Voluntary Movement: Computational Principles and Neural Mechanisms

Cognitive Neuroscience - 1997-05-01Georgopoulos AP
Movements of body parts comprise a large variety of motions (e.g. from small finger movements to locomotion) produced for various behavioural purposes (e.g. reach towards an object, manipulate an object, run away from a predator). Which of these movements are voluntary? Clearly, reaching to an object of interest is a voluntary motor act, but is arm swinging during walking, or running for life from a predator voluntary as well? In a way they are not because, for example, the arms swing while we walk without our intentionally willing them to do so, and we run away from a predator because our life is in imminent danger and, therefore, we have no choice. However, in both these cases, we can do otherwise if we choose to do so: we can walk without swinging our arms, and we can stay immobile when the predator approaches. However, we cannot stop other movements that are usually the result of brain damage, especially in a group of nuclei within the so-called basal ganglia. For example, people with lesions of a small nucleus in the basal ganglia, the subthalamic nucleus, frequently move their arms unwillingly in wild, throwing motions, a condition called hemiballismus; and the various dyskinesias consist of movements of body parts that happen without the patients willing them to, but also without the patients being able to stop them either. Therefore, these movements are called involuntary. Strangely, the best definition of voluntary movement seems to be the opposite of involuntary movement, that is a movement that can be suppressed (or not initiated at all) at will. This comes from the fact tat involuntary movements are well defined and possess the cardinal feature that they come and go by themselves, and that they cannot be suppressed or not initiated. This then serves as a good background against which to define voluntary movements.