ontogenesis and synaptogenesis

[Elizabeth Bates]

Just a small empirical correction to the discussion on ontogeny in
neural nets.  Synaptogenesis does NOT continue across the (human/primate)
lifespan, at least not on any kind of a large or interesting scale.
Research by Rakic, Huttenlocher and others suggests that there is a
huge burst in synaptogenesis between (roughly) human postnatal months
6 - 24.  There is some controversy about whether this "burst" takes
place across the whole brain at once (the Rakic position) or whether
different regions "burst" at different times (the Huttenlocher position),
but it is fairly clear from research on both sides that the "burst"
is over by the time the child is 2 years old.  And of course, as
already noted on the net, cell proliferation and migration is over with
well before that, at least a month or so prior to birth (with the
exception of a couple of areas like the olfactory bulb).  The bottom
line for "neurally inspired" connectionist models seems to be that
the block of marble is delivered to the studio for carving by age 2.
This of course does not exclude small-scale, very local changes that
do occur across the lifetime (c.f. research by Merzenich and others
demonstrating reorganization of somatosensory maps in adult primates --
but reorganization that appears to be restricted to within a millemeter
distance).  However, most developmental neurobiologists that I have
read recently argue that subtractive events (i.e. axon retraction,
cell death and above all synaptic degeneration) are the most interesting
candidates for a brain basis of behavioral change after the infant
years; there is (or so it seems right now) little likelihood that the
major behavioral changes we observe across the human lifetime can
be explained by recourse to additive events (cell formation, synaptogenesis,
and not even the peripatetic but poorly understood event of myelination).

My colleagues and I have written a chapter on the relevance of these
neural events for early language development, which I would be happy
to send out to anyone that is interested in this "consumer's perspective".
the reference is: Bates, E., Thal, D. & Janowsky, J. (in press0.
Early language development and its neural correlates.  In I. Rapin
& S. Segalowitz (Eds.), Handbook of Neuropsychology, Vol. 7. Holland:Elsevier.

I would be interested in hearing from anyone who has tried to model
higher cognitive processes in neural nets that are "exploding" (in a
fairly uncontrolled way) in number of connections.  Should be an
interesting problem!  It does appear to be the case that the first
stages of language learning (from first words through the early stages
of grammar) take place under precisely those circumstances.  Which
must be balanced against a second fact: a child who suffers a massive
left hemisphere lesion up to at least age 2 - 3 can apparently acquire
language at normal or near-normal levels after that point, presumably
in the undamaged hemisphere.  So whatever "investments" in neural
tissue are being made during this rapid phase of development apparently
can be "undone" and/or "redone" elsewhere. -liz bates