Stereopsis from contrast envelopes

Department of Computing and Information Science, Queens University, Kingston, K7L 3N6, Canada
Vision Research (Impact Factor: 1.82). 08/1999; 39(14):2313-2324. DOI: 10.1016/S0042-6989(98)00271-5
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We report two experiments concerning the site of the principal nonlinearity in second-order stereopsis. The first exploits the asymmetry in perceiving transparency with second-order stimuli found by Langley et al. (1998) (Proceedings of the Royal Society of London B, 265, 1837–1845) i.e. the product of a positive-valued contrast envelope and a mean-zero carrier grating can be seen transparently only when the disparities are consistent with the envelope appearing in front of the carrier. We measured the energy at the envelope frequencies that must be added in order to negate this asymmetry. We report that this amplitude can be predicted from the envelope sidebands and not from the magnitude of compressive pre-cortical nonlinearities measured by other researchers. In the second experiment, contrast threshold elevations were measured for the discrimination of envelope disparities following adaptation to sinusoidal gratings. It is reported that perception of the envelope’s depth was affected most when the adapting grating was similar (in orientation and frequency) to the carrier, rather than to the contrast envelope. These results suggest that the principal nonlinearity in second-order stereopsis is cortical, occurring after orientation- and frequency-selective linear filtering.

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    • "Secondly, Langley et al. (1999) showed that the effect of prior adaptation to a 1-D grating on the perceived depth of the envelope was also selective for orientation (and spatial frequency). Thirdly, Langley at al. (1999) found that the energy of the envelope frequency needed to null a depth asymmetry in the perceived transparency with 2 nd -order stimuli—previously described by Langley et al. (1998)—was much greater than predicted by the pre-cortical nonlinearity. "
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    ABSTRACT: Vergence eye movements were elicited in human subjects by applying disparities to square-wave gratings lacking the fundamental ("missing fundamental", mf). Using a dichoptic arrangement, subjects viewed gratings that were identical at the two eyes except for a phase difference of 1/4 wavelength so that, based on the nearest-neighbor matches, the features and the 4n+1 harmonics (5th, 9th, etc.) all had binocular disparities of one sign, whereas the 4n-1 harmonics (3rd, 7th, etc.) all had disparities of the opposite sign. Further, the amplitude of the ith harmonic was proportional to 1/i. Using the electromagnetic search coil technique to record the positions of both eyes indicated that the earliest vergence eye movements elicited by these disparity stimuli had ultra-short latencies (minimum, <65 ms) and were always in the direction of the most prominent harmonic, the 3rd, but their magnitudes fell short of those elicited when the same disparities were applied to pure sinusoids whose spatial frequency and contrast matched those of the 3rd harmonic. This shortfall was evident in both the horizontal vergence responses recorded with vertical grating stimuli and the vertical vergence responses recorded with horizontal grating stimuli. When the next most prominent harmonic, the 5th, was removed from the mf stimulus (creating the "mf-5" stimulus) the vertical vergence responses showed almost no shortfall-indicating that it had been almost entirely due to that 5th harmonic-but the horizontal vergence responses still showed a small shortfall, at least with higher contrast stimuli. This small shortfall might represent a very minor contribution from higher harmonics and/or distortion products and/or a feature-based mechanism. We conclude that the earliest disparity vergence responses-especially vertical-were strongly dependent on the major Fourier components of the binocular images, consistent with early spatial filtering of the monocular visual inputs prior to their binocular combination as in the disparity-energy model of complex cells in striate cortex [Ohzawa, I., DeAngelis, G. C., & Freeman, R. D. (1990). Stereoscopic depth discrimination in the visual cortex: neurons ideally suited as disparity detectors. Science, 249, 1037-1041].
    Full-text · Article · Nov 2006 · Vision Research
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    • "Perhaps we are unable do so in the absence of intermediate orientations in the scene to serve as common reference stimuli. Alternatively, second-order stimulus features (Hess and Wilcox, 1994; Schor et al., 1998; Langley et al., 1999; McKee et al., 2004) might play this role in naturalistic scenes. Another possibility is that normalization does occur, but only among the spatially overlapping components of a single object, such as the components of the plaids used here. "
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    ABSTRACT: The left and right eyes receive subtly different images from a visual scene. Binocular disparities of retinal image locations are correlated with variation in the depth of objects in the scene and make stereoscopic depth perception possible. Disparity stereoscopically specifies a stimulus; changing the stimulus in a way that conserves its disparity leaves the stimulus stereoscopically unchanged. Therefore, a person's ability to use stereo to see the depth separating any two objects should depend only on the disparities of the objects, which in turn depend on where the objects are, not what they are. However, I find that the disparity difference between two stimuli by itself predicts neither stereoacuity nor perceived depth. Human stereo vision is shown here to be most sensitive at detecting the relative depth of two gratings when they are parallel. Rotating one grating by as little as 10 degrees lowers sensitivity. The rotation can make a perceptible depth separation invisible, although it changes neither the relative nor absolute disparities of the gratings, only their relative orientations. The effect of relative orientation is not confined to stimuli that, like gratings, vary along one dimension or to stimuli perceived to have a dominant orientation. Rather, it is the relative orientation of the one-dimensional components of stimuli, even broadband stimuli, that matters. This limit on stereoscopic depth perception appears to be intrinsic to the visual system's computation of disparity; by taking place within orientation bands, the computation renders the coding of disparity inseparable from the coding of orientation.
    Full-text · Article · Oct 2006 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
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    • "Previously, a substantial number of psychophysical studies investigated binocular processing of contrast-envelope cues (Hess and Wilcox, 1994; Wilcox and Hess, 1995, 1996, 1997; Schor et al., 1998; Edwards et al., 1999, 2000; Langley et al., 1999; McKee et al., 2004). These studies have shown that we are able to perceive depth based purely on binocular disparities of contrastenvelope cues and suggested that such stereopsis is mediated by nonlinear mechanisms that are distinct from that for extracting luminance-based cues. "
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    ABSTRACT: Humans and animals use visual cues such as brightness and color boundaries to identify objects and navigate through environments. However, even when these cues are not available, we can effortlessly perform these tasks by using second-order cues such as contrast variation (envelope) of patterns on surfaces. Previously, numerous psychophysical studies examined properties of binocular depth processing based on the contrast-envelope cues and suggested the existence of a stereo system that uses these cues. However, its physiological substrate has not been identified yet. Here, we show that a subset of cortical neurons in cat area 18 show binocular interactions for the contrast-envelope stimuli. These neurons are capable of representing a variety of depths in the three-dimensional space based on the information available from contrast cues alone. Furthermore, these neurons show similar disparity-tuning curves for borders defined by both luminance and contrast cues. This cue-invariant tuning is consistent with a linear binocular convergence model for monocular luminance and contrast-envelope processing pathways.
    Preview · Article · May 2006 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
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