Robustness ?

[Jan Vorbrueggen]

[Note to moderator: I read connectionists via my collegue Rolf Wuertz 
(rolf at neuroinformatik.ruhr-uni-bochum.de), so that only one 
copy of connectionists needs to cross the Atlantic. We work
on the same stuff, however.)]

As I remember it, the studies showing a marked reduction in nerve
cell count with age were done around the turn of the century. The 
method, then as now, is to obtain brains of deceased persons, fix
them, prepare cuts, count cells microscopically in those cuts, and
then estimate the total number of cells by multiplying the sampled
cells/(volume of cut) with the total volume.

This method has some obvious systematic pitfalls, however. The study
was done again some (5-10?) years ago by a German anatomist (from Kiel,
I think), who tried to get these things under better control. It is
well known, for instance, that tissue shrinks when it is fixed; the
cortex's pyramid cells are turned into that form by fixation. The new
study showed that the total water content of brain does vary dramatically
with age; when this is taken into account, it turns out that the number
of cells is identical within error bounds (a few percent?) between quite
young children and persons up to 60-70 years of age.

All this is from memory, and I don't have access to the original source,
unfortunately; but I'm pretty certain that the gist is correct. So the
conclusion seems to be that cell loss with age in the CNS is much lower
than generally thought.

On the other hand, if you compare one "neuron" in your backprop net not
with a single neuron in the CNS, but with a group of them (a column in
striate cortex, for instance), which would be a better analogy functional-
ly, then it seems to be easier to understand the brain's robustness: The
death of a single neuron doesn't kill the whole processing unit, it merely
reduces, e.g., its output power or its resolution. If, however, you kill
off a whole region, i.e., all member cells of a column, your functionality
will suffer and degradation will be much less gracefull.

-- Jan Vorbrueggen, Institut f. Neuroinformatik, Ruhr-Universitaet Bochum, FRG
-- jan at neuroinformatik.ruhr-uni-bochum.de