Invariance of the slope of the psychometric function with spatial summation

The high-threshold probability summation model for improved detection of near-threshold gratings with increased spatial extent of the patterns assumes that the detection psychometric function is a Weibull function. This model predicts that the slope of the psychometric function should not change as the number of mechanisms stimulated increases, although other models predict that the slope should vary. We confirm for a two-alternative, forced-choice paradigm that the slope parameter does not vary systematically with spatial frequency or with number of periods of the grating, although there are reliable differences in average slope between observers.