The detection of modulation and the discrimination of modulation type

Psychometric functions were measured for the detection of amplitude modulation (AM) or frequency modulation (FM), using a two-alternative forced-choice (2AFC) task. Carrier frequencies (fc) were 125, 1000, and 6000 Hz, and modulation rates (g) were 2, 5, and 10 Hz. Then, pairs of values of AM and FM were selected that would be equally detectable, and psychometric functions were measured for the discrimination of AM from FM, again in a 2AFC task. For fc=125 and 1000 Hz and g=10 Hz, some subjects were essentially unable to discriminate AM from FM when the detectability of the modulation was relatively low. For g=2 Hz, some subjects identified the type of modulation as well as they detected the modulation. For a carrier frequency of 6000 Hz, the effect of modulation rate varied across subjects, but there was still a trend for poorer discrimination of modulation type at the highest modulation rate. We suggest that FM detection at a 10-Hz modulation rate is based largely on changes in excitation level for all carrier frequencies. For a 2-Hz modulation rate, and for the two lowest carrier frequencies, an extra mechanism, possibly based on phase locking, may play a role in the detection and discrimination of FM.