Neural Basis of Movement Representations

Abstract

In the framework of the cognitive neuroscience research strategy, data are accumulating which provide strong support for the central assumption of motor control studies : the organization of motor behavior would be based upon the utilization by the motor system of information stored in memory in the form of multiple, more or less abstract, hierarchically-organised representations of motor actions. Especially, studies conducted with single-neuron recording techniques in monkeys trained in tasks derived from those used in cognitive psychology, have demonstrated brain mechanisms which can be associated with three levels of representational processes. At the highest level, which may be called “semantic”, the action goal would be represented in a non-motoric, holistic, context-independent and symbolic mode, from which the “response” to be made is determined. At the middle level, which may be called “syntactic”, the motor features would be represented in a non-motoric, parametric, context-dependent and subsymbolic mode, from which the subroutines of the motor program would be specified and then assembled. At the lowest level, motor commands would be represented in a motoric, anatomical, biomechanically-constrained and neuromuscular mode which, when activated, results in a specific motor output. However, neurophysiological data appear to be increasingly incompatible with the traditional view in which, according to the hypothesis of a one-to-one mapping between functional processes and neural structures, these three representational levels would be implemented in association,premotor and motor cortical areas, respectively. The functional heterogeneity of cortical areas, which differ quantitatively more than qualitatively, as well as the continuum of function for individual neurons, between which differences are also more quantitative than qualitative, suggest another organization of the neocortex: each “behavioral” function would be implemented in a widely distributed neuronal network, which explains that the three different representational functions can be found closely intermixed in the same cortical region. A key point for future research is, thus, to understand how cognitive processes, as representations of action, are implemented in the microstructure of the cortical tissue.