Subthalamic nucleus deep brain stimulation improves somatosensory function in Parkinson's disease

An established treatment for the motor symptoms of Parkinson's disease (PD) is deep brain stimulation (DBS) of the subthalamic nucleus (STN). Mounting evidence suggests that PD is also associated with somatosensory deficits, yet the effect of STN-DBS on somatosensory processing is largely unknown. This study investigated whether STN-DBS affects somatosensory processing, specifically the processing of tactile and proprioceptive cues, by systematically examining the accuracy of haptic perception of object size. (Haptic perception refers to one's ability to extract object features such as shape and size by active touch.) Without vision, 13 PD patients with implanted STN-DBS and 13 healthy controls haptically explored the heights of 2 successively presented 3-dimensional (3D) blocks using a precision grip. Participants verbally indicated which block was taller and then used their nonprobing hand to motorically match the perceived size of the comparison block. Patients were tested during ON and OFF stimulation, following a 12-hour medication washout period. First, when compared to controls, the PD group's haptic discrimination threshold during OFF stimulation was elevated by 192% and mean hand aperture error was increased by 105%. Second, DBS lowered the haptic discrimination threshold by 26% and aperture error decreased by 20%. Third, during DBS ON, probing with the motorically more affected hand decreased haptic precision compared to probing with the less affected hand. This study offers the first evidence that STN-DBS improves haptic precision, further indicating that somatosensory function is improved by STN-DBS. We conclude that DBS-related improvements are not explained by improvements in motor function alone, but rather by enhanced somatosensory processing.