Generalized motor inhibitory deficit in Parkinson's disease patients who freeze

Freezing of gait is a disabling symptom of Parkinson's disease (PD) that involves failure to initiate and continue motor activity appropriately. PD disrupts fronto-basal ganglia circuitries that also implement the inhibition of responses, leading to the hypothesis that freezing of gait may involve fundamental changes in both initiation and inhibition of motor actions. We asked whether PD patients who show freezing of gait show selective deficits in their ability to inhibit upper and lower extremity reactions. We compared older healthy controls, older PD controls without freezing of gait, and older PD participants with freezing of gait, in stop-signal tasks that measured the initiation (go trials) and inhibition (stop trials) of both hand and foot responses. When only go trials were presented, all three groups showed similar initiation speeds across lower and upper extremity responses. When stop-signal trials were introduced, both PD groups slowed their reactions nearly twice as much as healthy controls. While this adjustment helped PD controls stop their actions as quickly as healthy controls, PD patients with freezing showed significantly delayed inhibitory control of both upper and lower extremities. When anticipating the need to stop their actions urgently, PD patients show greater adjustments (i.e., slowing) to reaction speed than healthy controls. Despite these proactive adjustments, PD patients who freeze show marked impairments in inhibiting both upper and lower extremity responses, suggesting that freezing may involve a fundamental disruption to the brain's inhibitory control system.