Why Is the Brain Wired Backwards?
AOne of the oddest facts about the vertebrate nervous system is that it is crossed. In humans, the left side of the brain receives most of its information from the right side of the body and sends commands back to it, while the right hemisphere looks after the left side. This arrangement is so familiar to doctors and students of biology that it is easy to forget how strange it is. After all, there is no obvious engineering reason why a signal from the right hand should have to cross the midline of the body before it reaches the part of the brain that interprets it.
BFor a long time, the usual assumption has been that such a peculiar design must bring some advantage, since otherwise natural selection would surely have removed it. Explanations have therefore tended to focus on the benefit that crossing might provide. A new paper in the journal Frontiers in Neuroanatomy takes a different view. Its authors, Martin Wessendorf, an emeritus associate professor of neuroscience at the University of Minnesota, and Daniel Wang, a specialist in cognitive neuroscience and neurodegenerative disease, argue that the crossing may not be an adaptation at all. Instead, they suggest it may be a historical legacy: a pattern established more than 500 million years ago in the visual systems of early fish, on which later circuits were simply built.
CThe story, according to this hypothesis, begins with vision. The lens of a vertebrate eye reverses the image it forms, so that objects on the right fall on the left side of the retina and objects on the left fall on the right. Nerve fibres leaving the eyes then meet at a junction called the optic chiasm, where they are sorted so that information about the right half of the visual world travels to the left side of the brain, and information about the left half travels to the right. Much of this information arrives at the optic tectum, a structure known in mammals as the superior colliculus. The tectum controls fast, reflex-like behaviour, such as turning towards a sudden movement. Each tectum holds a map of the opposite half of the visual field, and its deeper layers also contain maps of sensations from the body. Since every vertebrate possesses a tectum, it is regarded as one of the oldest structures in the vertebrate brain.
DWessendorf and Wang propose that once this crossed visual map was in place, it shaped the way that later systems developed. Rather than designing new circuits from the beginning, evolution tends to modify and extend what already exists. On this view, pathways that appeared later in the history of vertebrates may have followed the crossed layout of the older visual system instead of adopting an arrangement of their own.
EThe authors support this idea by comparing the pathways found in different groups of animals. The dorsal column pathway, which carries gentle touch, vibration and information about the position of the limbs, crosses to the other side in the brainstem, after a relay just above the spinal cord. It is found in amphibians, reptiles, birds and mammals but not in fish, which suggests that it appeared early in the evolution of four-limbed animals, known as tetrapods. Another sensory route, the spinothalamic tract, carries pain, itch and some kinds of touch. Unlike the dorsal column pathway, it crosses within the spinal cord itself before rising to a region of the brain called the thalamus. This crossing occurs in birds, mammals and reptiles, and also in some cartilaginous fish such as sharks and rays.
FMotor pathways, which carry commands from the brain towards the muscles, show a similar pattern. The tectospinal pathway, which produces quick orienting reactions to things that are seen, heard or felt, is present in most tetrapods and in some cartilaginous fish. The rubrospinal pathway begins in a region of the midbrain called the red nucleus and helps to control the limbs. It is most prominent in mammals, though it also occurs in other tetrapods and in some cartilaginous fish. Finally, the corticospinal tract allows primates, including humans, to make delicate movements of the fingers. In most other mammals, by contrast, this tract mainly regulates incoming sensations rather than movement. Only mammals have a six-layered cerebral cortex, although all vertebrates have a telencephalon, the front region of the brain.
GThe authors suggest that the fine motor control so characteristic of human beings was not created from scratch. Rather, evolution appears to have taken an existing pathway that originally helped to regulate sensory input and given it a new job. If that is correct, then even our most sophisticated physical skills, such as writing or playing a musical instrument, depend on wiring whose basic layout was fixed long before such abilities existed. In this account, the crossing has survived not because it is especially efficient but simply because it was there first.
HIt is important to stress that this is a hypothesis rather than a proven account. The paper presents no new experimental data; its argument rests on comparing the anatomy of living vertebrates. Nevertheless, the authors believe that their idea could be tested. They suspect that the optic chiasm and optic tectum were already present in primitive jawless fish during the Cambrian Period, roughly 538 to 486 million years ago. If so, fossilised skulls of such animals could be examined for evidence of these structures. Whatever the outcome, the paper is a useful reminder that not every feature of a living body needs to be explained as a clever solution to a problem. Some features may simply reflect the fact that evolution works with whatever materials happen to be available.