Cerebellar Involvement in Anticipating the Consequences of Self-Produced Actions During Bimanual Movements

Anticipatory postural adjustments (APA) during bimanual actions can be observed when participants hold an object in one hand and then lift it with the other hand. The postural force used to hold the object is reduced in anticipation of unloading, indicating an accurate prediction of the change in load. We examined patients with unilateral or bilateral cerebellar damage as well as two individuals lacking the corpus callosum on the bimanual unloading task. The acallosal patients showed an intact APA, suggesting subcortical integration of motor signals for anticipatory adjustments during bimanual actions. Contrary to the hypothesis that the cerebellum is critical for predicting and compensating for the consequences of our actions, we found that the well-learned APA in this task was largely intact in cerebellar patients. However, cerebellar damage abolished short-term adaptation of the APA, and the patients were unable to acquire an APA in a similar but previously untrained situation. These results indicate that while over-learned anticipatory adjustments are preserved after cerebellar lesions, adaptation of this response and the acquisition of a novel coordination requires the cerebellum ipsilateral to the postural hand. Furthermore, this structure appears to be essential for the accurate timing of previously learned behaviors. The patients with cerebellar damage showed poorly timed adjustments with the APA beginning earlier than in healthy participants.

Evaluation of central commands: Toward a theoretical basis for rehabilitation

The commentary focuses on the need for motor control modeling as a rationale for rehabilitation. We give examples in a bimanual unloading task and examine the potential consequences for recovery in patients with cerebral lesions. Hierarchical models of motor control lead to a distinction between “task optimization” and “motor optimization” and to a qualifier on Latash & Anson's “hands-off” position.

Temporal coordination in bimanual actions

The issue of bimanual temporal coordination in human subjects is discussed for three selected movement paradigms: (i) simple, symmetric, bimanual finger movements, (ii) bimanual unloading, and (iii) a complex bimanual pull and grasp task. Temporal synchronization was found for all three experiments and was least variable for the first experiment. In the second experiment, synchronization concerned unloading with the index finger of one hand (electromyographic activation of the first dorsal interosseus muscle) and the postural adjustment of the load-bearing index finger of the other hand (electromyographic deactivation of the first dorsal interosseus muscle). In the third experiment, a goal-related temporal invariance was observed, even in the absence of visual guidance. Possible neural mechanisms for the observed temporal coordination of the three types of bimanual movements are discussed, as well as the concepts of goal invariance and motor equivalence.Key words: bimanual coordination, temporal invariance, motor equivalence.