The effects of duration,area, and background intensity on the visual intensity difference threshold given by the forced-choice procedure: Derivations from a statistical decision model for sensory discrimination

A model for visual discrimination is outlined. This assumes that detection is based on separating signaland noise by means of a statistical decision procedure, and that the noise arises from physical and biological sources. When the model is applied to such traditional detection procedures as the method of constant stimuli, it successfully predicts the laws of spatial and temporal summation and the form of the Weber function. The model is extended to the forced-choice procedures. It is found that it predicts departures from the classical laws for spatial and temporal summation and an increase in the slope of the Weber function when the pulsed forced-choice procedure is used, and data are presented that agree with the predictions. It is shown that data obtained with forced choice against a continuous background are consistent with the assumption that the Seither covertly detects or fails to detect an increment at each ofthe two alternative locations and this determines his final overt response, rather than that he directly compares the inputs at the two locations. A comparison between results obtained with the forced-choice procedure with a pedestal, or with the continuous forced-choice procedure, provides further evidence that Ss employ a double detection strategy in the latter case.