Spatial sensitization and desensitization with small adapting fields: interactions of signals from different classes of cones

Red-green interactions in the spectral sensitivity of primates as derived from ERG and behavioral data

Different techniques were used to manipulate the inhibitory interaction between the red and green photoreceptors (R and G cones) of rhesus and human primates. The response techniques that were used were the corneal electroretinogram (ERG) and psychophysical increment-threshold spectral sensitivity functions. Red-green opponency, as measured by the depth of the notch at 580 nm, is removed by intravitreal injection of bicuculline but not by strychnine. Therefore, red-green opponency is mediated by GABA and not glycine. The depth of the notch is dependent upon stimulus size. Between 30' and 15' test light diameters, this sign of red-green opponency disappears. Psychophysical increment thresholds are shown to produce the notch while decrements do not and intravitreal APB is shown to reduce the notch, evidence that red-green opponency is carried by the "ON" and not the "OFF" bipolar pathways of the retina. Red and green annuli are shown to selectively reduce red and green inhibition, as though there were selective reduction of the surround response in center-surround organized red-green receptive fields.

Cone interaction occurs in the parafovea under pi 4 stimulus conditions

This study shows that stimulus parameters that isolate Stiles' pi 4 mechanism at the fovea do not isolate pi 4 parafoveally. Instead, the parafoveal test sensitivity peaks at 520 nm and is relatively depressed at longer wavelengths. This narrowed spectral sensitivity is not due to rod intrusion or interaction but rather suggests cone interaction for a relatively wide range of spatial and temporal parameters. The anomalous shape of the parafoveal spectral sensitivity is not found in a protanope, further supporting the view that the effect seen in normal trichromats is the result of interaction between L and M cones. The study provides a unifying explanation for discrepant results by investigators studying the chromatic properties of spatial sensitization and the contrast after-flash effect. Foveal studies found independence of pi mechanisms while parafoveal studies found interaction. The present study emphasizes the importance of retinal eccentricity on color threshold experiments.

Illusory contours induced by isoluminant chromatic patterns

An illusory-contour was induced by abutting colored gratings embedded in the white field under isoluminance condition. The present study specified the stimulus conditions that invoked the just-perceptible illusory contours for the isoluminant chromatic patterns. The results showed that the purity difference between the colored lines and the white field required for the illusory-contour perception gave a function closely resembling in shape the function obtained for saturation discrimination. Increasing the width of lines reduced threshold for the perception of illusory contours, while the line spacing had no significant effect. The dependence of the illusory-contour perception upon saturation could be accounted for by considering the extraction of the edge and colour information by means of the opponent-color processes observed for the cells in the retina and LGN. On the other hand, the effects of the spatial parameters met the response properties observed for the cells in the visual cortex. It is suggested that the perception of illusory contour may result from hierarchical organization from the retina to cortex in the visual system.

Effect of surround configuration on increment thresholds of a tiny violet flash

Increment thresholds were measured for a 5', 435-nm test flash (50 or 200 msec) on background fields of various wavelengths and spatial configurations. As the field diameter was increased, the detection threshold for the 50-msec test decreased rapidly and leveled off when the field diameter exceeded 20'. For short-to-middle wavelength fields (much less than or equal to 503 nm), the detection threshold for the 200-msec test varied with field diameter in a similar manner to those observed for the 50-msec tests, while for middle-to-long wavelength fields (much greater than or equal to 544 nm), the detection threshold continued to decrease even when the field diameter exceeded 20'. The temporal-integration properties and the threshold vs intensity curves were determined for 465-nm and 579-nm fields to specify the mechanisms responsible for the detection. It was shown that the detection thresholds of the test upon the large 579-nm field were characterized by the short-wavelength mechanism isolated by an intense yellow adapting field, whereas the detection thresholds of the test upon the small 579-nm or the 465-nm field showed an intrusion of a more sensitive alternate mechanism, presumably the middle-wavelength mechanism. It was suggested that the sensitivity of the S cone signals might be affected by spatially distributed signals from the longer-wavelength mechanisms.

Spatial extent of rod-cone and cone-cone interactions for flicker detection

Over a large range of light adaptation levels, sensitivity to 25 Hz flicker improves as the light level of the background increases. Using small background discs and annular surrounds, this effect was shown to be mediated by the surround and not the average luminance of the test region, in agreement with recent reports. The effect is due to two types of lateral interaction: at mesopic light levels (from 0.1 to 1.0 td), cone-mediated flicker resolution is enhanced by the stimulation of surrounding rods; at photopic light levels (above 10 td), flicker sensitivity improves with light stimulation of adjacent cones. The spatial zone, or extent, over which the surround contributes to the flicker threshold was measured. The spatial area over which rods influence the cone 25 Hz flicker threshold is larger than the analogous spatial area of cone influence. In the parafovea, at 5 deg eccentricity, cone flicker sensitivity for a 20' spot is influenced by cones in a 1 deg diameter area centered on the spot; the corresponding area of rod influence is about 3 deg. In the fovea, flicker sensitivity for a 10' spot is influenced by cone stimulation in an area of about 20' diameter. Rods which affect foveal flicker sensitivity appear to occupy an annular zone with about a 2 deg outer diameter and 1 deg inner diameter, centered on the fovea.

Adaptation of a color-opponent mechanism increases parafoveal sensitivity to luminance flicker

Sensitivity to 25 Hz luminance flicker is enhanced as the intensity of a background is increased. This effect is due, in part, to light adaptation of cones. Preliminary studies suggested that, in the parafovea, the enhancement of flicker sensitivity is due to the adaptation of a color-opponent mechanism rather than a single cone mechanism or achromatic mechanism. In the present study, action spectra and field additivity experiments demonstrated that the enhancing effect was opponent in nature. The field sensitivity function had a notch at 570 nm; fields on either side of the notch acted subadditively. This suggests that, in the parafovea, the sensitivity of an achromatic flicker mechanism is influenced by the adaptation of a chromatic mechanism.

Spatial and chromatic antagonism between the long- and middle-wavelength cones in the detection of long-wavelength flashes

Field additivity was tested for small red increments on small and large adapting fields. The results for a large adapting field showed the failure of field additivity of the cancellation type, providing strong evidence for cone-antagonistic coding, while the results for a small adapting field showed field additivity. The cancellation effect took place concurrently with the spatial sensitization effect, thus suggesting that the spatial and chromatic antagonism occur concomitantly at some stage in the detection pathway.