Higher Order Motor Control

The framework within which motor functions have been studied in the brain has been dominated by a presumed hierarchical organization of the brain mechanisms underlying motor function. Hughlings Jackson (1889) postulated three levels of "motor centres". He based the distinction among these levels on the degree of combinatorial representation of muscles to form complicated motor acts.Therefore, for Jackson, "higher" meant a larger number of degrees of freedom in combinations and re-combinations of motor elements. This view refers to the output stage of processing of the motor command. This stage is preceded by other stages, which include processing of the motor intention from its inception as a self-initiated or stimulus-guided movement to its translation to combinations of motor elements for its implementation. I use the term "higher motor control" in this review in the latter sense, that is in reference to processes that lead to the initiation of a motor response. For motor responses guided by visual signals, a more or less serial model can be considered: Given a relevant stimulus, events in the brain proceed from stimulus-processing areas to movement-producing areas. Although in a general sense this sequence is true, it is clear now from studies in behaving animals that there is activation in parallel of many motor areas and that a clear sequential activation is not observed. The concept then has emerged, based especially on the effects of lesion studies, that different areas might be related to different aspects of motor processing. The almost simultaneous activation of these areas reflects the parallel processing of the motor command from different aspects. This review deals with psychophysical and neural studies of aimed motor responses, e.g. isometric force pulses or movements; other movements (e.g. key presses) are not considered. In particular, I review below three aspects of motor function; namely, the specification of motor parameters, the question of coordinate transformations, and cognitive operations related to the planning of movement.