Tuesday, December 9, 2008

A primate genesis model of focal dystonia and repetitive strain injury

Byl, N. N., Merzenich, M.M., & Jenkins, W.M. (1996)
A primate genesis model of focal dystonia and repetitive strain injury: learning-induced dedifferentiation of the representation of the hand in the primary somatosensory cortex in adult monkeys.
Neurology 47, 508-520.

This article reports the findings of an experiment about the physiological causes and neurological manifestation of focal dystonia, a repetitive strain injury common to musicians and professionals who perform repetitive fine-motor skill movements.
Two adult female monkeys were engaged in this experiment and learned to hold a cylindrical handle and perform specific opening and closing movements with their fingers and thumb to obtain food.
Both monkeys mastered this hand grasping movement quickly. After an average of 6 weeks and half, they began to have difficulty removing their hand from the handle. Several weeks after voluntary adjustment of their grip force on the handle, one monkey displayed signs of tendinitis while the other did not. Later, both became unable to hold the handle with all fingers and failed in more than 50% of their attempts. When they reached this stage of physical state, they were scheduled for electrophysiologic mapping studies.
In their electrophysiologic mappings, this repetitive hand-use working experience showed substantial alteration in the representations of the surfaces of the hands in cortical area (changes were substantially more severe in the monkey that had developed clear signs of an enduring tendinitis). The dedifferentiation of neuronal responses and the degradation of cortical topographies were markedby a number of changes in cortical area:
1)a dedifferentiation of cortical representations of the skin of the hand manifested by receptive fields that were 10 to 20 times larger than normal;
2)the emergence of many receptive fields that covered the entire glabrous surface of individual fingers, or extended across the surfaces of two or more fingers;
3)a breakdown of the normally sharply segregated area of volar glabrous and dorsal hairy skin of the hand;
4)a breakdown in the normally separated representations of different fingers;
5)a breakdown of the local shifted-overlap receptive field topography in the cortex, with many digital receptive fields overlapping the fields of neurons sampled in cortical penetrations up to more than four times farther apart than normal.
This experiment proved the hypothesis that repetitive strain injury might arise because of an experience-induced degradation of sensory feedback in the cerebral cortex, which destroys the independent representation of fingers and reduces the information-bearing capacity of the cortex by expanding the dimensions or overlaps of input-specific cortical columns. The important conditions for work activities that must be achieved to generate this representational degradation are:
1)input activity patterns must be rigidly stereotyped (variations can lead to refinement, not degradation, of sensory input representations);
2)inputs must be repetitive;
3)repetitive motion events must nearly simultaneously engage normally differentiated sensory inputs (a relatively small statistically consistent separation of input events will result in the segregation-and not the integration-of inputs delivered into the cortex on a heavy behavioral schedule);
4)inputs must be attended in the work behavior (behaviors performed automatically-with low or no attention-drive weak or no significant plasticity changes in the cortex).

Review & Reflection
This is an excellent study on the behavioural patterns that may explain the causes of focal dystonia. It is a bit of a shock to read the last section about working conditions leading to the generation of representational degradation: those are the principles we instrumentalists follow in our daily practice, to a lesser or greater degree ...
However, knowing the signals when the body and mind are tired, taking regular breaks, doing lots of mental practice away from our instrument, and dividing daily practice into small sections can help us avoid robotic (and therefore, humanly impossible) routines for our systems. Understanding how our systems work definitely helps us figure out appropriate life styles. Everything takes time, but it is encouraging to know that many of us are continuously making an effort to improve the quality of our life.

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