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E Switkes

Publications and source records attributed to E Switkes.

At least 19 recordsLinked to original sources
PubMed1999

Comparison of color and luminance contrast: apples versus oranges?

Using a spatial, forced-choice, matching protocol, we have measured observers' ability to equate the contrasts of sinusoidal gratings which vary along differing directions in a 3-dimensional color space. In a given experiment, the observer obtained a perceptual match between the contrasts of two gratings whose chromaticities or luminances varied along differing chromatic directions which were selected from among five axes: an achromatic luminance axis (lum), an isoluminant axis where only S-cone activation varied (S-axis), an isoluminant axis where L- and M-cone activation varied in a complementary manner (LM-axis), an axis where only L-cone activation varied (L-axis), and an axis where only M-cone activation varied (M-axis). Even though these chromatic axes were chosen to activate independent mechanisms involved in the early stages of spatiochromatic visual processing, and despite the distinctly differing appearance of patterns from variations along differing directions, we find that observers can reliably make such pairwise contrast matches. Furthermore there is reasonable consistency of matching contrasts among observers and the pairwise contrast matches exhibit the properties of homogeneity and transitivity. This observed homogeneity and transitivity allows, for each color direction, the specification of a single scaling factor which relates perceptual contrast to physical contrast.

Color Perception↗
PubMed1997

Hue scaling of isoluminant and cone-specific lights.

Using a hue scaling technique, we have examined the appearance of colored spots produced by shifts from white to isoluminant stimuli along various color vectors in order to examine color appearance without the complications of the combined luminance and chromatic stimulation involved in most previous hue scaling studies, which have used flashes of monochromatic light. We also used spots lying along cone-isolating vectors in order to determine what hues would be reported with a change in activation of only single cone types or of only single geniculate opponent-cell types, an issue of direct relevance to any model of color vision. We find that: 1. Unique hues do not correspond either to the change in activation of single cone types or of single geniculate opponent-cell types. This is well known to be the case for yellow and blue, but we find it to be true for red and green as well. 2. These conclusions are not limited to the particular white (Illuminant C) used as an adapting background in most of the experiments. Shifts along the same cone-contrast vectors relative to different backgrounds lead to much the same hue percepts, independent of the starting white used. 3. The shifts of the perceptual colors from the geniculate axes are in the directions, and close to the absolute amounts, predicted by our [De Valois & De Valois (1993). Vision Research, 33, 1053-1065] multi-stage color model in which we postulate that the S-opponent cells are added to or subtracted from the M- and L-opponent cells to form the four perceptual color systems. 4. There are distinct asymmetries with respect to the extent to which various hues within each perceptual opponent system deviate from the geniculate opponent-cell axes. Blue is shifted more from the S-LM axis than is yellow; green is shifted more from the L-M axis than is red. There are also asymmetries in the angular extent of opponent color regions. Blue is seen over a larger range of color vectors than is yellow, and red over a slightly larger range than green. 5. Such asymmetries are not accounted for by any model that treats red-green and yellow-blue each as unitary, mirror-image opponent-color systems. Although red and green are perceptually opponent, the red and green perceptual systems do not appear to be constructed in a mirror-image fashion with respect to input from different cone types or from different geniculate opponent-cell types. The same is true for yellow and blue.

Color Perception↗
PubMed1997

Temporal response characteristics of the spatiochromatic visual evoked potential: nonlinearities and departures from psychophysics.

Although the visual evoked potential (VEP) for isoluminant stimuli has been characterized in terms of spatiochromatic parameters, temporal tuning along various chromatic directions has received less systematic attention. Additionally, there has been little categorical comparison of psychophysical appearance with VEP responses obtained for temporal variation of these patterns. At appropriate contrasts the VEP's for color axes (LM, S) show a robust and contrast-sensitive temporal tuning peak at 4 Hz. Contrast response functions at 4 Hz for the LM color axis are markedly nonmonotonic. However, there is a clear monotonicity with contrast for VEP latencies along these color axes. The anomalous behavior does not appear to be due to interactions between chromatic signals, to luminance artifact, or to rod intrusion. These anomalies in the temporal characteristics of the chromatic VEP may reflect interactions between chromatic responses and inherent cortical responsivity not linked to psychophysical behavior.

Color Perception↗
PubMed1997

Acute effects of blood glucose on chromatic visually evoked potentials in persons with diabetes and in normal persons.

PURPOSE: To determine whether specific chromatic pathways are selectively affected by short-term variations in blood glucose levels in observers with and without diabetes. METHODS: Ten subjects with diabetes, all with type 1 diabetes and no retinopathy, and eight age-similar normal subjects were tested. Cortical visually evoked potentials (VEPs) in response to stimuli designed to selectively activate the short-wavelength-sensitive (S) or long- and middle-wavelength-sensitive (LM) chromatic (isoluminant) pathways or the achromatic pathway were recorded over a period of several hours. Capillary blood glucose also was measured repeatedly over the same period. The relation between VEP latency and blood glucose was determined. RESULTS: The S-pathway VEP latency was correlated significantly with blood glucose in a slight majority (6/10) of persons with diabetes; S-pathway latency was longer at higher blood glucose levels. This association between S-pathway latency and blood glucose was not dependent on the pattern of blood glucose variation over time (i.e., significant correlations between blood glucose and latency were observed in persons for whom blood glucose increased, decreased, or rose and then fell over time). No dependence on blood glucose was observed for LM- or achromatic-pathway VEP latency in subjects with diabetes. CONCLUSIONS: Acute variations in blood glucose of subjects with diabetes over hours selectively affect the function of the short-wavelength-sensitive chromatic pathway. The findings are discussed within the context of known mechanisms by which elevated glucose affects cellular metabolism with a time course consistent with the transient nature of the effect observed.

Adult↗
PubMed1994

Visual evoked potentials in three-dimensional color space: correlates of spatio-chromatic processing.

Visual evoked potentials (VEPs) were measured for sinusoidal gratings with spatio-chromatic modulation defined in a three-dimensional color space. The spatio-chromatic modulation of the gratings can be decomposed into contributions from an achromatic luminance varying component, an isoluminant component which modulates only the activities of L cones and M cones, and an isoluminant component corresponding to modulation of only S-cone activity. The emphasis of this report is the nature of VEPs resulting from isoluminant spatio-chromatic modulation. The VEP response was characterized along a number of spatial, temporal, and chromatic stimulus dimensions: contrast, spatial frequency, chromaticity in the isoluminant plane, chrominance/luminance ratio, orientation, and temporal frequency. Isoluminant VEPs resulting from stimuli modulating L and M cones are compared with those from S-cone modulation. When appropriate spatiotemporal conditions are employed, both types produce robust VEPs; however, the S-pathway VEPs show considerably longer latencies than do those from LM-pathway activation. The VEP results are compared to psychophysical and single unit electrophysiological observations. VEP latencies exhibit the lowpass character of psychophysical chromatic contrast sensitivity functions but VEP amplitudes show bandpass tuning along both the S and LM axes. An oblique effect, i.e. shorter latencies for horizontal and vertical gratings than for diagonal, is observed in the isoluminant VEP. S-pathway VEPs are used to demonstrate an electophysiological correlate of transient tritanopia. Normative amplitude and latency data for VEPs from selectivity stimulated chromatic mechanisms provide a baseline for clinical electrodiagnostic applications.

Adaptation, Ocular↗
PubMed1993

Application of the spatiochromatic visual evoked potential to detection of congenital and acquired color-vision deficiencies.

Visual evoked potentials were recorded in response to spatiochromatic stimuli modulated in different directions in cone-activation color space from subjects with congenital and acquired color defects. This technique was effective for detection and classification of both mild and severe forms of congenital deficits. Results suggest that the visual evoked potential is useful for early identification of color abnormalities in acquired deficits such as diabetes and that it is sensitive enough to detect regional retinal losses of sensitivity (e.g., as in central serous choroidopathy). The spatiochromatic visual evoked potential provides a systematic and sensitive indication of different color-vision anomalies.

Adult↗
PubMed1992

Perceptual ambiguity and the short wavelength sensitive visual pathway.

Visual illusions and ambiguous figures are difficult to perceive when presented in a manner which stimulates only chromatic pathways. This perceptual limitation could indicate that these pathways are unable to mediate form perception, or it may reflect the relative visibility of the stimulus, regardless of the pathway utilized. To examine this issue, an ambiguous figure (illusory Necker cube) was presented to the short wavelength sensitive (S) cone pathway which is thought to convey only chromatic signals. Although few observers could recognize the cube when presented in isolation to S cones, the same reduction in recognition was found when the cube was detected by achromatic pathways at a level of visibility comparable to that conveyed by S cones. Our results indicate that for this object recognition task, visibility of the stimulus is the relevant variable, rather than the particular pathway traversed. Prior experience was also found to be important determinant of object recognition.

Color Perception↗
PubMed1992

Higher-order factors influencing the perception of sliding and coherence of a plaid.

The effect of several new stimulus parameters on the perception of a moving plaid pattern (the sum of two sine-wave gratings) were tested. It was found that: (i) the degree of perceived sliding is strongly influenced by the aperture configuration through which the plaid is viewed; (ii) the chromaticity of the sinusoidal components affects coherence in that more sliding is observed when the plaid components differ in hue, and there is less sliding when they are of the same hue; (iii) equiluminant plaids made of components equal in color almost never show any sliding; and (iv) sliding increases with viewing time. The coherence-sliding percept must therefore be influenced by color, by global interactions, and by adaptation or learning effects, thus suggesting a higher-level influence. These results are most easily modelled by separating the decision to carry out recombination from the process of recombination.

Attention↗
PubMed1992

Sites of age-related sensitivity loss in a short-wave cone pathway.

The age-related losses in the sensitivity of a short-wave (S) cone mechanism were determined by measuring the threshold-versus-radiance (t.v.r.) functions for nine older (mean age, 71.0 years) and six younger (mean age, 24.4 years) subjects. Measurements were obtained for a 250-ms, 1.03 degrees-diameter, 440-nm foveally viewed test light presented on 470-nm adapting fields and on a 570-nm auxiliary field. A comparison of t.v.r. functions revealed intensity-dependent sensitivity losses in the older group. By applying a quantitative model of an S-cone pathway to the t.v.r. data obtained from the older subjects, we found that the age-related changes in the ocular media and the receptor sensitivities can account for much but not all of the difference in the sensitivity of an S-cone mechanism demonstrated by eight of our nine older subjects.

Adolescent↗
PubMed1991

Spatial localization across channels.

We have studied vernier acuity for patterns in which the stimuli to be aligned either are similar in their spatial and color characteristics or differ in these properties. The question which we address is whether spatial localization is independent of the channels being stimulated by the patterns to be aligned. We found that the precision of vernier alignment of Gabor patches was very similar irrespective of whether the patches were the same or different in spatial frequency, orientation, or color. It appears that the visual system extracts very precise location information independent of the similarity or dissimilarity of the spatio-chromatic selectivity of the channels carrying that information.

Color Perception↗
PubMed1990

Readily visible changes in color contrast are insufficient to stimulate accommodation.

In an earlier study (Wolfe & Owens, 1981) it was reported that humans could not accommodate to an insoluminant red-green border. However, recent masking studies (Switkes, Bradley & DeValois, 1988) have shown that, using an appropriately normalized contrast metric, contrast decrements similar to those produced by defocus are equally visible for color or luminance modulated grating patterns. We have compared accommodative responses to 1.75 c/deg gratings that consisted of either isochromatic luminance modulations or isoluminant red-green color modulations. All four observers could accommodate accurately to luminance modulated gratings over a wide range of contrasts. However, no appropriate accommodative responses were obtained even for the highest contrast color modulated gratings. These results show the changes in color contrast are ineffective as stimuli for the human accommodative response even when the changes in chromatic contrast accompanying defocus are readily perceived.

Accommodation, Ocular↗
PubMed1990

Orientation and spatial-frequency discrimination for luminance and chromatic gratings.

We have examined the accuracy of orientation and spatial-frequency discrimination for sine-wave gratings that vary in either luminance or color. The equiluminant chromatic gratings were modulated along either a tritanopic confusion axis (so that they were detectable on the basis of activity in only the short-wavelength-sensitive cones) or an axis of constant short-wavelength-sensitive cone excitation (so that they could be detected on the basis of opposing activity in only the long- and medium-wavelength-sensitive cones). Grating contrasts ranged from the detection threshold to the highest levels that we could produce; the contrasts of the luminance and color patterns were equated for equal multiples of their respective detection thresholds. Discrimination thresholds for all patterns showed a similar dependence on stimulus contrast, rising sharply at low contrasts and becoming nearly asymptotic at moderate contrasts. However, even at threshold contrasts, observers could still reliably discriminate sufficiently large differences in the orientation or spatial frequency of all patterns, and they could also reliably identify the type of variation (luminance or which color) defining the grafting. For most conditions the discrimination thresholds did not differ from the two types of color grafting and reached values as low as 1 deg (orientation) or 4% (spatial frequency). Thus observers were able to make accurate spatial judgments on the basis of either type of chromatic information. However, these thresholds were slightly but consistently higher than the thresholds for comparable luminance graftings. This difference in the color and luminance discrimination thresholds may reflect somewhat coarser orientation and spatial-frequency selectivity in the mechanisms encoding the chromatic patterns.

Color Perception↗
PubMed1988

Orientation and spatial frequency selectivity of adaptation to color and luminance gratings.

Prolonged viewing of sinusoidal luminance gratings produces elevated contrast detection thresholds for test gratings that are similar in spatial frequency and orientation to the adaptation stimulus. We have used this technique to investigate orientation and spatial frequency selectivity in the processing of color contrast information. Adaptation to isoluminant red-green gratings produces elevated color contrast thresholds that are selective for grating orientation and spatial frequency. Only small elevations in color contrast thresholds occur after adaptation to luminance gratings, and vice versa. Although the color adaptation effects appear slightly less selective than those for luminance, our results suggest similar spatial processing of color and luminance contrast patterns by early stages of the human visual system.

Adaptation, Ocular↗
PubMed1988

Contrast dependence and mechanisms of masking interactions among chromatic and luminance gratings.

The contrast dependence of simultaneous masking has been measured using isochromatic yellow-black luminance sinusoids and isoluminant red-green chrominance gratings. Masking functions for all four combinations of chromatic and luminance masks and tests are reported. In the two same-on-same conditions (luminance mask/luminance test and chromatic mask/chromatic test) these functions (increment threshold contrast versus mask contrast) have the typical dipper shape and are almost identical when test and mask contrasts are normalized to the unmasked contrast thresholds. The contrast dependence of the luminance mask/color test and color mask/luminance test functions are quite different. The luminance mask/color test shows facilitation over a broad range of both subthreshold and suprathreshold contrasts of the luminance mask. In the color mask/luminance test condition facilitation is never observed, but at suprathreshold contrasts a 2-cycle/degree (c/deg) chromatic grating masks a 2-c/deg luminance grating as strongly as does a luminance mask. The luminance mask/chromatic test results are invariant over the 0.25-2-c/deg spatial-frequency range, whereas the robust masking of luminance by color at 2 c/deg diminishes at lower spatial frequencies. The spatial-frequency selectivity of the luminance-facilitates-color interaction is much broader than facilitatory interactions in either the color-color or luminance-luminance conditions. Possible mechanisms of color-luminance interactions are considered. The lack of facilitation in the color mask/luminance test condition precludes a simple pedestal interpretation of this masking interaction. The data are, however, consistent with models that invoke inhibitory or more elaborate excitatory masking interactions.

Color↗
PubMed1988

Functional anatomy of macaque striate cortex. I. Ocular dominance, binocular interactions, and baseline conditions.

A series of experiments was carried out using 14C-2-deoxy-d-glucose (DG) in order to examine the functional architecture of macaque striate (primary visual) cortex. This paper describes the results of experiments on uptake during various baseline (or reference) conditions of visual stimulation (described below), and on differences in the functional architecture following monocular versus binocular viewing conditions. In binocular "baseline" experiments, monkeys were stimulated either (1) in the dark, (2) with a diffuse gray screen, or (3) with a very general visual stimulus composed of gratings of varied orientation and spatial frequency. In all of these conditions, DG uptake was found to be topographically uniform within all layers of parafoveal striate cortex. In monocular experiments that were otherwise similar, uptake was topographically uniform within the full extent of the eye dominance strip, in all layers. Certain other visual stimuli produce high uptake in the blobs, and still another set of visual stimuli (including high-spatial-frequency gratings) produce highest uptake between the blobs at parafoveal eccentricities, even in an unanesthetized, unparalyzed monkey. Eye movements per se had no obvious effect on striate DG uptake. Endogenous uptake in the blobs (relative to that in the interblobs) appears higher in the squirrel monkey than in the macaque. The pattern of DG uptake produced by binocular viewing was found to deviate in a number of ways from that expected by linearly summing the component monocular DG patterns. One of the most interesting deviations was an enhancement of the representation of visual field borders between stimuli differing from each other in texture, orientation, direction, etc. This "border enhancement" was confined to striate layers 1-3 (not appearing in any of the striate input layers), and it only appeared following binocular, but not monocular, viewing conditions. The border enhancement may be related to a suppression of DG uptake that occurs during binocular viewing conditions in layers 2 + 3 (and perhaps layers 1 and 4B), but not in layers 4Ca, 4Cb, 5 or 6. Another major class of binocular interaction was a spread of neural activity into the "unstimulated" ocular dominance strips following monocular stimulation. Such an effect was prominent in striate layer 4Ca, but it did not occur in layer 4Cb. This "binocular" spread of DG uptake into the inappropriate eye dominance strip in 4Ca may be related to the appearance of orientation tuning and orientation columns in that layer. No DG effects were seen that depended on the absolute disparity of visual stimuli in macaque striate cortex.

Animals↗
PubMed1988

Functional anatomy of macaque striate cortex. II. Retinotopic organization.

Macaque monkeys were shown retinotopically-specific visual stimuli during 14C-2-deoxy-d-glucose (DG) infusion in a study of the retinotopic organization of primary visual cortex (V1). In the central half of V1, the cortical magnification was found to be greater along the vertical than along the horizontal meridian, and overall magnification factors appeared to be scaled proportionate to brain size across different species. The cortical magnification factor (CMF) was found to reach a maximum of about 15 mm/deg at the representation of the fovea, at a point of acute curvature in the V1-V2 border. We find neither a duplication nor an overrepresentation of the vertical meridian. The magnification factor did not appear to be doubled in a direction perpendicular to the ocular dominance strips; it may not be increased at all. The DG borders in parvorecipient layer 4Cb were found to be as sharp as 140 micron (half-amplitude, half width), corresponding to a visual angle of less than 2' of arc at the eccentricity measured. In other layers (including magnorecipient layer 4Ca), the retinotopic borders are broader. The retinotopic spread of activity is greater when produced by a low-spatial-frequency grating than when produced by a high-spatial-frequency grating. Orientation-specific stimuli produced a pattern of activation that spread further than 1 mm across cortex in some layers. Some DG evidence suggests that the spread of functional activity is greater near the foveal representation than near 5 degrees eccentricity.

Animals↗
PubMed1988

Functional anatomy of macaque striate cortex. III. Color.

Using spatially diffuse stimuli (or sinusoidal gratings of very low spatial frequency), levels of 14C-2-deoxy-d-glucose (DG) uptake produced by color-varying stimuli are much greater than those produced by luminance-varying stimuli in macaque striate cortex. Such a difference in DG results is consistent with previous psychophysical and electrophysiological results from man and monkey. In DG experiments with color-varying gratings of low and middle spatial frequencies, or with spatially diffuse color variations, DG uptake was highest in the cytochrome oxidase blobs, as was also seen with low-spatial-frequency luminance gratings. High-spatial-frequency, color-varying uptake patterns were shifted to cover both blob and interblob regions in a manner similar to that of the patterns obtained with middle-spatial-frequency luminance stimuli. However, in no instance did chromatic gratings produce uptake restricted to the interblob regions, as with the pattern seen with the highest-spatial-frequency luminance gratings. Thus, DG uptake is relatively higher in the interblob regions when comparing luminance with color-varying gratings that are otherwise similar. It was also possible to show DG evidence for receptive-field double-opponency in the upper-layer blobs, but color sensitivity in layer 4Cb appears single-opponent. The DG results suggest that color sensitivity is also high in the lower-layer (layers 5 + 6) blobs, and that many layer 5 receptive fields are double-opponent. Striate layers 4Ca and 4B-appeared color-insensitive in a wide variety of DG tests; this supports the idea of a color-insensitive stream running from the magnocellular LGN layers through striate layers 4Ca and 4B to extrastriate areas MT and V3. There was also a major effect due to wavelength: long and short wavelengths produced much more uptake than did middle wavelengths, even when all colors were equated for luminance and saturation. No variation with eccentricity was seen in cortical color sensitivity, at least between 0 degrees and 10 degrees.

Animals↗