Scientists at Vanderbilt University in Tennessee, US, are using new technology to more accurately establish where in the visual pathway stimuli from our two eyes begin to be processed together.
Contradicting previous Nobel Prize-winning work, they have found that monocular neurons in the primary visual cortex, which were previously believed to only respond to stimuli from a single eye, actually vary their response when the other eye is stimulated as well.
“Our data suggest that the two eyes are merged as they arrive in the neocortex and not at a later stage of brain processing, as previously believed,” says corresponding author Alexander Maier.
“This major leap in our understanding of how the brain combines information from the two eyes is promising for our search for therapeutic approaches to some of the most common eye diseases in children.”
The findings are reported in a paper published in the journal Current Biology.
Binocular processing is what allows us to see a single image through two separate eyes. If you look at something with one eye at a time, switching back and forth, the image will appear to jump.
Binocular processing makes into one what would otherwise appear in double vision. But where and how this occurs in the brain still isn’t clear.
In 1981, David Hubel and Torsten Wiesel, two Harvard University researchers, received a Nobel Prize in Physiology or Medicine for their work on the visual cortex. One of their conclusions had been that visual signals remain separated – monocular – into the visual cortex.
The visual cortex is multilayered. The “input layer” is in the middle, where stimuli is first processed and where most monocular neurons are found. Hubel and Wiesel believed that “binocular convergence” occurred later along the visual pathway.
“We show that these so-called monocular neurons are actually sensitive to both eyes,” says Kacie Dougherty, lead author of the Vanderbilt study.
To establish these findings, the team use electrodes capable of tracking the activity of a single neuron. Focusing on monocular neurons, the researchers display stimuli to monkeys using a mirror stereoscope.
They find that across all layers of the visual cortex “some monocular [visual cortex] neurons’ responses were significantly enhanced, or facilitated, when both eyes are stimulated”.
That response would not occur if binocular processing of stimuli occurred further downstream in the visual cortex.
“There are six functionally distinct layers in the primary visual cortex,” Dougherty explains. “We thought the initial processing happened in the upper layers, but it’s actually in the middle. That’s vital information for developing treatments.”
The findings could have implications for treating a variety of eye diseases, particularly, the researchers say, amblyopia, a condition where the brain ignores stimuli from one eye. Currently, there are limited treatment options for amblyopia, known as lazy eye, which begins in childhood.
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