Delineation of FM rate channels in man by detectability of a three-component modulation waveform

Adaptation studies have provided evidence for auditory mechanisms which operate on the modulation waveform of frequency-modulated sounds, and that these mechanisms behave like a series of channels selectively tuned to a limited range of modulation rates. In the present experiments, measurements of the modulation detectability threshold of a frequency-modulated tone, which had a modulation waveform spectrum consisting of three components: one component at the modulation rate and two sidebands which could be set to different frequency positions on either side of it, revealed that when the two sidebands were located within a limited bandwidth around the central component they contributed to the detection of the modulation. However, when the sidebands were more widely separated from the central component, the modulation detection threshold was dependent only on the magnitude of the central component. Within the area of sideband interaction, there was an inversely proportional relationship between the magnitude of the sidebands and the modulation threshold. Differences in the time-integrated sound spectra for the FM signals, although very complex, do not contain changes which adequately explain the data on the basis of critical band mechanisms, or a peak-to-peak detection model. The data provide further evidence in support of the hypothesis derived from adaptation studies that channels selectively sensitive to the rate of frequency modulation in frequency-modulated tones are concerned in the processing of frequency-modulated signals.