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Biomedical subjects

Q Zaidi

Publications and source records attributed to Q Zaidi.

At least 37 records · Page 2Linked to original sources
PubMed1995

Visual processing of motion boundaries.

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.

Fixation, Ocular↗
PubMed1993

Induced contrast from radial patterns.

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.

Color Perception↗
PubMed1993

Visual mechanisms that signal the direction of color changes.

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.

Color Perception↗
PubMed1993

Effect of spatial configuration on motion aftereffects.

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.

Adaptation, Ocular↗
PubMed1993

Chromatic and luminance sensitivity in diabetes and glaucoma.

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.

Adolescent↗
PubMed1993

Adaptive orthogonalization of opponent-color signals.

This paper concerns the processing of the outputs of the two opponent-color mechanisms in the human visual system. We present experimental evidence that opponent-color signals interact after joint modulation even though they are essentially independent under neutral steady adaptation and after exclusive modulation of each mechanism. In addition, prolonged modulation linearizes the response function of each mechanism. The changes in interaction serve to orthogonalize opponent signals with respect to the adapting modulation, and the changes in response functions serve to equalize the relative frequencies of different levels of response to the adapting modulation. Adaptive orthogonalization reduces sensitivity to the adapting color direction, improves sensitivity to the orthogonal direction, and predicts shifts in color appearance. Response equalization enhances effective contrast and explains the difference between the effects of adaptation to uniform versus temporally or spatially modulated stimuli.

Adaptation, Ocular↗
PubMed1992

The effects of prolonged temporal modulation on the differential response of color mechanisms.

The identification of three independent cardinal directions in color space suggests the existence of three independent post-receptoral mechanisms that can be desensitized by habituation to a temporally modulated light. In this paper, the differential response of each cardinal mechanism is estimated over a range of inputs before and after habituation. Simple mathematical considerations show that the threshold elevations following temporal modulation are not consistent with multiplicative gain changes; rather, these elevations require a change in the shape of each cardinal mechanism's response function. With this method, the effects of habituation can also be differentiated from the effects of a change in the steady adapting light.

Adaptation, Ocular↗
PubMed1992

Lateral interactions within color mechanisms in simultaneous induced contrast.

The perceived color of a region of visual space is a function not only of the spectral composition of the light incident from it, but also depends on the light incident from surrounding regions. The color contrast induced into a region is a result of lateral interactions between neural mechanisms. These interactions were studied by measuring the induced effect of circularly symmetric spatial sine-waves on a circular central test region. The phase of the surrounding sine-waves was changed uniformly in time, inducing a modulation in the appearance of the test. Observers adjusted the amplitude of real sinusoidal modulation in the test in order to null the induced modulation, and the nulling modulation was used as a measure of the induced effect. Spatial additivity was tested by using pairs of sine-waves of distinct spatial frequencies. The results showed that brightness induction can be characterized as a linear spatial process, i.e. the effects of parts of the surround at different distances from the test are summed, after the effect of each part is weighted by a negative exponential as a function of distance from the test. The magnitude of pure chromatic induction, however, is a result of nonlinear spatial interactions. Thus, these results have implications for the connections between visual mechanisms that process brightness and chromatic contrast.

Color Perception↗
PubMed1992

The effect of adaptation on the differential sensitivity of the S-cone color system.

This paper presents a psychophysical dissection of the S-cone color system. Experiments were guided by a skeletal model that assumed a first stage consisting of S-, M- and L-cones, and a second stage of the opponent combination of the S and L+M signals. The response of the S-cone system was isolated by measuring difference thresholds between lights that were equiluminant tritanopic confusion pairs and thus differed only in S-cone excitation. Two types of mechanisms that control sensitivity in the S-cone system were identified: (i) static mechanisms that have a restricted range and thus limit discrimination to a small range of inputs; and (ii) adaptive mechanisms that change the state of the system in response to changes in steady illumination, so that the system is sensitive to small changes from the adapting light. These mechanisms were localized by lights that stimulated the S-cone system while keeping the signal constant at either the S, the L+M, or the post-opponent stage. The response function of the static mechanism was estimated by measuring difference thresholds at judgment points other than the steady adapting light. This procedure was repeated at a number of adaptation lights to examine the properties of adaptive mechanisms. The data were consistent with an elaborated model that included identical multiplicative gain control mechanisms in the S and L+M pre-opponent branches, and a post-opponent static sigmoidal nonlinearity with different amounts of compression for positive and negative opponent inputs.

Adaptation, Ocular↗
PubMed1992

Chromatic and luminance signals in visual memory.

The efficiency of chromatic and luminance signals was studied in a set of tasks requiring the discrimination of two colors. Discrimination was measured around an adapting achromatic light and a number of other points in a three-dimensional color space. As a baseline, discrimination thresholds were measured under conditions permitting a side-by-side comparison of stimuli in space or time. For the spatiotemporal configurations used in these experiments, chromatic signals were more efficient than luminance signals in terms of the difference in cone excitation required at the discrimination threshold. When stimuli were separated in both space and time, so that memory was required for their comparison, the efficiency of luminance signals was attenuated further, while chromatic signals retained their efficiency. Further experiments showed that the addition of a memory requirement did not impair the accuracy of luminance discrimination when the two test colors could be placed in distinct perceptual categories with respect to the surround color. Our results indicate that chromatic signals are particularly efficient in simple color discrimination tasks requiring even the barest amount of memory, especially when the perceptual categorization scheme is not available for the comparison of stimuli.

Adaptation, Ocular↗
PubMed1992

Psychophysical evidence for post-receptoral sensitivity loss in diabetics.

Although numerous reports show that the sensitivity of the S cone system is decreased in diabetic patients, few studies have been directed toward identifying the possible sites of the sensitivity loss. In this study, a psychophysical technique was used to test hypotheses about sites of S cone system sensitivity loss in a group of patients with early diabetic retinopathy. A model of the S cone system was assumed and the experimental conditions were chosen to distinguish between explanations for S cone sensitivity loss at the receptor level from explanations for loss at a post-receptoral level. Within the context of the model, the data were consistent with S cone system sensitivity loss occurring at a post-receptoral level.

Adult↗
PubMed1991

Motion adaptation from surrounding stimuli.

When a narrow uniform gap was surrounded by a moving grating, the gap appeared as a grating in the opposite phase to that of the surround, moving in the same direction with the same speed. Contrast thresholds for moving test-gratings placed in the region of the uniform gap were found to be elevated after prolonged viewing of this pattern, thus demonstrating the existence of motion adaptation in a retinal region surrounded by, but not covered by, a moving pattern. The amplitude of the moving induced-grating was measured by nulling with a real grating moving in the same direction and with the same speed as the surround. When the speed of the inducing grating was varied, the amplitude of the induced effect did not correlate with the magnitude of the threshold elevation. Therefore, it is unlikely that motion adaptation in the uniform gap was due to induced gratings. In some conditions, the adaptation effect of surrounding gratings was no less than the adaptation effect of gratings covering the test region. This result rules out an explantation involving scattered light, and indicates that motion adaptation occurs at a later stage than that consisting of simple motion mechanisms which confound the contrast and velocity of a moving stimulus.

Attention↗
PubMed1991

Influence of shape and perimeter length on induced color contrast.

The magnitude of induced color contrast was measured for tests whose areas, perimeter lengths, and shapes were independently varied. Test shapes were smoothly contoured, multiple-lobed figures generated from unitary Fourier shape descriptors. The shapes had from 3 to 40 lobes and were equal in area to a disk of diameter 2 deg, with perimeter lengths of 1.25, 1.75, 2.25, and 2.75 times the circumference of a 2-deg disk. The surround was a 5-deg disk. The surround was modulated sinusoidally along one of the three cardinal directions of color space around an equal-energy white of 50 cd/m2. The observer nulled the modulation induced into the test by adjusting the amplitude of real modulation in the test. The amplitude of nulling modulation was the measure of induction. The main result was that the amount of induction was similar for all tests of equal area irrespective of the shape or the length of perimeter.

Color Perception↗
PubMed1989

Local and distal factors in visual grating induction.

When a uniform test field is surrounded by luminance or chromatic gratings, a grating is induced in the test field. The perceived spatial frequency and orientation of the induced grating can be different from the frequency and orientation of the inducing gratings. Local edge effects are the factors primarily responsible for visual grating induction. Distal parts of the inducing stimulus affect only the amplitude of the induced modulation.

Color Perception↗
PubMed1986

Induced desensitization.

Viewing of annuli modulated in color in a sawtooth fashion in time results in differential threshold elevations for the detection of color changes of inscribed disks. The elevations are of nearly the same magnitude as those resulting from viewing modulated disks. However, the differential effects on thresholds for complimentary colors are reversed. The differential effects, thus, are correlated with the variation in appearance of the test area.

Color Perception↗
PubMed1986

Adaptation and color matching.

Crawford's (1965) experiment [Vision Res. 5, 71-78], implies that there is a failure of linearity when maximum saturation color matches are compared to Maxwell matches. This implication was tested by making the same maximum saturation matches with and without the superposition of a monochromatic desaturating light. A nonlinearity in color matching for short wavelengths was measured without the possible computational or pre-receptoral artifacts in Crawford's design. The data presented are consistent with the hypothesis that this nonlinearity is due to post-receptoral interactions and not to a failure of spectral invariance or to the participation in the match of more than three types of photopigments.

Adaptation, Ocular↗
PubMed1986

Mechanisms of simultaneous color induction.

A new method of measuring simultaneous contrast, or chromatic induction, is introduced and used to test the hypotheses that induction results from either multiplicative or subtractive interaction of either like receptors or like second-stage, opponent mechanisms. Predictions derived from these hypotheses do not predict the outcome of the experiments as well as the traditional notion that induced colors are in the direction complementary to the inducing color with respect to the test color. We conclude that simultaneous contrast is a consequence of interaction within higher-level chromatic mechanisms.

Color Perception↗