Detecting coherent and incoherent frequency modulation.

Abstract

Three experiments investigated whether or not the auditory system contains a neural mechanism that is sensitive to differences in the pattern of frequency modulation imposed on widely separated carriers. Experiment 1 measured the discrimination between an unmodulated two-tone complex and one in which either coherent or incoherent frequency modulation was imposed on the two carrier frequencies. These two frequencies were either 1100+1925 Hz or 1100+2000 Hz, and the stimuli were presented against a pink-noise background. The method was based on that used in experiments by Furukawa and Moore (1996), which were previously interpreted as providing evidence in favour of a mechanism sensitive to FM coherence. Discrimination was sometimes better for coherent than for incoherent FM, as reported by Furukawa and Moore, but only for four out of the eight listeners tested. The remaining experiments excluded those subjects who had shown no effect of FM coherence in experiment 1. Experiment 2 showed that detection of a static shift on the carrier frequencies of the two components was better when the carriers were shifted in the same, compared to the opposite, direction. This difference occurred regardless of whether the carriers were modulated coherently, incoherently, or were unmodulated. The experiment also showed that performance was better when the 1100-Hz carrier was shifted down and the 1925-Hz carrier was shifted up, compared to when the 1100-Hz carrier shifted up and the 1925-Hz carrier shifted down. Experiment 3 showed that this difference also applied to dynamic changes: detection of quasi-linear frequency sweeps (0.5 cycles of sinusoidal FM) was better when the higher component glided up and the lower component glided down than vice versa. In the former condition, performance was as good as with same-direction sweeps. It is concluded that the effects observed in experiment 1 and by Furukawa and Moore result from the processing of a global percept arising from the perceptual fusion of the two carriers, and do not represent an across-frequency mechanism sensitive to FM coherence. In addition, it is argued that experiments 2 and 3 demonstrate the existence of perceptual asymmetries in hearing.