Human depth perception – a gripping story:
The need to coordinate eye and hand drove the evolutionary development of the brain to a considerably greater extent than scientists previously believed, according to Matz Larsson, researcher and senior physician at Örebro University Hospital, Sweden, who has published articles on this topic in Frontiers in Zoology and Brain, Behavior and Evolution.

In most animals, the optic nerve from the left eye crosses over to the right half of the brain, while the nerve from the right eye crosses over to the left. The site where the neurons cross is called the “optic chiasm.”

Uncrossed pathways provide visual depth perception
Isaac Newton concluded as early as 1682 that the optic nerves in primates take their own special route from the eye to the brain, with nearly half of the nerve fibres travelling an ipsilateral path (back to the same side) after passing through the optic chiasm. This means that, like other primates, humans have a large number of uncrossed pathways in this area and so both sides of the brain receive information from both eyes.

According to earlier theories, taking into account our forward-facing eyes, this aspect of the optic chiasm developed because it created good depth perception (stereopsis). The theory is that our two eyes view an object from slightly different angles, and the brain makes use of this angular difference to calculate distance.

This view is not necessarily wrong, but Matz Larsson holds that the specialized optic chiasm in primates in combination with forward-facing eyes resulted in something that was significantly more important.

“The uncrossed neural pathways likely came about to facilitate hand-eye coordination. The half-uncrossed optic nerves plus forward-facing eyes allow primates to receive visual information about the hand that always ends up in the correct half of the brain. By ‘correct half of the brain’ I mean that visual information about the hand ends up in the half of the brain that governs motor function—hand movements—and that also receives all sensory information about the hand. In principle, this applies to all situations, except when we carry out work with our arms crossed.”

“This ‘architecture’ quite simply results in fewer synapses for the nerve signals to cross, which results in faster motor function and improved precision. Visual control over the hand is important to humans, and of course also to tree-climbing species.

“Our depth perception (stereopsis) is also optimized within arm’s length.”

To prove this theory, he compared various vertebrates and examined the occurrence of crossed and uncrossed neural pathways.

“Primates, felines and tree-climbing marsupials have up to 50 percent uncrossed neural pathways. Moreover, their eyes face forward. Consequently, the eye and hand/paw become coordinated. Visual information reaches the correct half of the brain and has fewer synapses to pass.

“Crocodiles, most birds, fish and dolphins are exclusively equipped with crossed visual pathways. This is also consistent with the theory, since such animals seldom use their front limbs—paws, wings, fins—in a forward-directed position for both anatomical and functional reasons.”

The full articles are available at:
Frontiers in Zoology [Ref 1]

Brain, Behavior and Evolution [Ref 2]

Örebro University Hospital hospital [Ref 3], we offer modern and accessible healthcare on the best scientific foundation.

Matz Larsson, Senior Physician, Lung Clinic, Örebro University Hospital +46 (0)19-6025596, 073-809 02 28.