Summary: Researchers provide new evidence of brain plasticity. A new study reveals visual cortex neurons sprout new axons and shed some old ones as animals improve at perceptual learning tasks.
Source: Rockefeller University.
Life-changing moments are also brain-changing moments: everything from a first kiss to a last goodbye modifies cells within the skull. The capacity to learn and remember, however, extends beyond the profound experiences that we lament or treasure. In fact, a new study led by Rockefeller scientist Charles D. Gilbert suggests that even when we’re not consciously forming new memories, our brains can change in important ways, altering how we interpret and interact with the world.
“Some connections in brain are fixed after a period in early life, known as the critical period,” says Gilbert, the Arthur and Jane Ross Professor of Neurosciences and Behavior. “Others are changing throughout life, and play an important role in encoding information in our brain as we encounter new experiences.”
Perceiving is believing
The study, published in the Proceedings of the National Academy of Sciences, focused on perceptual learning, a process that involves tuning the senses to better perceive the subtleties of various sights, sounds, and smells. Most of the time, we’re not aware that we’ve become better at distinguishing between two different shapes, for instance; but over time, with repeated exposure, this type of unconscious learning indeed takes place.
Scientists have long known that the brain can remodel itself, both as a result of normal memory formation and as a way to recover from brain damage. Gilbert had previously shown, for example, that when recovering from brain injury, neurons grow new axons, the threads that transmit signals to other cells. And he has long believed that this type of change might also occur under normal circumstances.
“We had always suspected that this ability to form new connections in the adult brain isn’t something that just evolved as a way to recover after injury,” says Gilbert. “Rather, it’s a mechanism that we’re using in our brains all the time.”
What’s more, disruptions to that mechanism could underlie some brain disorders, Gilbert says. “This type of neuronal flexibility happens in areas outside of perception,” he notes. “So if that mechanism gets disturbed, it may account for behavioral disorders such as autism and schizophrenia.”
Can you see it?
To test whether remodeling occurs in brain regions devoted to vision, Gilbert and his colleagues challenged two macaque monkeys to perceive elements of order within otherwise disordered scenes. During experiments, the monkeys viewed two pictures side by side. Both contained a frenzy of dashes, but in one image some of these dashes came together to form a cohesive line. The monkeys’ task was to identify the line–and the fewer dashes it contained, the harder it was to spot.
Initially, the monkeys succeeded only during the easy rounds, when the lines consisted of nine dashes. With weeks of training, however, they mastered increasingly difficult trials, and one animal was eventually able to spot a line only three dashes long.
“The animal didn’t need a reward to move his eye in the direction of the image that contained a line,” says Gilbert. “Just by doing the same task over and over, he improved.”
Throughout the experiment, the researchers tracked cellular changes in the visual cortex, a brain region that receives and processes signals from the eye. They found that, as the animals got better at the task, visual cortex neurons sprouted new axons and shed some old ones. These findings show that the circuitry of the adult brain is constantly changing, a quality known as plasticity.
“We’ve always known the brain needs some degree of plasticity through adulthood so that we can acquire new memories,” says Gilbert. “But it turns out that plasticity is more ubiquitous than we initially thought.”
Funding: This research was supported by the NIH/National Eye Institute.
Source: Katherine Fenz – Rockefeller University
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to Laboratory of Neurobiology at The Rockefeller University.
Original Research: Abstract for “Axonal plasticity associated with perceptual learning in adult macaque primary visual cortex” by Timo van Kerkoerle, Sally A. Marik, Stephan Meyer zum Alten Borgloh, and Charles D. Gilbert in PNAS. Published October 9 2018.
Axonal plasticity associated with perceptual learning in adult macaque primary visual cortex
Perceptual learning is associated with changes in the functional properties of neurons even in primary sensory areas. In macaque monkeys trained to perform a contour detection task, we have observed changes in contour-related facilitation of neuronal responses in primary visual cortex that track their improvement in performance on a contour detection task. We have previously explored the anatomical substrate of experience-dependent changes in the visual cortex based on a retinal lesion model, where we find sprouting and pruning of the axon collaterals in the cortical lesion projection zone. Here, we attempted to determine whether similar changes occur under normal visual experience, such as that associated with perceptual learning. We labeled the long-range horizontal connections in visual cortex by virally mediated transfer of genes expressing fluorescent probes, which enabled us to do longitudinal two-photon imaging of axonal arbors over the period during which animals improve in contour detection performance. We found that there are substantial changes in the axonal arbors of neurons in cortical regions representing the trained part of the visual field, with sprouting of new axon collaterals and pruning of preexisting axon collaterals. Our findings indicate that changes in the structure of axonal arbors are part of the circuit-level mechanism of perceptual learning, and further support the idea that the learned information is encoded at least in part in primary visual cortex.