Mechanisms underlying the detection of frequency modulation

Frequency modulation detection limens (FMDLs) were measured for carrier frequencies ( f c ) of 1000, 4000, and 6000 Hz, using modulation frequencies ( f m ) of 2 and 10 Hz and levels of 20 and 60 dB sensation level (SL), both with and without random amplitude modulation (AM), applied in all intervals of a forced-choice trial. The AM was intended to disrupt excitation-pattern cues. At 60 dB SL, the deleterious effect of the AM was smaller for f m = 2 than for f m = 10 Hz for f c = 1000 and 4000 Hz, respectively, while for f c = 6000 Hz the deleterious effect was large and similar for the two values of f m . This is consistent with the idea that, for f c below about 5000 Hz and f m = 2 Hz, frequency modulation can be detected via changes in phase locking over time. However, at 20 dB SL, the deleterious effect of the added AM for f c = 1000 and 4000 Hz was similar for the two values of f m , while for f c = 6000 Hz, the deleterious effect of the AM was greater for f m = 10 than for f m = 2 Hz. It is suggested that, at low SLs, the auditory filters become relatively sharp and phase locking weakens, so that excitation-pattern cues influence FMDLs even for low f c and low f m .