Motor skill acquisition under environmental perturbations: on the necessity of alternate freezing and freeing of degrees of freedom

In a recent study on the pendulation of a small-sized humanoid robot (Lungarella & Berthouze,2002a,b), we provided experimental evidence that starting with fewer degrees of freedom enables a more efficient exploration of the sensorimotor space during the acquisition of a task. The study came as support for the well-established framework of Bernstein (1967), namely that of an initial freezing of the distal degrees of freedom, followed by their progressive release and the exploitation of environmental and body dynamics. In this paper, we revisit our study by introducing a nonlinear coupling between environment and system. Under otherwise unchanged experimental conditions, we show that a single phase of freezing and subsequent freeing of degrees of freedom is not sufficient to achieve optimal performance, and instead, alternate freezing and freeing of degrees of freedom is required. The interest of this result is twofold: (1) it confirms the recent observation by Newell & Vaillancourt (2001) that Bernsteins (1967) framework may be too narrow to account for real data; (2) it suggests that perturbations that push the system outside its postural stability or increase the task complexity may be the mechanism that triggers alternate freezing and freeing of degrees of freedom.