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Cochlear prosthetics
have proven to be extremely useful tools to provide
auditory sensations to profoundly deaf patients and have
restored hearing to tens of thousands of individuals
around the world. Although the electrical stimulation
poorly represents cochlear place information, temporal
information is reliably represented. This is obviously
sufficient for frequency discrimination over lower
frequencies and thus makes human speech comprehension
possible. In congenitally deaf children, cochlear
implants enable language development, but the outcome is
critically dependent on the age of implantation.
Congenitally deaf subjects implanted in adulthood show
a markedly poorer performance in speech discrimination
than subjects implanted in childhood. The optimum
implantation age appears to be under five years of age.
 Our long-term
goal is to understand and define the capabilities of the
auditory cortex to establish acoustic function following
cochlear implant. By understanding how the cerebral
cortex can adapt to process signals generated by a
cochlear prosthetic, it will be possible to alter
cochlear prosthetics to better serve the needs of the
cerebrum. This endeavor requires four steps: 1) The
elucidation of the essential contributions that primary
and non-primary auditory cortex make to fundamental
acoustically-guided behaviours in the hearing condition,
2) A determination of the acoustic abilities of
congenitally deaf subjects following cochlear implant,
3) An assessment of the behavioural capabilities of
primary and non-primary auditory cortex following
cochlear implant in congenitally deaf subjects, and 4)
An examination of the plastic changes that can be made
by the auditory cortex when the age at time of cochlear
implant is altered. We are presently examining the first
three steps.
To that
end, we are examining auditory cortical function in the
hearing animal and in the congenitally-deaf (CD) animal
following cochlear implant. Specifically, we will test
the animals on a battery of tasks that require the
subjects to: 1) discriminate temporal patterns of the
same duration; 2) discriminate acoustic stimuli that
differ only in their temporal duration; 3) discriminate
different frequencies; 4) discriminate different natural
vocalizations; and 5) detect the presence of an acoustic
stimulus. Each auditory area is reversibly deactivated
with cooling. As multiple bilateral pairs of cooling
loops are implanted in the same animal, it is possible
for double and even triple dissociations to be
performed. The second aim in the study is to determine
what acoustic abilities CD animals with cochlear
prosthetics implanted early in development (2 months of
age, correlating to 2-5 years old in humans) can
establish. We hypothesize that the animals will be able
to attain high performance levels on the tasks examined
in Aim 1. The results of the first two aims are brought
together in Aim 3, where we combine the CD animals with
cochlear implants examined in Aim 2 with the reversible
cooling deactivation technique used in Aim 1 to
determine how the normal functional organization of
primary and non-primary auditory cortex differs from
that established following early cochlear implant. To
accomplish this we reversibly deactivate each auditory
area in the CD animals with cochlear implants while
performing the battery of auditory tasks and compare the
results with those from the intact subjects in Aim 1.
The results from these studies are directly applicable
to clinical studies presently investigating the
functional outcomes of cochlear implants in young
children.
These experiments are conducted in collaboration with
Dr. Andrej Kral (University of Hamburg). |
