ABSTRACT
The historical development of some ideas concerning the hierarchical organization of the brain is traced back to Spencer and Jackson. Concepts such as humunculus representation, upper motor neuron, and higher motor deficits, all still widely used in neurological thinking, emerged from the original ideas of Jackson. The process of movement initiation and execution is seen as a cascadelike activation, beginning in the "highest centers" with propagation to "lower centers." This view, in its extreme, has been challenged in recent years as it became apparent that many structures at all levels of the neuraxis may operate in parallel, with many interconnections and interactions as well as with many opportunities for back-propagation. In a second part of this chapter the evidence for parallel processing in motor cortical areas is reviewed and compared with more advanced insights in the domain of the visual system.
