Monitoring for One Kind of Signal in the Presence of Another: The Effects of Signal Mix on Detectability

Subjects in a 1-h visual vigilance task looked for the occasional presence of a signal inserted into a sequence of single items which consisted of either (a) homogeneous nonsignals or (b) a mixture of nonsignals and signals of another kind. The experiment also manipulated the nature (and discriminability) of the signal to be detected (two types) and its probability of Occurrence (0.05 or 0.15). Results indicate that signal detectability is impaired by the presence of another kind of signal, the impairment tending to be greater the lower the detectability of the signal considered on its own. But, due to a concomitant increase in response bias, the impairment is not reflected in correct detection. The presence of the second signal did not influence the extent of the within-session decline in detection, and the effect of signal probability on detection levels failed to emerge. The practical implications of the findings are discussed.