Decision behavior with changing signal strength

The Theory of Signal Detectability (TSD) has nearly replaced classical notions of the threshold because of its ability to separate sensory and decision processes in weak-signal detection and recognition paradigms. The primary emphasis of recent work has concentrated on the sensory rather than the decision aspects and almost all work has been exclusively at one signal strength. We propose a model to describe behavior at different signal strengths based on subjective rather than objective distributions. The model predicts performance at a constant likelihood ratio (LR) criterion (even though subjective distributions are the basis for determining cutoff criteria) unless the observer adopts a Subjective Neyman-Pearson (SNP) objective. Results from an experiment in visual discrimination suggest that some observers in fact operate at constant objective LR's as signal strength is varied randomly over a wide range. The objective LR's of the other subjects changed dramatically with signal strength, but this behavior is consistent with the use of a Subjective Neyman-Pearson (SNP) decision rule and the linear relation between subjective and objective log LR's found in the studies of subjective probability.