The rhythmic nature of central motor control and its derangement in neurological disease

Many studies have indicated that the output from the central nervous system in controlling motor activity is rhythmic in nature. Such rhythms may be important in motor control and their derangement may result in the tremors of pathological conditions. Central rhythmicities may be investigated in the human by looking for their manifestation in the periphery. However, partly because of the poor agreement between results using different investigation techniques, there has been no clear agreement on the central origin of peripheral rhythms and therefore little progress in developing a hypothesis on the nature and role of this widespread rhythmic activity. This thesis has three main aims: 1) Conflicting results in the literature on peak frequencies of peripheral oscillation during upper limb muscle activity are clarified using a novel experimental arrangement. It is shown that these oscillations originate in the CNS and are present in a variety of different muscles. Similar rhythms are also shown to be manifest during anticipatory eye movements. 2) The oscillations are studied in neurological diseases characterized by problems with motor control and by the production of pathological oscillations in the form of tremor. 3) A hypothesis is proposed that the oscillation frequencies have an important role in coding of motor commands. This is investigated by studying how oscillations and their distribution change according to the task performed. The discovery that the abnormally strong rhythmic modulations of different limb muscles in primary orthostatic tremor have complex phase relationships specific for particular postures suggests that such rhythms may be involved in coordination of the combined activity of postural muscles. The synchronization of eye and limb oscillations that is found to occur specifically during visually-guided manual tracking tasks indicates that the oscillations may also have a role in hand-eye coordination. In addressing these questions, experiments are designed to measure tremor oscillation, electromyogram and muscle vibration in limb movements and to measure eye movements and generate targets for visual tracking. To quantify and compare such oscillations, use is made of techniques such as spectral, coherence and phase analysi