GPi is Involved at Multiple Levels in Selection and Inhibition of Competing Motor Behaviors

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Neurobiology and Anatomy, 2014.To test the hypothesis that the basal ganglia are involved in facilitating desired behaviors and inhibiting competing ones, we have recorded GPi neurons in two monkeys trained to perform a wrist movement task with four levels of potential competition: 1) turning on vs. turning off muscles, 2) movement toward an illuminated target (automatic) vs. movement away from that target (non-automatic), 3) suppression of any movement (NoGo), 4) performance according to over-learned rules vs. reverse rules. We then focally inactivated GPi with muscimol to determine the effects of inactivation on the motor and cognitive performance of the task. We have recorded 98 GPi neurons from two animals. 52% of recorded neurons carried signals specifically related to the task type, rule, or their interaction. 82% of these cells modulated their firing rates prior to target presentation, mostly with increases in firing rate when the animals had to suppress prepotent responses. Another subset of GPi neurons (18%) signaled suppression of movement (NoGo), predominantly with an increase of firing rates. Muscimol (0.5 ug in 0.5 ul) was injected at 40 separate sites in GPi of four monkeys where task-related neurons had been recorded. Inactivation caused substantial impairment of task performance. Kinematic deficits comprised of: 1) slowing of movement, 2) greater difficulty performing eccentric movements, 3) compromised movement in the direction of extension. Task-related deficits included: 1) overall increase of errors post inactivation, 2) increased impulsivity, 3) difficulty holding the cursor against the load (with loaded extensors), 4) increased perseveration in task types requiring suppression of prepotent response. The most prominent deficits were observed when the animals had to suppress ongoing muscle activity, inhibit prepotent responses or switch between competing tasks types. These findings support the hypothesis that the basal ganglia are involved in multiple levels of action control, with an important role in suppression and facilitation of competing motor behaviors