Page 29 - Fall 2007

TECHNICAL COMMITTEE ON PSYCHOLOGICAL AND PHYSIOLOGICAL ACOUSTICS:
AN ADJUSTABLE AUDITORY SYSTEM
Elizabeth A. Strickland
and
Skyler G. Jennings
Department of Speech, Language, and Hearing Sciences, Purdue University West Lafayette, Indiana 47907
If a person is said to have “cat-like
reflexes,” one might envision a star
athlete or martial artist. Although
reflexes are often associated with auto-
matic neural feedback to muscles in the
arms or legs they can also appear in the
most unlikely of places, such as the ear.
For several years neural feedback from
the brain to the ear has been well understood from a physiological per-
spective; however, from a psychophysi-
cal or “hearing” perspective it remains a mystery.
The auditory system not only sends information to the brain, but also receives feedback from it. This is also true of other sensory systems. For example, feedback to the visual system adjusts pupil diameter in response to changes in light. In the auditory system, one feedback loop projects from the brainstem to the outer hair cells in the cochlea, and is called the medial olivocochlear bundle (MOCB). MOCB neurons in the brainstem are activated by afferent neural fibers from the cochlea. When activated by sound or electric shock, MOCB neurons send neural impulses to the cochlea’s outer hair cells, which control cochlear amplification (or gain) and are related to the “active process.” The anatomy of the MOCB involves two fiber pathways, which vary in size and projec- tion. The largest branch of the MOCB feeds back to the same ear from which the afferent fibers came. A smaller branch goes to the other ear. Physiological evidence in animals sug- gests that this feedback loop responds to sound by decreasing gain in the cochlea in a frequency-specific way, an effect
1
In addition to the physiological data from anesthetized
animals, there is some evidence of the activity of this feed-
back loop in awake animals and humans. Otoacoustic emis-
sions (OAEs), a byproduct of the active process in the
cochlea, are backwards-propagated disturbances measured in
the ear canal in response to sound. OAE level decreases with
the duration of a sound, or with the addition of a contralat-
1
eral sound. This decrease largely disappears when the
MOCB is cut, and is correlated with vulnerability to acoustic injury in animals, suggesting that the MOCR may function to
2
hearing in noisy environments. Evidence for this hypothesis comes from a psychophysical phenomenon called “the tem- poral effect.” The temporal effect refers to the fact that a short signal presented at the beginning of a longer masker
called the medial olivocochlear reflex (MOCR).
protect the cochlea.
Another possible role for the MOCR is to improve
“In general the temporal effect seems to be consistent with a behavioral effect that could be caused by the medial olivocochlear reflex.”
is easier to detect if it follows a precur- sor (a separate sound or an extension of the masker). Quantitatively, the temporal effect is the difference between thresholds measured with and without the precursor. Although the temporal effect was first described over 40 years ago,3 it received renewed interest when studies tied it to the active process in the cochlea. For example, the temporal effect is
reduced in adults with temporary hearing loss caused by excess noise exposure or aspirin ingestion, both of which
4,5
precursor may be mediated by the MOCR.
Although the temporal effect appears to be related to
outer hair cell function, additional evidence is necessary to link it to the MOCR. For example, a precursor intended to activate the MOCR should decrease gain and frequency selectivity. Moreover, the magnitude of this reduction should increase with increasing precursor level. In an effort to tie behavioral findings to the MOCR, studies have been designed which allow the results to be analyzed in terms of these expectations. These have shown that the presence of a precursor does appear to decrease the gain of the input-out- put function, and that this effect increases with the level of the precursor. The presence of a precursor at the signal fre-
6
quency also decreases frequency selectivity. If the precursor
is off-frequency, the results support the idea that another
byproduct of the active process, suppression of one sound by
7
another, is decreased. This would seem to be consistent with
a decrease in gain at the suppressor frequency, although it does not fit with current theories of suppression. Thus in general the temporal effect seems to be consistent with a behavioral effect that could be caused by the MOCR. To show this conclusively, physiological and behavioral meas- ures need to be obtained from the same subjects, either humans or animals.
Interestingly, the temporal effect decreases with hearing loss because thresh- old improves in the no precursor condition. The improve- ment suggests that reduced cochlear gain (as seen with hearing loss) may enhance detection in some circum- stances. It follows that the large temporal effect seen in nor- mal hearing listeners may be caused by a precursor-induced reduction in gain; thus leaving thresholds (and gain) high in the no precursor condition while lowering them in the con- dition with a precursor. This gain reduction effect by the
decrease the gain of the active process.
Psychological and Physiological Acoustics 27