Coordination of the hand – a significant driver of the brain’s development
The body's need for eye and hand coordination has played a significant role in the evolution of the brain, more so than previously believed, research suggests. These findings by Matz Larsson, researcher at Örebro University and Örebro University Hospital, Sweden, were recently published in the Deutsche Zoologische Gesellschaft's journal Frontiers in Zoology.
In 1682, Isaac Newton proposed that the human and primate optic nerve, in terms of how it stretches from the eye to the brain, has a very particular architecture compared to that of other animals. Almost half of the nerve fibres from the human retina display hemidecussation, i.e. they turn back to the brain hemisphere on the same side, while in most animals all, or almost all, nerve fibres cross to the opposite side of the brain.
- Since the time of Isaac Newton, the general assumption among researchers has been that the arrangement of nerve fibres in the optic chiasm in primates and humans is intended to create accurate depth perception, also known as stereopsis. The eyes perceive an object from slightly different angles and the difference in angle helps the brain to estimate distance, says Matz Larsson.
There are however many uncertainties in terms of how the arrangement of nerve fibres in the optic chiasm has evolved.
New theory
The article in Frontiers in Zoology, along with a previous article in the journal Brain, Behavior and Evolution, presents a new theory on how our optic chiasm came to assume its special architecture.
- There is evidence to suggest that there have been small, gradual changes to the direction of the nerve pathways in the optic chiasm. The direction of these pathways may change in either direction. What has decided whether the change has brought advantages to the animal in question is whether it has resulted in an increased or reduced distance between the nerve cells used to control the hand or forelimb, says Matz Larsson.
With the human variant of optic chiasm, nerve cells that control hand movement, nerve cells that receive sensory impressions from the hand, and nerve cells that receive visual information about the hand, end up in the same side of the brain. However, this only applies as long as the hand is working on the "right" side, for example when the right hand is working to the right in the field of vision. The opposite applies to the left hand.
- If we move the hand to the opposite side of our field of vision, visual control of the hand slows down, which has been demonstrated by many researchers – the first one already a hundred years ago. Unlike today however, they did not possess the knowledge that nerve pathways may change direction as a result of evolutionary processes.
New and old discoveries
It is the combination of old and new discoveries that has paved the way for the new eye-forelimb hypothesis.
- Proving how evolutionary changes have occurred is difficult, but by comparing visual systems from various vertebrates and analysing how the forelimb (hand, front paw, fin, wings etc.) works and how it is controlled by the eye in various ecological niches, an interesting pattern emerged, along with strong support for the new theory.
The eye-forelimb hypothesis applies to essentially all vertebrates while the older theory (on depth perception) generally only applies to mammals, and even then there are important exceptions.
One older hypothesis claims that predatory animals generally have frontally placed eyes to enable them to estimate the distance to their prey, while animals preyed upon have laterally positioned eyes allowing them to scan their surroundings and detect the enemy in time. There are however flaws to this logic; most predatory animals may also become prey to other predators, and many predatory animals, for example the crocodile, have laterally situated eyes.
The crocodile only has crossed nerve pathways, and under the new eye-forelimb hypothesis, this optic chiasm architecture would have evolved to provide short nerve connections and optimal control of the crocodile's front foot.
The theory on depth perception is problematic in more senses than one, for instance in the fact that birds, most of which have laterally situated eyes, have a good ability to estimate distance and the fact that they often manage to fly through a dense wood without crashing.
Eye and hand coordination
The article in Frontiers in Zoology demonstrates how finely tuned the human eye and hand are to each other:
•There are nerve cells that react to sensory information from the hand as well as to visual information about the hand.
•The eye functions optimally in the lower part of the field of vision – where the hand is primarily working.
•In the past it was believed that one particular eye in a person, for example the right eye, was dominant in any given situation. New research shows that which eye is the dominant one is rather determined by which hand it is that is currently working.
•Our depth perception is outstanding, and the fact that the eyes are working from different angles is indeed an important principle, but it is worth noting that it is generally within an arm's length that this applies.
- The eye-forelimb hypothesis does not rule out the theory on depth perception. On the contrary, it may provide us with a better understanding of how humans' excellent ability to estimate distance has developed.