The Slope of the Psychometric Function at Different Wavelengths

Many current models of visual detection predict that the slope of psychometric functions for detection will be independent of the spectral power distribution of the test light once the spatial and temporal characteristics of the light are fixed. This article examines reports that the slope of the psychometric function depends on the wavelength of a spectrally-narrowband test light of fixed size and duration. Three sources of difficulty that any experimental measurement of slope must encounter are addressed: (1) the different spatial distributions of the photoreceptor classes across the retina; (2) possible variations in threshold over the course of the experiment; and (3) the large variability of estimates of slope and the need for a method of assessing this variability. Measurements of slope with 2 and 4 deg test flashes against a bright 510 nm field show no significant trend with the wavelength of the test. A novel statistical test bounds the magnitude of possible variations in slope across the visible spectrum.