Parkinsonism disrupts neuronal modulation in the pre-supplementary motor area during movement preparation

Multiple studies suggest that Parkinsons disease (PD) is associated with changes in neuronal activity throughout the basal ganglia-thalamocortical motor circuit. There are limited electrophysiological data, however, describing how parkinsonism impacts neuronal activity in the pre-supplementary motor area (pre-SMA), an area in medial frontal cortex involved in movement planning and motor control. In this study, single unit activity was recorded in the pre-SMA of two non-human primates during a visually cued reaching task in both the naive and parkinsonian state using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of parkinsonism. In the naive state neuronal discharge rates were dynamically modulated prior to the presentation of the instructional go-cue. In a subset of these modulated cells, the magnitude of modulation correlated linearly with reaction time (RT). In the parkinsonian state, however, modulation of discharge rates in the pre-SMA was disrupted and the predictive encoding of RT was significantly diminished. These findings add to our understanding of the role of pre-SMA in motor behavior and suggest that disrupted encoding in this cortical area contributes to the alteration of early preparatory and pre-movement processes that are present in Parkinsons disease. SIGNIFICANCE STATEMENTGoal-directed movements, such as reaching for an object, necessitate temporal preparation and organization of information processing within the basal ganglia-thalamocortical motor network. Impaired movement in people with Parkinsons disease is thought to be the result of pathophysiological activity disrupting information flow within this network. This work provides neurophysiological evidence linking altered motor preplanning processes encoded in pre-supplementary motor area (pre-SMA) neuronal firing to the pathogenesis of motor disturbances in parkinsonism.