Beverly A. Wright
Keck Ctr., Box 0732, Univ. of Calif., San Francisco, CA 94143-0732
Huanping Dai
Univ. of Florida, Gainesville, FL 32611
The detectability of sinusoidal amplitude modulation having unexpected rates was assessed using a probe-signal method. With this method, three listeners were led to expect a target modulation rate (4, 32, or 256 Hz) by presenting the signal most often at that rate, and sensitivity to modulation at six other unexpected rates between 4 and 256 Hz was measured via occasionally presented probe modulation rates. The modulation phase was random on each 2IFC trial and the overall level of the 500-ms wideband carrier was randomly varied between 55 and 75 dB SPL across intervals. The modulation depth at each rate was set so that the modulation was detected on about 90% of the trials when only that rate was presented. Performance at the unexpected rates depended upon the target rate. For the 4-Hz target, modulation at all rates was detected on about 80% of the trials. For the 32- and 256-Hz targets, unexpected modulation rates of 16 Hz and above were detected on 80%--90% of the trials, but modulation rates below 16 Hz were detected nearly at chance. Thus, contrary to the predictions of several popular models, the ability to detect modulation was clearly influenced by expectation of the modulation rate. [Work supported by NIH.]