For a number of decades, neuroscientists were of the opinion that there was a “critical period” during which the brain could learn to make sense of visual input. This window was thought to end around the age of six or seven.
The picture is more nuanced than that, as Professor Pawan Sinha of MIT has demonstrated in recent research. He has found that older children in India who have had surgery to remove congenital cataracts after the age of 7 can learn visual skills like recognizing faces, separating objects from a background, and recognizing motion.
In another review, Sinha and his associates have now found physical changes that happen in the minds of these patients after their sight is reestablished. Some of the visual improvements that the researchers also observed in these patients appear to be caused by these changes in the structure and organization of the brain’s white matter.
“Given the striking degree of brain structure change that is being shown, it supports the argument we have been attempting to make with our behavioral findings that all children should receive treatment.“
Pawan Sinha, an MIT professor of brain and cognitive sciences and one of the authors of the study.
The results add to the evidence that, at least for some visual tasks, the window of brain plasticity is much wider than previously thought.
One of the study’s authors, MIT professor of brain and cognitive sciences Pawan Sinha, states, “Given the remarkable level of remodeling of brain structure that we are seeing, it reinforces the point that we have been trying to make with our behavioral results, that all children ought to be provided treatment.”
The senior author of the study, which was published this week in the Proceedings of the National Academy of Sciences, is Bas Rokers, an associate professor at New York University Abu Dhabi and the director of the Neuroimaging Center there. Caterina Pedersini, a postdoc at New York University Abu Dhabi, is the paper’s lead author; Nathaniel Miller is a student at the University of Minnesota Medical School who is studying medicine; and Tapan Gandhi is a former Sinha Lab postdoc who is now an associate professor at the IIT. Sharon Gilad-Gutnick, a MIT research researcher, and Vidur Mahajan, overseer of the Middle for Cutting-edge Exploration in Imaging, Neuroscience, and Genomics, are additionally authors of the paper.
White matter versatility
In developed countries, for example, the US, newborn children brought into the world with waterfalls are treated within half a month of birth. Be that as it may, in non-industrial countries, for example, India, a higher percentage of these cases go untreated.
Sinha started a project called Project Prakash nearly two decades ago with the goal of providing medical care to children in India who are blind or have vision problems. The project screens thousands of kids every year, and many of them get eyeglasses or more advanced treatments like cataract surgery. With permission from their families, some of these kids also take part in research into how the brain’s visual system reacts when sight is restored.
In this new study, the researchers wanted to see if they could find any brain anatomical changes that could be linked to the behavioral changes they had previously observed in children who had received treatment. Following surgery to remove congenital cataracts, 19 participants, ranging in age from 7 to 17 years, were scanned at various points.
Diffusion tensor imaging, a specialized form of magnetic resonance imaging, was used by the researchers to examine brain anatomical changes. Changes in how the white matter—bundles of nerve fibers that connect different parts of the brain—is organized can be seen with this kind of imaging.
Two measurements are produced by diffusion tensor imaging, which follows the movement of hydrogen nuclei in water molecules: fractional anisotropy, which shows how much water is forced to move in one direction over another, and mean diffusivity, which measures how freely water molecules can move.
Because nerve fibers in white matter are oriented in a particular direction, an increase in fractional anisotropy suggests that water molecules are more constrained.
“You can infer that what’s happening is that the nerve fibers are growing in volume and they’re getting more organized in terms of their alignment,” Sinha says. “If you see increasing fractional anisotropy and decreasing mean diffusivity, then you can infer that’s what’s happening.” Certain white matter bundles exhibit precisely these kinds of changes when we examine the brain’s white matter.
These changes were specifically observed by the researchers in white matter pathways in the later stages of the visual system, which are thought to be involved in higher-order functions like face perception. After the surgery, these improvements came slowly over several months.
According to Gilad-Gutnick, “you see anatomical changes in the white matter, but in separate studies using functional neuroimaging, you also see increasing specialization as a function of visual experience, similar to what happens in typical development.” Gilad-Gutnick is referring to the findings of these studies.
The participants’ ability to distinguish faces from other objects was found to be correlated with the amount of structural change in the white matter pathways associated with higher-order visual function, according to the researchers, who also tested the participants’ performance on a variety of visual tasks.
In examination, while the treated kids showed enhancements in visual keenness — the capacity to plainly see subtleties of items a ways off — their sharpness never completely recovered, and they showed just negligible changes in the white matter association of the early visual pathways.
“The thought that pliancy is a period-restricted asset and that beyond a specific window we can’t expect a lot of progress appears to turn out as expected for low-level visual capability like keenness,” Sinha says. “But when we talk about a higher-order visual skill like telling a face from a non-face, we do see behavioral improvements over time. We also find a correlation between the behavioral improvements and the anatomical changes.”
Benefits of treatment
The researchers also discovered that younger children with cataracts had faster and greater gains in their ability to perceive faces than older children. However, in addition to alterations in the structure of the white matter, all of the children demonstrated at least some improvement in this skill.
According to Sinha, the findings provide additional evidence that older children should be offered this type of surgery and suggest that they can benefit from it.
He states, “If the brain has such exceptional abilities to reconfigure itself and even change its structure, then we really ought to capitalize on that plasticity and provide treatment to children, regardless of age.”
Presently, additional imaging data from Project Prakash patients is being analyzed in Sinha’s lab. In one study, the researchers are looking into whether patients’ gray matter thickness changes after treatment, particularly in the areas of the brain responsible for sensory processing. Additionally, the researchers are attempting to localize visual functions like face perception using functional MRI to see if they develop in the same regions of the brain as people born with normal sight.
More information: Pedersini, Caterina A. et al, White matter plasticity following cataract surgery in congenitally blind patients, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2207025120
