How does the brain develop in a person who has never received visual input? Using modern imaging techniques, a Polish-German team of researchers from the Institute of Psychology at the Jagiellonian University in Krakow and the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig investigated how the lack of visual experience affects the maturation of the brain. Their findings were published in Science Advances.
The scientists were able to map brain tissue microstructure at sub-millimeter scale, revealing several key patterns.
© MPI CBS
The development of the human brain is one of the most complex biological processes. During the first years of life, enormous changes take place in the brain that help optimize its functioning. In the first year alone, the surface area of the cerebral cortex increases by about three-quarters, while its thickness grows by roughly one-third. At the beginning of development, the brain creates far more connections between neurons than it actually needs. Over the course of life, frequently used connections are strengthened, while others are eliminated in a process called pruning.
At the same time, another process - myelination - progresses throughout the brain. During this process nerve fibers become surrounded by a fatty sheath that functions much like insulation around electrical wires, speeding up signal transmission. This "insulation" develops differently depending on the needs of each brain region. Neurons in sensory cortices - such as the visual and auditory cortices - are highly myelinated, whereas neurons in the frontal lobe, responsible for higher cognitive functions, have less of it.
"We already know from animal studies that sensory experiences have a tremendous influence on these developmental processes. However, the exact mechanisms behind this influence remain the subject of ongoing research", explains the study's lead author, Anna-Lena Stroh from the Max Planck Institute for Human Cognitive and Brain Sciences. "People who are born blind are particularly important in this context, as their brains develop without visual input. This allows us to study the effects of sensory experiences on brain development in a targeted way".
For many years, scientists have observed that the visual cortex of people born blind appears thicker than that of sighted people. The dominant explanation was that blindness disrupts pruning, leaving excess neural connections in the visual cortex. To test this idea directly, the researchers from Poland collaborated with a team in Leipzig who have the expertise in in vivo histology and access to advanced imaging technology capable of characterizing brain tissue in exceptional detail. The study was therefore also a major organizational undertaking, requiring the recruitment and coordination of blind participants from across Poland. In total, 24 people born blind traveled to Leipzig to take part in neuroimaging measurements, alongside 24 sighted participants matched for age and sex.
"Thanks to high-resolution data from our 7T and 3T Connectom MRI scanners, we were able to map tissue microstructure at sub-millimeter scale, revealing several key patterns", emphasizes Nikolaus Weiskopf, director of the Neurophysics Department at the Max Planck Institute for Human Cognitive and Brain Sciences. "As in previous studies, we observed that the visual cortex appeared thicker in blind participants. Importantly, we discovered that the visual cortex was less myelinated in people born blind, which may help explain the apparent thickening. We did not see clear evidence for altered cortical pruning, although pruning may still play a role. Our results point to reduced myelination as a central factor in the structural differences observed in blindness."
"A reduced amount of myelin may also affect how the boundary between the gray and white matter appears in MRI scans", says Anna-Lena Stroh. "This can make the cortex appear thicker in MRI measurements." Additional analyses revealed that these changes also extended into the white matter, which is responsible for transmitting information between different brain regions. At the same time, the researchers did not observe similar differences in brain areas associated with hearing or touch. This suggests that the changes described are directly linked to the absence of visual experience.
The researchers emphasize that these findings should not be interpreted as evidence that the blind persons' brain is "impaired" or "immature". In blind individuals, the visual cortex is involved in functions such as language processing, working memory, and cognitive control. It is well established that brain regions supporting these functions in sighted people are less myelinated. "Therefore, the differences we observe in blind individuals are entirely consistent with what this cortex does in people who are blind. The visual cortex in blind individuals is not impaired. It's perfectly fine. It is just doing something else," notes Marcin Szwed, group leader at the Jagiellonian University in Krakow.
