Aleksander Sek
Brian C. J. Moore
Dept. of Exp. Psychol., Univ. of Cambridge, Downing St., Cambridge CB2 3EB, England
Previously [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 92, 3119--3131 (1992)], psychometric functions were measured for the detection of amplitude modulation (AM) alone and frequency modulation (FM) alone, using a 1-kHz carrier and a modulation rate of 10 Hz. Detectability (d') was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. Significant effects of relative modulator phase were found. Further, d' for combined AM and FM was better than would be predicted if the two types of modulation were coded completely independently. However, the phase effects were not correctly predicted by excitation pattern models. Moore and Sek suggested that the good performance for combined AM and FM may have occurred because FM is coded by changes both in excitation level and in phase locking. This idea was tested by repeating the earlier experiment with a carrier frequency of 6 kHz, a frequency where phase locking does not occur. The pattern of results was similar to that obtained for the 1-kHz carrier. [Work supported by The Royal Society and the MRC, UK.]