Pulse and steady-pedestal contrast discrimination: effect of spatial parameters

The goal of this study was to establish the spatial summation properties as sociated with inferred PC- and MC-pathway mediated psychophysical contrast discrimination. Previous work has established two paradigms that reveal cha racteristic signatures of these pathways. In the pulse paradigm, a four-squ are array was pulsed briefly, on a constant background. In the steady-pedes tal paradigm, the stimulus array was presented continuously as a steady-ped estal within a constant surround. In both paradigms, one square differed fr om the others, giving the observer a forced choice spatial discrimination t ask. Area summation functions derived for the pulse paradigm decreased with area, with a slope of - 0.25 on a log-log axis. Area summation functions d erived for the steady-pedestal paradigm decreased as a power function of ar ea, approaching an asymptote above one square degree. The latter are consis tent with the classical data of threshold spatial summation. (C) 2001 Elsev ier Science Ltd. All rights reserved.