"Paradoxical kinesis" is not a hallmark of Parkinson's disease but a general property of the motor system

Abstract

Although slowness of movement is a typical feature of Parkinson's disease (PD), it has been suggested that severely disabled patients remained able to produce normal motor responses in the context of urgent or externally driven situations. To investigate this phenomenon (often designated "paradoxical kinesis"), we required PD patients and healthy subjects to press a large switch under three main conditions: Self Generated, produce the fastest possible movement; External Cue, same as Self Generated but in response to an acoustic cue; Urgent External Cue, same as External Cue with the switch controlling an electromagnet that prevented a ball falling at the bottom of a tilted ramp. Task difficulty was equalized for the two experimental groups. Results showed that external cues and urgent conditions decreased movement duration (Urgent External Cue < External Cue < Self Generated) and reaction time (Urgent External Cue < External Cue). The amount of reduction was identical in PD patients and healthy subjects. These observations show that paradoxical kineses are not a hallmark of PD or a byproduct of basal ganglia dysfunctions, but a general property of the motor system.