Detection of quasitrapezoidal frequency and amplitude modulation

It has been proposed that the detection of frequency modulation (FM) of sinusoidal carriers can be mediated by two mechanisms; a place mechanism based on FM-induced amplitude modulation (AM) in the excitation pattern, and a temporal mechanism based on phase locking in the auditory nerve. The temporal mechanism appears to be "sluggish" and does not play a role for FM rates above about 10 Hz. It also does not play a role for high carrier frequencies (above about 5 kHz). This experiment provided a further test of the hypothesis that the effectiveness of the temporal mechanism depends upon the time spent close to frequency extremes during the modulation cycle. Psychometric functions for the detection of AM and FM were measured for two carrier frequencies, 1 and 6 kHz. The modulation waveform was quasitrapezoidal. Within each modulation period, P, a time Tss was spent at each extreme of frequency or amplitude. The transitions between the extremes, with duration Ttrans had the form of a half-cycle of a cosine function. The modulation rate was 2, 5, 10, or 20 Hz, giving values of P of 500, 200, 100, and 50 ms. TSS varied from 0 ms (sinusoidal modulation) up to 160, 80, 40, or 20 ms, for rates of 2, 5, 10, and 20 Hz, respectively. The detectability of AM was not greatly affected by modulation rate or by the value of TSS, except for a slight improvement with increasing TSS for the lowest modulation rates; this was true for both carrier frequencies. For FM of the 6-kHz carrier, the pattern of results was similar to that found for AM, which is consistent with an excitation-pattern model of FM detection. For FM of the 1-kHz carrier, performance improved markedly with increasing TSS, especially for the lower FM rates; there was no change in performance with TSS for the 20-Hz modulation rate. The results are consistent with the idea that detection of FM of a 1-kHz carrier is partly mediated by a sluggish temporal mechanism. That mechanism benefits from greater time spent at frequency extremes of the modulation cycle for rates up to 10 Hz.