Scientists discover brain mechanism that compensates for vision loss
A team of KU Leuven researchers have discovered a molecular on-off switch that can control the brain’s reaction to vision loss
What eyes beneath
Researchers have long known that the brain responds to sensory loss by combining input from other senses, like vision or touch. They call this cross-modal neuroplasticity. “Behavioural and fMRI experiments have shown that blind people have enhanced tactile skills or auditory localisation,” explains biochemist Julie Nys from KU Leuven’s Lab for Neuroplasticity and Neuroproteomics.
“Subsequent studies have employed deprivation of one sense from birth in test animals to understand these intriguing flexibilities,” she says. “However, in the last decade, it has become clear that the adult brain is also able to express cross-modal changes when deprivation is imposed later in life.”
Previous experiments with mice revealed two types of neuroplasticity in response to vision loss. When a mouse loses sight in one eye, the remaining eye starts sending additional signals to the area in the brain that used to be served by the lost eye. After a while, the mouse’s whiskers – so its sense of touch – step in as well.
And a couple of weeks later, the researchers found, the “lost” area in the brain was entirely reclaimed and brain activity almost as high as it was before the loss of sight.
An on-off switch
The Leuven project, which was helmed by Lut Arckens and funded by Research Foundation Flanders (FWO) and the Flemish government’s agency for innovation through science and technology (IWT), differs from these previous experiments in that it examined the brains of older mice and also studied the molecular level. “That’s crucial since most medication acts on specific molecules,” Nys explains.
We didn’t really expect that the adult mouse brains would still be so flexible
In their own mice experiments, the team discovered that cross-modal neuroplasticity could be suppressed with Indiplon, a sedative that affects communication between brain cells by acting on effects of the activity-reducing neurotransmitter GABA – much like an on-off switch in the brain.
When the switch is on, the loss of sight in one eye is compensated for by the other eye, but also by tactile input from the whiskers. When the switch is off (so after the sedative was administered), the other eye alone takes over.
Another unexpected but important finding was that these changes were age-dependent. “Most research indicates that the younger brain is far more flexible than the adult brain,” says Nys. “Our study shows that this doesn’t hold true for every type of plasticity. In adult mice, both the remaining eye and the whiskers compensate for the lack of vision in one eye.”
The finding was surprising, she says, “because we didn’t really expect that the adult mouse brains would still be so flexible. In adolescent mice, only the functioning eye took over.”
The researchers also found that the Indiplon sedative suppressed cross-modal plasticity in adult mice: the lack of vision in one eye was compensated by the remaining eye, but not by the whiskers.
Vivid images
“You could say that we managed to ‘turn off’ the whisker influence, similar to what we observe in adolescent mice,” Nys explains. “We also discovered that by exposing adult mice to darkness before removing their eye, they recovered differently. Again, their other senses took over to a smaller degree, similar to what happens in adolescent mice.”
Brain plasticity is a dynamic and diverse process
According to the researcher, their findings may help improve patient response to sensory prosthetics. She gives the example of a young child with ear damage. “If you wait too long with cochlear implants, there is a chance that the child’s brain has already adapted to the new situation – meaning auditory areas in the brain have already been taken over by other senses. As a result, the implant doesn’t affect the corresponding auditory brain regions adequately.”
Results of the Leuven group’s study was published in The Journal of Neuroscience, and the team has received positive feedback from international colleagues.
One New Yorker, for instance, sent an email saying that ever since he lost his eye due to cancer when he was a child, he has been seeing vivid images in the dark. “Even though he is not adding any scientific proof, such stories are always intriguing to hear,” says Nys, who hopes this new study will motivate scientists to think outside the box. “Brain plasticity is a dynamic and diverse process; it’s just not as simple as: ‘Young brains are more flexible than old brains’.”
Nys says that additional research might offer clues on what’s best – turning the molecular switch on or off – while experiments with more visually oriented mammals, like cats, could be a further step towards finding out whether the mice findings can be extrapolated to the human brain.
Photo by Mycroyance/Flickr
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