Mathematical Tractability of Neural Nets

[Elizabeth Bates]

Do I think the brain is cottage cheese, all the same everywhere? No, of
course not.  And to be sure, there are clearcut differentiations by
modality (visual cortex, etc.).  But I indeed insist, based on all
we now know, that EVEN IF WE COULD PINPOINT BROCA'S AREA (notice
the spelling, only on "c") WE COULD NOT NECESSARILY PREDICT THE
PATIENT'S BEHAVIOR.  that is EXACTLY what the current data suggest.
For example, there are age-related changes that occur WITHIN individuals,
as follows: up to some time between 7 and 12 years of age (no one knows
the cutoff), anterior and posterior lesions both result in a non-fluent
aphasia; then things stabilize into the usual correlation between
lesion site and aphasia type (a loose correlation at that); then again,
some time after 50 (no one knows the cutoff), the pattern changes
again, with the probability of a FLUENT aphasia going up even with
an anterior lesion.  As for the other issues: in fact there is no
real evidence (not anymore...) linking grammar with a particular
region within the left hemisphere.  Grammatical errors occur in
fluent and nonfluent patients, with lesions all over the left half
of the brain.  Your belief in the right hemisphere's role in prosody
(note the spelling ) is also an oversimplification.  It isn't clear
at all whether the current prosody results are more than a by-product
of a right hemisphere bias for certain kinds of emotional signals.
In short, the whole story is and remains MUCH less differentiated
that you have been taught to believe.  Is there specialization of
some sort?  Yes, of course, but we are so far off from mapping it
out for language that it is a poor time to recommend that connectionists
pied-pipe after neurologists (by the way, the people doing the best
experimental work on aphasia tend not to be neurologists anyway; they
are usually experimental psychologists working with some neurologist
nearby to read the CT scans....). -liz bates