Of course since there are people who function "normally" with vast
regions of their brain completely missing, I suspect having a few more
neurons either way makes very little difference.
R. McPherson wrote:
> On Sun, 17 Aug 1997, James Howard wrote:
>> :I started this thread as an example of the effects of testosterone on
>> :human brain evolution.
>> :J Comp Neurol 1997 Jul 28;384(2):312-320, "Neocortical neuron number
> :humans: effect of sex and age." Pakkenberg B, Gundersen HJ
> : estimate the total number of neocortical neurons in the
> :normal human brain
> : The average numbers of neocortical neurons were 19
> :billion in female brains and 23 billion in male brains, a 16%
> :equation predicting the total neocortical neuron number in any
> :individual in which sex and age are known is provided."
>> I would like to respond to this if I may.
>> The body size to brain size correlation is an inter-specific
> comparison and is not useful for comparing within a species so it is
> not at all surprising that the study cited above did not find a
> relationship between human body size and brain size. A body size to
> brain size correlation can be drawn only when comparing between
> species of animals.
>> Previous to the study above, neuroscientists have explored potential
> differences between female vs. male brain mass and have concluded that
>> while males have larger brains, females have more invagination of the
> cortex. Thus female brains have essentially the same cortical surface
>> area as do male brains. The cortex in female brains is more folded
> (invaginated) enabling it to fit into the smaller cranium
> (smaller body = smaller skull).
>> If the estimation technique used by the above study is legitimate, and
>> if the number of samples is sufficient, then perhaps they have
> identified a characteristic difference in total neocortical neuron
> numbers. Before this characteristic is meaningful, we would need to
> be able to establish the 1. distribution of those additional neurons,
> 2. demonstrated activity in those additional neurons, 3. overall
> differences in neuronal activity during cognitive function in those
> neurons, and 4 demonstration that more neurons = more function; as
> opposed to the notion that trimming of neuronal connectivity
> constitutes refinement of function. The answer to all of these
> questions are bound to be sometimes yes and sometimes no and it
> depends upon which sensory modality is involved and whether you are
> examining developmental or adult functioning. So more neurons doesn't
>> mean anything right now.
>> I would ask how body size was determined. If males have a larger
> somatic body surface, then perhaps more neurons will be required to
> simply represent more body skin surface (or muscles etc...). If this
> were true, then more neurons would certainly be expected and would not
>> betray any particular propensity for increased cognitive capacity.
>>>rjmcpher at uci.edu Ron McPherson