Current Investigations
Intensity coding in acoustic
and electric hearing
Speech processor for auditory prostheses
Hearing deficits due to auditory neuropathy
(Do you have a temporal processing deficit? Try our Online
Hearing Test)
1. Intensity coding in acoustic and electric hearing
Funding agency:
National Institutes of Health (DC02267)
PI: Fan-Gang Zeng
The long-term objectives of the
proposed study are to (1) develop a
unified theory of the encoding of sound intensity (i.e., psychophysical
laws) in normal-hearing, hearing-impaired, and implant listeners, (2)
differentiate the functional deficits that hearing-impaired and implant
listeners may have due to the delivery of altered peripheral excitation patterns
to the brain, and (3) design rehabilitative devices that can maximally
compensate for the functional deficits in these listeners with hearing
impairment. This proposal addresses the dynamic range problem for the encoding
of intensity in both acoustic and electric hearing. On one hand, the acoustic
dynamic range problem refers to the discrepancy between the large psychophysical
loudness range and the limited physiological range demonstrated by the majority
of individual auditory neurons. On the other hand, the electric dynamic range
problem is a practical and harsh reality facing clinicians in fitting speech
processors: implant listeners who have typically 6-20 dB range must accommodate
normal speech and environmental sounds containing important information over a
40-60 dB range. The specific aims are to (1) understand the underlying neural
mechanisms of encoding the enormous dynamic range in acoustic hearing, (2)
quantify loudness growth and discrimination functions in electric hearing, and
(3) determine which normal mechanisms are missing in electric hearing and
whether it is possible to increase the electric dynamic range by compensating
for these missing normal mechanisms. Our hypothesis is that loudness growth and
discrimination functions are primarily determined by the peripheral excitation
pattern and the central system responds in a similar fashion regardless of the
changes in the peripheral inputs. Our experimental design is to measure loudness
growth and discrimination functions in conditions that alter the peripheral
inputs to the central system. Methods used to alter the peripheral inputs
include acoustic high-pass noise, low- and band-pass noise, forward masking,
amplitude or phase-modulated stimuli, vestibular-nerve-section, electric
stimulation of the auditory nerve and cochlear nucleus. Corresponding
physiological mechanisms to be examined are: spread of excitation along the
basilar membrane, suppressive and excitatory masking, adaptation and
low-spontaneous rate auditory neurons, synchronization of neural firing,
contributions of efferent neurons, nonlinear vibration of the basilar membrane
and its associated phenomena (e.g. lateral suppression). A unique feature of
this proposal is that it uses cochlear
implant, auditory brainstem implant, and vestibular neurectomy patients to
better understand the neural mechanisms of encoding the large dynamic range of
sound intensity. The results obtained from this proposal could lead to better
designs of auditory prostheses and hearing aids that will compensate for the
reduced dynamic range that occurs with both electric stimulation of the auditory
system and hearing loss.
This is one of the two contracts funded by the Neural Prostheses Program, NIH to improve the design of speech processors in cochlear implants. One goal of this contract is to design, develop and evaluate both laboratory based and wearable speech processors for use in auditory prostheses. Another goal is to develop new speech test materials for more effective evaluation of the speech processors in deaf humans with implanted auditory prostheses. Dr. Shannon and Dr. Zeng have developed acoustic simulations of cochlear implants, which not only serve as a research tool, but also allow normal-hearing people to experience what a cochlear implant user is able to get through his or her device. In addition, Dr. Zeng serves as a consultant on the other speech processors contract led by Blake Wilson at RTI/Duke University.
Quarterly progress reports
generated from this contract are available (in .pdf format):
Dr. Zeng's involvement in this project has been in the study of functional aspects of auditory neuropathy, which is part of a multidisciplinary study including: (1) clinical features of patients with auditory neuropathy , (2) audiological measures; (3) physiology of cochlear and auditory brainstem functions; (4) psychophysics of temporal processing of acoustic signals; (5) genetics of auditory neuropathy; (6) neuropathology of peripheral neuropathies accompanying auditory neuropathy; (7) vestibular abnormalities. More information regarding auditory neuropathy can be found on the website at Louisiana State University Medical Center.
Auditory neuropathy affects the normal synchronous activity in the auditory nerve, without affecting the amplification function in the inner ear. Patients with auditory neuropathy often complain that they can hear sounds, but cannot understand speech. Here we report psychophysical tests indicating that these patients' poor speech recognition is due to a severe impairment in their temporal processing abilities. We also simulate this temporal processing impairment in normally hearing listeners and can produce similar speech recognition deficits. This study demonstrates the importance of neural synchrony for auditory perceptions including speech recognition in humans. The results should contribute to better diagnosis and treatment of auditory neuropathy.
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