Detecting coherent and incoherent frequency modulation

Three experiments investigated whether or not the auditory system contains a neural mechanism that is sensitive to differences in the pattern of frequ ency modulation imposed on widely separated carriers. Experiment 1 measured the discrimination between an unmodulated two-tone complex and one in whic h either coherent or: incoherent frequency modulation was imposed on the tw o carrier frequencies. These two frequencies were either 1100+1925 Hz or 11 00+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 (19 96), which were previously interpreted as providing evidence in favour of a mechanism sensitive to FM coherence. Discrimination was sometimes better f or 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 experime nt 1. Experiment 2 showed that detection of a static shift on the carrier f requencies 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 bette r when the 1100-Hz carrier was shifted down and the 1925-Hz carrier was shi fted up; compared to when the 1100-Hz carrier shifted up and the 1925-Hz ca rrier shifted down. Experiment 3 showed that this difference also applied t o dynamic changes: detection of quasi-linear frequency sweeps (0.5 cycles o f sinusoidal FM) was better when the higher component glided up: and the lo wer component glided down than vice versa. In the former condition, perform ance was as good as with same-direction sweeps. It is concluded that the ef fects observed in experiment 1 and by Furukawa and Moore result from the pr ocessing of a global percept arising from the perceptual fusion of the two carriers, and do not represent an across-frequency mechanism sensitive to F M coherence. In addition, it is argued that experiments 2 and 3 demonstrate the existence of perceptual asymmetries in hearing. (C) 2000 Elsevier Scie nce B.V. All rights reserved.