Introduction to Acoustics

452 Part D Hearing and Signal Processing
Part D 12.4
In terms of processing sound, the major steps taken in
the cochlea are frequency analysis, compression, and
suppression. Frequency analysis is essential to all of
the processing that follows in the brain. Indeed central
auditory centers are all organized by frequency and
most properties of the perception of sound appear to include
frequency analysis as a fundamental component.
Compression is important for extending the dynamic
range of hearing. Its importance is illustrated by the
problems of loudness abnormality and oversensitivity to
loud sounds in persons with damaged cochleae that are
missing compression [12.127]. Suppression is important
for maintaining the quality of the spectrotemporal
representation, at least at moderate sound levels, and
is the first example of interaction across frequencies
in the auditory system. In the following paragraphs,
some of the central nervous system’s secondary analyses
on the cochlear spectrotemporal representation are
described.
The first function of the central auditory system is
to stabilize the spectrotemporal representation provided
in the AN. As shown in Fig. 12.14, the representation
changes across sound level and the quality of the representation
is generally lower at high sound levels and
at low sound levels. In the cochlear nucleus the representation
is more stable as sound level changes, e.g.,
for speech [12.128]. Presumably the stabilization occurs
through optimal combination of information across SR
groups in the AN and perhaps also through inhibitory
interactions.
A second function of the lower central auditory
system is binaural interaction. The location of sound
sources is computed by the auditory system from small
12.4 Summary
Research on the auditory system has defined the means
by which the system efficiently captures sound and
couples it into the cochlea, how the frequency analysis
of the cochlea is conducted, how transduction
occurs, how nonlinear mechanisms involving the OHCs
sharpen the frequency tuning, increase the sensitivity,
and compress the stimulus intensity, and how
suppression acting at the level of AN fibers main-
differences in the sounds at the two ears [12.84, 129].
Essentially, a sound is delayed in reaching the ear on the
side of the head away from the source and is less intense
there. Differences in interaural stimulus timing are small
and require a specialized system of neurons in the first
and second stages of the central auditory system to allow
accurate representation of the interaural timing cues.
A similar system is present for interaural differences in
sound level. The representation of these cues is elaborated
in higher levels, where neurons show response
characteristics that might explain perceptual phenomena
like binaural masking level differences [12.130]and
the precedence effect [12.131].
A third aspect of central processing is that the
representation of sound switches away from a spectrotemporal
description of the stimulus, as in the AN,
and moves to derived representations, such as one that
represents auditory objects [12.132]. For example, the
representation of speech in the auditory cortex does not
take a recognizable tonotopic form, in that peaks of activity
at BFs corresponding to the formant frequencies
are not seen [12.133]. The nature of the representation
used in the cortex is unknown; it is clearly based
on a tonotopic axis, but neurons’ responses are not
determined in a straightforward way by the amount
of energy in their tuning curves, as is observed in
lower auditory nuclei [12.134]. In some cases, neurons
specialized for particular auditory tasks have been
found; neurons in one region of the marmoset cortex
are tuned to pitch, for example [12.135]. Perhaps the
best-studied cases are neurons that respond specifically
to species-specific vocalizations in marmosets [12.136]
and songbirds [12.137, 138].
tains the sharpness of the frequency analysis for
complex, multicomponent stimuli. Over the next decade,
research on the auditory system will increasingly
be focused on the organization and function of the
central auditory system, with particular reference to
the way in which central neurons derive information
from the spectrotemporal display received from
the AN.