Objective: To study the role of STN neurons in modifying an ongoing movement.

Background: Patients with Parkinson’s Disease (PD) have difficulty starting or modifying a learned movement sequence, but can complete it once started. For example, they may have freezing of gait only when turning or stopping at a doorway. The subthalamic nucleus (STN) is the main hub of the hyperdirect pathway of the basal ganglia, which is presumed to be involved in interrupting an ongoing movement or decision. We hypothesized that the STN plays a role when the brain actively reacts to the visual environment to modify a movement sequence, as opposed to when planning the same sequence in advance and carrying it out.

Method: In an ongoing project, so far 6 patients with Parkinson’s Disease undergoing STN DBS surgery performed a visuo-motor task during the microelectrode recording. They were instructed to move their contralateral hand in a straight line toward a target on the monitor (Go Trials). In “Turn” trials, a different cue prompted them to make a two-step movement: first towards the target then turn away from it. “Swerve” Trials started similar to Go Trials, but in the midst of the movements, they were cued to turn, resulting in an L- shaped hand movement similar to Turn trials. The 3D hand trajectory was recorded and the turning time was identified in a semi-automated fashion.

Results: We identified a total of 21 single units, 17 of which modulated their firing in a 500 msec window around the time of turning. The majority of them, 14 of 17 cells, did so before the turning time. We did not find power modulation of beta frequency around the time of turning, but spike entrainment to beta frequency changed in 12 of 17 cells. Some cell showed this modulation for delta and gamma frequencies as well.

Conclusion: As hypothesized, STN cells change their firing rate or entrainment to delta and beta frequencies when the task requires a change to anticipated movement sequences. Since patient moved their hands similarly in both Turn and Swerve trials, the shape of movement does not fully explain this difference. Rather, we argue that the hyperdirect pathway gets selectively activated to interrupt the sequence and allow the appropriate modification to planned movement. STN overactivity in PD likely interferes with this mechanism, contributing to difficulties navigating unanticipated changes in environment.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/subthalamic-neurons-fire-differently-in-planned-versus-reactive-movements/