For many decades, neuroscientists believed that there was a “critical period” in which the brain could learn to make sense of visual information, and that this window closed around age 6 or 7.
Recent work by MIT professor Pawan Sinha has shown that the picture is more nuanced than that. In many studies of children in India who underwent surgery to remove congenital cataracts over the age of 7, he found that older children can learn visual tasks such as recognizing faces, distinguishing objects from a background, and discerning movement.
In a new study, Sinha and his colleagues have now discovered anatomical changes that occur in the brains of these patients after their sight is restored. These changes, seen in the structure and organization of the brain’s white matter, appear to underlie some of the visual improvements the researchers also observed in these patients.
The findings further support the idea that the window of brain plasticity, at least for some visual tasks, extends much further than previously thought.
“Given the remarkable level of remodeling of brain structure that we’re seeing, it reinforces the point that we’ve been trying to make with our behavioral results, that all children should receive treatment,” says Pawan Sinha, a professor of brain at MIT. and cognitive sciences and one of the study authors.
Bas Rokers, associate professor and director of the Center for Neuroimaging at New York University in Abu Dhabi, is the lead author of the study, which appears this week in the Proceedings of the National Academy of Sciences. The paper’s lead authors are Caterina Pedersini, a postdoc at New York University in Abu Dhabi; Nathaniel Miller, who is studying medicine at the University of Minnesota Medical School; and Tapan Gandhi, a former postdoc at Sinha Lab who is now an associate professor at the Indian Institute of Technology. Sharon Gilad-Gutnick, a research scientist at MIT, and Vidur Mahajan, director of the Center for Advanced Research in Imaging, Neuroscience, and Genomics, are also authors of the paper.
white matter plasticity
In developed countries like the United States, babies born with cataracts receive treatment within a few weeks of birth. However, in developing countries like India, a higher percentage of these cases go untreated.
Nearly 20 years ago, Sinha launched an initiative called Project Prakash, with the mission of providing medical treatment to blind and partially sighted children in India. Each year, the project evaluates thousands of children, many of whom receive glasses or more advanced interventions, such as surgical removal of cataracts. Some of these children, with the permission of their families, also participate in studies on how the brain’s visual system responds after their sight is restored.
In the new study, the researchers wanted to explore whether they could detect anatomical changes in the brain that might correlate with behavioral changes they had previously seen in children who received treatment. They scanned 19 participants, ranging in age from 7 to 17 years, at various times after undergoing surgery to remove congenital cataracts.
To analyze the anatomical changes in the brain, the researchers used a specialized type of magnetic resonance imaging called diffusion tensor imaging. This type of image can reveal changes in the organization of the white matter: bundles of nerve fibers that connect different regions of the brain.
Diffusion tensor imaging, which tracks the movement of hydrogen nuclei in water molecules, produces two measurements: mean diffusivity, a measure of how freely water molecules can move, and fractional anisotropy, which reveals the extent to which water is forced to move in one direction. About another.
An increase in fractional anisotropy suggests that the water molecules are more constrained because the nerve fibers in white matter are oriented in a particular direction.
“If you see an increase in fractional anisotropy and a decrease in mean diffusivity, then you can infer that what’s happening is that the nerve fibers are growing in volume and becoming more organized in terms of their alignment,” Sinha says. “When we look at the white matter of the brain, we see precisely these kinds of changes in some of the white matter bundles.”
The researchers looked at these changes specifically in white matter pathways that are part of the later stages of the visual system, which is thought to be involved in higher-order functions such as face perception. These improvements occurred gradually over several months after surgery.
“You see anatomical changes in white matter, but in separate studies using functional neuroimaging, you also see increasing specialization, based on visual experience, similar to what happens in typical development,” says Gilad-Gutnick.
The researchers also assessed the participants’ performance on a variety of visual tasks and found that their ability to distinguish faces from other objects was correlated with the amount of structural change in white matter pathways associated with higher-order visual function.
By comparison, although the treated children showed some improvements in visual acuity (the ability to clearly see details of objects at a distance), their acuity never fully recovered and they showed only minimal changes in the organization of the white matter of the eyes. first visual pathways. .
“The notion that plasticity is a time-limited resource and that beyond a certain window we can’t expect much improvement, that seems to be true for low-level visual functions like acuity,” says Sinha. “But when we’re talking about a higher-order visual ability, like distinguishing a face from a non-face, we do see improvements in behavior over time, and we also find that there’s a correlation between the improvement we’re seeing in behavior and the changes we see anatomically”.
Treatment Benefits
The researchers also found that children who had their cataracts removed at a younger age showed greater and faster gains in face perception ability than older children. However, all of the children showed at least some improvement in this ability, along with changes in the structure of the white matter.
The findings suggest that older children may benefit from this type of surgery and offer further evidence that it should be offered to them, Sinha says.
“If the brain has such outstanding abilities to rewire itself and even change its structure, then we really should capitalize on that plasticity and provide treatment to children, regardless of their age,” he says.
Sinha’s lab is now analyzing additional imaging data from Project Prakash patients. In one study, researchers are investigating whether patients show any changes in the thickness of their gray matter, especially in the sensory processing areas of the brain, after treatment. Researchers are also using fMRI scans to try to localize visual functions such as face perception, to see if they arise in the same parts of the brain as in people born with normal eyesight.
The research was funded by the National Eye Institute.
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