Optimal response biases and the slope of ROC curves as a function of signal intensity,signal probability,and relative payoff

Rats performed in a two-lever analogue of the yes-no psychophysical procedure. The signal consisted of of an increment in the intensity of a random noise. Correct responses were reinforced with single bursts of brain stimulation; incorrect responses produced brief periods of time-out. Receiver-operating-characteristic curves were generated at each of several signal intensities by varying either the signal probability (0.1–0.9) or the relative number of brain stimulations for correct responses (1:3–3:1). The index d’ (or d’_e) increased with the signal intensity and was independent of response bias. When the signal probability was varied, the animals optimized the number of correct trials. and hence the number of brain stimulations obtained at each level of detection. They approximated this optimum more closely as the signal intensity was reduced. When the ratio of brain stimulations was varied, the animals compromised between optimizing the number of correct trials and optimizing the number of brain stimulations obtained. The slopes of the ROC curves plotted on normal-normal coordinates frequently departed from unity, but did not change systematically with either the signal intensity or the method by which they were generated.