Evolution of the opposed cerebral hemisphere control
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Parts of the visual system and the motor control can be explained by the evolution of the brain and an additional stage in the evolution of the eye. Today, in the visual system the left visual field is represented in the right hemisphere and the right visual field in the left hemisphere (Figure 1, Right). In addition, the left hemisphere controls the right part of the body, and the right hemisphere controls the left part of the body.
Evolution of the retina
During eye evolution there existed an additional convex stadium of the retina (Figure 1, Left), 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.
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 form of the optic vesicle (Figure 2). Also insects have convex eyes which supports the notion of a 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 grew into the body. But after the retina did sink in, the light reached the retina vice versa (Figure 1). 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.
The efferent motor neurons on the other hand adapted to the new situation and crossed. The reason is that the visual input on one side still controls the corresponding part of the body and hence the motor control. So the right hemisphere controls the left part of the body and vice versa, which has in addition the advantage that the motor control can be maintained after injury on the injured side.
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 sinking through the afferent nerves.
Principle of the evolution of senses
The evolution of the retina also hints to a principle of the evolution of senses, namely the retina and the ear: First they are turned to the environment, but for their protection they grow 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.