Adaptation to a modified peripheral frequency map

Mario A. Svirsky, Ph.D.
Dept. of Otolaryngology-HNS, Indiana University School of Medicine
ECE and BME Department, Purdue University

Thursday, November 6,2003
11:00 to 12:00 P.M.
Pharmacology Conference Room 369 A&B, Med Surge II

Abstract:
Part 1: Long term results and longitudinal studies

Cochlear implants (CI's) attempt to mimic the tonotopicity of the normal ear
by stimulating more basal regions of the cochlea in response to higher
frequencies.  However, there may be a mismatch between the normal
place-frequency map and that implemented by a CI.  Aiming to measure this
potential mismatch and its changes over time, the present study used a
method-of adjustment procedure where CI users and normal hearing listeners
selected synthetic vowels to match pre-specified vowel targets.  Data from
CI users was obtained longitudinally, starting the day of initial
stimulation and continuing for two years.  Results were compared to data
obtained from listeners with normal hearing.  CI users showed a significant
amount of initial mismatch with respect to the normal hearing listeners, but
they also showed significant learning and adaptation over time and achieved
nearly normal performance after some experience with the CI.  In general,
this adaptation process took several months, suggesting that some CI users
may benefit from alternative signal processing or rehabilitation procedures
designed to facilitate perceptual learning after cochlear implantation

Part 2: A method to facilitate adaptation- gradual is better.
Some authors believe that CI's impose a basalward shift to the acoustic
input, that is, sounds stimulate neurons with higher characteristic
frequency than the acoustic frequency of the original stimulus. This
frequency misalignment may have a negative influence on speech perception by
CI users.  However, a perfect frequency-place alignment may result in the
loss of important low frequency speech information. A trade-off may involve
a gradual approach: start with correct frequency-place alignment to allow
listeners to adapt to the spectrally degraded signal first, and then
gradually increase the basalward shift to allow them to adapt to it over
time.

Four pairs of normal hearing listeners underwent 15 hours of speech
perception training and testing using a real-time acoustic model of a CI.
This 8-channel model simulated a 6.5 mm basalward shift.  Subjects were
randomized either to the "gradual" group (where the 6.5 mm basalward shift
was introduced gradually over the course of ten sessions) or to the "fixed"
group, who were exposed to the full 6.5 mm shift since the beginning.  Both
groups underwent 15 1-hour sessions using audiovisual speechtracking as well
as vowel, consonant and sentence recognition tests.  For the last three
pairs of subjects, three fMRI recordings were conducted at the beginning,
middle and end of the study to assess changes in cortical activation in
response to the CI acoustic simulations with 6.5 mm shift.

Speech perception scores were initially much higher for the "gradual" group,
but by the end of the 15 sessions the "fixed" group had almost caught up
with them.  Imaging results showed some differences that were consistent
with the behavioral data.  Taken together, these results suggest that
gradual exposure to basalward shift may result in faster speech perception
improvement by CI users.


[Supported by NIH-NIDCD grant R01-DC03937.  Collaborators: Heidi Neuburger,
Thomas M. Talavage, Alicia Silveira, Hamlet Suárez, David Pisoni, Su-Wooi
Teoh]

Main suggested reading:

Harnsberger, J. D., Svirsky, M.A., Kaiser A.R., Pisoni D.B., Wright R., &
Meyer T. A .  (2001). Perceptual "vowel spaces" of cochlear implant users:
implications for the study of auditory adaptation to spectral shift. Journal
of the Acoustical Society of America, 109, 5(1), 2135-2145.  

Optional reading:

Svirsky, M. A., Silveira, A., Suarez, H., Neuburger, H., Lai, T. T.,
Simmons, P.M. (2001). Auditory learning and adaptation after cochlear
implantation: A preliminary study of discrimination and labeling of vowel
sounds by cochlear implant users.  Acta Oto-Laryngologica, 121(2), 262-265.


Kaiser, A. R., & Svirsky, M. A. (2000).  Using a personal computer to
perform real-time signal   processing in cochlear implant research.
Proceedings of the IXth IEEE-DSP Workshop. October 15-18, 2000.  Hunt, TX.
Also in
http://spib.ece.rice.edu/SPTM/DSP2000/submission/DSP/papers/paper123/paper12
3.pdf