Planning and executing sequential motor acts in Parkinson Disease

Many experimental evidences highlighted that Parkinson Disease (PD) patients are particularly impaired in performing a sequence of movements. In the present experiment we focused to study how PD patients plan and execute an action constituted by the sequence of reaching-grasping an object (first motor act) and placing it on a second target (second motor act). We wished to determine whether properties of the target of the second motor act influenced the kinematics of first motor act. The results showed that PD patients modified, as the controls, the initial ballistic phase of reaching according to variation of second target position (object extrinsic properties). In particular, peak acceleration was higher for farther position of the second target. However, in the subsequent phase the patients, differently from the controls, modified their reaching kinematics removing the effects of second target distance. In other words the patients re-programmed the reaching component by taking into account only properties of the first target. This effect was obtained by varying the duration of the acceleration phase. These results indicate that PD patients are able to compute the general program of an action that takes into account extrinsic properties of the final target. However, this program decays during its execution inducing premotor areas to reprogram it. This finding provides a further evidence of the role of the basal ganglia as structure involved in the storing motor program and in controlling the action.