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A pioneering study by J. M. Harris and A. J. Parker (1995) found that disparity judgments using random-dot stereograms were better for stimuli composed of mixed bright and dark dots than when the dots were all bright or all dark. They attribute this to an improvement in stereo correspondence. This result is hard to explain within current models of how stereo correspondence is achieved. However, their experiment varied task difficulty by adding disparity noise. We wondered if this might challenge mechanisms subsequent to the solution of the correspondence problem rather than mechanisms that solve the correspondence problem itself. If so, this would avoid the need to modify current models of stereo correspondence. We therefore repeated Harris and Parker's experiment using interocular decorrelation to vary task difficulty. This technique is believed to probe stereo correspondence more specifically. We observed the efficiency increase reported by Harris and Parker for mixed-polarity dots both using their original technique of disparity noise and using interocular decorrelation. We show that this effect cannot be accounted for by the stereo energy or by simple modifications of it. Our results confirm Harris and Parker's original conclusion that mixed-polarity dots specifically benefit stereo correspondence and point up the challenge to current models of this process., The Royal Society, Medical Research Council, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub

Vertical disparities influence the perception of 3D depth, but little is known about the neuronal mechanisms underlying this. One possibility is that these perceptual effects are mediated by an explicit encoding of two-dimensional disparity. Recently, J. C. A. Read and B. G. Cumming (2006) pointed out that current psychophysical and physiological evidence is consistent with a much more economical one-dimensional encoding. Almost all relevant information about vertical disparity could in theory be extracted from the activity of purely horizontal-disparity sensors. Read and Cumming demonstrated that such a 1D system would experience Ogle's induced effect, a famous illusion produced by vertical disparity. Here, we test whether the brain employs this 1D encoding, using a version of the induced effect stimulus that simulates the viewing geometry at infinity and thus removes the cues which are otherwise available to the 1D model. This condition was compared to the standard induced effect stimulus, presented on a frontoparallel screen at finite viewing distance. We show that the induced effects experienced under the two conditions are indistinguishable. This rules out the 1D model proposed by Read and Cumming and shows that vertical disparity, including sign, must be explicitly encoded across the visual field., Royal Society, Medical Research Council, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub

Because our eyes are set apart horizontally in our head, most disparities between the retinal images are horizontal. However, vertical disparities also occur, and can influence depth perception. The classic example is Ogle's induced effect (K. N. Ogle, 1938), in which applying a uniform vertical magnification to one eye's image produces the illusion that the surface has been rotated around a vertical axis. This is thought to be because uniform vertical magnifications can be produced in natural viewing when the eyes are in eccentric gaze (J. E. Mayhew, 1982; J. E. Mayhew & H. C. Longuet-Higgins, 1982). Thus, vertical magnification is taken by the visual system as indicating that the viewed surface is slanted away from the line of sight. Here, we demonstrate that the induced effect becomes stronger when the sign of the magnification alternates across the visual field. That is, as one moves horizontally across the screen, the left eye's image is alternately stretched and squashed vertically relative to the right eye's image, producing the illusion of a surface folded into triangular corrugations (H. Kaneko & I. P. Howard, 1997). For most subjects, slant judgments in this folded surface have lower thresholds and greater reliability than the classic induced effect, where magnification is applied uniformly across the whole visual field. This is remarkable, given that the disparity pattern of the classic induced effect can be produced by real surfaces with the eyes in eccentric gaze, whereas it is not clear that stripes of alternating vertical disparity could be produced by any physically realizable situation. The analogous improvement for alternating horizontal magnification is attributed to neuronal mechanisms which detect the jumps in horizontal disparity that occur at object boundaries. Our results suggest that a similar, previously unreported system may exist for vertical disparity. Jumps in vertical disparity do occur at object boundaries, and we suggest that our sur, Royal Society, Medical Research Council, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub

The contrast detection threshold of a grating located in the periphery is increased if a surrounding grating of the same frequency and orientation is present. This inhibition between center and surround has been termed surround suppression. In this work we measured the spatial frequency bandwidth of surround suppression in the periphery for different spatial frequencies (0.5, 1.1, 3, and 5 cycles/deg) of a sinusoidal grating (target) surrounded by a grating with different spatial frequencies (surround). Using a Bayesian adaptive staircase, we measured contrast detection thresholds in an 8AFC detection task in which the target (grating with a 2.3-deg Butterworth window) could appear in one of eight possible positions at 4° eccentricity. The target was surrounded by a grating (with a 18° Butterworth window) with the same or an orthogonal orientation. In each session we fixed the spatial frequency of the target and changed the spatial frequency and the orientation of the surround. When the surround was orthogonal to the target, the thresholds were similar to those obtained without surround, independent of the surrounding spatial frequency. However, when the target and surround had the same orientation and spatial frequency, the contrast threshold was increased by a factor ranging from 3 to 6 across subjects. This suppression reduced rapidly as the spatial frequency of the surround moved away from that of the target. The bandwidth of the suppressive effect depended on spatial frequency, declining from 2.9 octaves at 0.5 c/deg to 1 octave for frequencies above 3 c/deg. This is consistent with the bandwidth of individual simple cells in visual cortex and of spatial frequency channels measured psychophysically, both of which decline with increasing spatial frequency. This suggests that surround suppression may be due to relatively precise inhibition by cells with the same tuning as the target., Ministerio de Ciencia e Innovación (MICINN), Royal Society, Medical Research Council, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub

At high contrast, duration thresholds for motion direction discrimination deteriorate with increasing stimulus size. This counterintuitive result has been explained by the center-surround antagonism present in the neurons of visual area MT. Conversely, at very low contrast, direction discrimination improves with increasing size, a result that has been explained by spatial summation. In this investigation, we study the effects of stimulus shape and contrast on center-surround antagonism. Using adaptive Bayesian staircases, we measured duration thresholds of 5 subjects for vertically oriented Gabor patches of 1 cycle/deg with two types of oval Gaussian windows, one vertically elongated (Sx = 0.35, Sy = 2.5 deg) and other horizontally elongated (Sx = 2.5, Sy = 0.35 deg) moving rightward or leftward at a speed of 2 deg/s. We found that at high contrast (92%) duration thresholds were lower for vertically than horizontally elongated windows. However, at low contrast (2.8%), we found that duration thresholds were lower for horizontally than vertically elongated windows. These asymmetric results mirror the spatial non-homogeneity of the antagonistic surround found in MT neurons and suggest that the underlying center-surround antagonism is stronger along the direction of motion., Royal Society, Medical Research Council, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub

PURPOSE Human vision has a puzzling stereoscopic anisotropy: horizontal depth corrugations are easier to detect than vertical depth corrugations. To date, little is known about the function or the underlying mechanism responsible for this anisotropy. Here, we aim to find out whether this anisotropy is independent of age. To answer this, we compare detection thresholds for horizontal and vertical depth corrugations as a function of age. METHODS The depth corrugations were defined solely by the horizontal disparity of random dot patterns. The disparities depicted a horizontal or vertical sinusoidal depth corrugation of spatial frequency 0.1 cyc/deg. Detection thresholds were obtained using Bayesian adaptive staircases from a total of 159 subjects aged from 3 to 73 years. For each participant we computed the anisotropy index, defined as the log10-ratio of the detection threshold for vertical corrugations divided by that for horizontal. RESULTS Anisotropy index was highly variable between individuals but was positive in 87% of the participants. There was a significant correlation between anisotropy index and log-age (r = 0.21, P = 0.008) mainly driven by a significant difference between children and adults. In 67 children aged 3 to 13 years, the mean anisotropy index was 0.34 ± 0.38 (mean ± SD, meaning that vertical thresholds were on average 2.2 times the horizontal ones), compared with 0.59 ± 0.55 in 84 adults aged 18 to 73 years (vertical 3.9 times horizontal). This was mainly driven by a decline in the sensitivity to vertical corrugations. Children had poorer stereoacuity than adults, but had similar sensitivity to adults for horizontal corrugations and were actually more sensitive than adults to vertical corrugations. CONCLUSIONS The fact that adults show stronger stereo anisotropy than children raises the possibility that visual experience plays a critical role in developing and strengthening the stereo anisotropy., Ministerio de Economía y Competitividad (MINECO), Health Innovation Challenge Fund, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub

Psychophysical surround suppression is believed to reflect inhibitory neuronal mechanisms in visual cortex. In recent years, two psychophysical measures of surround suppression have been much studied: (i) duration thresholds on a motion-discrimination task (which are worse for larger than for smaller stimuli) and (ii) contrast thresholds on a contrast-detection task (which are worse when grating stimuli are surrounded by a stimulus of the same orientation than when they are presented in isolation or surrounded by a stimulus of orthogonal orientation). Changes in both metrics have been linked to several different human conditions, including aging, differences in intelligence, and clinical disorders such as schizophrenia, depression, and autism. However, the exact nature of the neuronal correlate underlying these phenomena remains unclear. Here, we use an individual-differences approach to test the hypothesis that both measures reflect the same property of the visual system, e.g., the strength of GABA-ergic inhibition across visual cortex. Under this hypothesis we would expect the two measures to be significantly positively correlated across individuals. In fact, they are not significantly correlated. In addition, we replicate the previously reported correlation between age and motion-discrimination surround suppression, but find no correlation between age and contrast-detection surround suppression. We conclude that the two forms of psychophysical surround suppression arise independently from different cortical mechanisms., Ministerio de Economía y Competitividad, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub

Stereo vision displays a well-known anisotropy: disparity-defined slant is easier to detect for rotations about a horizontal axis than about a vertical axis, and low-frequency sinusoidal depth corrugations are easier to detect when the corrugations are horizontal than when they are vertical. Here, we determined disparity thresholds for vertically and horizontally oriented depth corrugations with both sinusoidal and square-wave profiles. We found that the orientation anisotropy for square waves is much weaker than for sine waves and is almost independent of frequency. This weaker anisotropy for square waves can be explained by considering the Fourier harmonics present in the stimulus. Using linear models imported from the luminance and texture perception domain, the disparity thresholds for square waves can be very well predicted from those for sine waves, for both horizontally and vertically oriented corrugations. For horizontally oriented corrugations, models based on the root mean square of the output of a single linear channel or the output of multiple linear channels worked equally well. This is consistent with previous evidence suggesting that stereo vision has multiple channels tuned to different spatial frequencies of horizontally oriented disparity modulations. However, for vertically oriented corrugations, only the root mean squared output of a single linear channel explained the data. We suggest that the stereo anisotropy may arise because the stereo system possesses multiple spatial frequency channels for detecting horizontally oriented modulations in horizontal disparity, but only one for vertically oriented modulations., The Royal Society, Medical Research Council, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub

PURPOSE To investigate the cortical mechanisms that prevent diplopia in intermittent exotropia (X(T)) during binocular alignment (orthotropia). METHODS The authors studied 12 X(T) patients aged 5 to 22 years. Seventy-five percent had functional stereo vision with stereoacuity similar to that of 12 age-matched controls (0.2-3.7 min arc). Identical face images were presented to the two eyes for 400 ms. In one eye, the face was presented at the fovea; in the other, offset along the horizontal axis with up to 12° eccentricity. The task was to indicate whether one or two faces were perceived. RESULTS All X(T) patients showed normal diplopia when the nonfoveal face was presented to nasal hemiretina, though with a slightly larger fusional range than age-matched controls. However, 10 of 12 patients never experienced diplopia when the nonfoveal face was presented to temporal hemiretina (i.e., when the stimulus simulated exodeviation). Patients showed considerable variability when the single image was perceived. Some patients suppressed the temporal stimulus regardless of which eye viewed it, whereas others suppressed a particular eye even when it viewed the foveal stimulus. In two patients, the simulated exodeviation might have triggered a shift from normal to anomalous retinal correspondence. CONCLUSIONS Antidiplopic mechanisms in X(T) can be reliably triggered by purely retinal information during orthotropia, but the nature of these mechanisms varies between patients., Royal Society, Medical Research Council, Depto. de Psicología Experimental, Procesos Cognitivos y Logopedia, Fac. de Psicología, TRUE, pub