Detection of frequency modulation at low modulation rates: evidence for a mechanism based on phase locking.

These experiments tested the hypothesis that detection of frequency modulation (FM) at very low rates depends mainly on temporal information (phase locking to the carrier) for carriers below about 5 kHz, whereas FM detection at higher rates (10 Hz and above) depends mainly on changes in the excitation pattern (a "place" mechanism). In experiment 1, thresholds for detecting FM were measured for a wide range of carrier frequencies (0.25-6 kHz) for modulation rates, fm, of 2, 5, 10, and 20 Hz. Thresholds were determined when FM only was present and when the carriers in both intervals of a forced-choice trial were amplitude modulated at the same rate as the FM with a modulation index of 0.333. The phase of the amplitude modulation (AM) relative to the FM was randomly selected on each trial, in order to disrupt cues for FM detection based on changes in the excitation pattern. For carrier frequencies up to 4 kHz, the deleterious effect of the added AM increased with increasing fm. For the 6-kHz carrier, the deleterious effect was independent of fm. In experiment 2, psychometric functions were measured for detecting combined FM and AM of a 1-kHz carrier, with fm = 2 Hz, as a function of the relative phase of the modulators. The modulation depths for AM and FM were chosen so that each would be equally detectable if presented alone. This was done both in quiet and in the presence of noise designed to mask either the lower or the upper side of the excitation pattern. In contrast to earlier results obtained with fm = 10 Hz [Moore and Sek, J. Acoust. Soc. Am. 96, 741-751 (1994)], only small effects of relative modulator phase were found. Experiment 3, was similar to experiment 2, except that all measurements were done in quiet, and carrier frequencies of 0.25, 1.0, and 6.0 kHz were used. There were no effects of relative modulator phase for the 0.25-kHz carrier, small effects for the 1-kHz carrier, and large effects for the 6-kHz carrier. The pattern of results is consistent with the hypothesis that both temporal and place mechanisms are involved in FM detection. The temporal mechanism dominates for carriers below about 4 kHz, and for very low modulation rates. The place mechanism dominates for high carrier frequencies, and for lower carrier frequencies when stimuli are frequency modulated at high rates.