An Overview of the Future Study of Audition 143important clues for understanding normal audition, the ge-
netic revolution has made significant strides in identifying the
genetic basis for several different forms of inheritable deaf-
ness. A gene that may control the motile response of the outer
hair cells has been identified (Zheng, Shen, He, Long, &
Pallos, 2000), opening a whole array of possibilities (e.g.,
genic manipulation) for better understanding outer hair-cell
function. In many areas, perceptual research should provide
improved ways to determine different phenotypes in order to
better define structure-function auditory relationships.
The development of better hearing aids, both amplification
hearing aids and the cochlear prosthesis, has stimulated new
knowledge about audition; these technologies have benefitted
from the past research on hearing as well. The cochlear pros-
thesis in particular offers unique opportunities to study the
hearing process (Miller & Spelman, 1989). The cochlear
prosthesis is a wire with multiple electrodes that is surgically
inserted into the cochlear partition of a patient with a hear-
ing loss. The electrodes stimulate selected portions of the
cochlear partition, based on the transduction of sound into
electrical current via a sound processor worn by the patient.
The success achieved by thousands of cochlear prosthetic
users worldwide suggests that these devices provide a useful
means of aural communication for many people with hearing
impairments. Because the use of cochlear prostheses by-
passes the biomechanical properties of the inner ear, under-
standing the auditory abilities of successful implant users
provides valuable information about the early neural stages of
the auditory process. Many successful users of the cochlear
prostheses had been deprived of useful hearing for many
years before their implantation. The significant improvement
in auditory abilities achieved by these cochlear prostheses
users, after implementation and training, suggests a degree of
auditory plasticity that is receiving a great deal of attention.
The importance of this issue has increased now that young
children are being implanted.
In addition to providing potential utility for hearing aids,
research on spatial hearing and the HRTF have provided
improvements for devices used in many sound localization
situations (Gilkey & Andersen, 1997). For instance, many
traditional hearing aids (especially if only one hearing aid is
used) do not allow users to accurately localize sound sources.
Proper use of HRTF technology may enable hearing aid users
to more accurately localize sound sources, and such accuracy
may also improve their ability to detect sounds in noisy envi-
ronments (the aforementioned cocktail party effect). HRTF
technology has also been adopted in the audio entertainment
and other industries.
The use of the HRTF offers complete control of the sound
cues that are important for sound localization. Such control
offers several advantages for studying hearing (Gilkey &
Andersen, 1997). One interesting use of HRTF-transformed
sound is in the study of auditory adaptation and neural plas-
ticity to alterations of the normal cues for sound localization
(Hofman, Van Riswick, & Van Opstal, 1998). If the HRTF is
altered such that the location of a sound source is now per-
ceived at a new location, listeners can adapt to the change and
after a few days demonstrate near-normal sound localization
abilities. When the nonnormal alterations are removed, lis-
teners quickly return to being able to accurately localize
as they had before the alteration. Such sound localization
adaptation research with human and animal (e.g., the barn
owl) listeners is revealing and will continue to reveal impor-
tant insights about the plasticity of neural sound localization
processes (Knudsen, Esterly, & Olsen, 1994).
In the late 1980s and early 1990s, several authors
(Hartmann, 1988; Moore, 1997; Yost, 1992a), most notably
Bregman (1990), suggested that our ability to determine the
sources of sounds, especially in multisource acoustic environ-
ments, was a major aspect of hearing about which very little
was known. Although the early history of the study of hearing
suggests that so-called object identification was an important
aspect of hearing, for most of the last century and a half the
study of audition focused on the detection and discrimination
of the attributes of sound—frequency, level, and timing—and
how those attributes were coded in the auditory periphery
(Yost, 1992a). While there is still not a lot known about how
the auditory scene is achieved, current research in hearing is
no longer focused on processing in narrow frequency bands
and over very short temporal durations. Psychophysical and
physiological investigators have examined and will continue
to investigate auditory mechanisms that integrate acoustic in-
formation across the spectrum and over time, because such
processing is crucial for sound source determination.
The progress in understanding auditory scene processing
may be hindered by a lack of appropriate techniques to study
these problems. New correlation techniques in psychophysics,
multiple electrode technology, new physiological techniques,
new ways of extracting information from neural data, and
neural imaging are some of the new methods that may open up
opportunities for understanding sound source determination
and audition. Auditory science also knows very little about the
functional purposes of the auditory nuclei in the ascending
auditory pathway and within the auditory cortex. With a few
notable exceptions of several animal models (e.g., bats and
echo processing; barn owls and sound localization), very little
is known about the roles various neural centers play in hearing.
A great deal is known about the anatomy of many neural cir-
cuits and the physiological properties of many types of fibers
in most neural centers, but far less is known about what