The effect of rods on perceptive field size at 10° eccentricity in the four retinal quadrants

Abstract

This research was conducted to determine the effect that rod photoreceptors have on the size of perceptive fields in the peripheral retina. Three observers used the "4+1" color-naming technique (Abramov and Gordon, 1977) to judge their perceptions of hue and saturation for test fields of varying size at 10° eccentricity in the temporal, nasal, superior, and inferior quadrants of the retina. Data were also collected in the fovea using a 1° stimulus. At each peripheral eccentricity, stimuli were judged both on the cone plateau following a rod bleach and after 30 minutes of dark adaptation to maximize cone and rod signals, respectively. Results were analyzed for the effects of adaptation, stimulus size, and retinal location. In the no-bleach condition, with maximal rod activation, 1) colors viewed in the peripheral retina appeared less saturated at shorter wavelengths than in the fovea and rod-bleach condition, and 2) percent hue of all four basic hue terms differed from that in the fovea and the rod-bleach condition. Percent blue was lower and its spectral range was less in the no-bleach condition compared to the fovea; less red was reported at both short and long wavelengths in the no-bleach condition. Rods had their largest effects between 490 and 500 nm, increasing yellow perception and decreasing green perception as well as shifting the peak of the green function to shorter wavelengths. Several of these differences persisted under conditions where rod signals were minimized, indicating that some effects may be due to peripheral cones functioning differently from foveal cones, and not to rod signals. Decreasing stimulus size corresponded to lower percentages of hue and saturation. Blue and red were the most well-preserved hues at smaller stimulus sizes. Yellow and green were more affected by variations in stimulus size, showing both decreases in the amplitude of the response function and changes in the spectral range of the response functions. Percent hue generally grew to an asymptote as a function of increasing stimulus size under all adaptation conditions. Perceptive field sizes were calculated by fitting the Michaelis-Menten growth function to color-naming data. Rod signals increased the size of perceptive fields with the largest effect on the perceptive field for green. Not surprisingly, rod effects were most notable in the area of the visible spectrum where they are most sensitive. Under the rod-bleach condition, where rod signals were minimized, the perceptive field for blue was the smallest at all retinal locations, followed by the size of the perceptive field for red. Perceptive fields were smallest in the temporal retina under dark-adapted conditions, and perceptive field sizes were largest in the inferior retina when measured on the cone plateau. In general, results from the temporal retina were most like those of the fovea, and results from the inferior retina deviated the most from foveal results. Taken together, these results demonstrate rods differentially affect perceptive field sizes. In addition, evidence is provided that the neural processing of color in the peripheral retina differs from that in the fovea and that perceptive fields are most likely to be cortical in origin.