Re: Why does one hand want to mirror the other hand's movements?

Dear Greg, 

          the condition which causes one hand to want to mirror the other 
might be referred to as 'normal and healthy functioning'. 

Motor learning is the name we give to the process of acquiring skills in 
movement. Much of it goes on beneath our attention. Certainly many skills 
that we learn involve the hands performing mirror symetric movements. 

Example (1) You reach for a box with two hands. Both approach in symmetric 
fashion. You pick up the box and begin to rotate it towards you or away 
from you. You hands execute mirror symmetric movements. (If you rotate the 
box about an axis drawn from, say, your stomach to a point far in front of 
you, your hands will not be operating mirror sysmetrically.)

Example (2) You wish to pull a shopping cart (trolley) from the back of a 
line of them. To do this you must raise you arms and rotate your hands so 
that they are aligned and then place them on the push-bar at the back of 
the cart. Again you have executed a mirror symmetric hand movement / 
rotation. 

Of course there are many examples of non-symmetric movements (e.g. hands on 
a car's steering wheel) But many everyday two-handed movements involve 
mirror symmetric movement. 

So what? Just because we can make these movements, why should we develop a 
general tendency to do so. 

The details of the operation of the human motor system are not known. 
Exactly what is stored and / or computed in such important parts of it as 
the motor cortex and cerebellum are subjects which are still hotly debated 
at scientific meetings. For example, the motor cortex may store or compute 
motor commands in 'muscle activation coordinates' or it may store vector 
representations of hand movement direction and extent. The cerebellum's 
operation is even more controversial. But few scientists would reject the 
following explanation as completely implausible. 

When you are learning a new motor skill, your conscious mind is greatly 
involved. The cerebellum 'listens in' to what you are telling your body to 
do and gradually learns to take over the motor operation. (Example: 
remember first learning to drive a car: how you couldn't talk while you 
were concentrating. An experienced driver can talk and drive, because the 
cerebellum has taken over most of the task of driving the car.) As you go 
about your daily life you do many movements which involve mirror symmetric 
hand movements. Your cerebellum is listening to what you do and it learns 
that if the right hand goes one way, then, generally, the left does the 
opposite. Of course you retain the ability to override this rule. (Example: 
you scald yourself in a shower in which the fawcets (taps) don't 
counterrotate for on-off.) But in the absence of noxious consequences, the 
cerebellum and other motor structures are free to generate mirroring 
commands to the contra-later hand. 


Finally, there is a story about the physicist and bongo player Dr. Richard 
P. Feyman, who is said to have avoided the draft with the following ploy. 
The army psychiatrist said: "show me your hands". Feynman did: one face up, 
the other face down. The pschiatrist decided he was psychologically 
unsuitable for combat. 

Best wishes.

Dhugal 

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  Dhugal F. Bedford, M.A. D.Phil.

   http://pubweb.nwu.edu/~dfb611/

         e-mail: db@nwu.edu

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