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PubMed1986

Shared pathways for rod and cone vision.

We have used heterochromatic gratings falling on 10 deg temporal retina to measure the spatial contrast sensitivities of the isolated rod and cone systems in the mesopic range. As the level of illumination was raised within this range, the contrast sensitivity of the rod system increased, reaching a peak of about 50 (and providing an acuity of 6 c/deg) at 20 scot. td, whereupon the rod system began to saturate. Over most of the mesopic range the sensitivity of the cone system was lower than that of the rod system, although it provided better acuity (up to 15 c/deg). Within the range of spatial frequencies capable of exciting both rod and cone systems, a grating that excited only rods was indistinguishable from a grating of the same spatial frequency that excited only cones. Moreover, contrast adaptation to gratings that excited either rods or cones raised threshold for gratings that excited rods or cones. From these results we conclude that signals from rods and cones travel together in pathways subserving the detection of low spatial frequencies, while only signals from cones travel in pathways subserving the detection of high spatial frequencies.

Adaptation, Ocular↗
PubMed1986

Increment spectral sensitivities for spatial periodic grating patterns: evidence for variable tuning of the chromatic system.

Increment spectral sensitivities were measured for spatial periodic grating patterns. First, the increment threshold was determined as a function of wavelength, for various spatial frequencies and white-background intensities. Second, the additivity of test mixture was tested. Thirdly, the threshold vs intensity curves were determined for various spatial frequencies of test monochromatic stimuli. Finally, the increment threshold was determined as a function of spatial frequency. The following results were obtained: the background intensity and the spatial frequency affected the increment spectral sensitivity functions in different manners; the result of test mixture showed subadditivity for high background intensity and approximately linear additivity for low background intensity; the resultant t.v.i. curves converged toward Weber's law at high background intensity; and the sensitivity vs spatial frequency curve showed a loss of the sensitivity at low spatial frequency for high background intensity. These results called in question the existing hypothesis of separate chromatic and achromatic systems whose spectral tunings are invariant with a spatial parameter, but favored the hypothesis of variable tuning of the chromatic system.

Color Perception↗
PubMed1986

Temporal properties of brightness and color induction.

With a matching procedure, we studied the temporal properties of direct brightness (or lightness) and chromatic changes (produced by modulation of the region being matched) and induced brightness and chromatic changes (produced by modulation of the surround of the region being matched). The amount of direct brightness and color change was found to vary only slightly with temporal frequency over the 0.5-8 Hz range studied, whereas induced changes were found to occur only at low temporal frequencies, below about 2.5 Hz. With high temporal-frequency modulation of the surround, the induced patterns appeared to flicker but not to change in brightness or color. Despite the fact that chrominance and luminance temporal contrast sensitivity functions are very different, the temporal induction curves for color and brightness were very similar. However, brightness induction was found to increase approximately linearly with increasing surround modulation up to very high levels, whereas the amount of color induction was much less dependent on the modulation depth of the surround.

Color Perception↗
PubMed1986

Pre-stereoptic binocular vision in infants.

In a preferential looking experiment, identical patterns (vertical stripes) were presented to both eyes on one of two screens while orthogonal patterns (vertical stripes in one eye and horizontal stripes in the other) were presented on the other screen. Most infants younger than 3.5 months of age originally showed a preference for the dichoptic (interocularly orthogonal) pattern. At an average age of 3.5 months, however, they showed a sudden shift of preference from this pattern to the interocularly identical pattern. The full shift from a preference for one stimulus to the other (both statistically significant) occurred within a few weeks in most cases. The onset age of the shift in preference agreed with the onset age of fusion-rivalry discrimination found in a previous study (Birch et al., 1985). The original preference for the bincularly orthogonal patterns may be interpreted as a preference for a grid (interocularly emergent intersections) over a grating, judging from results of two control experiments. These data suggest that the pre-stereoptic system non-selectively combines information from the two eyes without regard to edge orientation because it loses eye-of-origin information at a relatively early stage of binocular visual processing. Thus, the pre-stereoptic system does not have the capability of interocular suppression. The theoretical and clinical significance of the new findings are discussed along with a neuronal model of cortical development of ocular segregation and binocular pathways.

Adult↗
PubMed1987

Perception of random-dot symmetry and apparent movement at and near isoluminance.

There have been conflicting reports on whether apparent movement in random-dot kinematograms is abolished at isoluminance. The present results suggest that it is, provided that dynamic (uncorrelated) surrounds are used, and the subject has to report the shape of the target rather than the presence of movement in an isolated portion of the target. On the other hand, perception of random-dot symmetry is still possible at isoluminance. The reason for this difference appears to be the need for exact-position information in movement but not symmetry perception. Control experiments suggest that the effects are not due to artefacts such as chromatic aberration in the eye.

Adult↗
PubMed1987

Detecting the displacements of spatial beats: a monocular capability.

Sensitivity to the sudden displacement (phase shift) of a single monocularly presented sinusoidal grating is increased when a static grating of similar spatial frequency is presented to the same eye. If the static grating is presented to the other eye instead sensitivity is, at best, halved. This demonstration implies that monocular and binocular visual pathways differ in their sensitivity to spatial variations of contrast. In addition it provides another example in which the monocular visual pathways are more sensitive to spatial displacements than the binocular pathways.

Depth Perception↗
PubMed1987

Spatial influences on colour opponent contributions to pattern detection.

The contribution of colour opponent mechanisms to detection thresholds is investigated at different spatial frequencies by presenting monochromatic, sinusoidal gratings on a uniform white background. Colour opponent mechanisms, characterised by a triple peaked spectral sensitivity function, determine threshold at low spatial frequencies (below 1 c/deg) and their contribution flattens the Weber function. They display low pass spatial frequency characteristics, becoming relatively more sensitive than non-opponent mechanisms as spatial frequency decreases. Colour opponent contributions are not revealed when the test grating and background are presented dichoptically.

Color Perception↗
PubMed1987

Stereoscopic contours and optokinetic nystagmus in normal and stereoblind subjects.

Moving stereoscopic contours in a dynamic random-dot stereogram have been previously shown to induce optokinetic nystagmus in subjects with normal stereopsis. For this to be validated as an objective test of stereopsis, stereoblind subjects must also be shown not to develop OKN, especially since it has been shown that the optomotor system of stereoblind individuals retains sensitivity to some cyclopean stimuli. In this report we verify that stereoblind subjects do not have an optomotor response to stereoscopic contours--regardless of the alignment angle at which the stereo image pair is presented.

Depth Perception↗
PubMed1988

Illusory contours induced by isoluminant chromatic patterns.

An illusory-contour was induced by abutting colored gratings embedded in the white field under isoluminance condition. The present study specified the stimulus conditions that invoked the just-perceptible illusory contours for the isoluminant chromatic patterns. The results showed that the purity difference between the colored lines and the white field required for the illusory-contour perception gave a function closely resembling in shape the function obtained for saturation discrimination. Increasing the width of lines reduced threshold for the perception of illusory contours, while the line spacing had no significant effect. The dependence of the illusory-contour perception upon saturation could be accounted for by considering the extraction of the edge and colour information by means of the opponent-color processes observed for the cells in the retina and LGN. On the other hand, the effects of the spatial parameters met the response properties observed for the cells in the visual cortex. It is suggested that the perception of illusory contour may result from hierarchical organization from the retina to cortex in the visual system.

Color Perception↗
PubMed1988

Shape similarity and distance disparity as apparent motion correspondence cues.

Apparent motion is perceived when two spots of light are presented successively in different locations. When more than one element is present in each frame, there is a correspondence problem in matching the elements in one frame with those in the other. We report the effects of shape similarity and distance disparity on the correspondence process. Twenty subjects were tested using a 2-AFC design. We found that both shape and distance cues are used by the correspondence process: when distance is the only cue the motion which is usually perceived is that involving the shorter distance; when shape is the only cue the motion involving two elements of the same shape is preferred. We also studied the interaction between the two cues when both were present. Quantitative measures of the relative strengths of these effects and of their interaction are reported. A Signal Detection Theory model is used to analyze these apparent motion correspondence effects.

Cues↗
PubMed1988

Integrating stereopsis with monocular interpretations of planar surfaces.

Experiments are reported that involved spatial judgments of planar surfaces that had contradictory stereo and monocular information. Tasks included comparing the relative depths of two points on the depicted surface and judging the surface's apparent spatial orientation. It was found that for planar surfaces the 3D perception was dominated by the monocular interpretation, despite the strongly contradictory stereo information. We propose that stereo information is effectively integrated only where the surface exhibits curvature features or edge discontinuities, i.e. where the second spatial derivatives of disparity are nonzero. Planar surfaces induce constant gradients of disparity and are thus effectively featureless to stereopsis. Further observations are reported regarding nonplanar surfaces, where contradictory monocular information can still be effectively rivalrous with that suggested stereoscopically.

Depth Perception↗
PubMed1989

Kinetic depth effect and optic flow--I. 3D shape from Fourier motion.

Fifty-three different 3D shapes were defined by sequences of 2D views (frames) of dots on a rotating 3D surface. (1) Subjects' accuracy of shape identifications dropped from over 90% to less than 10% when either the polarity of the stimulus dots was alternated from light-on-gray to dark-on-gray on successive frames or when neutral gray interframe intervals were interposed. Both manipulations interfere with motion extraction by spatio-temporal (Fourier) and gradient first-order detectors. Second-order (non-Fourier) detectors that use full-wave rectification are unaffected by alternating-polarity but disrupted by interposed gray frames. (2) To equate the accuracy of two-alternative forced-choice (2AFC) planar direction-of-motion discrimination in standard and polarity-alternated stimuli, standard contrast was reduced. 3D shape discrimination survived contrast reduction in standard stimuli whereas it failed completely with polarity-alternation even at full contrast. (3) When individual dots were permitted to remain in the image sequence for only two frames, performance showed little loss compared to standard displays where individual dots had an expected lifetime of 20 frames, showing that 3D shape identification does not require continuity of stimulus tokens. (4) Performance in all discrimination tasks is predicted (up to a monotone transformation) by considering the quality of first-order information (as given by a simple computation on Fourier power) and the number of locations at which motion information is required. Perceptual first-order analysis of optic flow is the primary substrate for structure-from-motion computations in random dot displays because only it offers sufficient quality of perceptual motion at a sufficient number of locations.

Depth Perception↗
PubMed1990

"Colour constancy" in Mondrian patterns: a partial cancellation of physical chromaticity shifts by simultaneous contrast.

Edwin Land's Mondrian demonstrations (Land 1977, 1983, 1986a) are striking examples that the perceived colours of objects are largely independent of the chromaticity of the light incident upon them. Attempts to implement this independence in artificial vision systems have renewed interest in colour constancy and contrast, and the explanation of these phenomena in the Retinex theory. We use colour matches to demonstrate that departures from "colour constancy" are large and that it is possible to obtain the same colour shifts when the complex Mondrian pattern is replaced by a homogeneous grey field surrounding a test patch. A given patch has the same colour when surrounded by the Mondrian as when set in a grey background, provided that the grey represents the spatially weighted average of the Mondrian. Neither the colour shifts nor the equivalence of this neutral surround are correctly predicted by the Retinex theory. The phenomenon of partial cancellation of physical chromaticity shifts with changes of illuminant thus reduces to one of simultaneous contrast and adaptation where a spatio-chromatic and luminance average over a Mondrian pattern is the same as for a grey surround. Experiments with simultaneous contrast demonstrate that spatial weighting factors need to be applied in computations of the effect of the separate areas of a complex Mondrian pattern.

Algorithms↗
PubMed1990

da Vinci stereopsis: depth and subjective occluding contours from unpaired image points.

Distant surfaces are occluded by nearer surfaces to different extents in the two eyes, leading to the existence of unpaired image points visible in one eye and not the other. An ecological analysis of the real world situation that could have given rise to such unpaired points indicates the presence of a depth constraint zone, defined by visibility lines between which possible real world points must lie. The leading edge of this zone starts at the edge of a fused binocular occluding surface and recedes linearly with increases in horizontal distance to the unpaired point. Psychophysical evidence indicates that the human visual system makes use of this unpaired information in a remarkably adaptive manner, showing an increase in perceived depth for increasing horizontal separations between the unpaired target and fused edge, at least over a significant angular range (approx. 25-40 min arc). We also show that unpaired points in binocular images can lead to the formation of subjective occluding contours and surface having the qualitatively appropriate sign of depth. Furthermore, we show that the visual system could not recover depth of unpaired points camouflaged from the other eye against silhouettes. Our findings indicate that the visual system makes use of occlusive relations in the real world to recover depth, contour, and surface from unpaired points. The fact that such processes must utilize eye-of-origin information implies that they share this essential characteristic with classical or Wheatstone stereopsis. The necessity of eye-of-origin information also suggests that the processing may begin relatively early in cortical visual processing, possibly as early as V1. Finally, the novel emergence of subjective occluding contours from unpaired monocular stimuli raises the possibility that this process is mediated by visual experience, built up by the association of unpaired points and occluding contours.

Depth Perception↗
PubMed1995

Perception of three-dimensional shape from ego- and object-motion: comparison between small- and large-field stimuli.

We compare the performance in the detection of the shape of concave, planar and convex surfaces for small-field (8 deg) and large-field (90 deg) stimuli. Shape is perceived from head translations, object translations and object rotations. We find large differences between small-field and large-field stimulation. For small-field stimulation performance is best for object rotation, intermediate for self-motion and worst for object translation. For large-field stimulation performance is similar across conditions. Few errors on the sign of the curvature are found for self-motion for both field sizes, indicating that self-motion information disambiguates the curvature sign. For object rotation with small-field stimulation, the concave-convex ambiguity is strong with many apparent deformations. In contrast, large-field curvature signs are always accurately reported, suggesting that the weight of the rigidity hypothesis depends on field size.

Depth Perception↗
PubMed1995

Measurement and modeling of depth cue combination: in defense of weak fusion.

Various visual cues provide information about depth and shape in a scene. When several of these cues are simultaneously available in a single location in the scene, the visual system attempts to combine them. In this paper, we discuss three key issues relevant to the experimental analysis of depth cue combination in human vision: cue promotion, dynamic weighting of cues, and robustness of cue combination. We review recent psychophysical studies of human depth cue combination in light of these issues. We organize the discussion and review as the development of a model of the depth cue combination process termed modified weak fusion (MWF). We relate the MWF framework to Bayesian theories of cue combination. We argue that the MWF model is consistent with previous experimental results and is a parsimonious summary of these results. While the MWF model is motivated by normative considerations, it is primarily intended to guide experimental analysis of depth cue combination in human vision. We describe experimental methods, analogous to perturbation analysis, that permit us to analyze depth cue combination in novel ways. In particular these methods allow us to investigate the key issues we have raised. We summarize recent experimental tests of the MWF framework that use these methods.

Bayes Theorem↗