Explanation for the representation of the visual fields in the brain

Evolution of the retina

During eye evolution there existed an additional convex stadium of the retina (Figure 1, Left), presumably, which explains why today the left visual field is represented in the right hemisphere and vice versa. With the change from convex to concave retinas, the visual input changed sides on the retina, and was represented upside down in addition.

Embryonal development of the retina

Evolutionary stages are sometimes conserved in developmental stages, which serve as a proof for stages during evolution. The convex stadium is also mirrored in the embryonal development in the form of the optic vesicle (Figure 2). Also insects have convex eyes which supports the notion of a previous convex stadium too (**). Originally, when the retinas were turned outside, the left visual field of the eye went to the left part of the retina and the left hemisphere, and the right to the right (Figure 1, Left); back then the motor control must have been direct. For the protection of the retina, the retina was retracted into the body. But after the retina was retracted, the light reached the retina vice versa (Figure 1) and upside down. Now the right visual field of the eye reaches the left part of the retina but still the left hemisphere, and vice versa. This explains why the right visual field of the eye connects today to the left hemisphere and vice versa. Furthermore, it additionally explains why everything is represented upside down in the brain.

Motor control

The efferent motor neurons on the other hand adapted to the new situation and crossed. Like that the visual input on one side still controls the corresponding side of the body and hence the motor control on that side. So the right hemisphere controls the left part of the body and vice versa, which has in addition the advantage that the motor control and self defense can be maintained after injury on the injured side. It would make sense that the same mechanism also leads to the change from a convex to a concave retina, at least at the beginning of eye evolution. The change of sides could be caused by a protein like Myosin D1, which causes asymmetry, or a similar protein.

Eye architecture

The change in the architecture within the eye, from convex to concave, also explains why afferent nerves are located on the retina. It is due to the photo receptors retracting through the afferent nerves.

Principle of the evolution of senses

The evolution of the retina could also hint at a principle about the evolution of senses of bigger animals, namely the retina and the ear: First they are turned to the environment, but for their protection they are retracted into the body.
Today the eyes are our most important sense with 90% of our perception being visual. Since most efferent motor neurons are crossed it seems that the CNS evolution is strongly influenced by the visual input.
(*) This leads to an inverted representation of the visual fields in the brain - they crosse sides and are represented upside down.
(**) They only remained convex because insects can flee and bring their eyes into security fast enough. Our ancestors were bigger, less mobile and hence could not save their eyes fast enough by movement; because of that the retina was retracted into the body for its protection - during evolution (and development).