The functions of the basal ganglia and the paradox of stereotaxic surgery in Parkinson's disease.

The basal ganglia play a role in controlling movement. The motor circuits within the striato-pallidal complex are thought to facilitate desired movement and inhibit unwanted movement through their influence via thalamus, mainly on precentral motor cortical regions. Lesions in the motor thalamus, or in the globus pallidus, therefore might be expected to impair voluntary movement. But stereotaxic lesions in patients with Parkinson's disease directed at the motor thalamus verified at autopsy, and lesions in the globus pallidus, which improve rigidity and tremor, apparently do not worsen parkinsonian hypokinesia and bradykinesia; nor do they regularly cause dyskinesias. Reasons for this discrepancy are reviewed. It is concluded that the motor circuits of the basal ganglia are part of a distributed motor system which can operate, albeit imperfectly, in the absence of striato-pallido-thalamo-cortical feedback. There may, however, be subtle defects in motor performance after thalamic and pallidal lesions which have escaped attention. Further consideration leads to two hypotheses concerning normal basal ganglia motor function. First, it seems most likely that it is a pause in firing of medial pallidal and substantia nigra reticulata neurons that, by disinhibition of thalamic targets, permits movements generated by cortical motor areas. An increase in firing of medial pallidal neurons, which so far has been the major focus of attention, may be more concerned with inhibition of unwanted movement. Secondly, we suggest that the basal ganglia play a particular role in motor control. A change in firing of medial pallidal neurons appears to occur too late to initiate a new movement. However, the motor circuit within the striato-pallidal system routinely receives a continuous delayed read-out of cortical motor activity and issues an output directed via thalamus mainly to premotor cortical regions. This may permit the routine automatic execution of sequences of movements generated in cortical motor areas. There is evidence that other regions of the striatum respond to significant external or internal cues as dictated by their cortical inputs, the significance being determined by memory, novelty, emotional and other contexts. We suggest that such events capture the attention of the non-motor striatum, which then interrupts the routine operation of the motor circuit, perhaps at the level of the medial pallidum and substantia nigra pars reticulata, to permit new cortical motor action.