Perceptual Coherence : Hearing and Seeing

290 Perceptual Coherence
Figure 6.18. The magnitude of comodulation release depends on the degree to
which the noise forms a separate perceptual entity. The simple detection experiment
is illustrated in (A). A tone (narrow line) is embedded in the middle of a noise band
(narrow rectangle). The basic comodulation release experiment is illustrated in (B).
All of the noise bands are modulated coherently (wiggly lines under noise bands)
and the tone is much easier to detect (by 20 dB). The interference of deviant bands is
illustrated in (C). The two deviant noise bands (modulated at different rate) disrupt
the segregation of the noise bands, and the tone remains embedded in the deviant
plus the original noise bands. The formation of the deviant noise bands into a perceptual
entity is illustrated in (D). Adding more deviant bands at that modulation frequency
and depth makes it easier to detect the tone. Three sounds emerge. Adapted
from “Comodulation Masking Release and Auditory Grouping,” by J. W. Hall and
J. H. Grose, 1990, Journal of the Acoustical Society of America, 88, 119–125.
masking the tone to segregate with the comodulated bands. Perceptually,
there are two sounds: the tone masked by three noise bands and the six remaining
coherently modulated bands. If we add additional noise bands that
match the modulation of the deviant bands (figure 6.18D), that creates a
stronger tendency for the deviant bands to form their own noise source and
a stronger tendency for the noise band at the tone frequency to rejoin the
coherently modulated bands. Perceptually, there now are three sounds: the
coherently modulated deviant noise bands, all the original coherently modulated
noise bands, and the tone target. The detectability of the tone increases
due to the greater degree of comodulation masking release.
Another temporal factor that determines the degree of masking release
is the onset of the noise bands. Paralleling the outcomes for profile analysis,
if the onsets of the surrounding noise bands are not synchronous with
the onset of tonal noise band (greater than 50 ms), the degree of masking
release is reduced (Grose & Hall, 1993).