Why does the right side of the brain control the left side of the body? An evolutionary history

Dr Martin Wessendorf is a neuroscientist and an associate professor emeritus at the University of Minnesota. His work focuses on how the brainstem controls the function of neurons in the spinal cord. Daniel Wang also researches the brain, with his focus on cognitive neuroscience and neurodegenerative diseases.

They are the authors of a new Frontiers in Neuroanatomy article that explores several brain circuits in four-limbed animals that are cross-wired (meaning one side of the brain connects with the opposite side of the body). It proposes a novel hypothesis about how they might have evolved.

by Daniel Wang and Martin Wessendorf

For those who have studied neuroanatomy, you might have wondered about the brain’s cross-wiring and its evolutionary history. We’ll go over different pathways and then propose our hypothesis on why most of these contralateral (midline-crossing) pathways evolved.

How information gets from the eyes into the brain

The story begins with sensory input from the eyes. Because the lens of the eye reverses the image on the retina, what the eye sees to the right of us forms an image on the left side of the retina and vice versa. The optic nerve carries this information to the optic chiasm, where the nerve fibers responsible for the right visual field are separated out and directed to the left half of the brain. This process is mirrored for the left visual field. Our visual inputs target, among other things, an ancient structure called the optic tectum. 

The optic tectum (which in mammals is called the superior colliculus) is a structure in the midbrain that generates rapid responses to what we see. Information from each visual field is mapped to the opposite side of the optic tectum. In its deeper layers, physical sensations (somatosensory information) are mapped as well. The optic tectum is present in all vertebrate species and, as a result, appears to be among the oldest neuroanatomical structures in vertebrates.

How information about the body gets to the brain

We address two neural pathways, which mediate physical sensations in most of the body. Both cross the midline of the central nervous system (the brain and spinal cord) and both appear to be present in all tetrapod species (the animals with four limbs, including mammals, birds, reptiles, and amphibians).

The dorsal column pathway mediates light touch, vibration, and body position. Like the name suggests, it transmits sensations from the limbs and body by ascending the dorsal column in the spinal cord. It then connects to another neuron and crosses the midline in the brainstem, which is located just above the spinal cord. Although fishes don’t have this pathway, it has been observed in amphibians, birds, mammals, and reptiles – demonstrating a highly conserved presence in tetrapods.

Meanwhile, the spinothalamic tract pathway carries information about pain, itch, and some forms of touch from the body to the brain. Besides carrying different types of sensation, this tract differs from the dorsal column pathway by crossing the midline in the spinal cord itself. After this, it ascends and conveys its sensory information from the body to the thalamus: the part of the brain that sits just above the midbrain and processes much movement and sensory information. A contralaterally organized spinothalamic tract has been found in birds, mammals, reptiles, and even in some cartilaginous fishes: sharks and rays. 

How information gets from the brain to the body

We also looked at three contralaterally organized pathways that control skeletal muscle.

First is the tectospinal pathway. The optic tectum receives visual input, but it also receives auditory and somatosensory inputs, which are mapped in registration with the visual inputs to the contralateral tectum. If you’re about to enter a crosswalk and from the corner of your eye see an oncoming car, you turn towards it. This is one example of how the tectospinal pathway mediates responses to visual stimuli. It can mediate similar responses to auditory and tactile stimuli. The motor component of this pathway originates from the deepest layers of the optic tectum and projects its axons to the contralateral spinal cord. This pathway is present in most tetrapods and some cartilaginous fishes.

The rubrospinal pathway is thought to facilitate limb control in animals. This pathway, large in most mammals, originates in the red nucleus of the midbrain before it projects to the contralateral spinal cord. It has been reported to exist in tetrapods and some cartilaginous fishes.

The corticospinal pathway, which in humans allows for our fine motor control, is a special case.  While all vertebrates have a telencephalon – the front part of the brain making up most of its mass in humans – only in mammals does it contain a six-layered cerebral cortex. The six-layered cortex is responsible for higher-order functions like integration of different sensations with memory, motor control, and language. The corticospinal tract can modulate either somatosensory input or, in the case of primates and a few other species, motor output. In most mammals, the corticospinal pathway travels from the cerebral cortex of the brain, projects to the part of the spinal cord where sensations enter, and modulates somatosensory input. In primates (including humans) and some other dexterous species, this pathway projects to the part of our spinal cord that controls our muscles and allows, for instance, fine control of the fingers. Thus, depending on the species, the corticospinal pathway can be either a motor pathway or a pathway that modulates somatosensation. In both cases, the corticospinal tract crosses the brain midline. This suggests that we humans obtained our fine motor control by co-opting a circuit whose ancestral function was somatosensory control. 

Evolution, the visionary engineer

All considered, we propose that evolutionarily ancient visual pathways biased our central nervous systems towards being organized contralaterally – at least in tetrapods. Given that all living vertebrates have both an optic chiasm and a contralaterally organized optic tectum, we think that Cambrian-era jawless fishes that lived roughly 538 to 486 million years ago had both an optic chiasm and an optic tectum. It may be possible to test this hypothesis by examining fossilized remains of their skulls.

Since evolution is conservative in its innovations – it often retains and builds upon existing structures, pathways, and mechanisms rather than inventing new ones – the organization of the visual system (and in particular, the optic tectum) may have defined the organizational principles required of the sensory and motor pathways that evolved later. We propose that contralateral organization didn’t necessarily evolve as an adaptation, but because evolution is an engineer that builds with the foundation and materials it has at hand.