Temporal and spatial alterations in GPi neuronal encoding might contribute to slow down movement in Parkinsonian monkeys.

Although widely investigated, the exact relationship between changes in basal ganglia neuronal activity and parkinsonian symptoms has not yet been deciphered. It has been proposed that bradykinesia (motor slowness) is related either to a modification of the activity of the globus pallidus internalis (GPi), the main output structure, or to a loss of spatial selectivity of the extrapyramidal motor system. Here we investigate the relationship between movement initiation and GPi activity in parkinsonian non-human primates. We compare neuronal encoding of movement in the normal and pathological conditions. After dopamine depletion, we observe an increased number of neurons responding to movement, with a less specific somato-sensory receptive field and a disruption of the selection mechanism. Moreover, the temporal order of the response of GPi neurons in parkinsonian animals is reversed. Indeed, whereas muscle activity and movement are delayed in parkinsonian animals, GPi neuronal responses to movement occur earlier and are prolonged, compared with normal conditions. Parkinsonian bradykinesia could thus result from an impairment of both temporal and spatial specificity of the GPi response to movement.