Male Brains slightly larger than Female Brains [but not proportionately]

Bart Janssen bjanssen at ag.arizona.edu
Mon Aug 18 14:19:59 EST 1997


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.

cheers
Bart


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
> in
> :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%
> :difference.
> :An
> :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






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