If one eye is temporarily or permanently damaged, the visual cortex will rewire itself, devoting more resources to the remaining functional eye. This change is reversible if binocular vision is restored. But until a recent study published in Science, we didn’t know how the brain manages to reallocate its resources. This new study shows that this change happens at the level of individual cells, which can shift their attention to eyes as needed.

The visual cortex functions by integrating information from the neurons that are wired to one of an animal’s two eyes. If you cover one eye up for long enough, there’s a shift where the other eye becomes dominant, and more of the visual cortex is devoted to the working eye. This occurs in many animals, including carnivores, primates, and rodents—in mice, this shift in dominance is reversible. What we haven't known is how it takes place. Does the visual cortex contain entire tissues devoted to different eyes that it repurposes, or do individual cells change their connections to follow different eyes?

The scientists used a technique called ratiometric calcium imaging, which allowed them to see excitatory changes in calcium concentration within the neurons, an indication that they're busy processing signals. The team used this to follow the sight-driven activity in the binocular visual cortex of adult mice.

The researchers began by determining the baseline of the neurons’ specificity for a single eye. In the area that they were studying, there was a high level of stability, with neurons consistently responding to just one of the two eyes. This showed that, under normal conditions, neurons don’t spontaneously switch affiliations from one eye to another—this phenomenon only happens in unusual circumstances.

Next, they set up the experimental conditions, causing one eye to no longer be able to provide visual input but then restoring it to full functionality. They found that the neurons that changed their attention to the one functional eye were able to re-orient a second time when the original eye they responded to was restored to functionality. They found that the precision of single-cell neuron recovery was significantly greater than you'd expect if the neurons were simply following some global cue that told them to switch eyes.

It had been previously hypothesized that changes in synaptic connections during a period of single-eye functionality would be reactivated during a second period of single-eye functionality. This means that the same neuronal connections switch back and forth as eyes lose and gain functionality rather than form new connections each time. The researchers tested this idea by inducing a second period of single-eye functionality in the mice after they had recovered from the first trial, and their binocular vision had been restored.

The team found the same synaptic changes that occurred in the brains of the mice during the first single-eye period were reactivated during a second period of single-eye functionality. These findings demonstrated that individual neurons associated with the visual cortex experience highly reproducible plastic changes in response to modifications in eye-specific input.

The researchers found that entire brain structures undergo periods of intense plasticity in response to changes in visual input, and these are rewired when only receiving input from a single eye. However, the team also found that after binocular vision is restored, the post-recovery brain is indistinguishable from its pre-experimental status. So, the plasticity probably isn't the result of a complete rewiring.

These experiments are the most recent in a long history of experiments that demonstrate the incredible flexibility and resilience of the brain. As neuroscientists learn more about this process, we may come closer to being able to intervene when there’s damage to our brain or sensory organs. Though that may be a long way off, studies like this one could be laying the groundwork for future advances.

Science, 2016. DOI: 10.1126/science.aad3358 (About DOIs).