The impact of deep brain stimulation on executive function in Parkinson's disease

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) or the intern al segment of the globus pallidus (GPi) improves Parkinson's disease and in creases frontal blood flow. We assessed the effects of bilateral DBS on exe cutive function in Parkinson's disease patients, seven with electrodes impl anted in the STN and six in the GPi, Patients were assessed off medication with stimulators off, on and off again. The groups showed differential chan ge with stimulation on the Reitan Trail-Making test (TMT B) (STN more impro ved) and on some measures of random number generation and Wisconsin Card So rting (STN improved, GPi worse with stimulation). Across the groups, stimul ation speeded up responding (Stroop control trial, TMT A) and improved perf ormance on paced serial addition and missing digit tests. Conversely, condi tional associative learning became more errorful with stimulation across th e two groups. In general, change in performance with stimulation was signif icant for the STN but not the GPI group. These results support two opposite predictions. In support of current models of Parkinson's disease, 'releasi ng the brake' on frontal function with DBS improved aspects of executive fu nction. Conversely, disruption of basal ganglia outflow during DBS impaired performance on tests requiring changing behaviour in novel contexts as pre dicted by Marsden and Obese in 1994.