Dr. Martin Wessendorf and Daniel Wang, researchers at the University of Minnesota, propose a new explanation for why the left side of the brain controls the right side of the body. They published their hypothesis in Frontiers in Neuroanatomy.
The researchers examined neural circuits in four-limbed animals and suggest that cross-wired connections between opposite sides of the brain and body may have developed because evolution built new circuits around an ancient visual system.
The visual system establishes the pattern. When light enters the eye, the image on the retina reverses: objects on the right appear on the left side of the retina, and vice versa. Visual information travels to the optic chiasm, where fibers organize so that the right visual field reaches the left brain and the left visual field reaches the right brain.
This visual information reaches the optic tectum (called the superior colliculus in mammals), an ancient midbrain structure present in every vertebrate species. Each side of the optic tectum contains a map of the opposite visual field. The optic tectum also receives information about physical sensations and helps produce rapid reactions to visual events.
The crossed organization extends to sensory pathways. The dorsal column pathway, which carries information about touch and body position, crosses to the opposite side in the brainstem. This pathway appears in amphibians, reptiles, birds, and mammals but not fish, suggesting it emerged early in tetrapod evolution. The spinothalamic tract, which transmits pain and certain touch sensations, crosses within the spinal cord itself and appears in birds, mammals, reptiles, and some cartilaginous fish.
Movement pathways also cross sides. The tectospinal pathway helps the body react rapidly to environmental information. The rubrospinal pathway helps control limbs. The corticospinal tract, which supports precise finger movements in humans and other primates, extends from the cerebral cortex to the spinal cord and crosses the nervous system midline.
Wessendorf and Wang propose that the earliest visual circuits established a pattern in which sensory and movement pathways organized across opposite sides of the nervous system. Later circuits may have inherited this arrangement as they evolved, particularly in tetrapods. This pattern may reach back hundreds of millions of years.