Why is the Brain Cross Wired?

When the anatomy of the brain is discussed in my classes, students often ask why a particular cerebral hemisphere (left or right) controls the sensory and motor functions on the opposite side of the body (e.g., the left motor cortex controls your right hand). We do not know for sure why the brain is cross wired but there are three theories for explaining it. The first theory is that visual images would be perceived discontinuously if the brain was not cross wired. Stereoscopic vision, the perception of depth, is partly the result of being able to see the left and right visual field on both sides of the brain and then combining the images.

A second theory is that cross wiring permits invertebrates to escape from danger. The only sensory input that is not crossed is the olfactory system. Since smell is used for getting towards food, it initiates a movement towards something. This is contrasted with other sensory input which makes the animal go away.

The third theory is that during the evolutionary development from invertebrates to vertebrates, the orientation of the body rotated 180 degrees with regard to the head. This is based on recent findings suggest that an early ancestor, perhaps transitional between something resembling an early worm and a chordate, underwent a 180-degree torsion or twist half-way along its body and nerve chord, rotating previously ventral structures to a dorsal position, and resulting in a cross-over between much of the body and controlling brain regions. However, this explanation does not account for why some intertebrates such as the octopus and squid have uncrossed nervous systems. So, we still do not know why certain animals went on to develop an uncrossed or crossed nervous system. However, this theory provides a physiological basis for why cross wiring in the brain may have occurred in the first place.