Detection and discrimination of frequency modulation of complex signals

Detection and discrimination of frequency modulation were studied for harmonic signals with triangular spectral envelopes. The center frequency of the stimuli was near 2 kHz; the fundamental frequency was near 100 Hz. To prevent the possibility that the discrimination was based on differences of initial or final frequencies, these frequencies were equal within and across modulations in each individual experiment. Differences between modulations consisted of differences in the trajectories between the initial and final frequencies. Performance worsened as the slopes of the spectral envelopes decreased. Addition of noise also impaired modulation discrimination. The dependence on the signal-to-noise ratio was similar to what is found for stationary stimuli: Discrimination of frequency modulation deteriorated more rapidly with decreasing signal-to-noise ratio when stimuli had shallow spectral slopes than when they had steep spectral slopes. In spite of the precautions taken (i.e., initial and final frequency the same), the discrimination of these stimuli was more likely based on quasistationary frequency discrimination than on discrimination of modulation rate. This conclusion is consistent with previous findings for pure tones presented in quiet that frequency discrimination is more acute than modulation-rate discrimination.