DETECTION OF SINUSOIDAL AMPLITUDE-MODULATION AT UNEXPECTED RATES

The detectability of sinusoidal amplitude modulation at unexpected mod ulation rates was assessed using a probe-signal method. With this meth od. 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 sen sitivity to modulation at six other unexpected rates between 4 and 256 Hz was measured via occasionally presented probe modulation rates. Th e modulation phase was random on each two-interval forced-choice trial and the overall level of the 500-ms broadband carrier was randomly va ried between 55 and 75 dB SPL across intervals. The modulation depth a t each rate was set so that the modulation was detected on about 90% o f the trials when only that rate was presented. Performance at the une xpected rates depended upon the target rate. For the 4-Hz target, modu lation at all rates was detected on about 80% of the trials. For the 3 2- 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. The influence of expectation of mod ulation rate on che detection of sinusoidal amplitude modulation is no t readily predicted by current models of modulation detection. (C) 199 8 Acoustical Society of America. [S0001-4966(98)04611-6].