Masking in the modulation rate domain

This study examines the amplitude modulation (AM) detection
in the presence of a masking modulating signal. The detection thresholds of
sinusoidal amplitude modulation were measured for a 4 kHz sinusoidal carrier.
The masking modulators were a 16 Hz tone, a bandwidth of a low-noise noise
centered at 16 Hz with a bandwidth of 2 or 8 Hz, and the gaussian noise centered
at 16 Hz with a 8 Hz bandwidth. A 3AFC procedure was used. The results obtained
suggest the existence of a masking effect in the modulation rate domain. This
form of masking is the most effective one when the modulation frequency of the
masking signal is close to the masker or, in the spectral range of masking, to
the modulation signal. These results are consistent with experimental data,
which suggest the existence of frequency selectivity and tuning in the amplitude
modulation domain. The results obtained are consistent with the idea of a second
stage of filtering in the auditory system by means of so-called modulation
filters. It seems that the auditory system performs a limited resolution
spectral analysis of the signal amplitude envelope. However, it is necessary to
stress that the frequency selectivity in the amplitude modulation domain is not
so evident as the selectivity in the audio frequency domain.