Origins of asymmetry in the CNS.

Vertebrate ancestors had extreme asymmetry of the CNS, largely imposed by bodily asymmetry. In the zebrafish a key asymmetry is that of the habenulae. Their major outflow on the left is concerned with visual control of sustained response to targets, and on the right with response to potent releasers of innate responses. Mammals retain comparable outflows but without obvious asymmetry. Behavioural asymmetries associated with the processing of perceptual information are, if anything, exaggerated. Evidence from insects suggests that these latter asymmetries are of great value in any complex CNS. Bodily asymmetry may therefore not be essential for their evolution.