Deoxyglucose analysis of retinotopic organization in primate striate cortex.

We have anatomically analyzed retinotopic organization using the 14C-labeled 2-deoxy-D-glucose method. The method has several advantages over conventional electrophysiological mapping techniques. In the striate cortex, the anatomical substrate for retinotopic organization is surprisingly well ordered, and there seems to be a systematic relationship between ocular dominance strips and cortical magnification. The 2-deoxyglucose maps in this area appear to be largely uninfluenced by known differences in long-term metabolic activity. This method should prove useful in analyzing retinotopic organization in various visual areas of the brain and in different species.

A comparison of color and luminance discrimination in amblyopia.

Using isochromatic luminance gratings and isoluminant red-green color gratings, examined luminance and color discrimination in amblyopia were examined. Complete color and luminance contrast sensitivity functions were measured monocularly from each eye of six normal and six amblyopic observers. Similar spatial frequency-specific color and luminance discrimination deficits were found in five of the amblyopes. One amblyope with a slight luminance deficit showed no color deficit. It appears that color and luminance discrimination are similarly affected in most amblyopes when spatial factors are effectively controlled.

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.

Picture processing techniques applied to autoradiographic studies of visual cortex.

Picture processing techniques are applied to 2-deoxyglucose autoradiographs of sections from striate cortex and to patterns resulting from staining these sections for cytochrome oxidase. This procedure allows computer identification of deoxyglucose active and inactive regions in the autoradiographs and cytochrome active and inactive regions in the stain patterns. Subsequently, the topographical relationship between these patterns can be quantitatively analyzed by means of overlap and density distribution measures and can be displayed using color enhanced graphics. The processing techniques have been applied in studies of the functional organization of visual cortex in primates. Computer graphic techniques have allowed implementation of split-field presentations of stimuli in deoxyglucose experiments. An application of this split-field technique for presenting multiple-stimuli to distinct parts of the visual field is described and an autoradiograph from a split-field experiment is shown.

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.

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.

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.

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.

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.

Glass pattern studies of local and global processing of contrast variations.

Using Glass patterns [Nature 223 (1969) 578; Nature 246 (1973) 360; Perception 5 (1976) 67], we have studied the role of contrast differences in local and global processes of form perception. The virtue of these patterns (composed of a set of randomly distributed elements combined with a geometrically transformed copy) for studying object formation is that they allow ready isolation of local processes, the combination of dots to form a perceptual pair, from global processes, the combination of dipoles into the percept of an overall rotational or translational pattern. We find that a contrast difference within dot-pairs reduces the ability to resolve local features; large differences totally abolish the perception of the pattern. Contrast differences between dot-pairs lessen, but do not abolish, the global integration among local features. In both cases the effect is proportional to the ratio of the two contrast levels employed. Effects which differ for rotations and translations, are consistent with the greater areal integration required to resolve rotational patterns.

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.

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.

Spatial frequency specific interaction of dot patterns and gratings.

Adaptation to patterns of paired random dots produces loss of contrast sensitivity to sinusoidal luminance gratings oriented perpendicularly to the dot-pair direction. This adaptation loss is spatial frequency- and orientation-specific and varies with dot-pair separation in a manner predictable from the Fourier spectra of the stimuli and observed characteristics of the visual system. These results support the idea that the visual system acts as a periodicity analyzer with known restrictions and cannot be accounted for by a feature-detector model. When the bars of the test gratings are aligned in the dot-pair direction, there is no adaptational loss at any frequency despite the fact that the adaptation pattern contains significant spectral power at all frequencies in this orientation. This lack of adaptation may be due to inhibitory interactions among channels or to nonlinear effects within local receptive fields.

Parallel processing of motion and colour information.

When the two eyes are confronted with sufficiently different versions of the visual environment, one or the other eye dominates perception in alternation. A similar situation may be created in the laboratory by presenting images to the left and right eyes which differ in orientation or colour. Although perception is dominated by one eye during rivalry, there are a number of instances in which visual processes nevertheless continue to integrate information from the suppressed eye. For example the interocular transfer of the motion after-effect is undiminished when induced during binocular rivalry. Thus motion information processing may occur in parallel with the rivalry process. Here we describe a novel example in which the visual system simultaneously exhibits binocular rivalry and vision that integrates signals from both eyes. This apparent contradiction is resolved by postulating parallel visual processes devoted to the analyses of colour and motion information. Counterphased gratings are viewed dichoptically such that for one eye the grating is composed of alternating yellow and black stripes (luminance) while for the other it is composed of alternating red and green stripes (chrominance). When the gratings are fused, a moving grating is perceived. A consistent direction of motion can only be achieved if left and right monocular signals are integrated by the nervous system. Yet the apparent colour of the binocular percept alternates between red-green and yellow-black. These observations demonstrate the segregation of processing by the early motion system from that affording the perception of colour. Although, in this stimulus, colour information in itself can play no part in the cyclopean perception of motion direction, colour is carried along perceptually (filled in) by the moving pattern which is integrated from both eyes.

Simultaneous masking interactions between chromatic and luminance gratings.

Simultaneous masking using test and mask gratings composed of isochromatic luminance variations and isoluminant chromatic variations was studied. Masking of chromatic gratings by chromatic gratings shows less spatial-frequency specificity than does masking of luminance gratings by luminance gratings. Luminance gratings mask chromatic gratings of identical space-average luminance and chromaticity little and only when the spatial frequencies of the test and mask gratings are similar. Chromatic gratings, however, profoundly mask luminance gratings with a degree of spatial-frequency specificity akin to that of luminance-luminance masking. The insensitivity of the luminance-color masking results to the relative phase of the chromatic and luminance gratings indicates that the observed asymmetry is not due to local interactions.

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.