Biomedical subjects
Publications and source records attributed to Q Zaidi.
Simple rigid objects are presented that appear to bend when viewed from certain angles. These illusions illustrate that perspective information is used by the stereo system, that projective distortions can override rigidity constraints in motion perception, and that touch only corrects the illusion for a local region.
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.
In everyday scenes, from perceived colors of objects and terrains, observers can simultaneously identify objects across illuminants and identify the nature of the light, e.g., as sunlight or cloudy. As a formal problem, identifying objects and illuminants from the color information provided by sensor responses is underdetermined. It is shown how the problem can be simplified considerably by the empirical result that chromaticities of sets of objects under one illuminant are approximately affine transformations of the chromaticities under spectrally different illuminants. Algorithms that use the affine nature of the correlation as a heuristic can identify objects of identical spectral reflectance across scenes lit simultaneously or successively by different illuminants. The relative chromaticities of the illuminants are estimated as part of the computation. Because information about objects and illuminants is useful in many different tasks, it would be more advantageous for the visual system to use such algorithms to extract both sorts of information from retinal signals than to discount either automatically at an early neural stage.
We examined the spatial integration of simultaneously induced achromatic contrast and compared it to the spatial integration of simultaneously induced brightness. This study extends the work of Zaidi et al. [(1992). Vision Research, 32, pp. 1695-1707], who showed that the total magnitude of induced brightness can be described as the weighted sum of the brightness induced by individual elements of the surround. The results show that contrast induction, though weaker than brightness induction, occurs over greater distances, and that a weighted spatial summation model for contrast induction requires an additional static non-linear compression, which is not required to model brightness induction. The analysis indicates that the contrast compression occurs prior to the lateral interactions that generate induced contrast.
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.
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 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.
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.
For a large sample of broadband lights reflected from natural and man-made objects, the correlation between L- and M-cone absorptions was found to be 0.99. The correlation between L + M and L - M signals was 0.21. The early recombination of cone signals in the visual system thus leads to a substantial decorrelation.
The purpose of this study was to compare the effects of glaucoma, at different stages of the disease process, on the two color-opponent system and on the luminance system. Discrimination thresholds were measured along the two equiluminant cardinal color axes (RG and YV) and along an achromatic luminance axis (LD) in 27 patients with open-angle glaucoma (OAG) and in 13 glaucoma suspects. Patients with OAG showed increased thresholds along all three axes. The threshold increases correlated significantly with the level of visual field loss. For glaucoma suspects, thresholds were also increased along all three axes. A subgroup of patients with OAG, those with pigmentary glaucoma, showed minimal increases in threshold along the RG axis. To further investigate this finding an additional 15 patients, seven with primary OAG and eight with pigmentary glaucoma were run in a two-alternative forced-choice experiment. For patients with pigmentary glaucoma, thresholds were increased less along the RG axis. The results of the study for OAG patients and glaucoma suspects are consistent with deficits in the two color-opponent systems, and in the luminance system.
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 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.
Psychometric functions for motion detection were measured for various spatial velocity profiles made of independently moving lines of random dots. In the first experiment, sensitivity was greater for square-wave velocity profiles than for sine waves of the same fundamental spatial frequency. Sensitivity for square waves depended on the phase of the waveform with respect to the fixation point, which precludes a characterization of the processes underlying the detection of shearing motion as a translation-invariant system. The second experiment, using velocity fields created by spatial super-position of sine waves, showed that motion boundaries facilitate detection of motion due to the steepness of the velocity gradient, and not simply because of added power at higher harmonics. In the third experiment, fluted velocity waveforms were created by subtracting the fundamental sinusoidal component from square waves, retaining sharp motion boundaries between opposing directions but removing the regions of uniform motion. Subtracting the fundamental from low-frequency square waves did not lower sensitivity to motion, indicating that sensitivity was largely determined by the presence of motion boundaries. In the final section of this article, a model is presented that can account for the data by using linear center-surround velocity mechanisms whose sizes increase with eccentricity while their sensitivity for shearing motion decreases.
Explore the source record for details and available documents.
The perceived color of a region of visual space depends both on the spectral composition of the light incident from it and the light incident from surrounding regions. We studied the induced effect of combinations of colors surrounding a central test disk. Surround colors varied in a radially sinusoidal fashion around the perimeter of the test. Using a modulation nulling technique we show that when DC adaptation is controlled, the combined effect of such surrounds is equal to the sum of contrasts induced by individual elements of the surround. This result holds for both lightness and chromatic induction.
Evidence is presented for a visual capacity specialized to sense the chromatic direction of change in colors over time. Discrimination thresholds were measured between pairs of suprathreshold color changes presented in consecutive intervals. In one interval, the color of a spatially uniform disk was changed at a constant speed along the circumference of a circle in an equiluminant color plane. In the other, an instantaneous change, which can be described as a vector in the equiluminant plane, was added to the circular color modulation. Averaging across conditions showed that the threshold for discriminating between a pair of purely temporal color changes was approximately proportional to the cosine of the color angle between them. The model that is presented to account for these results is based on parallel directional-color mechanisms that are tuned to different directions in color-space and are responsive to change in one color direction but not its opposite.
Sensitivity to motion was measured by the percentage of trials on which an observer reported seeing motion of briefly presented high-contrast sinusoidal gratings moving over a range of velocities. The psychometric curve was remeasured following adaptation to a grating moving in one direction for an extended period of time. Adaptation shifted the minimum of the psychometric curve toward the direction of the direction of the adapting stimulus. The shift was smaller when the adapting field was larger than the test. In a second set of experiments we measured the effect of motion adaptation on contrast thresholds for moving gratings of different sizes. Threshold elevation was maximal when adapting and test sizes matched. We present a mechanistic model of the motion aftereffect that consists of independent multiplicative gain controls in motion-sensing mechanisms tuned to different rates of motion. In addition, we discuss a model of size effects in motion adaptation that invokes diffuse inhibitory connections among motion-sensing mechanisms.
The effects of glaucoma and diabetes on the sensitivities of the opponent and achromatic systems were investigated by measuring thresholds along theoretically defined axes in a three-dimensional color space. Thresholds were measured along two equiluminant chromatic axes and one achromatic axis in patients with diabetes or glaucoma and in glaucoma suspects. The results were compared with measures of sensitivities of short- and middle-wavelength-sensitive-cone pathways [S (Stiles pi 1) and M (Stiles pi 4), respectively] and with measures of hue discrimination by use of the Farnsworth-Munsell 100-hue test. The glaucoma suspects and diabetic patients showed preferential S-cone-pathway sensitivity losses. For glaucoma patients, however, these losses were associated with significant decreases in the sensitivity of the L-M opponent system and with decreased sensitivity to achromatic contrast.