Biomedical Engineering Reference
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of early deprivation on early visual cortex are anything other than permanent
after adolescence [86, 100].
These results clearly have important implications for sight restoration in the
early or congenitally blind. Even if it were possible to implant a “perfect”
prosthesis - i.e. one that would result in perfect vision within a normal visual
system - it is still the case that within early blind patients this would result in
only limited useful vision.
Deterioration within Auditory Cortex in Deaf Individuals
This same process of deterioration also seems to occur after early deafness.
The success of cochlear implants depends greatly upon the age of implantation.
A variety of studies find that intervention during the first 6 months of life
(whether with a hearing aid or a cochlear implant) significantly increases the
level of language development, speech intelligibility, and emotional stability
compared with children with later diagnosis and intervention. Even though the
critical period for language development is thought to last until 7 years of
age, it has generally been found that infants implanted at a very early age
(younger than 18 months) have better outcomes than children implanted later in
life [101-103]. Neurophysiological markers show a similar pattern: in children
implanted after the age of 7 years, the cortical auditory evoked potential is
aberrant and shows slower latencies as compared to children implanted before
the age of 3 years [104].
There is some evidence that this deterioration in the ability to process auditory
input may be due to cross-modal plasticity. Lee et al. [105] found that the resting
metabolic rate within auditory areas was inversely related to years of deafness.
Children who had been deaf for a relatively short period of time showed a large
drop in the resting metabolic rate within auditory cortex. In contrast, young
deaf adults who had been deaf for many years had a resting metabolic rate
within auditory cortex that was close to normal levels. Performance on speech
perception after cochlear implantation was inversely related to the metabolic
rate pre-implantation. Speech processing outcomes were worst in those patients
who showed near-normal resting metabolic activity in auditory cortex. This is
consistent with the hypothesis that the lack of a reduction in metabolic rate
may be due to cross-modal plasticity, which in turn interferes with the ability to
recover auditory processing abilities post-implantation.
Interestingly, the “critical period” for auditory development seems to depend
on the task used to assess post-implantation performance. Better performance on
simple sound identification within a closed set of alternatives seems to depend
on whether implantation occurred before the age of 4 1/2 years. More difficult
tasks involving identifying a sound from an open set of alternatives depends on
whether implantation occurred before the ages of 5 1/2-8 1/2 years [106]. This is
consistent with work examining the effects of visual deprivation which suggests
that different regions of visual cortex vary in their developmental time courses,
with higher-level visual areas continuing to retain plasticity at later ages [17, 86].
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