'Rat vision' gives humans best sight ever

Sydney: Humans have the best of all possible visual worlds because our full stereo vision combines with primitive visual pathways to quickly spot danger, a study led by University of Sydney has discovered.

The surprising finding shows that in humans and other primates, information from the eyes is not only sent to the visual cortex for the processing that allows stereoscopic vision but can also feed directly into deep brain circuits for attention and emotion.

"The brain cells that we identified suggest that human and other primates retain a visual pathway that traces back to the primitive systems of vertebrates like fish and frogs," said professor Paul Martin who led the team.

"These connections may not have been lost during evolution of humans and other primates after all," Martin noted, who speculates that primates have the best of all possible visual worlds - full stereo vision and the ability to quickly spot and respond to danger.

The ability of the primate visual system to generate 3D pictures of its surroundings is well recognised.

This enables humans to play a game of tennis and enjoy other fine motor skills such as threading a needle.

To do this, primates have two forward-facing eyes that capture the same view from slightly different angles.

They have a visual system that keeps information from each eye separate until it reaches the brain's visual cortex.

Rodents, on the other hand, are more preoccupied with detecting and avoiding predators and their visual systems reflect this.

Their eyes are on each side of their head, scanning different fields of view, and stereo vision is poor.

In both primates and rodents, messages from the two eyes enters the brain through a small structure which is made of slivers of nerve cells, arranged like sponge in a layer cake.

Now, the team has found a subset of cells, squeezed in between the main layers in marmoset monkeys that fire in response to inputs from both eyes.

The properties and connections of these 'two-eye' cells resemble cells in the rodent's small structure.

"At first we thought we will made a mistake but we repeated the experiment and we were right. The cells responded to inputs from either eye," noted Natalie Zeater, PhD Student and lead author on the paper.

"There is no doubt that processing of complex visual information in the cerebral cortex is what enables uniquely human behaviours," Martin pointed out.

The researchers plan to delve deeper into the function of the two-eye cells, said the study published in the journal Current Biology.