Psychometric functions in the central visual field

Abstract

We consider the nature of the luminance (L) and chromatic (RG or BY) psychometric functions in the central visual held Stimuli were generated on a colour TV monitor with spatiotemporal characteristics that minimized transients (3 degrees Gabor - 1 c/deg sine wave, 1 sec raised-cosine duration). Testing was performed along the vertical meridian in 3 degrees steps between 0 degrees and 12 degrees. Thresholds were expressed as cone contrasts (%) for probes from a white background (CIE 1931 x = 0.31, y = 0.32; L = 33 cd/m(2)). Two observers performed a method of constant stimuli, with data fit by a Weibull function (base 2). Foveal RG thresholds were significantly more sensitive than others (RG = 0.29%; L = 1.03%; BY = 10.02%: p < 0.05). The average eccentricity-related fall off in sensitivity (deci-Log/10 degrees: L = 2.81 +/- 0.08; RG = 6.13 +/- 0.07; BY = 5.96 +/- 0.64) was significantly steeper for both colour mechanisms than for luminance. The average slope (beta) for RG = 3.61 +/- 0.33 was significantly greater than Lum = 2.97 +/- 0.19 or BY = 2.63 +/- 0.20 (p < 0.05). Both colour processes showed a complex eccentricity-related trend for beta with low values in the fovea. The opponent RG process showed low variability and the highest sensitivity up to 6 degrees. Beyond 6 degrees, the non-opponent luminance mechanism has the highest sensitivity, but high variability.