In email message <Pine.SUN.3.91.950312224022.25230C-100000 at kon
ichiwa.cc.columbia.edu> Theodore H Schwartz writes:
> So then the next question is why did the left hemisphere specialize in
> fine motor control. Perhaps functions localized to one or the other
> hemipshere to avoid the temporal delay of sending information across the
> corpus callosum. But why left for language and fine motor control.
> Here's another question, why does the right hemisphere control
> the left half off the body and vice versa?
>
Speculative answers to the three questions:
1. Why LH for fine motor controL? The short but not final answer
may be: Because hand and eye must be controlled together and
you have the optic chiasma with the dominant right-eye controlled
by the left hemisphere. So LH fine motor control reduces to right-hand
preference, essential to minimise confusion and delay in the
co-ordinated action-response of the organism to what it perceives.
2. But why righthand rather than lefthand preference? This goes far
beyond the human case. See MacNeilage's survey of the increasing
information about handedness in other creatures [in Biological and
Behavioral Determinants of Language Develpment 1991 ed. by Krasnegor,
Schiefelbusch,and Studdert-Kennedy]. He offers a postural explanation
for neurobiological asymmetries, essentially apes using the right arm
to hold on to the tree and the left to gather food. There are also
other suggestions, for example, the tendency for human and ape mothers
to hold infants on the same side as the heart.
Such speculations may be plausible but no more than evolutionary
Just So stories. What seems more likely is that the prevalence of
right-handedness (plus LH for fine motor control as a result of the
optic cross-over) is just a product of a chance event early in the
evolutionary process. This is much the same as the explanation given
by Thomas Huxley for the prevalence of five digits throughout a huge
range of the animals. There is no obvious particular reason for it
- it just happened. Or it may go back at the most fundamental level
to chemical chirality.
3. Why RH controlling left body and LH controlling right body?
I don't know the answer and indeed had thought myself of posing
the question on the Internet. It is perhaps again something to do with
the relation between control of the eyes and control of the limbs but
the position differs between species.
Robin Allott email: rmallott at percep.demon.co.uk
