Neuronal activity and information processing in motor control: from stages to continuous flow.

Some years ago, we proposed, along with others, that the isomorphism between models of information processing by stages and the organization of neural pathways connecting functionally specialized neuronal networks was a guideline for conducting experiments in which the integration of methods and concepts of cognitive psychology and of neurophysiology was a promising approach to increase our knowledge of the processes responsible for motor control. At a time when models of serially organized information processing stages are being increasingly challenged, the deciphering of the underlying brain processes increasingly suggests that current views about the linkage between neural structures and behavioural functions must be reconsidered. First, at the "molar" level, the notion of a functional specialization of neuronal networks as, for example, being "sensory", "sensorimotor" or "motor", has to be viewed as a quantitative and not as a qualitative concept. Second, at the "molecular" level, the notion of a clear-cut functional differentiation between neuronal units, or between small sets of neurons, must similarly be revised: a neuron may be more or less "sensory" or "motor" and, moreover, may share both these functional properties to varying degrees. When the brain processes responsible for movement control are reconsidered in the light of these two concepts--that is a functional heterogeneity of structurally defined neuronal networks, as well as a continuum in functional specification of isolated neuronal units--data collected by using single-cell recording of neuronal activity fit well into the model of a continuous flow of information processing: neural pathways from the cortical parietal association areas to the corticospinal apparatus appear as a privileged sensorimotor information stream along which the amount of neuronal activity responsible for movement planning progressively decreases, while the amount of neuronal activity involved in movement execution progressively increases.