Back to the Future

In 1981, Hubel and Wiesel received the Nobel Prize for their groundbreaking work in the primary visual cortex. They proposed that signals from our eyes merge at a point beyond the cortical input stage, but due to technical limitations of the time, they were unable to tell whether these monocular neurons might alter their activity if both eyes were stimulated at the same time. Well, it has taken 38 years but the mystery has been solved. A team at Vanderbilt University has discovered that monocular neurons do alter their activity if both eyes are stimulated, moving the point of binocular convergence further along that previously thought to where visual signals enter cortical processing. “Combining signals is a challenging task because each eye has a slightly different perspective,” explains Alex Maier, assistant professor at the Department of Ophthalmology and Visual Sciences, who led the study. “So it was surprising to find out that this process starts just one synapse away from the retina – at the input stage of visual cortex.”

Unlike Hubel and Wiesel, Maier and his team had access to modern technology, namely, a means of measuring neuronal activity, to discover when neurons were active and where they were located within each of the six layers of the visual cortex. This was combined with a stereoscope consisting of two pairs of mirrors, positioned in such a fashion that each eye could only see one half of a computer monitor, to act as visual stimulation. This allowed the team to show images to the left eye or right eye in isolation, or both eyes simultaneously. “Using this technique we confirmed that many neurons in the (middle) input layers of visual cortex respond to only one eye, which is why they are called monocular neurons” explains Maier. “We then found that these monocular neurons change their activity when both eyes see a stimulus at the same time. Thus, those so-called monocular neurons are actually sensitive to what both eyes view.”

This process, whereby the brain combines visual signals into a single coherent view, is known as binocular combination or integration, and is commonly disturbed in amblyopia. “By improving our understanding of how binocular integration works in individuals with normal binocular vision, we will better understand how it can go awry, as in amblyopia” says Maier. “Knowing the neural sites and mechanisms of binocular integration may provide cortical targets for future therapies for amblyopia, and we hope that our work can help pave the way.”