The effect of image fractal properties and its interaction with visual discomfort on gait kinematics.

Exposure to images of urban environments affords higher cognitive processing demands than exposure to images of nature scenes; an effect potentially related to differences in low-level image statistics such as fractals. The aim of the current study was to investigate whether the fractal dimensions of an abstract scene affect cognitive processing demands, using gait kinematics as a measure of cognitive demand. Participants (n = 40) were asked to walk towards different types of synthetic images which were parametrically varied in their fractal dimensions. At the end of each walk, participants rated each image for its visual discomfort (n = 20) or for its likability (n = 20) as potential confounding factors. Fractal dimensions were predictors of walking speed. Moreover, the interaction between fractal dimensions and subjective visual discomfort but not liking predicted velocity. Overall, these data suggest that fractal dimensions indeed contribute to environmentally induced cognitive processing demands.

Seeing shapes in seemingly random spatial patterns: Fractal analysis of Rorschach inkblots.

Rorschach inkblots have had a striking impact on the worlds of art and science because of the remarkable variety of associations with recognizable and namable objects they induce. Originally adopted as a projective psychological tool to probe mental health, psychologists and artists have more recently interpreted the variety of induced images simply as a signature of the observers' creativity. Here we analyze the relationship between the spatial scaling parameters of the inkblot patterns and the number of induced associations, and suggest that the perceived images are induced by the fractal characteristics of the blot edges. We discuss how this relationship explains the frequent observation of images in natural scenery.

Perceptual and physiological responses to the visual complexity of fractal patterns.

Fractals have experienced considerable success in quantifying the complex structure exhibited by many natural patterns and have captured the imagination of scientists and artists alike. With ever widening appeal, they have been referred to both as "fingerprints of nature" and "the new aesthetics." Our research has shown that the drip patterns of the American abstract painter Jackson Pollock are fractal. In this paper, we consider the implications of this discovery. We first present an overview of our research from the past five years to establish a context for our current investigations of human response to fractals. We discuss results showing that fractal images generated by mathematical, natural and human processes possess a shared aesthetic quality based on visual complexity. In particular, participants in visual perception tests display a preference for fractals with mid-range fractal dimensions. We also present recent preliminary work based on skin conductance measurements that indicate that these mid-range fractals also affect the observer's physiological condition and discuss future directions based on these results.

Degraded illusory contour formation with non-uniform inducers in Kanizsa configurations: the role of contrast polarity.

The neuronal and computational mechanisms involved in illusory contour formation are thought to be sensitive to the orientation and magnitude of contrast at luminance discontinuities, but insensitive to the sign of contrast at such discontinuities. It is shown here that boundary formation in Kanizsa-type illusory figures exhibits sensitivity to the spatial distribution of inducing elements of opposite contrast polarity. Shape discrimination was used as an objective measure of the saliency of illusory figures, revealing pronounced degradation of illusory boundaries when contrast polarity reversed at the intersections of orthogonally oriented edges within each inducer. These results suggest the previously unsuspected importance of image properties related to environmentally relevant constraints in perception of illusory contours and occlusion.

A functional angle on some after-effects in cortical vision.

The question of how our brains and those of other animals code sensory information is of fundamental importance to neuroscience research. Visual illusions offer valuable insight into the mechanisms of perceptual coding. One such illusion, the tilt after-effect (TAE), has been studied extensively since the 1930s, yet a full explanation of the effect has remained elusive. Here, we put forward an explanation of the TAE in terms of a functional role for adaptation in the visual cortex. The proposed model accounts not only for the phenomenology of the TAE, but also for spatial interactions in perceived tilt and the effects of adaptation on the perception of direction of motion and colour. We discuss the implications of the model for understanding the effects of adaptation and surround stimulation on the response properties of cortical neurons.

An anchoring theory of lightness perception.

A review of the field of lightness perception from Helmholtz to the present shows the most adequate theories of lightness perception to be the intrinsic image models. Nevertheless, these models fail on 2 important counts: They contain no anchoring rule, and they fail to account for the pattern of errors in surface lightness. Recent work on both the anchoring problem and the problem of errors has produced a new model of lightness perception, one that is qualitatively different from the intrinsic image models. The new model, which is based on a combination of local and global anchoring of lightness values, appears to provide an unprecedented account of a wide range of empirical results, both classical and recent, especially the pattern of errors. It provides a unified account of both illumination-dependent failures of constancy and background-dependent failures of constancy, resolving a number of long-standing puzzles.

Adaptation to textured chromatic fields.

Probe-flash threshold curves were used to show that adaptation to textured fields consists not only of adaptation to the steady local constituents but also of a process that is similar to habituation to prolonged temporal modulation, which in this case could be caused by miniature eye movements across element boundaries. The response curves derived from probe-flash thresholds are compressive on both sides of the adaptation level after adaptation to spatially uniform fields but have an accelerating form when they are measured after adaptation to textured backgrounds. This change is suggestive of a response equalization process, which modifies the response function of each mechanism to match the cumulative frequency distribution of its inputs.

Color constancy in variegated scenes: role of low-level mechanisms in discounting illumination changes.

For a visual system to possess color constancy across varying illumination, chromatic signals from a scene must remain constant at some neural stage. We found that photoreceptor and opponent-color signals from a large sample of natural and man-made objects under one kind of natural daylight were almost perfectly correlated with the signals from those objects under every other spectrally different phase of daylight. Consequently, in scenes consisting of many objects, the effect of illumination changes on specific color mechanisms can be simulated by shifting all chromaticities by an additive or multiplicative constant along a theoretical axis. When the effect of the illuminant change was restricted to specific color mechanisms, thresholds for detecting a change in the colors in a scene were significantly elevated in the presence of spatial variations along the same chromatic axis as the simulated chromaticity shift. In a variegated scene, correlations between spatially local chromatic signals across illuminants, and the desensitization caused by eye movements across spatial variations, help the visual system to attenuate the perceptual effects that are due to changes in illumination.

Surround effects on the shape of the temporal contrast-sensitivity function.

The shape of the temporal contrast-sensitivity function at low temporal frequencies is sensitive to the relative luminance of the test and the surround. We show that this effect is due to greater sensitivity, in different conditions, either to the internal luminance modulation in the test or to temporal changes in the spatial contrast at the edge of the test. We measured temporal contrast sensitivity in tests at various luminance levels combined with surrounds at levels of higher, lower, or equal luminance as the test; compared the sensitivity for contrast modulation to luminance modulation at different temporal frequencies; and compared temporal contrast sensitivity in uniform and textured surround of equal mean luminance. Temporal contrast sensitivity was similar on equiluminant steady and out-of-phase modulating surrounds, indicating that the measured sensitivity for small tests in equiluminant surrounds is based on the detection of the temporal modulation of the spatial contrast at the edge of the test field. For all temporal frequencies, contrast sensitivity decreased as a monotonic function of the absolute magnitude of the Michelson contrast between test and surround. When small test fields of moderate to high intensities are embedded in dark surrounds, the sensitivity at lower spatial frequencies is similar to the sensitivities measured for a large test and may reflect sensitivity for luminance modulation within the test.

Induced effects of backgrounds and foregrounds in three-dimensional configurations: the role of T-junctions.

In three-dimensional configurations, and two-dimensional pictures of such configurations, simultaneous contrast induction from proximate backgrounds affects perceived brightness, color, and internal contrast to a greater extent than induction from coplanar or occluding surrounds or from more distant backgrounds. In the projected image the presence of occluding flanks or retinally adjacent distant backgrounds is indicated by T-junctions. However, the presence of T-junctions inhibits induced contrast irrespective of the three-dimensional percept. The configurations in this paper refute the notions that perceived coplanarity or perceptual belonging necessarily enhance induced contrast.

New configurational effects on perceived contrast and brightness: second-order White's effects.

Novel phenomena of perceived contrast and brightness in spatial configurations are presented, which are of the type used by White but consist exclusively of contrast variations or of combined contrast and luminance variations. As with White's effect, perceived contrast and brightness in these displays do not correspond to predictions based on low-level mechanisms of contrast and brightness induction, and it is suggested that spatial organisation influences the appearance of first-order and second-order stimuli in a similar fashion.

The enhanced S cone syndrome: an analysis of receptoral and post-receptoral changes.

The purpose of the study was to test the hypothesis that the retinae of patients with enhanced S cone syndrome (ESCS) have more S cones than the normal retina and these cones have replaced some of the L and M cones. Standard and spectral full-field electroretinograms, measurements of L, M, and S cone system sensitivities and S cone acuity were obtained from three patients with ESCS. The results were qualitatively consistent with the presence of more S cones and more S cone ganglion cells. To test this hypothesis further, a model of the receptoral and post-receptoral components of the S cone system was used in conjunction with psychophysical measurements of S cone system sensitivity under flashed and steady-state adaptation conditions. Within the context of the model, the data were consistent with an increase in the number of S cones and S - (L + M) ganglion cells and with a decrease in the total L + M cone input to each S - (L + M) ganglion cell.

Brightness induction from uniform and complex surrounds: a general model.

We studied the brightness induced from complex non-figural achromatic surrounds. A spatially uniform test field was surrounded by a random texture composed of two sets of dots. The luminance of each set of dots was modulated sinusoidally at 0.5 Hz. The mean luminance, phase and amplitude of modulation of each set were controlled independently so as to modulate the luminance and/or the contrast of the surround. Brightness induction was measured by a modulation nulling technique. The results were fit by a model in which the total brightness induced by a surround is equal to a weighted spatial summation of the induced effects from each point in the surround. The model incorporates local luminance gain controls in the test and surround fields and assumes that the magnitude of induction from each surround element is gain controlled by the difference between the mean luminance of the test and the individual surround elements.

The critical role of relative luminance relations in White's effect and grating induction.

It has been proposed that both White's effect and the grating induction effect are examples of brightness induction phenomena modeled in terms of local spatial filters. We have shown that for these illusions to occur it is necessary that the luminance of the gray target elements falls between that of the inducing stripes of the square-wave pattern. This critical role of luminance relationships is not predicted by existing models of these illusions.

Lightness, brightness, and brightness contrast: 1. Illuminance variation.

Changes of annulus luminance in traditional disk-and-annulus patterns are perceptually ambiguous; they could be either reflectance or illuminance changes. In more complicated patterns, apparent reflectances are less ambiguous, letting us place test and standard patches on surrounds perceived to be different grays. Our subjects matched the apparent amounts of light coming from the patches (brightnesses), their apparent reflectances (lightnesses), or the brightness differences between the patches and their surrounds (brightness contrasts). The three criteria produced quantitatively different results. Brightness contrasts matched when the patch/surround luminance ratio of the test was approximately equal to that of the standard. Lightness matches were illumination invariant but were not exact reflectance matches; the different surrounds of test and standard produced a small illumination-invariant error. This constant error was negligible for increments, but, for decrements, it was approximately 1.5 Munsell value steps. Brightness matches covaried substantially with illuminance.

Lightness, brightness, and brightness contrast: 2. Reflectance variation.

Changes of annulus luminance in traditional disk-and-annulus patterns can be perceived to be either reflectance or illuminance changes. In the present experiments, we examined the effect of varying annulus reflectance. In Experiment 1, we placed test and standard patch-and-surround patterns in identical Mondrian patchworks. Only the luminance of the test surround changed from trial to trial, appearing as reflectance variation under constant illumination. Lightness matches were identical to brightness matches, as expected. In Experiment 2, we used only the patch and surround (no Mondrian). Instructions said that the illumination would change from trial to trial. Lightness and brightness-contrast data were identical; illumination gradients were indistinguishable from reflectance gradients. In Experiment 3, the patterns were the same, but the instructions said that the shade of gray of the test surround would change from trial to trial. Lightness matches were identical to brightness matches, again confirming the ambiguity of disk-and-annulus patterns.