Asymmetries in the body, brain, and cognition: a systems-theory approach

Lateral asymmetries in body, brain, and cognition are ubiquitous among organisms. Asymmetries in motor-action patterns are a central theme of investigation, among others, as they are likely to have shaped primate evolution, and more specifically, their motor dexterity. Using an adaptationist approach one would argue that these asymmetries were evolutionarily selected because no bilateral organism can maneuver in three-dimensional space unless any one side becomes dominant and always takes the lead. However, which side becomes dominant is beyond the scope of this hypothesis as there is no apparent advantage or disadvantage associated with either the left or the right side. Both the evolutionary origin and adaptive significance of asymmetries in motor-action patterns remain largely unexplored. In the present study, we mathematically model how an asymmetry at a lower level could stimulate as well as govern asymmetries at the next higher level, and this process might reiterate; ultimately lateralizing the whole system. We then show by comparing two systems: one incorporating symmetric and the other incorporating asymmetric motor-action patterns, that (a) the asymmetric system performs better than the symmetric one in terms of time optimization, and (b) as the complexity of the task increases the advantage associated with asymmetries in the motor-action patterns increases. Our minimal model theoretically explains how lateral asymmetries could appear and evolve in a biological system using a systems theory approach.