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The body is complicated, and often the origins of a condition are all but obvious. Focal dystonia of the hand is one disorder whose underlying cause has been found in the more recent past. Although it can be genetic (1), the form of focal dystonia of the hand I look at here is caused by environmental factors (2).

Focal dystonia of the hand is a condition characterized by a loss in motor control of one or more fingers. A single muscle or group of muscles is involved: muscles in the hand and forearm tense and tighten, with the result of making the hand (or part of it) curl (2). Musicians who have intensively practiced their instruments over a number of years are a group most affected by this condition. The reason is that focal dystonia can be caused by the repetitive movement of the fingers over a significant period of time. The condition was long known as “occupational hand cramp.” (3). It can easily be misdiagnosed as simple overuse or stress of the hand (1). Although it may not be obvious at first sight of the symptoms, the level at which the problem is caused is not the hand, but the brain.

Researchers at the University of Konstanz report “overlap or smearing of the homuncular organization of the representation of the digits in the primary somatosensory cortex” (3). Given that functions such as motor control cross over from the right side of the body to be represented in the left hemisphere, they found that the distance between the representations of individual fingers was smaller in the somatosensory cortex side corresponding to the hand that had undergone continued repetitious training (the left hand in case of violin players for example).

What does all this mean in terms of the brain? Looking at the central nervous system as an input-output system, in very simple terms we can observe that a specific input is presented over and over again – in this case the stimulation of the fingers that play the violin – and as a result the organization within the box changes. More specifically, there is a one-to-one correspondence between input and internal representations of this input: all fingers are individually represented on the somatosensory cortex. But somehow, as these regions of representation begin to smear or overlap, the one-to-one correspondence is blurred.

And the result? The problem turns into one of perception and motor control. Subjects with dystonia of the hand consistently are unable to localize light pressure stimuli applied to the tips of the fingers on the correct finger(4). Further, fingers in question cannot be moved individually, and specific movements or movement sequences cannot be controlled any longer (5). We get similar effects in blind people who read Braille with several fingers at once: they develop a single representation of all these fingers on the somatosensory cortex, but are not able to determine which part of the information received in the brain comes from which finger (6). Psychologist Thomas Elbert further points out a parallel of this in all of us: our toes are generally stimulated only simultaneously as we walk, and most of us have trouble telling which of the middle toes has been touched upon application of a light pressure stimulus. Indeed, our toes are not individually represented on the somatosensory cortex as our fingers are (6).

As we can see, through some process, representations in the brain have become reorganized in a way that makes it difficult to distinguish between certain afferent inputs. These inputs enter the peripheral nervous system close to each other, say via two adjacent fingers, and have been presented to the system simultaneously over and over again. The examples are stimulation of the first and the second fingers in quick musical sequences – so quick that stimulation can be seen by the system to happen simultaneously – or continuous use of several fingers in reading Braille (6).

This continuous pairing of two (or more) stimuli, until their representations in the brain merge, is not a new idea. It has been established that if two connected neurons are stimulated at the same time, the connection between them is strengthened (7), and rules of associative learning have been based on this idea. Fittingly, Dr Merzenich of the University of California San Francisco calls focal dystonia of the hand a “learning-based catastrophe” and a “failure of the brain’s learning processes” (5). Consequently, he focuses on developing techniques that will help to “re-normalize the learning system”, in helping to newly distinguish the areas on the somatosensory cortex that have become blurred. Although this approach is very new, Merzenich claims some good results in training children with linguistic impairments, such as dyslexia, which show similar blurring of representations in the brain (5).

The form of focal dystonia of the hand that is caused by repetitious training of two or more inputs is a clear indicator of the brain’s ability to restructure itself. Traditionally, the belief has been that the capacity to restructure is present during childhood, and that the adult brain shows little room for change (6). However, as we can see, the adult brain is still capable of rather far-reaching change, and the old view of a hard-wired adult brain is thus being challenged. Further, we have seen that the central nervous system reacts to the kind of input it receives from the outside world, and the context this input is presented in (what are other impulses presented at the same time?). According to what these inputs are, there seems to be at least some leeway for the brain to adjust, in order to process these inputs in a way that may be more efficient. Indeed, this movement towards efficiency is fulfilled in the combining of various fingers into one larger representation in blind readers of Braille. Those skilled at reading Braille with multiple fingers are quicker than readers of Braille that use only a single finger. However, focal dystonia of the hand also suggests that there may be some rare inputs (such as long-term repetitious practicing of fast musical sequences) that “trick” the central nervous system into making changes that are clearly less beneficial.

 

Source:http://serendip.brynmawr.edu/

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