Q: Why left hemisphere language?

rmallott rmallott at percep.demon.co.uk
Fri Mar 17 09:42:15 EST 1995

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

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