Abstract

Assuming a binomial sampling of LWS and MWS cones in the fovea and Poisson-distributed quantal stimulus fluctuation, we can estimate three important parameters from threshold detection data: the minimum number of quanta absorbed per cone (C ~ 3–6), the number of cones illuminated by C quanta (N~1–2), and the proportion of LWS cones in the fovea (P_L).¹,² We combined the method of constant stimuli with a rating response to examine the effect of observer criterion to the parameters C and N. The test stimulus was a 1′ point source of 520-620-nm wavelength, pulsed for 0.5 ms at an eccentricity of 8.5′. A different foveal location was sampled on each trial. Observers responded using one of four rating criteria: (1) definitely present; (2) probably present; (3) possibly present; and (4) definitely not present. Three psychometric functions were constructed corresponding to the cumulation of strict, medium, and loose criteria. The data were fit by the model¹,² and showed that the quantal requirement per cone C decreased systematically as criterion was relaxed.

© 1988 Optical Society of America