[Dynamic focusing of informational convergence in basal ganglia].

Data in the literature support two apparently contradictory hypotheses: that of parallel processing and that of informational convergence in the basal ganglia. We present electrophysiological data supporting one and the other. Thus, at the output of the basal ganglia, in the intact monkey, neurons of the pallidum respond almost exclusively and in small number to passive limb movement. The specificity of the responses supports the parallel processing hypothesis. However, when the monkey is rendered parkinsonian by the neurotoxin MPTP, the dopaminergic deficit discloses a strong convergence upon pallidal neurons of information originating from different body parts. Such a convergence is also observed in the case of pallidal responses to electrical stimulation of the striatum. According to data in the recent literature, it is more likely that this information converges into the pallidum through the subthalamo-pallidal rather than the striato-pallidal pathway. On the other hand, pallidal responses to electrical stimulation of the striatum display a topological antagonistic center-surround organization, probably resulting from lateral inhibition. Moreover, changes in neuronal activities induced in the external pallidal segment by local injection of bicuculline display a similar organization and suggest the occurrence of a wide and powerful lateral inhibition in this pallidal segment. Such focusing mechanisms would be useless in an exclusively parallel system. One may therefore think that, in the normal individual, the level of dopamine dynamically focuses the system on the appropriate information. This would allow parallel processing in neuronal channels that are only relatively independent, since they can also demonstrate informational convergence.