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PubMed1979

Cognitive impairment of aphasics in a colour-to-picture matching task.

Broca's and Wernicke's aphasics, non-aphasic brain-damaged patients and chronic schizophrenics were tested in a colour-to-picture matching task. For each of 42 black-and-white drawings of objects the most "typical" colour was to be selected from a set of 24 colour strips. Separate scores for the appropriateness of the choices as to hue, brightness, and saturation were obtained by comparison with the data from 50 normal Ss. The choices of both aphasic groups were more deviant than those of brain-damaged controls for chromatically coloured objects with respect to all three colour dimensions, and for achromatically coloured objects with respect to saturation and brightness. Wherever the aphasics' performance was worse than that of the controls, the deviancy-scores correlated significantly with the Token Test. Results are interpreted as an impairment of aphasics in the ability to isolate analytically specific features of objects or concepts which might be conceived of as a prerequisite of propositional language.

Aphasia↗
PubMed1983

Color "amnesia" without aphasia.

Following an apparent left parietal CVA, a patient developed a severe and nearly complete color amnesia which was not associated with any disturbance in color vision or color perception. Like all previously reported cases with color amnesia, this patient was alexic but, unlike most previously reported cases, he was not aphasic.

Agnosia↗
PubMed2001

Surface color from boundaries: a new 'watercolor' illusion.

A colored line flanking a darker border will appear to assimilate its color onto the enclosed white area over distances of up to 45 deg (the Watercolor Effect). This coloration is uniform and complete within 100 ms. We found that thin (6 arcmin), winding inducing lines with different contrasts to the ground are generally more effective than thick, straight, and equiluminant lines. Blue and red lines induce the strongest effects, but watercolor spreading may also be seen with green and yellow. On a white background, color spreading is stronger than on chromatic, gray or black backgrounds. Little or no color is perceived when a narrow white zone (gap) is inserted in between the two inducing lines. However, chains of colored dots instead of continuous lines suffice to produce spreading. Edge-induced color is also observed when the two colored lines are presented dichoptically, suggesting a cortical origin. The Watercolor Effect described here may serve to enhance figure-ground segregation by imparting surface color onto the enclosed area, and to promote grouping between distant stimulus elements. As a grouping factor, watercolor coloration wins over proximity. Assimilative color spreading may arise in two steps: First, weakening of the contour by lateral inhibition between differentially activated edge cells (local diffusion); and second, unbarriered flow of color onto the enclosed area (global diffusion).

Color Perception↗
PubMed2001

First-order structure induces the 3-D curvature contrast effect.

A 3-D curvature contrast effect has been reported in shading-and-texture-defined (Curran & Johnson (1996). Vision Research 36, 3641-3653) and in stereoscopically defined (te Pas, Rogers, & Ledgeway (2000). Current Psychology Letters: Brain, Behaviour and Cognition 1, 117-126) stimuli. Our experiments show that a clear 3-D curvature contrast effect also occurs in motion-defined stimuli. The magnitude of the effect is similar in motion-, stereo- and shading-and-texture defined stimuli, suggesting that the 3-D curvature contrast effect is shape-based. We find a distinct contrast effect that is similar in the case of inducers that contain second-order (curvature) information and in the case of inducers that contain only first-order (slant and tilt) information. The effect with inducers that contain only zeroth-order (depth) information is very small. We conclude that the first-order structure is sufficient to induce a 3-D contrast effect.

Depth Perception↗
PubMed2001

Motion-surface labeling by orientation, spatial frequency and luminance polarity in 3-D structure-from-motion.

A compelling percept of three-dimensionality is attainable from a purely motion-defined simulation of a transparent rotating cylinder, referred to as 3-D structure-from-motion (SFM). Interestingly, subjects rarely perceive reversals of the cylinder's direction of rotation when they are introduced. Treue, Andersen, Ando, and Hildreth (Vision Res. 35 (1995) 139-148) have argued that this reflects the visual system's insensitivity to the textural detail on the cylinder's motion surfaces. We have recently shown however that with cylinders made from oriented micropatterns, motion reversals are perceived when the orientations of the micropatterns are different on the cylinder's front/back surfaces, suggesting that the visual system is sensitive to the type of feature in these stimuli (Vision Res. 39 (1999) 881-886). In the present study we extended this finding by testing for feature-sensitivity along other dimensions besides orientation, specifically spatial frequency, colour and luminance polarity. We found that subjects perceived more rotation direction reversals when the front/back surfaces of the cylinder were segregated, as opposed to non-segregated by feature-type, along all of these dimensions except, notably, colour. We also investigated the stage at which the feature-sensitivity is incorporated in 3-D SFM. We reasoned that if 3-D SFM mechanisms were tuned, or labeled for feature-type, swapping of features during the cylinder's rotation would result in illusory reversals in just the feature-segregated condition, whereas if grouping of like-features preceded the formation of 3-D motion surfaces, no such illusory reversals would be expected. We found that feature-swapping resulted in more illusory reversals in the feature-segregated compared to non-segregated conditions, supporting the mechanism tuning, or labeling, hypothesis.

Color Perception↗
PubMed2003

Luminance mechanisms mediate the motion of red-green isoluminant gratings: the role of "temporal chromatic aberration".

In this paper we use a dynamic noise-masking paradigm to explore the nature of the mechanisms mediating the motion perception of drifting isoluminant red-green gratings. We compare contrast thresholds for the detection and direction discrimination of drifting gratings (1.5 cpd), over a range of temporal frequencies (0.5-9 Hz) in the presence of variable luminance or chromatic noise. In the first experiment, we used dynamic luminance noise to show that direction thresholds for red-green grating motion are masked by luminance noise over the entire temporal range tested, whereas detection thresholds are unaffected. This result indicates that the motion of nominally isoluminant red-green gratings is mediated by luminance signals. We suggest that stimulus-based luminance artifacts are not responsible for this effect because there is no masking of the detection thresholds. Instead we propose that chromatic motion thresholds for red-green isoluminant gratings are mediated by dynamic luminance artifacts that have an internal, physiological origin. We have termed these "temporal chromatic aberration". In the second experiment, we used dynamic chromatic noise masking to test for a chromatic contribution to red-green grating motion. We were unable to find conclusive evidence for a contribution of chromatic mechanisms to the chromatic grating motion, although a contribution at very high chromatic contrasts cannot be ruled out. Our results add to a growing body of evidence indicating the presence of dynamic, internal luminance artifacts in the motion of chromatic stimuli and we show that these occur even at very low temporal rates. Our results are compatible with our previous work indicating the absence of a chromatic mechanism for first order (quasi-linear) apparent motion [Vision Res. 40 (2000) 1993]. We conclude that previous conclusions based on the motion of chromatic red-green gratings should be reassessed to determine the contribution of dynamic luminance artifacts.

Color Perception↗
PubMed1998

Two segmentation mechanisms with differential sensitivity for colour and luminance contrast.

In a texture-segregation paradigm, subjects were asked to detect figures whose elements were segregated from background either because of temporal offset or because of differing orientations. Texture elements were either isoluminant or had high or low luminance contrast. At high luminance contrast, figures could be segregated both on the basis of orientation and temporal cues whereby temporal offsets as short as 10 msec supported detection. At isoluminance, orientation defined figures were as readily distinguishable as in the high contrast condition but temporally defined figures were perceived only for offset intervals > 50 msec. With low luminance contrast, performance for orientation defined figures was impaired relative to the high contrast condition, but for temporally defined figures, it was superior to the isoluminant condition; detection was possible for offset intervals as short as 22 msec. These results suggest that the temporal and orientation cues which support scene segmentation are transmitted by both the luminance and colour sensitive pathways. However, if temporal offsets are < 50 msec, segmentation of temporally defined figures is supported only by the luminance sensitive system.

Adult↗
PubMed1998

Effects of contrast and temporal frequency on orientation discrimination for luminance and isoluminant stimuli.

We compared the mechanisms responsible for orientation discrimination of stimuli defined by luminance and red/green isoluminant contrast. A four-alternative forced-choice (4-AFC) paradigm was used to determine thresholds for discriminating 1 cpd sinewave gratings differing in orientation, contrast, or both. When measuring orientation thresholds as a function of stimulus contrast, we found a decrease in thresholds with increasing stimulus contrast. For three temporal frequencies (0, 1, and 8 Hz) the functions relating orientation thresholds to stimulus contrast had similar shapes for luminance and isoluminant gratings, indicating similar processing mechanisms. Thresholds for stationary or slowly moving gratings were consistently lower for isoluminant than for luminance gratings, when contrast was expressed on an absolute RMS-cone-contrast scale. When contrast was defined as multiples of detection thresholds, discrimination was slightly better for luminance gratings. Thresholds for fast moving gratings were similar, irrespective of the definition of contrast. In contrast to previous work, we found a marked "oblique-effect" for both luminance and isoluminant gratings, when measuring discrimination thresholds as a function of standard orientation. Finally, we measured discrimination thresholds for gratings that varied in contrast and orientation simultaneously. The shapes of the resulting two-dimensional threshold contours were similar for luminance and isoluminant gratings, indicating again that these stimuli undergo similar neuronal processing. Performance of the observers could be described by probability summation of the orientation and contrast cues, resulting in an elliptical shape of the two-dimensional threshold contours. In conclusion, our results show similar performance for luminance and isoluminant gratings in several orientation discrimination tasks. The similarity in shape of the different threshold functions presents strong evidence that similar mechanisms underlie orientation discrimination of luminance and isoluminant stimuli.

Color Perception↗
PubMed1998

Why the visual recognition system might encode the effects of illumination.

A key problem in recognition is that the image of an object depends on the lighting conditions. We investigated whether recognition is sensitive to illumination using 3-D objects that were lit from either the left or right, varying both the shading and the cast shadows. In experiments 1 and 2 participants judged whether two sequentially presented objects were the same regardless of illumination. Experiment 1 used six objects that were easily discriminated and that were rendered with cast shadows. While no cost was found in sensitivity, there was a response time cost over a change in lighting direction. Experiment 2 included six additional objects that were similar to the original six objects making recognition more difficult. The objects were rendered with cast shadows, no shadows, and as a control, white shadows. With normal shadows a change in lighting direction produced costs in both sensitivity and response times. With white shadows there was a much larger cost in sensitivity and a comparable cost in response times. Without cast shadows there was no cost in either measure, but the overall performance was poorer. Experiment 3 used a naming task in which names were assigned to six objects rendered with cast shadows. Participants practised identifying the objects in two viewpoints lit from a single lighting direction. Viewpoint and illumination invariance were then tested over new viewpoints and illuminations. Costs in both sensitivity and response time were found for naming the familiar objects in unfamiliar lighting directions regardless of whether the viewpoint was familiar or unfamiliar. Together these results suggest that illumination effects such as shadow edges: (1) affect visual memory; (2) serve the function of making unambigous the three-dimensional shape; and (3) are modeled with respect to object shape, rather than simply encoded in terms of their effects in the image.

Adolescent↗
PubMed2000

Tilt dependency of slant aftereffect.

Slant aftereffect (SAE), the negative aftereffect of slant induced after prolonged observation of a surface, is considered as evidence that slant is encoded in the visual system. Because slant and tilt are mathematically independent dimensions, Stevens (Stevens, K. A. (1983a). Biological Cybernetics, 46, 183-195) assumed that slant and tilt are processed independently in the visual system. To confirm this assumption, we investigated whether SAE is induced independently of the difference in tilt between the adapting and test stimuli. The stimuli were displayed by simulating the motion disparity of rotating disks. After adaptation to a surface of 60 degrees slant, the subjective 0 degree slants of the test stimulus were measured with the tilt differences of 0, 45, 90, 135 and 180 degrees. The magnitude of SAE was greatest when the tilt difference was zero, and decreased with increasing tilt difference. The results suggest that slant and tilt are not processed independently in the visual system and that the slant detector in the visual system is sensitive not only to slant but also to tilt.

Computer Simulation↗
PubMed2006

In honour of Lothar Spillmann - filling-in, wiggly lines, adaptation, and aftereffects.

I have studied a number of visual phenomena that Lothar Spillmann has already elucidated. These include: Neon spreading: when a small red cross is superimposed on intersecting black lines, the red cross seems to spread out into an illusory disk. Unlike the Hermann grid, neon spreading is relatively unaffected when the black lines are curved or wiggly. This suggests that the Hermann grid, but not neon spreading, involves long-range interactions. Neon spreading can be shown in random-dot patterns, even without intersections. It is strongest when the red crosses are equiluminous with the gray background. Adaptation, aftereffects, and filling-in: direct and induced aftereffects of color, motion, and dimming. Artificial scotomata and filling-in: the "dam" theory is false. Staring at wiggly lines or irregularly scattered dots makes them gradually appear straighter, or more regularly spaced. I present evidence that irregularity is actually a visual dimension to which the visual system can adapt. Conjectures on the nature of peripheral fading and of motion-induced blindness. Some failed experiments on correlated visual inputs and cortical plasticity.

Adaptation, Physiological↗
PubMed2004

Development of configural 3D object recognition.

There is evidence for the late development in humans of configural face and animal recognition. We show that the recognition of artificial three-dimensional (3D) objects from part configurations develops similarly late. We also demonstrate that the cross-modal integration of object information reinforces the development of configural recognition more than the intra-modal integration does. Multimodal object representations in the brain may therefore play a role in configural object recognition.

Adolescent↗
PubMed2003

Integration process of contours defined by different attributes.

We examined how contours defined by different attributes are integrated into a shape using the visual search paradigm and found that the search for the bar orientation is particularly difficult when each bar is composed of motion-defined and luminance- or color-defined contours. This does not support the view that the shape integration process is based on the unified contour representation entirely common to all attributes, but suggests that motion-defined contours are integrated through a different process.

Color Perception↗
PubMedDate not supplied

How long range is contour integration in human color vision?

We quantified and compared the effect of element spacing on contour integration between the achromatic (Ach), red-green (RG), and blue-yellow (BY) mechanisms. The task requires the linking of orientation across space to detect a contour in a stimulus composed of randomly oriented Gabor elements (1.5 cpd, sigma = 0.17 deg), measured using a temporal 2AFC method. A contour of ten elements was pasted into a 10 x 10 cells array, a ndbackground elements were randomly positioned within the available cells. The effect of element spacing was investigated by varying the mean interelement distance between two and six times the period of the Gabor elements (lambda = 0.66 deg) while the total number of elements was fixed. Contour detection was measured as a function of its curvature for jagged contours and for closed contours. At all curvatures, we found that performance for chromatic mechanisms declines more steeply with the increase in element separation than does performance for the achromatic mechanism. Averaged critical element separations were 4.6 +/- 0.7, 3.6 +/- 0.4, and 2.9 +/- 0.2 deg for Ach, BY, and RG mechanisms, respectively. These results suggest that contour integration by the chromatic mechanisms relies more on short-range interactions in comparison to the achromatic mechanism. In a further experiment, we looked at the combined effect of element size and element separation in contour integration for the Ach mechanism. We found that the critical separation decreases linearly with the spatial frequency, from about 5 deg at low spatial frequency (larger elements) to about 1 deg at high spatial frequency (smaller elements) suggesting a scale invariance in contour integration. In both experiments we also found no differences between closed and open jagged contours detection in terms of element separation. The neuroanatomical implications of these findings relatively to area V1 are discussed.

Color Perception↗
PubMedDate not supplied

Information limits on neural identification of colored surfaces in natural scenes.

If surfaces in a scene are to be distinguished by their color, their neural representation at some level should ideally vary little with the color of the illumination. Four possible neural codes were considered: von-Kries-scaled cone responses from single points in a scene, spatial ratios of cone responses produced by light reflected from pairs of points, and these quantities obtained with sharpened (opponent-cone) responses. The effectiveness of these codes in identifying surfaces was quantified by information-theoretic measures. Data were drawn from a sample of 25 rural and urban scenes imaged with a hyperspectral camera, which provided estimates of surface reflectance at 10-nm intervals at each of 1344 x 1024 pixels for each scene. In computer simulations, scenes were illuminated separately by daylights of correlated color temperatures 4000 K, 6500 K, and 25,000 K. Points were sampled randomly in each scene and identified according to each of the codes. It was found that the maximum information preserved under illuminant changes varied with the code, but for a particular code it was remarkably stable across the different scenes. The standard deviation over the 25 scenes was, on average, approximately 1 bit, suggesting that the neural coding of surface color can be optimized independent of location for any particular range of illuminants.

Color Perception↗
PubMed1984

Detection of three-dimensional structure in moving optical patterns.

Three experiments examined parameters affecting human observers' ability to detect the global three-dimensional (3D) organization of a random-dot display corresponding to the polar projection of a rotating sphere. Results indicate that the detection of 3D structure is critically dependent on the detectability of motion, is disrupted by increased redundant information specifying the two-dimensional location of points in the display, and undergoes a rapidly increasing resistance to the disruptive effects of noise with increasing numbers of frames. These results, in conjunction with earlier findings, are inconsistent with existing theories concerning the perception of three-dimensionality in moving displays, in that they indicate a high degree of visual sensitivity to stimulus organizations with unique topological representations.

Attention↗