Simulations of adaptation and color appearance in observers with varying spectral sensitivity.

A model of adaptation and visual coding was used to simulate how color appearance might vary among individuals that differ only in their sensitivity to wavelength. Color responses to images were calculated for cone receptors with spectral sensitivities specific to the individual, and in postreceptoral mechanisms tuned to different combinations of the cones. Adaptation was assumed to normalize sensitivity within each cone and postreceptoral channel so that the average response to an ensemble of scenes equaled the mean response in channels defined for the reference observer. Image colors were then rendered from the adapted channels' outputs. The transformed images provide an illustration of the variations in color appearance that could be attributed to differences in spectral sensitivity in otherwise identical observers adapted to identical worlds, and examples of these predictions are shown for both normal variation (e.g. in lens and macular pigment) and color deficiencies (anomalous trichromacy). The simulations highlight the role that known processes of adaptation may play in compensating color appearance for variations in sensitivity both within and across observers, and provide a novel tool for visualizing the perceptual consequences of any variation in visual sensitivity including changes associated with development or disease.

Variations in normal color vision. V. Simulations of adaptation to natural color environments.

Modern accounts of color appearance differ in whether they assume that the perceptual primaries (e.g., white and the unique hues of red, green, blue, and yellow) correspond to unique states determined by the spectral sensitivities of the observer or by the spectral statistics of the environment. We examined the interaction between observers and their environments by asking how color perception should vary if appearance depends on fixed responses in a set of color channels, when the sensitivities of these channels are adapted in plausible ways to different environments. Adaptation was modeled as gain changes in the cones and in multiple postreceptoral channels tuned to different directions in color-luminance space. Gains were adjusted so that the average channel responses were equated across two environments or for the same environment during different seasons, based on sets of natural outdoor scenes (Webster et al., 2007). Because of adaptation, even observers with a shared underlying physiology should perceive color in significantly and systematically different ways when they are exposed to and thus adapted by different contexts. These include differences in achromatic settings (owing to variations in the average chromaticity of locations) and differences in perceived hue (because of differences in scene contrasts). Modeling these changes provides a way of simulating how colors might be experienced by individuals in different color environments and provides a measure of how much color appearance might be modulated for a given observer by variations in the environment.

Looking at perspective pictures from too far, too close, and just right.

A central problem for psychology is vision's reaction to perspective. In the present studies, observers looked at perspective pictures projected by square tiles on a ground plane. They judged the tile dimensions while positioned at the correct distance, farther or nearer. In some pictures, many tiles appeared too short to be squares, many too long, and many just right. The judgments were strongly affected by viewing from the wrong distance, eye height, and object orientation. The authors propose a 2-factor angles and ratios together (ART) theory, with the following factors: the ratio of the visual angles of the tile's sides and the angle between (a) the direction to the tile from the observer and (b) the perpendicular, from the picture plane to the observer, that passes through the central vanishing point.

Blind man draws using diminution in three dimensions.

Can the principle of convergence in three spatial dimensions be reflected in drawings by the congenitally blind? A man who had been totally blind since birth was asked to draw scenes such as a tabletop with three cubes receding to the observer's left side. He used converging lines to show the tops of the cubes receding in depth. He drew the cubes to the left smaller than the cube in front of the observer. He drew faces of cubes to the left with tilted lines, pointing to below the front face of the cube in front. The result approximates three-point perspective. We note that the directions of objects from a vantage point in touch converge much as they do in vision.

Selectivity of face aftereffects for expressions and anti-expressions.

Adapting to a facial expression can alter the perceived expression of subsequently viewed faces. However, it remains unclear whether this adaptation affects each expression independently or transfers from one expression to another, and whether this transfer impedes or enhances responses to a different expression. To test for these interactions, we probed the basic expressions of anger, fear, happiness, sadness, surprise, and disgust, adapting to one expression and then testing on all six. Each expression was varied in strength by morphing it with a common neutral facial expression. Observers determined the threshold level required to correctly identify each expression, before or after adapting to a face with a neutral or intense expression. The adaptation was strongly selective for the adapting category; responses to the adapting expression were reduced, while other categories showed little consistent evidence of either suppression or facilitation. In a second experiment we instead compared adaptation to each expression and its anti-expression. The latter are defined by the physically complementary facial configuration, yet appear much more ambiguous as expressions. In this case, for most expressions the opposing faces produced aftereffects of opposite sign in the perceived expression. These biases suggest that the adaptation acts in part by shifting the perceived neutral point for the facial configuration. This is consistent with the pattern of renormalization suggested for adaptation to other facial attributes, and thus may reflect a generic level of configural coding. However, for most categories aftereffects were stronger for expressions than anti-expressions, pointing to the possible influence of an additional component of the adaptation at sites that explicitly represent facial expressions. At either level our results are consistent with other recent work in suggesting that the six expressions are defined by dimensions that are largely independently normalized by adaptation, possibly because the facial configurations conveying different expressions vary in independent ways.

Visual discomfort and natural image statistics.

Images with excessive energy at medium spatial frequencies (Fernandez and Wilkins, 2008 Perception 37 1098-1113), or that have high color contrast and little or no luminance contrast (Wilkins et al, 2008 Perception 37 Supplement, 144-145) appear uncomfortable or aversive and can induce headaches in hypersensitive observers. Such stimuli are uncharacteristic of natural images, and we examined whether visual discomfort more generally increases with deviations from the spatial and chromatic properties of natural scenes. Observers rated the level of discomfort or artistic merit in color images generated from noise or random overlapping rectangles (Mondrians). In one set, the slopes of the amplitude spectra for luminance or chromatic contrast were varied independently to create images ranging from strongly blurred to sharpened relative to a 'natural' 1/f spectrum. Perceived blur was dominated by the luminance slopes, with discomfort rated lowest for approximately 1/f spectra. In a second set of focused Mondrians, color was varied along different axes in the L-M versus S-LM chromatic plane. Discomfort ratings were lowest around bluish yellowish axes that are again typical of natural outdoor scenes. A final set varied in the levels of both luminance and chromatic contrast. Discomfort increased with increasing color contrast or decreasing luminance contrast, and tended toward lower ratings for images with balanced levels of luminance and color contrast. Notably, these ratings of discomfort were not related to judgments of artistic merit. Thus, for both spatial and chromatic content, the least aversive images corresponded to characteristic properties of the natural visual environment, and may reflect a normalization of visual coding to the natural world.

Foreshortened tiles in paths converging on an observer viewing a picture: elevation and visual angle ratio determine perceived size.

Observers were shown wide-angle pictures of tiles on a ground plane and were asked about the aspect ratios of the tiles. The observers viewed the pictures from a fixed center of projection. Some of the tiles were in a path coming straight toward the observer. In one picture, the path came from the center of the picture, and in two others the path came from the left side of the picture (one from 30 degrees and one from 45 degrees to the left of the center, from the observer's point of view). The apparent aspect ratios were a function of the elevations of the tiles and the ratios of visual angles of the sides of the tiles. Judgments were identical for all three paths. The local slant of the picture surface was not a significant factor.

Angle illusion on a picture's surface.

Shapes on picture surfaces are not seen accurately (Arnheim, 1954). In particular, if they depict 3-D forms, angles between lines on a picture surface are misperceived. To test four theories of the misperception, subjects estimated acute and obtuse internal angles of quadrilaterals. Each quadrilateral was shown alone or as part of a drawing of a cube. The drawings showed the tops of the cubes, tilted at various angles around a horizontal axis. This generated different acute and obtuse angles in the drawings. Compared to a quadrilateral on its own, judgments of the acute and obtuse angles in the cube drawings were biased towards 90 degrees . The bias was present over a wide range of intermediate tilts. The results support a perspective convergence theory and run counter to 'Extreme Foreshortening', Gestalt and Cognitive theories.

Ellipses on the surface of a picture.

Shapes on the surface of a perspective picture may be misperceived. Subjects picked a match for an ellipse depicting the circular top of a cylinder. The top was depicted as tilted forward from 5 degrees to 85 degrees, generating a series of ellipses on the picture surface. The matches were biased towards a circle over a wide range of midrange tilts, which suggests that, influenced by features of perspective, they were seen as in-between the shape on the surface and the shape they depicted.

Foreshortening, convergence and drawings from a blind adult.

Esref is a congenitally totally blind man, practiced in drawing. He was asked to draw solid and wire cubes situated in several places around his vantage point. He used foreshortening of receding sides and convergence of obliques, in approximate one-point perspective. We note that haptics provides information about the direction of objects--the basis of perspective.

Looking for inhibition of return in pigeons.

We conducted four experiments in order to investigate whether pigeons' responses to a recently attended (i.e., recently pecked) location are inhibited. In Experiments 1 and 2, stimulus displays were similar to those used in studies of inhibition of return (IOR) with humans; responses to cued targets tended to be facilitated rather than inhibited. In Experiments 3 and 4, birds were presented with stimulus displays that mimicked clusters of small grains and were relatively localized, which should have been more appropriate for detecting IOR in pigeons. The results from these experiments again provided evidence for facilitation of responding to cued targets, rather than for IOR.