No subject

Moshe Abeles in Corticonics (Cambridge Univ. Press, 1991) 
writes on page 208 that:

"Comparisons of neural densities in the brain of people
who died at different ages (from causes not associated
with brain damage) indicate that about a third of the
cortical cell die between the ages of twenty and eighty
years (Tomlinson and Gibson, 1980). Adults can no longer
generate new neurons, and therefore those neurons that
die are never replaced.
	The neuronal fallout proceeds at a roughly steady 
rate throughout adulthood (although it is accelerated when 
the circulation of blood in the brain is impaired). The rate
of neuronal fallout is not homogeneous throughout all
the cortical regions, but most of the cortical regions
are affected by it.
	Let us assume that every year about 0.5% of the
cortical cells die at random...."

and goes on to discuss the implications for network robustness.

Reference:

Gearald H,  Tomlinson BE and Gibson PH (1980) "Cell counts
in human cerebral cortex in normal adults throughout life
using an image analysis computer" J. Neurol., 46, pp. 113-136.


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