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Abstract

A basic fact of vertebrate neurology is the crossed representation of the world in the brain: an external stimulus at one side of the animal has its most marked physiological effect in the brain of the opposite side, and electrical stimulation of one side of the brain has its most evident motor effect in the musculature of the opposite side. One of the most general rules of functional neuroanatomy is the crossed representation of the world in the brain. The origin of this puzzling phenomenon has been explained in the past on the basis of a primary optic decussation, which was interpreted as a necessary correction of the image inversion in lens eyes. The validity of this scheme is doubted in this chapter and an alternate hypothesis is presented. The supposition of a vertebrate ancestor governed mainly by olfactory taxes allows one to derive the crossed representation under the assumption that (a) positive kinesis associated with positive taxis (and rice rersa) is advantageous, and (b) an evolutionary transition has occurred from a chiefly taxic chordate to one that is governed by the principle of the representation of objects in “internal maps” of the environment within the brain.