People born blind have a thicker visual cortex than those who are sighted. The prevailing consensus is that these differences result from pruning. That is, when the brain discards neural connections that are no longer useful.
However, new research suggests that there is no direct evidence that disrupted pruning was the primary explanation. Findings published in Science Advances suggest myelination (a process where protective sheaths form around neurons) is a “central factor.”
Experts hope this research can provide a better understanding of myelination's impact on the visual cortex and lead to further findings.
Investigating Sensory Experience’s Influence On Brain Development
During the first few years of life, the brain undergoes rapid development, starting with a period of dramatic expansion.
In the first 12 months, the surface area of the outermost layer of the brain (cerebral cortex) increases 76 percent while its thickness grows by a third. By some estimates, over a million new neurons are produced every second, according to a report from Harvard University.
The result is that the brain produces way more connections between neurons than it needs. To compensate, the brain undergoes processes that improve efficiency by strengthening the connections used more often and weakening those used less often.
To investigate how sensory experience influences brain development, researchers at the Max Planck Institute and Jagiellonian University in Krakow compared the brain structures of 24 blind participants with those of 24 sighted participants using high-resolution MRI. This enabled the team to map tissue microstructures at a highly detailed scale.
“People who are born blind are particularly important in this context, as their brains develop without visual input,” the study’s lead author, Anna-Lena Stroh from the Max Planck Institute for Human Cognitive and Brain Sciences, said in a statement. “This allows us to study the effects of sensory experiences on brain development in a targeted way.”
Read More: The Mystery of Why There Hasn't Been a Confirmed Case of Schizophrenia in People Born Blind
Myelination Is “A Central Factor”
The MRI images show the visual cortex appears thicker in blind participants — a result that mirrors earlier research. But while this difference was previously attributed to lower levels of pruning, Stroh’s team found no evidence to suggest that this was the case. (Note: in the words of Carl Sagan, “absence of evidence is not evidence of absence.”) But the results did suggest lower rates of myelination are “a central factor.”
Myelination is the formation of a fatty sheath around nerve fibers, which speeds up communication between neurons. It’s commonly compared to the plastic casing you might find on electrical wires.
The extent of myelination varies across the brain. Neurons in sensory cortices, for example, tend to display high levels. Areas like the frontal lobe display less. In brain images of blind participants, lower levels of myelination in the visual cortex made it appear thicker.
This, according to the researchers, is consistent with animal studies, which have found that rearing animals in the dark reduces developmental myelination of the optic nerve. In contrast, premature eye-opening appears to accelerate the process.
The Visual Cortex Is “Just Doing Something Else”
As the name suggests, the visual cortex plays a vital role in visual processing. This does not mean that this area of the brain is impaired in people born blind. Instead, it is repurposed to support functions such as language processing, working memory, and cognitive control.
In those who are sighted, these functions are associated with brain regions that tend to be less myelinated.
“Therefore, the differences we observe in blind individuals are entirely consistent with what this cortex does in people who are blind,” Marcin Szwed, group leader at the Jagiellonian University in Krakow, said in a statement. “The visual cortex in blind individuals is not impaired. It’s perfectly fine. It is just doing something else.”
Moving forward, the team hopes future research can provide greater clarity on the exact relationship between brain structure and brain function in those born blind.
Read More: The Brain Sparks Sudden "Aha Moments" As We Try to Decipher Tricky Visual Puzzles
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