X-Message-Number: 1109
From:	Ralph Merkle <>
Subject: Estimates of Neural Hardware
Date:	Fri, 7 Aug 1992 14:09:45 PDT

"The Bounded Brain: Toward Quantitative Neuroanatomy"
by Christopher Cherniak
Journal of Cognitive Neuroscience
Volume 2, Number 1
pages 58-68

Quoted from the abstract:

"An idea that human cognitive resources are virtually without limit
turns up at all levels of mind/brain science.  This tacit
unbounded-resource assumption has paradoxical consequences
in neuroscience, particularly involving the quantitative incoherence
of some key anatomical studies of cortical connectivy resources:
cortical sheet area, synaptic density there, and giant axonic
arborizations in visual cortex.  This inattention to quantitative
consistency checking in neuroanatomy appears to stem from, as a
notable instance, something of the nonspatial character of the
Cartesian concept of mind being extended to the brain as physical

Cherniak discusses various estimates of important neural hardware counts.
He estimates that "...(1) Total cortical sheet area falls in the
100,000-200,000 mm^2 range.  (2) Mean cortical synapse density should
not run higher than about 4000/neuron; at 4000/neuron, an average of
approximately 200 million synapses/mm^3 of cortex would be available.
(3) The mean total gray-matter connectivy available per cortical neuron
will be around half a centimeter of dendrite and/or axon -- yielding
about 1/4 km of connections/mm^3 of cortex.  These estimates, as
emphasized earlier, can only be a provisional starting point."  Extensive
references are provided.  He also points out the failure to adopt a
consistent set of estimates of various numerical parameters, e.g.,
synapse volume times number of synapses should not occupy too large
a percentage of the brain.  He makes other volume estimates, with
corresponding "reality checks" involving summing the volumes to
insure they equal the total volume of the human brain.

Recommended for anyone interested in good estimates of the available
"neural hardware."

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