Operating characteristics and a priori probability of the signal

Two experiments in auditory signal detection produced changes in the operating characteristic as the a priori probability of signal occurrence, p(SN), was varied. The signal was a sinusoid of 1,000 Hz presented for 250 msec against a continuous background of noise. In Experiment 1 three values of p(SN)—0.25, 0.50, and 0.75—were paired with each of three signal intensities. In Experiment 2 the signal intensity was fixed and p(SN) was assigned values of 0.10, 0.25, 0.50, 0.75, and 0.90. On normal-normal coordinates, operating characteristics were fitted to the points obtained from the 4-point rating scale used by the listeners. Such operating characteristics may be specified by two parameters: d_s, an index of detectability related to d’, and m, its slope. While ds was found to be independent ofp(SN), m was found to be a joint function of p(SN) and signal intensity. These results are discussed against the background of the theory of signal detectability.