A Polish-German research team has used advanced imaging techniques to study how the brain develops in people who are born blind, finding that structural differences in the visual cortex are linked to reduced myelination rather than clear changes in the process of neural pruning.
The findings, published in the journal Science Advances, provide new insight into how the human brain adapts when it develops without visual input.
Human brain development is one of the most complex biological processes. Changes that help optimise its functioning occur mainly during the first years of life. During the first year, the surface area of the cerebral cortex increases by about three-quarters, and its thickness by about one-third.
Early in development, the brain forms significantly more connections between neurons than it ultimately needs. Throughout life, frequently used connections become stronger, while others are eliminated in a process called synaptic pruning.
At the same time, another process — myelination — progresses throughout the brain. It involves surrounding nerve fibres with a fatty sheath, which acts similarly to the insulation of electrical wires, speeding up signal transmission. This "insulation" develops differently depending on the needs of individual brain regions. Neurons in sensory cortices, such as the visual and auditory cortex, are heavily myelinated, while neurons in the frontal lobe, responsible for higher cognitive functions, contain less myelin.
"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. 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," said Anna-Lena Stroh, PhD, from the Max Planck Institute for Cognitive and Brain Sciences in Leipzig, the first author of the study.
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, researchers from Poland collaborated with a team in Leipzig with expertise in in vivo histology and access to advanced imaging technology capable of characterising brain tissue in exceptional detail.
The study included 24 people born blind who travelled to Leipzig for neuroimaging measurements, alongside 24 sighted participants matched for age and sex.
"Thanks to high-resolution data (...) we were able to map tissue microstructure at sub-millimetre scale, revealing several key patterns. 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," said Professor Nikolaus Weiskopf, director of the Neurophysics Department at the Max Planck Institute for Human Cognitive and Brain Sciences.
"A reduced amount of myelin may also affect how the boundary between the grey and white matter appears in MRI scans. This can make the cortex appear thicker in MRI measurements," added Stroh.
Additional analyses showed that these changes also extended into the white matter, which is responsible for transmitting information between different brain regions. At the same time, researchers did not observe similar differences in brain areas associated with hearing or touch.
This suggests that the changes described in the study are directly linked to the absence of visual experience.
The researchers said that in blind individuals, the visual cortex does not remain inactive but is involved in functions such as language processing, working memory and cognitive control. Previous studies have shown 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 is perfectly fine. It is just doing something else," said Professor Marcin Szwed, group leader at the Jagiellonian University.
For decades, much of the scientific knowledge about blindness and the brain came from animal research, including experiments in which macaque monkeys were blinded.
"I recognise what research on monkeys has brought us in the past. But my personal conviction is that today, the +need to know+ does not justify primate research for the sake of basic knowledge. What we show in this paper is that we can now look directly into the living human brain, with extraordinary precision, and ask questions that once seemed accessible only through invasive animal experiments. I like to think of what we did in this paper as a way forward that is scientifically sound, that does not harm any human or animal," said Professor Marcin Szwed.
Ewelina Krajczyńska-Wujec (PAP)
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