Discrimination of the amplitude modulation rate

This study examines the amplitude modulation rate
discrimination for sinusoidal and noise carriers. It was shown that the
discrimination of AM rates is a monotonically growing function of modulation
rate. Higher values of the discrimination thresholds were observed for a
narrowband carrier. It appears that in the case of a narrowband noise carrier,
the spectral range of the noise envelope is similar to that of the modulation
rates of the signal (up to 120 Hz). It results in a masking in the modulation
rate domain and in a much higher threshold growth than that observed for a
wideband noise carrier or a sinusoidal carrier. The results are consistent with
the idea of the so-called second stage of filtering acting on the envelope of
the acoustic signal. This hypothesis postulates the existence of a so-called
modulation filter bank, (MFB), responsible for the frequency selectivity
observed in the amplitude modulation rate domain. The existence of the MFB
suggests that a certain form of the spectral analysis of any acoustic signal
envelope may be performed in the auditory system after initial filtering in the
auditory filter bank. A model of the modulation rate discrimination based either
on the classical concept of the excitation patterns or on the modulation
excitation patterns has not accounted for our experimental data. According to
both the models, an increase in the frequency discrimination threshold versus
modulation rate should be slower than that measured in the experiment.