PubMed · 5866518
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R L Gregory. 1965-07-03. Seeing in depth.. https://doi.org/10.1038/207016a0
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We investigated whether vertical-shear disparity was extracted from the whole visual field or from a more local area and how global estimates of vertical disparity are derived. We also investigated the role of cyclovergence in processing shear disparity. Random-dot stereoscopic displays in various configurations were presented with horizontal-shear disparity, vertical-shear disparity or same-sign horizontal- and vertical-shear (rotation) disparity. Vertical-shear disparity introduced into only the right half of a 60 deg-wide display produced perceived inclination of the whole display when the center of shear was on the fovea, but did not produce inclination, either of the whole display or of a local area when the centre of shear was in an eccentric retinal position. A display containing dots with vertical-shear disparity mixed with dots with zero-disparity produced one inclined surface. Horizontal-shear disparity always produced inclination confined to the local area of disparity. Rotation disparity produced no inclination when introduced into the whole display, but when introduced with zero-disparity dots. It produced an inclined plane distinct from the plane defined by the zero-disparity dots. These results could be attributed to cyclovergence, which we therefore eliminated in our last experiment. We conclude that the perception of surface inclination is based on the difference between local horizontal-shear disparity and global vertical-shear disparity averaged over the whole visual field.
Random dot patterns and white-noise luminance textures are widely used in psychophysical experiments to study low-level visual processes. Because these noise patterns are broadband, bandpass filtered versions are employed to limit their frequency content. It is not recognized, however, that bandpass noise patterns have amplitude-modulation (AM) components. The AM signal is not present in the Fourier spectrum, nonetheless, it is a valid signal for second-order mechanisms. We characterize the properties of the AM signal in bandpass noise textures: the relevant periodicities of the AM signal are always much lower than the actual passband; the upper frequency limit of the AM signal increases with the linear bandwidth. We present psychophysical data to demonstrate the perceptual significance of the AM signal in bandpass noise. We provide a method for obtaining AM-free bandpass patterns, and compare psychophysical performance in experiments employing AM-present and AM-free bandpass noise patterns as stereoscopic stimuli. The results show that the AM component contributes to stereoscopic discrimination performance at large disparities. We suggest that the low-frequency AM signal is a possible confounding factor in experiments employing bandpass noise textures, and that linear filtering can isolate spatial scales effectively only for linear systems.
To better understand the role of disparity in contour integration we compared detection performance of "paths" composed of elements confined either to a single depth plane, or spanning multiple depth planes. In both cases paths defined by alignment of elements were embedded in a noise background-field made up of similar, but randomly positioned, elements covering the same depth range as the path elements. We show that a systematic disparity cue can enhance the detectability of paths which traverse depth, but that this detectability is weak compared to paths made up of elements of the same disparity. These results suggest that the outputs of disparity detectors tuned to different disparities can be linked to define contours.