terça-feira, 14 de abril de 2009

Proprioception is a delicate clockwork




"The
kinesthetic sense is the sense by which muscular motion, weight, position in space, etc. are perceived. The organs concerned with this sense are the proprioceptive nerve endings diffused all over the body. The coordinating center is the labyrinth. 
The general law at work is as follows:
All sensations are related to the stimuli producing them in a fixed manner. 
For example , if you hold a 20 pound weight in your hand and you shut your eyes, and if,
noiselessly, a certain weight is added on to the weight you already carry, you will not become aware of it unless the additional weight is big enough to produce the least detectable difference in sensation. 
You won't notice a fly sitting on the 20 pounds, but you will certainly become aware of the addition of a pound weight; actually, you will probably detect half a pound, or even less, for the ratio for this kind of sensation is of 1/20 or 1/40 of a pound. 
In simple words, the Weber- Fechner law means that the smaller the weight you are holding, the smaller is the added or subtracted portion that you will be able to notice.
This law applies roughly to all sensations: light, sound, heat, pain, smell, different kinds of touch, muscular effort,...

In precise terms, we say that there is a logarithmic relationship between sensation and stimulus, namely: 

"The difference in stimulus (dS) that produces the least detectable difference in sensation (dp) is always in the same ratio to the whole stimulus (S)."

Thus:





where dp is the differential change in perception, dS is the differential increase in the stimulus and S is the stimulus at the instant. A constant factor k is to be determined experimentally.
Integrating the above equation gives
 p = k \ln{S} + C,  \,\!
where C is the constant of integration, and ln is the natural logarithm.
To determine C, put p = 0, i.e. no perception; then
 C = -k\ln{S_0},  \,\!
where S0 is that threshold of stimulus below which it is not perceived at all.
Therefore, our equation becomes
 p = k \ln{\frac{S}{S_0}}.  \,\!

(source for equations: wikipedia)


This is known as the Weber-Fechner law, which holds over the normal range of sensations.
All sensations in which muscular activity is involved are largely dependent on the smallest amount of tonus persistent in the musculature. When the tonus is the smallest possible for an action, you sense the finest increase in effort. Easy and smooth action is obtained when  the aim is achieved by the smallest amount of exertion. (...)"

In  Body and Mature Behavior, by Moshe Feldenkrais



Resource paper on Proprioception, by IADMS


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