Biomedical subjects

E R Charles

Publications and source records attributed to E R Charles.

5 recordsLinked to original sources

PubMed1991

Parallel pathways in the visual system: their role in perception at isoluminance.

It has been proposed that the functions of the two major parallel channels of the primate visual system, the color-opponent and the broad-band, can be determined in psychophysical experiments by eliminating luminance but maintaining chrominance information (isoluminance), since under such conditions the broad-band channel is believed to be silenced. To test this proposition we examined the visual functions of monkeys after blocking either of these channels and we also assessed the responses of neurons to isoluminant stimuli in the lateral geniculate nucleus. We show that color, texture, stereopsis and pattern perception in the absence of the color-opponent channel, and flicker and motion perception in the absence of the broad-band channel are compromised. Yet isoluminance functions for stereopsis and texture in the absence of the broad-band channel and for motion in the absence of the color-opponent channel are indistinguishable from normal. Our recordings show that the neuronal responses of the broad-band cells for isoluminant exchange of red and green lights are reduced but not eliminated and that the color-opponent cells also become similarly less responsive under these conditions. We conclude that perceptual losses at isoluminance are not specific for either channel.

Animals↗

PubMed1990

Perceptual deficits and the activity of the color-opponent and broad-band pathways at isoluminance.

The deficits in texture, motion, and depth perception incurred in monkeys at isoluminance were compared with the responses of neurons of the color-opponent and broad-band systems in the lateral geniculate nucleus. Texture perception, assumed to be carried by the color-opponent system, and motion and depth perception, ascribed to the broad-band pathway, were all found to be compromised but not abolished at isoluminance. Correspondingly, both the color-opponent and the broad-band systems were affected at isoluminance, but the activity of the neurons in neither system was abolished. These results suggest that impairment of visual capacities at isoluminance cannot be uniquely attributed to either of these systems and that isoluminant stimuli are inappropriate for the psychophysical isolation of these pathways.

Animals↗

PubMed1990

Functions of the colour-opponent and broad-band channels of the visual system.

The colour-opponent and broad-band channels of the primate visual system originate in the retina and remain segregated through several neural stations in the visual system. Until now inferences about their function in vision have been based primarily on studies examining single-cell receptive field properties which have shown that the colour-opponent retinal ganglion cells have small receptive fields, produce sustained responses and receive spatially segregated inputs from different cone types; the broad-band cells have large receptive fields, respond transiently and receive cone inputs that are not spatially separated. We have now examined the visual capacities of rhesus monkeys before and after interrupting either of these channels with small lesions at the lateral geniculate nucleus. Here we report that the colour-opponent channel is essential for the processing of colour, texture, fine pattern and fine stereopsis, whereas the broad-band channel is crucial for the perception of fast flicker and motion. Little or no deficits were found in brightness and coarse-shape discrimination, low spatial frequency stereopsis and contrast sensitivity after the disruption of either of the channels.

Animals↗

PubMed1990

The minimum motion technique applied to determine isoluminance in psychophysical experiments with monkeys.

Isoluminance ratios for red/green, red/blue and blue/green sinusoidal gratings were determined in macaque monkeys using the minimum motion heterochromatic photometry technique (Anstis & Cavanagh, 1983), in which the motion of specially constructed test grating reverses at the point of isoluminance. The point of luminance equality between the two colors was determined using (1) overt responses of monkeys trained to discriminate motion direction and (2) reversals in the direction of the nystagmic eye movements. Both measures yielded essentially identical isoluminance points. Both the spatial and the temporal frequency of the grating patterns were found to influence the isoluminance settings in all animals tested. The amount of blue equating a preset red or green value was found to increase consistently with increased spatial frequency and with increasing eccentricity. These results show the necessity of spatial and temporal frequency dependent adjustment of visual patterns used in psychophysical and physiological experiments dealing with the processing of color. Furthermore, eye movement measurements appear to be sufficiently in order to determine isoluminance in foveally presented moving patterns with nonhuman primates.

Animals↗

PubMed1990

Role of the color-opponent and broad-band channels in vision.

The functions of the primate color-opponent and broad-band channels were assessed by examining the visual capacities of rhesus monkeys following selective lesions of parvocellular and magnocellular lateral geniculate nucleus, which respectively relay these two channels to the cortex. Parvocellular lesions impaired color vision, high spatial-frequency form vision, and fine stereopsis. Magnocellular lesions impaired high temporal-frequency flicker and motion perception but produced no deficits in stereopsis. Low spatial-frequency form vision, stereopsis, and brightness perception were unaffected by either lesion. Much as the rods and cones of the retina can be thought of as extending the range of vision in the intensity domain, we propose that the color-opponent channel extends visual capacities in the wavelength and spatial-frequency domains whereas the broad-band channel extends them in the temporal domain.

Animals↗