Why Is Our Eyesight Not Shaky ?

Our eyes have a constant very quick movement, which is crucial for land sight (fish don't have it, amphibians also, that's way they see only moving things), but we watch a coherent scene and not a shaky image.

Scientists have recently uncovered what puzzled them for decades: the brain circuit responsible behind this.

"People have been searching for a circuit to accomplish this stability for the last 50 years, and we think we've made good progress with this study," said Marc Sommer of the University of Pittsburgh.

Each neuron in the visual cortex of the brain has a "receptive field": it sees a tiny part of the world.

Just before the eye sight changes the location, the receptive field shifts to that location.

"The neuron can sample the same absolute position in space both before and after movement. So in this way, if the visual information at that same part of space is the same before and after the movement, then the neuron knows the world has been stable," Sommer told.

This phenomenon had been known but what was undiscovered was the link between the visual cortex and the motor zone responsible for the movement of the eye.

"What they didn't know is how in the world do these visual neurons know that the eye is about to move, and how do the visual neurons know where the eye is going to move," Sommer explained.

A pathway between the motor regions of the brain up to the visual cortex was found in 2002.

The scientists cut this pathway in two rhesus monkeys in order to check if it transmits messages about eye movement.

The receptive fields of the neurons decreased by more than half, thus this pathway has an important role on shifting the receptive fields of the neurons.

As visual perception in humans and monkeys is identical, we can assume that the same circuit is found in the human brain.

Researchers think that the same principle applies in other sensory systems like hearing.

When you move your head, you keep on hearing sounds coming from the same place, thus a circuit similar to the visual one must be responsible for the steady sound "image".