Why most neural pathways are crossed over?

In Principles of Neural Science, edited by E. Kandel, et al., 4^th Edition, (page 324), it can be reading: “An important, but as yet unexplained, feature of the organization of the central nervous system is that most neural pathways are bilaterally symmetrical and cross over to the opposite (contralateral) side of the brain or spinal cord.”

I think this could be the explanation:

Sensory and motor activity of one side of the body should be handling together and exclusively by one of the two cerebral hemispheres, but not for both hemispheres.

Sensory activity from both eyes, for example, as a consequence of images of objects in the right visual field fall on half of both retina, and then projects their axons exclusively to the left hemisphere. Half of those axons had to cross over the middle line in the optic chiasm.

Continuing with the right side, as sensory and motor activities should be handled together; the motor activity emerging in the left cerebral hemisphere should cross over to reach the muscles of the right side of the body, nearest to the right visual field where objects were seen.

There is some exception to this rule. As the sternocleidomastoid is the only muscle in the body whose action is toward the opposite side of space (it turns the head to the opposite side), its motor innervations does not cross the middle line! (K´s page 880).

As sensory and motor activity from one side of the body are processing exclusively together in only one hemisphere, damages of some selective cortical areas produce amazing results. For example, damage to the right inferior parietal lobe usually produces sensory neglect of the left side of space. It means that the patient may fail to recognize the existence of contralateral space and deny their left limbs as their own!

Some patients will not dress, undress, or wash the effected side (personal neglect syndrome), even deny or disown their left arm or leg, going so far as to say, “Who put this arm in bed with me?

Because the idea of having a left limb is completely foreign to them, patients also appear to deny the existence of any paralysis in this limb and may attempt to leave the hospital prematurely since they believe nothing is wrong with them.

These patients, then, seem to lose a discrete part of self-awareness (K´s page 393).

It doesn’t mean that both hemispheres don’t communicate each other, but only when sensory and motor information of one of the side of the body were completely processed. That is why these patients cannot use their healthy hemisphere to compensate the damaged one.

These kind of impairs were call by Freud, agnosia (Greek, “not knowing”). There are some interesting kinds of agnosia after stroke; manifested by inability to perceive or recognize motion, faces, depth, color, etc. (K´s page 498-99).

http://www.propeller.com/story/2010/03/20/euro-murzi-why-most-neural-pathways-are-crossed-over/