Biomedical Engineering Reference
In-Depth Information
reason for it to be difficult to be fluent in both languages. Nonetheless, a child
implanted with a cochlear implant, especially one born to hearing parents, will
tend to associate with hearing rather than deaf children in school and play, and
will often be encouraged to rely on spoken English rather than sign language.
As a consequence, children with cochlear implants are less likely to learn fluent
sign language. For those deaf children that learn fluent spoken English, this is no
bad thing. But for the minority of children that never become fluent in spoken
English, this strategy can result in a failure to become fluent in any language.
When cochlear implants first came on the market, the question of whether the
improvement in auditory function was worth the potential loss of cross-modal
abilities became a hotly debated issue. We can expect to see analogous discus-
sions in the blind community in the future as visual prosthetics begin to reach the
marketplace.
Second, prosthetics may directly compete for cortical resources. For example,
Kauffman et al. [107] have demonstrated that TMS to early visual cortex
disrupts Braille reading in blind, and that sighted subjects who are (over 5 days)
temporarily deprived of vision and taught Braille begin to show responses to
Braille within visual cortex. Subjects who were taught Braille but were not
deprived of vision were less fluent with Braille at the end of the 5-day period, and
showed no responses in visual cortex. Interestingly, sighted teachers of Braille
tend not to read by touch, but instead read Braille “visually” by looking at the
shadows cast by the paper indentations. These data, while not conclusive, do
suggest that lack of visual input is necessary for visual cortex to respond to
Braille, and that this cross-modal plasticity plays an important functional role.
As well as considering the possible impact of a prosthetic on the ability to
develop cross-modal skills, it is also important, when evaluating outcomes in
adult patients, to consider possible deleterious effects on existing cross-modal
skills. Given that the ability to make use of a restored sense may be limited
in adulthood, any deterioration in a patients' ability to navigate with a cane,
read Braille, or understand sign language can have serious consequences. Take
for example, the description of SB's ability to deal with crossing the road after
a sight recovery operation [82], “He found the traffic frightening, and would
not attempt to cross even a comparatively small street by himself. This was in
marked contrast to his former behavior, as described by his wife, when he would
cross any street in his own town by himself. In London, and later in his home
town, he would show evident fear, even when led by a companion whom he
trusted, and it was many months before he would venture alone.” Clearly new
visual information interfered with SB's ability to use cross-modal skills that he
had relied on very effectively pre-operatively.
Currently the default assumption is that the decision to implant a prosthetic
device (or carry out some other type of restorative surgery) should be based
on the potential to restore useful vision or audition, as compared to the risk
of losing any residual sensory function and the discomfort and risks of the
operation. The data described above shows that cross-modal plasticity is likely
to play a very significant role in compensating for sensory loss. This suggests
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