Synergies and other behaviors in the context of the referent control theory of action and perception.

The hindlimb of the spinalized frog when supported on a flat surface, can produce a precise wiping reflex to remove a small piece of paper soaked in a weak acidic solution placed on the forelimb. By passively changing the forelimb position, one can modify the body schema and the coordinates of the skin irritant relative to the body, but the frog still manages to remove the irritant and accurately wipe the affected skin spot several times. Similarly, healthy humans are able to preserve movement precision of hammering despite a broad variability in arm movement leading to variability in the hammer trajectory (Bernstein's example of synergy). An explanation of the neural mechanisms underlying these synergies, heretofore unknown, is proposed in the present paper. This is done in the framework of the referent control theory (RCT), an extension of the equilibrium-point hypothesis that is applicable to many members of the biological kingdom including humans and frogs. The RCT is also helpful in solving the classical posture-movement problem and in explaining the directional tuning of neurons and the role of proprioception in action-perception coupling. Experimental tests of the RCT in future studies are proposed in comparison with other theories of action and perception. It is suggested that the RCT represents a paradigm shift in the understanding of action and perception by implying that they are controlled by specific parameters of physical and physiological laws.