How the Brain Solves Redundancy Problems

1 In the review, Latash summarizes the many achievements of his group and numerous collaborators in clarifying and experimentally supporting the synergy concept redefined in the framework of the uncontrolled manifold (UCM) concept. This concept is the basis of an effective method of analysing motor actions, including their anticipatory (" feedforward ") aspects and their deficiencies in the case of neuropathology. In combination with the synergy concept, the UCM notion also represents a theoretical framework that provides some insights on how the brain controls motor actions. I will comment on the second, theoretical aspect of the UCM-synergy approach. Admittedly, this approach is not sufficient to understand how the brain solves redundancy problems in motor control. Specifically, redundancy in the number of mechanical degrees of freedom (DFs) is the basis for flexible motor behaviour allowing the brain to produce different motor actions to meet motor demands despite changes in the properties of neuromuscular elements (e.g. resulting from muscle fatigue) or external, environmental conditions. Such flexibility likely played a key role in human survival in evolution. On the other hand, flexibility might not be helpful without the ability to quickly initiate a unique action each time the motor goal is clear, while disregarding many other actions leading to the same motor goal. This ability often takes place in the absence of any possibility of repeating and comparing different actions, for example, in sports and especially in threatening situations (Bernstein 1996). This implies that the redundancy (abundance) problem cannot be solved without addressing the question of how the uniqueness of actions is achieved without repetitions. Latash suggests that the UCM-synergy approach should be combined with some optimality criterion or principle to enable a specific action for each motor goal. Formulated in terms of " elemental " variables that predominantly describe effect, rather than cause, the UCM-synergy approach leaves the question of how motor actions are actually controlled and executed unanswered. To overcome this limitation, Latash makes an attempt to place the UCM and synergy concepts into the context of a theory that directly addresses this question. The choice of the advanced formulation of the equilibrium-point (EP) theory to address the control question seems natural. Unlike other conventional theories, it offers a physiologi