Detection of signals having expected and unexpected temporal structures

Two experiments used a variant of the ‘probe-signal’ method [Greenberg and Larkin, J. Acoust. Soc. Am. 44 (1968) 1513–1523] to examine the effect of expectedness on the detection of signals having fixed frequencies but uncertain temporal structures. Expectedness was manipulated by presenting one signal on 75% of the trials and the other on only 25% of trials. Experiment 1 measured sensitivity ( d′) to 4000-Hz sinusoidal signals having durations of 10 ms and 295 ms. The results confirmed the finding by Wright and Dai [J. Acoust. Soc. Am. 95 (1994) 931–938] that sensitivity was poorer when the duration of the signal was unexpected. The second experiment used two signals having the same overall duration: six 10-ms 4000-Hz tone pulses with a 0-ms inter-pulse interval, and two 10-ms 4000-Hz pulses separated by 40 ms. Again, sensitivity was lower when the temporal structure of the signal was unexpected. It is argued that this finding is inconsistent with ‘long time constant’ models of temporal integration, but can be accounted for by assuming that subjects combine information from a number of short ‘looks’ at the signal, with the number and temporal location of these looks being influenced by its expected temporal structure.