The cone threshold: spatial interactions of rod and cone adapting signals

Differences in adaptation between on- and off-centre ganglion cells and rod-mediated cone sensitization in cat retina

1. Response properties of on- and off-centre retinal ganglion cells were investigated in cats. The stimulus parameters were selected so as to demonstrate interactions between the rod and the cone systems. 2. Response versus log stimulus intensity (R-log I) functions were determined for the receptive field centres while both test stimulus irradiance and the background illumination were varied over a range up to 7 log units. In order to determine the course of adaptation to chromatic stimuli, threshold versus intensity (t.v.i.) functions were measured over a wide range of adaptation levels. 3. An increase in background illuminance produced a shift of the R-log I functions to higher irradiances of test stimuli in most ganglion cells, indicating a desensitization of the centre response in the presence of background lights. Using test stimuli which most efficiently stimulate the rods (501 nm), clear differences could be seen in the adaptation behaviour of on- and off-centre ganglion cells. Chromatic backgrounds (blue-green and orange) reduced the responses of off-centre cells more than those of on-centre cells (the difference between them amounting to as much as 2 log units). Simultaneously, equivalent t.v.i. functions had significantly steeper slopes (0.94 and 1.1) in the linear proportions of off-centre cells compared to on-centre cells (0.76 and 0.75) under light levels mediated by rods. Such differences were not observed when a test stimulus of 575 nm was used which resulted primarily in stimulating the long-wavelength cone (L-cone) system. 4. In a subpopulation of off-centre cells (20% of the total number of off-centre cells recorded), a strikingly different adaptation behaviour was observed. Here, the presentation of a dim short-wavelength background produced a shift of R-log I functions to lower test stimulus irradiances. The receptive field centre became even more sensitive, by up to 1.5 log units, in the presence of dim adapting backgrounds rather than in the dark-adapted state. Accordingly, the t.v.i. function did not increase monotonically but showed a 'dip' in the presence of dim backgrounds. Only at photoic levels, the t.v.i. functions revealed a response behaviour similar to the other ganglion cells. The sensitization with dim backgrounds was only observed in the case of test stimuli designed to stimulate the cone system (575 nm) and in the presence of a rod-adapting blue-green background.(ABSTRACT TRUNCATED AT 400 WORDS)

Rod and cone contribution to adaptation processes in cat retinal ganglion cells

Extracellular ganglion cell responses were recorded to investigate mechanisms of light adaptation. Monochromatic test spots (575 nm) were projected onto the receptive field center of off-center cells and superimposed on a steady blue-green Ganzfeld background (Schott Filter BG 28), the strength of which was increased in steps of 0.5 log units to adapt rods. Response vs. log intensity functions were determined over a range of 7 log units of test light irradiance at each background level. At higher adaptation levels response thresholds followed the typical Weber function. Surprisingly at lower adaptation levels the sensitivity of the cell increased by about 0.7 log units, most markedly in a range of 1 log unit of moderate light adaptation when the background was changed from dark to the dimmest detectable background (10(-5) lm/m2). In the dark-adapted state a small off-response of long latency (40-100 ms at 10(2) quanta.s-1.microns-2) is observed at low rod stimulating test light irradiances. A transition to a cone-dominated transient response of 2 to 5 ms duration occurred at high intensities (10(5) quanta.s-1.microns-2). At mesopic levels the two responses seem to cancel each other, rendering a delayed off-response that is probably the result of rod-cone interaction. As in psychophysics, saturation can be observed at very high background intensities (10(6) quanta.s-1 microns-2). These data suggest interactions between rods and cones that determine the sensitivity of cat retinal ganglion cells at low levels of adaptation for suprathreshold stimuli.

Pathways in type-B (U-shaped) metacontrast

Rod and cone targets were crossed, in every combination, with rod and cone masks in flanking-bars metacontrast. Strong type-B (U-shaped) metacontrast was obtained in each condition, contrary to the claim that rod and cone masking are independent. In each condition, visibility declined steadily with stimulus-onset asynchrony (SOA) in trials in which target and mask appeared to be simultaneous, and increased with SOA in trials in which they appeared to be successive. The 'U' results from collapsing across these different types of trials, which may reflect distinct monotonic processes in masking. Under the light adaptation conditions used the time, Tmax, at which metacontrast was at a maximum was delayed by about 25 ms if rods, rather than cones, detected the target. Whether rods or cones detected the mask hardly altered Tmax.

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.

Increment spectral sensitivities for spatial periodic grating patterns: evidence for variable tuning of the chromatic system

Increment spectral sensitivities were measured for spatial periodic grating patterns. First, the increment threshold was determined as a function of wavelength, for various spatial frequencies and white-background intensities. Second, the additivity of test mixture was tested. Thirdly, the threshold vs intensity curves were determined for various spatial frequencies of test monochromatic stimuli. Finally, the increment threshold was determined as a function of spatial frequency. The following results were obtained: the background intensity and the spatial frequency affected the increment spectral sensitivity functions in different manners; the result of test mixture showed subadditivity for high background intensity and approximately linear additivity for low background intensity; the resultant t.v.i. curves converged toward Weber's law at high background intensity; and the sensitivity vs spatial frequency curve showed a loss of the sensitivity at low spatial frequency for high background intensity. These results called in question the existing hypothesis of separate chromatic and achromatic systems whose spectral tunings are invariant with a spatial parameter, but favored the hypothesis of variable tuning of the chromatic system.

Cone-rod interaction over time and space

Scotopic background stimulation can elevate photopic increment thresholds by more than 2 log units. This cone-rod interaction is greatest on small backgrounds (less than 1 degree diameter), but is found consistently on large backgrounds as well. Interaction develops and disappears quickly as backgrounds are turned on or off, respectively. The onset, and in some cases the offset, of a background stimulus can produce an additional, transitory interaction that augments the interaction that is maintained by continued presentation of the same background. The majority of the present findings lend support to a simple center-surround model of cone-rod interaction: nearby scotopic excitation raises photopic thresholds and more distant scotopic stimulation primarily antagonizes this interaction.

Rod-cone interaction in flicker detection

There is considerable evidence in the literature that rod-cone interaction occurs when both rods and cones simultaneously detect a test target. More recent evidence, however, has shown a parafoveal rod-cone interaction during dark adaptation for a purely cone-detected flickering test stimulus; this influence on cone threshold appears to be mediated by surrounding rods. In this study, we demonstrate a similar rod-mediated influence on parafoveal cone-detected flicker threshold. More surprisingly, foveal cone-detected thresholds are also influenced by rods. This effect occurs over at least a 2 log unit intensity range of mesopic background level; cone-detected 25 Hz flicker sensitivity is enhanced by increasing the radiance of the background. The action spectrum of this effect fits the scotopic spectral sensitivity curve. At higher background levels, this rod-cone interaction disappears and surrounding cone activity then influences the cone flicker threshold. The results suggest that, as rods recover sensitivity, they reduce cone-detected flicker sensitivity, even at the fovea. The rod influence on cone flicker is most apparent for long wavelength test stimuli. Our results, in agreement with recent reports, suggest that the rod-cone interaction is laterally-mediated and may be specific for the long wavelength-sensitive cone type.