A temporal analogue of MacAdam ellipses. Description of rod-cone interaction.
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Color Perception↗
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Hyperacuity in the detection of absolute and differential displacements of random dot patterns.
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Depth Perception↗
Colour selectivity in the tilt after-effect: comments upon Wade and Wenderoth.
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Color Perception↗
Discrimination and detection thresholds: the effect of observer criterion on the spatial properties of chromatic and achromatic mechanisms.
Spectral sensitivity functions were determined for structured test targets presented on a white background field. Data obtained for threshold detection of the targets are consistent with results obtained from previous studies which used unstructured test fields showing functions, with three maxima, dependent on opponent colour mechanisms. Data obtained from threshold discrimination measurements show a marked reduction in the blue sensitivity when 1 degree test targets are used. This effect decreases significantly when larger targets are used. A change of the observer's criterion can significantly alter the relative contributions of the achromatic and chromatic channels at threshold.
Color Perception↗
The spatial requirements for fine stereoacuity.
The finest human stereoacuity, which in some gifted individuals amounts to the detection of binocular disparities of less than 5 arc sec, is found with isolated vertical target lines 10-15 min of arc in length. Summation along the vertical dimension of the lines is physiological in origin, and is not due to probability summation of disparity signals from multiple point targets. What is being summed is not the quantity of light, but rather information about its distribution--positional signals leading to finer judgments of disparity. Increasing target length beyond 20 arc min produces little improvement in disparity thresholds because stereoacuity decreases at even small eccentricities. The threshold at 30 min away from the fixation point is nearly twice its value at the very center of the fovea. Fine stereoacuity has one additional constraint: the compared features should be disjoint. Connecting lines between test and reference targets can increase the stereo threshold markedly.
Depth Perception↗
Texture changes versus size changes as stimuli for motion in depth.
As an object approaches the eye, its retinal image size grows larger and its surface texture appears to grow coarser. We compare these two visual correlates of motion in their effectiveness as stimuli for motion in depth. In some experiments texture and object size both expanded or both contracted; in other experiments the two stimuli were pitted against each other. When texture and size change as for a rigid, nonrotating real world object an untextured square can be a more effective stimulus for motion in depth than the same square with texture. On way of describing this finding is to calculate the departure from linear summation of texture and size contributions. The departure is greatest when texture is static, being even greater than when texture changes in the opposite direction to size.
Adaptation, Ocular↗
The low level motion system has both chromatic and luminance inputs.
Adaptation to moving isoluminant gratings induces a motion after-effect (MAE). Isoluminant gratings are less effective at inducing and at nulling MAEs than are luminance gratings. These results are consistent with a low-level motion detection system which operates on signals from mechanisms which show both spatial and chromatic opponency.
Adaptation, Ocular↗
Detection threshold differences to crossed and uncrossed disparities.
Using a sample of 85 subjects measurements were made of minimum stimulus durations necessary for detection of crossed and uncrossed disparity stimuli which were presented in five positions in the visual field: centre, lower, upper, right, and left field. The results indicated large detection duration differences between the two disparity conditions, with a marked superiority for crossed disparity detection at all positions. A left-right visual field anisotropy was demonstrated for crossed disparity stimuli.
Depth Perception↗
Is edge information for stereoacuity spatially channeled?
Models of stereopsis generally assume that binocular correspondence is achieved through alignment of luminance edges in the two eyes. Yet the stimulus properties which constitute edge information for stereopsis have not been defined. Three experiments explored the nature of these stimulus properties. The first two experiments tested whether local luminance gradient and the relative phase of spatial components supply information about the position of edges which influences stereosensitivity. In Expt 1, stereothresholds were reduced with increased spatial frequency or contrast of sinusoidal luminance gratings, but no simple relationship between target luminance gradient and stereosensitivity was found. In Expt 2, stereothresholds were equivalent for targets having identical spatial frequency components, but differing in maximum luminance gradient and the relative spatial phase of their components. In addition, stereothresholds were lower for the target having the higher contrast in pairs of unequal-contrast targets having equal maximum luminance gradients. These results suggest that the properties of luminance gradient and relative spatial phase do not influence stereosensitivity independently of spatial frequency and contrast. Experiment 3 directly tested whether stereosensitivity depends on edge information whose disparity is detected independently at different spatial scales. Stereothresholds for IF + 5F compound targets were found to be equivalent to thresholds obtained separately with the more sensitive of the two components. Taken together with a compressive nonlinearity in the relationship between contrast and stereothreshold obtained by others (Halpern and Blake, 1989; Legge and Gu, 1989) and replicated in Expt 1, the results of Expt 3 indicate that, whatever the exact nature of the luminance discontinuity information utilized in disparity detection, it is processed independently at different spatial scales.
Contrast Sensitivity↗
Vernier acuity: effects of chromatic content, blur and contrast.
Offset thresholds were measured for targets whose horizontal profiles were either Gaussian or odd-symmetric Gabor functions. The targets were defined either by variation along the constant B or the constant R & G axes of color space or by luminance variation. Blur was varied in the case of the Gaussian targets by varying the standard deviation of the distribution and in the case of the Gabor functions by varying the spatial frequency of the sinusoidal component. Detection thresholds for all the stimuli were measured. The contrast of the targets used in the measurement of offset thresholds was varied from just above detection threshold to the maximum that could be produced. The offset thresholds obtained with targets of different chromatic composition are nearly identical when blur and contrast relative to detection threshold are held constant. We attribute the slight advantage held by luminance targets over chromatic targets for narrow Gaussians to the detectability of low frequency components of the chromatic targets which are of little use in the assessment of offsets. This conjecture is supported by the complete absence of such an advantage in the case of Gabor targets.
Color Perception↗
Brightness perception and filling-in.
Three experiments were performed in which a stimulus with homogeneous color and luminance was masked by a second stimulus containing contours. In the first experiment the target was a large white disk and the mask was a white circle concentric with the disk but of smaller radius. We found that the mask had a large (up to 2 log unit) suppressive effect on the brightness of the target, but only inside the radius of the mask. With monoptic presentation of target and mask, the greatest suppression was observed with an SOA of 50-100 msec. With dichoptic presentation the strongest suppression was obtained with simultaneous stimuli. The second experiment demonstrated that the latest time at which masking was effective was correlated with the distance between the edge of the target stimulus and the contour in the mask. One possible explanation of the results from these two experiments is that the masking contour is interfering with the propagation of a brightness signal from the target's border. In the third experiment gaps were introduced into the masking circle. Surprisingly, even with rather large gaps there was significant suppression of brightness in the center of the target. We have encountered difficulties attempting to account for these findings with known physiological mechanisms such as lateral inhibition. A qualitative explanation of the results that looks promising is a two-component process involving brightness filling-in and smoothing to satisfy fixed boundary conditions at contours.
Color Perception↗
Surface segmentation mechanisms and motion perception.
Two studies are presented which explore how depth information is determined from conditions of transparency and whether this information combines with other depth information to determine the segmenting of motion information on the basis of likely surface boundaries. The first study explored how binocular disparity combines with monocular depth cues associated with transparency to determine whether subjects see one or multiple surfaces in depth in static displays. When transparency provided a depth cue that was consistent with binocular disparity, depth discrimination thresholds were at normal levels. However, if transparency was inconsistent with the binocular disparity, depth discrimination thresholds were elevated, indicating that subjects had difficulty seeing distinct surfaces lying in separate depth planes. Moreover, threshold elevations were found to correspond to the reductions in contrast between the intersecting contours of the stimulus, suggesting that the strength of perceived depth from transparency is the result of attenuated responses from competing contrast sensitive T-junction mechanisms responsible for the detection of opaque occlusion. A second experiment explored whether the grouping of local motion signals relied on surface interpretations that result from the interaction of transparency and disparity. Surface interpretations were manipulated in moving plaids by combining transparent layering and binocular disparity to show that the motion arising from contours is grouped together (pattern motion) when these cues support the existence of a single surface, and is segregated (component motion) when they support separate surfaces. When these cues were consistent, only small disparity differences were required for the gratings to appear as separately moving surfaces. However, when they were inconsistent, greater disparities were required (about a factor of 2 greater). Taken together, these studies demonstrate that the grouping of local motion information is not resolved within the motion system alone. Information seemingly unrelated to motion processing, namely surface segmentation cues, is used to determine whether or not motion information arising from various contours is pooled together to determine a single motion.
Cues↗
Object classification for human and ideal observers.
We describe a novel approach, based on ideal observer analysis, for measuring the ability of human observers to use image information for 3D object perception. We compute the statistical efficiency of subjects relative to an ideal observer for a 3D object classification task. After training to 11 different views of a randomly shaped thick wire object, subjects were asked which of a pair of noisy views of the object best matched the learned object. Efficiency relative to the actual information in the stimuli can be as high as 20%. Increases in object regularity (e.g. symmetry) lead to increases in the efficiency with which novel views of an object could be classified. Furthermore, such increases in regularity also lead to decreases in the effect of viewpoint on classification efficiency. Human statistical efficiencies relative to a 2D ideal observer exceeded 100%, thereby excluding all models which are sub-optimal relative to the 2D ideal.
Depth Perception↗
Independent encoding of surface orientation and surface curvature.
Marr [(1982) Vision, San Francisco, Calif.: Freeman] proposed that we represent surface geometry in terms of a viewer-centred description of surface orientation and distance. This description is computed by a range of independent processing systems which take as input particular kinds of information present in images, like surface texture, shading, retinal disparity and motion parallax. The outputs of these modules are integrated in order to provide a unitary representation of the layout of visible surfaces. Higher order properties of surface geometry, like surface curvature, might be computed from this symbolic representation or might be encoded independently from the visual information available at the retinae. We measured surface slant and surface curvature discrimination thresholds for surface patches defined by shading, texture and retinal disparity as a function of the elevation of the illumination. We found that observers judgements about the curvature of local surface patches were too precise to be based on a symbolic representation of surface orientation and we conclude that surface curvature is computed directly from depth cues present in the retinal images.
Depth Perception↗
The visual perception of three-dimensional shape from self-motion and object-motion.
To evaluate the influence of egomotion on the three-dimensional visual processing of structure-from-motion (SFM), we compared the visual discrimination between planar and spherical surfaces during subject-translation, object-translation, or rotation of the object in depth. Performance was the best for object-rotation, intermediate for subject-translation, and the poorest for object-translation--and thus increased with the quality of retinal image stabilization achieved in the different conditions. This suggests that the major role of self-motion information was to stabilize retinal images. In view of previous results, we propose that the interactions between self-motion information and SFM are reduced to functional complementarity, in the sense that self-motion can lift visual ambiguities but does not improve the sensitivity of SFM processes.
Adult↗
Influence of surface attitude and curvature scaling on discrimination of binocularly presented curved surfaces.
We report on the ability of human observers to discriminate local second-order shape of quadratic stereo-defined surfaces. Local second-order shape can be specified by two parameters: the curvedness (a scale-dependent quantity describing overall curvature of a shape) and the shape index (a scale-independent quantity describing the shape's appearance). We measured shape index discrimination thresholds of shapes subject to attitude and curvedness transformations. Results show that neither slant nor curvedness manipulations affect or bias observers' judgements of surface shape. Furthermore, ideal detector simulations show that observers do not perform optimally: they do not exploit all available information.
Depth Perception↗
Stimulus discriminability in visual search.
We measured the probability of detecting the target in a visual search task, as a function of the following parameters: the discriminability of the target from the distractors, the duration of the display, and the number of elements in the display. We examined the relation between these parameters at criterion performance (80% correct) to determine if the parameters traded off according to the predictions of a limited capacity model. For the three dimensions that we studied, orientation, color, and spatial frequency, the observed relationship between the parameters deviates significantly from a limited capacity model. The data relating discriminability to display duration are better than predicted over the entire range of orientation and color differences that we examined, and are consistent with the prediction for only a limited range of spatial frequency differences--from 12 to 23%. The relation between discriminability and number varies considerably across the three dimensions and is better than the limited capacity prediction for two of the three dimensions that we studied. Orientation discrimination shows a strong number effect, color discrimination shows almost no effect, and spatial frequency discrimination shows an intermediate effect. The different trading relationships in each dimension are more consistent with early filtering in that dimension, than with a common limited capacity stage. Our results indicate that higher-level processes that group elements together also play a strong role. Our experiments provide little support for limited capacity mechanisms over the range of stimulus differences that we examined in three different dimensions.
Color Perception↗
Integration of shading and texture cues: testing the linear model.
One of the first attempts to develop a formal model of depth cue integration is to be found in Maloney and Landy's [(1989) Proceedings of the SPIE: Visual communications and image processing, Part 2 (pp. 1154-1163)] "human depth combination rule". They advocate that the combination of depth cues by the visual system is best described by a weighted linear model. The present experiments tested whether the linear combination rule applies to the integration of texture and shading. As would be predicted by a linear combination rule, the weight assigned to the shading cue did not vary as a function of its curvature value. However, the weight assigned to the texture cue varied systematically as a function of the curvature values of both cues. Here we describe a non-linear model which provides a better fit to the data. Redescribing the stimuli in terms of depth rather than curvature reduced the goodness of fit for all models tested. These results support the hypothesis that the locus of cue integration is a curvature map, rather than a depth map. We conclude that the linear combination rule does not generalize to the integration of shading and texture, and that for these cues it is likely that integration occurs after the recovery of surface curvature.
Cues↗