evidence shows that the SFAE and SAAE are mediated by mechanisms sensitive to curvature [Gheorghiu, E., & Kingdom, F. A. A. (2007a). The spatial feature underlying the shape-frequency and shape-amplitude after-effects. Vision Research, 47(6), 834-844]. Therefore we have used the SFAE and SAAE as a tool to study curvature processing. We examined whether curvature-encoding mechanisms are selective for (i) shape-phase, (ii) curvature polarity (or sign) and (iii) local orientation. We also investigated whether (iv) the two orthogonal dimensions of a curve, the sag and the cord, are encoded independently, and (v) whether curvature encoders are organized in an opponent manner. SFAEs/SAAEs were measured for adapting and test contours that differed or not in a given spatial property, the rationale being that if the after-effects were smaller when adaptor and test differed in a particular spatial property then curvature-encoding mechanisms must be selective for that spatial property. Our results reveal that SFAEs and SAAEs show (i) a degree of selectivity to curves that are mirror symmetric (in our stimuli half-cycle sine-wave contours in cosine (0/180deg) shape-phase); (ii) a degree of selectivity to the sign or polarity of curvature; (iii) a degree of selectivity to local orientation; (iv) independent coding of the sag and the cord of the curve, and (v) no evidence for opponent-curvature coding. The results agree with neurophysiological studies showing that simple shape dimensions are encoded independently.

Latent stereopsis for motion in depth in strabismic amblyopia.

PURPOSE To investigate the residual stereo function of a group of 15 patients with strabismic amblyopia, by using motion-in-depth stimuli that allow discrimination of contributions from local disparity as opposed to those from local velocity mechanisms as a function of the rate of depth change. METHODS The stereo performance (percentage correct) was measured as a function of the rate of depth change for dynamic random dot stimuli that were either temporally correlated or uncorrelated. RESULTS Residual stereoscopic function was demonstrated for motion in depth based on local disparity information in 2 of the 15 observers with strabismic amblyopia. The use of a neutral-density (ND) filter in front of the fixing eye enhanced motion-in-depth performance in four subjects randomly selected from the group that originally displayed only chance performance. This finding was true across temporal rate and for correlated and uncorrelated stimuli, suggesting that it was disparity based. The opposite occurred in a group of normal subjects. In a separate experiment, the hypothesis was that the beneficial effect of the ND filter is due to its contrast and/or mean luminance-reducing effects rather than any interocular time delay that it may introduce and that it is specific to motion-in-depth performance, as similar improvements were not found for static stereopsis. CONCLUSIONS A small proportion of observers with strabismic amblyopia exhibit residual performance for motion in depth, and it is disparity based. Furthermore, some observers with strabismic amblyopia who do not display any significant stereo performance for motion in depth under normal binocular viewing may display above-chance stereo performance if the degree of interocular suppression is reduced. The authors term this phenomenon latent stereopsis.

Global shape processing involves a hierarchy of integration stages

Radial Frequency (RF) patterns can be used to study the processing of familiar shapes, e.g. triangles and squares. Opinion is divided over whether the mechanisms that detect these shapes integrate local orientation and position information directly, or whether local orientations and positions are first combined to represent extended features, such as curves, and that it is local curvatures that the shape mechanism integrates. The latter view incorporates an intermediate processing stage, the former does not. To differentiate between these hypotheses we studied the processing of micro-patch sampled RF patterns as a function of the luminance polarity of successive elements on the contour path. Our first study measures shape after effects involving suprathreshold amplitude RF shapes and shows that alternating the luminance polarity of successive micro-patch elements disrupts adaptation of the global shape. Our second study shows that polarity alternations also disrupt sensitivity to threshold-amplitude RF patterns. These results suggest that neighbouring points of the contour shape are integrated into extended features by a polarity selective mechanism, prior to global shape processing, consistent with the view that for both threshold amplitude and suprathreshold amplitude patterns, global processing of RF shapes involves an intermediate stage of processing.

Orientation tuning of curvature adaptation reveals both curvature-polarity-selective and non-selective mechanisms.

We have used a curvature after-effect, or CAE, to explore whether curvature detectors are tuned for the overall orientation of a curve. CAEs were measured for half-cycle cosine-shaped contours as a function of adaptor contour orientation for a fixed test contour orientation. CAEs (i) were greatest when the adaptor and test contours had the same orientation, (ii) decreased rapidly as the orientation of the adapting contours rotated away from the test, the data being well fit by a Gaussian function with a standard deviation of 16 degrees , (iii) increased again to a secondary peak when the adapting contours were rotated 180 degrees relative to the test. Control experiments showed that the shape of the curvature-orientation tuning function could not be explained by local orientation adaptation, and that instead curvature encoding mechanisms are tuned for orientation. The secondary peak in the CAE at 180 degrees is argued to be inconsistent with curvature opponency and instead a result of the combination of polarity-selective and polarity-non-selective curvature mechanisms. The results are discussed in relation to recent psychophysical and physiological models of form processing and the possible significance of the findings with regard to symmetry processing.

Binocular properties of curvature-encoding mechanisms revealed through two shape after-effects.

We investigated the binocular properties of curvature-encoding mechanisms using the shape-frequency and shape-amplitude after-effects (or SFAE and SAAE). The SFAE and SAAE refer to the shifts observed in, respectively, the shape-frequency and shape-amplitude of a sinusoidal test contour following adaptation to a contour with different shape-frequency/shape-amplitude. We examined (i) the contribution of monocular versus binocular mechanisms to the SFAE and SAAE by measuring the interocular transfer of these after-effects, (ii) the stereo-depth selectivity of the after-effects and (iii) the depth selectivity of the reduction in the after-effects from texture-surround inhibition. Our results reveal that (i) both SFAE and SAAE have a high degree of interocular transfer (on average >90%), suggesting that they are mediated primarily by binocular mechanisms, (ii) neither SFAE nor SAAE are selective to stereo-defined depth and (iii) the reduction in the SFAE and SAAE from texture-surround inhibition is selective for stereo-depth. We conclude that the SFAE and SAAE are mediated by binocularly driven curvature-selective neurons that are not disparity selective in themselves but which receive inputs from neurons that are subject to depth- and orientation-selective texture-surround inhibition.

Differences in perceived depth for temporally correlated and uncorrelated dynamic random-dot stereograms

We investigated the influence of temporal frequency on binocular depth perception in dynamic random-dot stereograms (DRS). We used (i) temporally correlated DRS in which a single pair of images alternated between two disparity values, and (ii) temporally uncorrelated DRS consisting of the repeated alternation of two uncorrelated image pairs each having one of two disparity values. Our results show that disparity-defined depth is judged differently in temporally correlated and temporally uncorrelated DRS above a temporal frequency of about 3 Hz. The results and simulations indicate that (i) above about 20 Hz, the complete absence of stereomotion is caused by temporal integration of luminance, (ii) the difference in perceived depth in temporally correlated and temporally uncorrelated DRS for temporal frequencies between 20 and 3 Hz, is caused by temporal integration of disparity.

Temporal properties of disparity processing revealed by dynamic random-dot stereograms

In studies of the temporal flexibility of the stereoscopic system, it has been suggested that two different processes of binocular depth perception could be responsible for the flexibility: tolerance for interocular delays and temporal integration of correlation. None has investigated the relationship between tolerance for delays and temporal integration mechanisms and none has revealed which mechanism is responsible for depth perception in dynamic random-dot stereograms. We address these questions in the present study. Across five experiments, we investigated the temporal properties of stereopsis by varying interocular correlation as a function of time in controlled ways. We presented different types of dynamic random-dot stereograms, each consisting of two pairs of alternating random-dot patterns. Our experimental results demonstrate that (i) disparities from simultaneous monocular inputs dominate those from interocular delayed inputs; (ii) stereopsis is limited by temporal properties of monocular luminance mechanisms; and (iii) depth perception in dynamic random-dot stereograms results from cross-correlation-like operation on two simultaneous monocular inputs that represent the retinal images after having been subjected to a process of monocular temporal integration of luminance.

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.

Chromatic properties of texture-shape and of texture-surround suppression of contour-shape mechanisms.

Contour-shape coding is color selective (Gheorghiu & Kingdom, 2007a) and surround textures inhibit the processing of contour shapes (Gheorghiu & Kingdom, 2011; Kingdom & Prins, 2009). These two findings raise two questions: (1) is texture-surround suppression of contour shape color selective, and (2) is texture-shape processing color selective? To answer these questions, we measured the shape-frequency aftereffect using contours constructed from strings of Gabors defined along the red-green, blue-yellow, and luminance axes of cardinal color space. The stimuli were either single sinusoidal-shaped contours or textures made of sinusoidal-shaped contours arranged in parallel. We measured aftereffects for (A) single-contour adaptors and single-contour tests defined along the same versus different cardinal directions, (B) texture adaptors and single-contour tests in which the central-adaptor contour/single-contour test and surround adaptor contours were defined along the same versus different cardinal directions, and (C) texture adaptors and texture tests defined along same versus different cardinal directions. We found that color selectivity was most prominent for contour-shape processing, weaker for texture-surround suppression of contour-shape processing, and absent for texture-shape processing.