Some reasoning is proposed to explain why nerve pathways are crossing. The leading role of vision is suggested to be the basic reason for the phenomena as considered in the process of the nervous system evolution.
The immediate reason for this paper has been a remark in the Dr’s. David H. Hubel’s and T.N.Wiesel’s paper in “Scientific American”
“…It is worth remarking that no one has the remotest idea, why there should be this amazing tendency for nervous system pathways to cross…” (Hubel DH, Wiesel TN, 1979).
In the Dr. D. Hubel’s book “Eye, Brain and Vision”, one can find the following:
“…Incidentally, no one knows why the right half of the world tends to project to the left half of the cerebral hemispheres…” (Hubel DH., 1988).
The question is still unanswered. No need to explain the importance of the problem.
The nerve pathways’ crossing is the basic structural feature of vertebrate brain.
I shall take a risk to put forward an idea, which in my opinion gives the simplest explanation for the phenomena.
Methods And Materials
The leading role of sight in life evolution
The light perception started from a primitive light sensitive spot on the skin. But nevertheless this extremely important ability to distinguish light from dark played the tremendous role in evolution. The species, which possessed this ability, had an advantage over the ones not being able to get a light signal. Next step in vision development has been a transformation of light sensitive spot into lens which widened the field of vision and made an eye to be a powerful perception mechanism leading at the final step to images forming and analyzing ability. It should be emphasized that we are talking for now only about light perception, not forming and analyzing images. But even this ability meant a new epoch in the evolution. Oxford based scientist Andrew Parker in his book “In the blink of an eye” considers vision appearance as the basic reason for so called “Cambrian explosion”, “Big Bang” of evolution, happened about 543 million years ago. It is during this period of time, which lasted for about 50 million years,
“…roughly fifty separate major groups of organisms or "phyla" (a phylum defines the basic body plan of some group of modern or extinct animals) emerged suddenly, in most cases without evident precursors.”(Parker A., 2003).
Andrew Parker concludes his really magnificent book by the phrase:
“I am convinced that the evolution of that very first eye must have been a monumental event in the history of Life on Earth” (Parker A., 2003).
Detailed description of images perception and analyzing is beyond the scope of the paper. One can find it in a number of sources.
Why nerve pathways cross
We will concentrate now on the optics of the phenomena of nerve pathways crossing.
First, I will provide a short review of the optic pathways organization in a human being.
The part of the optic pathways starting from the retinal ganglion and going up to the optic pathways crossing point is called an “optic nerve”. The crossing point itself is called an “optic chiasm”, and the part behind the optic chiasm up to the cortical structures is called an “optic tract”.
At the chiasm, approximately 25%-30% (for human being) of optic nerve axons cross the chiasm to the contralateral (opposite) side of the brain where they merge with the uncrossed axons from another eye forming an optic tract on a contralateral side of the brain, while the rest 70%-75% of optic nerve axons continue their way through the same side of the brain – unilaterally - as an optic tract. As a result, after chiasm all axons representing left part of the vision field constitute right optic tract and visa versa.
This means that the left part of the visual field is projected on the right part of the cortex and visa versa.
Majority of the sensory and motion control systems follow this contralateral organization. This is a well known fact and I will not go further with it.
So, the question is why the evolution chose this contralateral way to manage a species perception-reaction controls? To approach the possible explanation let us consider a simple model, i.e. how light penetrates a lens with sensitive layer at the bottom.
Due to the lens light refracting property, left-right object’s configuration is inverted to the right-left configuration of this object’s image on the sensitive layer. Note that the up-down sides are reversed as well.
I suggest that the lens refracting property is the basic reason for the contralateral nervous system organization.
Body structures have been developed gradually during millions of years, vision structures included, and it could be that the first lens leading mutation happened in one light sensitive spot first or in a pair of them at once - it really makes no difference as both convert the object’s image in the same way - leading to the right / left images coming from the two eyes to merge into left / right images contralaterally.
It is logic to assume that to provide a fast reaction, like eye and head movement toward the moving object, it is better to originate this reaction circuit to the visual signal from the point closer to the point to where visual information arrives, that is preferably on the same left-right side.
I would correlate this image left/right visual reaction mechanism with the tectopulvinar system in the first place, which remarkably works even with the damaged primary visual cortex (Ratey J, 2002), being responsible to turn an eye in the direction of the moving object without actually seeing it. In non-mammalsthe tectopulvinar systemis the dominant structure for fast reaction to the visual stimulus. The fact witnesses that the tectopulvinar system preceded geniculostriate system and is dedicated to providing a fast reaction to the visual stimulus.That is, we can consider a tectopulvinar system as a blue print of much more sophisticated geniculostriate system.
Accepting that the visual system has been developed contralaterally, it is also logic to assume that another nerve structures have been developed also contra laterally to be in sync with the vision system organization which plays the leading role in perception. If not, there would be lots of confusion and disintegration between different sensory systems.
I will try to confirm this by a number of facts about sensory system architecture.
The degree of axon’s splitting is different for the different species.
To get an idea let us take a look at the visual system schema of a bird compared to visual system of a human being:
Now I assume it is not so surprising that almost 100 percent of bird’s nerve fibers crosses the chiasm and goes to the contralateral side of the bird’s brain in contrast to the human’s 25-30 percent of axons proceeding to the opposite side of the brain and merging with the 70-75 percent of axons originating from another eye and thus forming a full one side field of vision. This could be explained by the obvious fact we that bird’s eyes are located on the opposed sides of the head, which means that the two vision fields have nothing in common and full half of the world accumulated in the cortex is produced by the axons coming from one eye only.
“ … In birds … optical nerve in each eye crosses over almost completely in the optic chiasm. …”.(Springer SP, Deutch G., 1997).
Another remarkable proof could be obtained from the comparison of another sensory organs organization with the vision system organization.
Let us take a look at human’s olfactory system for example.
“…olfactory receptor neurons, like taste buds, wind up represented in the ipsilateral (unilateral – MK) cerebral hemisphere. Hence the chemical senses are at the opposite end of the spectrum from somatic sensory and motor systems, in which a given part of the body is represented primarily in the contralateral cerebral hemisphere.” (Nolte J., 1988).
And more:…“All chemically initiated sensory organs are unilateral” (Nolte J., 1988).
The only reasonable explanation for that in my opinion could be that chemically initiated sensory organs were developed before vision structures. It sounds quite reasonable as the first primitive species had a very primitive contact with the environment via direct physical contacts: tactile, odor, taste.
The interesting trend could arise from here. That is, investigating the uni- contra-lateral nerve structures organization and their relations we can trace the order in which those structures have been developed in the process of evolution with regard to the vision system development.
Summing up all the above, I’d like to give a support to this contra laterality idea from the philosophical point of view, namely Ockham’s razor principle, which states that the explanation of any phenomenon should make as few assumptions as possible, eliminating, or "shaving off", those that make no difference in the observable predictions of the explanatory hypothesis or theory. Thiscould be paraphrased as "all the things being equal, the simplest solution tends to be the best one."
In other words, when multiple competing theories are equal in other respects, the principle recommends selecting the theory that introduces the fewest assumptions and postulates the fewest hypothetical entities.
In this sense the idea of the leading role of vision in contralateral brain structures organization ideally fits Ockham’s razor principle, being a simple one and at the same time not requiring any additional assumptions.
All kinds of critics are most welcome.
Hubel DH, Wiesel TN (1979) Brain mechanisms of Vision. Sci.Am.,Vol. 241, N3, 150-62.
Hubel DH (1988) Eye, Brain and Vision, Sci.Am.Lib.
Nolte J (1988) The Human Brain: an introduction to its functional anatomy, Mosby.
Parker A (2003) In the blink of an eye. Perseus publishing.
Ratey J (2002) A user’s guide to the brain. Vintage books.
Springer SP, Deutch G.(1997) Left Brain Right Brain, W.H.Freeman.N.Y.