VIGILANCE FOR 2 KINDS OF SIGNAL WITH UNEQUAL PROBABILITIES OF OCCURRENCE

Seventy-two subjects each performed for one hour on a visual vigilance task incorporating two kinds of signal, A and B, of which A was the easier to recognize. For 36 subjects, signal A occurred with a probability of 0. 05, signal B with 0. 15; for the remaining subjects these probabilities were reversed. Subjects were instructed either to look for one signal only (A or B), or to look for both. Results indicate that, whereas the additional requirement of looking for a second signal had a differential effect, reducing the discriminability of signal A, but depressing the differential response bias towards signal B, signal probability predictably influenced only the response bias towards signals A and B.