Point where brain unites eyes' double vision found

Researchers have identified the point in the brain where separate images seen by our eyes are combined into a single image.
The combination of the two images from our two eyes makes for a much more useful impression of the world. With one eye shut, catching a ball or parking a car become more difficult.
"Two eyes are giving you two images that don't by themselves tell you where things are relative to your hand. It's the integrated information that tells you where things are," said Bas Rokers, psychology professor at the University of Wisconsin-Madison in US.
Using prisms and an advanced brain scanner, Rokers and collaborators at Utrecht University in the Netherlands have found the point in the human brain - very early in image processing in the visual cortex - in which the transformation to a cyclopean view of the world takes place.

According to Rokers, a group of neurons in the visual cortex called the striate cortex, or V1, is handling two sets of pictures from our eyes - one view each from the left eye and the right eye.
On moving to an area called V2, part of the extrastriate cortex, the neurons largely shift to a single picture. The research clears up unsettled questions as to what purpose V2 serves in visual processing.

The researchers took Magnetic resonance imaging (MRI) scans of people looking into a prismatic device that showed each eye a different image.
For example, the left eye would see a vertical black bar slightly to the right of centre, while the right eye saw the bar slightly to the left of centre.

"The brain processes the two presented images like it would with any normal pair of images, and perceives them as a single bar in the centre of the field of vision, but shifted slightly backwards in depth," Rokers said.
Since the MRI results could discern the different brain activity signatures for each vertical bar, the researchers could compare brain activity when the bars were presented to each eye separately or both eyes together.
"What we show is that in V1, that activity goes with the presented location - some neurons see the left eye image, some the right eye image," he said.

"But in V2, the activity matches the perceived, centred location. V2 is working with the combined, cyclopean image," Rokers said.
He expects that a better understanding of the way images are processed will help with ongoing research into disorders like lazy eye.
In amblyopia, the most common cause of vision problems among children around the world, the brain favours the images of a stronger eye over those of the weaker or misaligned eye.
"Now that we know where to look in amblyopia, we can focus on these brain regions and see if the representation has shifted towards the dominant eye," he said.

The study was published in the journal Current Biology.