ontogenesis

[Tom Shultz, Dept. of Psychology, McGill Univ.]

Relating the ongoing discussion of ontogeny in neural nets to her
recent review of neural correlates of language development, Liz
Bates writes:
 
         Synaptogenesis does NOT continue across the (human/primate)
         lifespan, at least not on any kind of a large or interesting
         scale.
 
At least after
          a huge burst in synaptogenesis between (roughly) human
          postnatal months 6 - 24.
 
Apparently, there is somewhat of a consensus that, through most of
human development, a dieback model accounts for many more brain
changes than does a synaptogenic model. However, there are some
dissenting views and evidence on this. For example, Greenough and
his colleagues at U. of Illinois have evidence that rats in enriched
environments add around 20% more synapses than control rats. This
is a process they call synapse-on-demand and it apparently occurs
throughout rat life, not only in early development.
 
And, as Scott Fahlman points out, algorithms like Cascade-
Correlation, although often described as performing recruitment of
new hidden units, can alternatively be described in ways that are
more compatible with a synaptic change model. (Candidate hidden
units are always in the network; it is just that they are not listened
to until they start to do something useful.)
 
This is essentially a caution that we shuld not impose a premature
closure on the issue of how best to characterize either the
connectionist or neurological literature on these issues of changing
network topology.
 
Tom Shultz