Biomedical Engineering Reference
In-Depth Information
Compensatory Hypertrophy
Blind Individuals Show Compensatory Hypertrophy within Both
Somatosensory and Auditory Cortices
There is a significant amount of evidence for compensatory hypertrophy in
blind subjects. For example, stimulation of the reading finger of blind Braille
readers results in expanded areas of scalp-recorded somatosensory-evoked poten-
tials [44] and in changes within the representation of reading fingers within
somatosensory cortex, measured using magnetic source imaging [45, 46].
Analogous topographic changes have been noted in animals trained with tactile
discrimination tasks [1, 2]. These changes include a doubling of the size of the
hand representation in blind subjects who read with three fingers, as well as
a topographic disorganization that seems to result in perceptual “smearing” or
uncertainty about which finger was stimulated. This “smearing” of information
across fingers may be due to the need to fuse input transmitted over different
fingers, so that the incoming information can be processed as a whole [46]. This
result provides the first evidence to demonstrate that dramatic reorganization
of an area has the potential to undermine the ability of the reorganized region
of cortex to perform its primary functions. Presumably, the improved ability to
integrate information across the three fingers while reading Braille outweighs any
functional disadvantages resulting from deterioration in the ability to determine
which finger was stimulated.
Analogous changes have also been found in the auditory cortex of blind
subjects using magnetic source imaging to measure responses within primary
and secondary auditory areas for tone bursts. Regions responding to auditory
stimulation, measured using tonotopic mapping [47], were expanded by
almost a factor of 2. N1m latencies were also considerably shorter in blind
individuals.
Compensatory Hypertrophy in Deaf Individuals
As described above, deaf subjects perform better at a variety of visual tasks.
Might this better performance be mediated by changes within visual cortex?
Two independent studies that measured the size of early retinotopic areas using
fMRI have failed to find any difference in either the overall size of these visual
areas or the relative amounts of cortex allocated to the fovea and the periphery
(Dougherty and Wandell, personal communication, [48]). It seems that there are
no major differences in the size or responsivity of primary visual areas (V1-V4)
between deaf and hearing subjects.
There is some evidence that attentional effects on fMRI responses to motion
within MT
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(the cortical region associated with motion processing) may
be slightly different in deaf subjects. Bavelier et al. found larger attentional
modulation in the visual cortex of deaf than hearing subjects, and also reported
an asymmetry in the extent of activity in area MT
for deaf and hearing
subjects [34, 49]. In these experiments the stimulus consisted of a full field of
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