Scientists discover anatomical changes in the brains of the newly sighted

Newswise — CAMBRIDGE, MA —

For numerous years, neuroscientists held the belief that there existed a "critical period" during which the brain could acquire the ability to comprehend visual input, and that this timeframe terminated around the age of 6 or 7.

The latest research by Professor Pawan Sinha from MIT has revealed a more intricate picture. Sinha's investigations on Indian children who underwent surgery to remove congenital cataracts after the age of 7 have demonstrated that older children can acquire visual abilities such as face recognition, object discrimination from the background, and motion perception through learning.

In a recent research endeavor, Sinha and his team have identified structural alterations in the brains of these patients following the restoration of their vision. These changes, evident in the organization and composition of the brain's white matter, seem to account for some of the enhancements in vision that the researchers observed in these individuals.

These findings reinforce the notion that the period of neuroplasticity, at least for certain visual abilities, extends much beyond what was previously believed.

Pawan Sinha, one of the authors of the study and a professor of brain and cognitive sciences at MIT, emphasizes that the significant restructuring of brain organization observed in their study reinforces their argument for providing treatment to all children. He 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, published this week in the Proceedings of the National Academy of Sciences, is Bas Rokers, an associate professor and director of the Neuroimaging Center at New York University Abu Dhabi. The paper's lead authors are Caterina Pedersini, a postdoctoral researcher at New York University Abu Dhabi, Nathaniel Miller, who is pursuing medical studies at the University of Minnesota Medical School, and Tapan Gandhi, a former postdoctoral researcher at the Sinha Lab and now an associate professor at the Indian Institute of Technology. The paper's other authors include Sharon Gilad-Gutnick, an MIT research scientist, and Vidur Mahajan, director of the Center for Advanced Research on Imaging, Neuroscience, and Genomics.

White matter plasticity

In developed countries like the United States, infants born with cataracts receive treatment within weeks of birth. However, in developing nations like India, a greater proportion of these cases remain untreated.

Around 20 years ago, Sinha founded Project Prakash, an initiative aimed at providing medical care to visually-impaired and blind children in India. The project screens thousands of children annually, offering corrective glasses or advanced interventions, such as the surgical removal of cataracts. With the consent of their families, some of these children also participate in studies that investigate the response of the brain's visual system after their vision is restored.

The researchers aimed to investigate whether they could identify any structural alterations in the brain that correlated with the behavioral changes observed in children who received treatment in their new study. They conducted brain scans on 19 participants aged between 7 to 17 years old, at different intervals after they underwent surgery to remove congenital cataracts.

The researchers utilized diffusion tensor imaging, a specialized form of magnetic resonance imaging, to examine the anatomical changes in the brain. This imaging method is capable of revealing modifications in the organization of the white matter, which is made up of nerve fibers that connect various areas of the brain.

Diffusion tensor imaging involves tracking the movement of hydrogen nuclei in water molecules and generates two measurements: mean diffusivity and fractional anisotropy. Mean diffusivity is a measure of how freely water molecules can move, while fractional anisotropy reveals the extent to which water is forced to move in one direction over another.

An increase in fractional anisotropy indicates that water molecules are more restricted in their movement, suggesting that nerve fibers in the white matter are oriented in a particular direction.

According to Sinha, if fractional anisotropy increases while mean diffusivity decreases, it indicates that nerve fibers are growing in volume and becoming more organized in terms of their alignment. Sinha adds that when they examined the white matter of the brain, they observed precisely these types of changes in some of the white matter bundles.

The researchers observed these changes specifically in white matter pathways that are part of the later stages of the visual system, which are believed to be involved in higher-order functions such as face perception. These improvements occurred gradually over several months following the surgery.

"You can 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," says Gilad-Gutnick.

The researchers evaluated the participants' performance on various visual tasks and discovered that the extent of structural changes in the white matter pathways associated with higher-order visual functions correlated with their ability to differentiate faces from other objects.

The children who received treatment showed some enhancements in their visual acuity, which refers to the ability to perceive details of objects at a distance, but their acuity did not fully recover. Additionally, they demonstrated only slight alterations in the organization of the white matter pathways associated with the early stages of the visual system.

According to Sinha, the idea that plasticity is limited to a certain time period and that beyond that, there can be little improvement, is applicable to low-level visual function such as acuity. However, for higher-order visual skills, such as the ability to distinguish a face from a non-face, the researchers observed behavioral improvements over time, and there was also a correlation between these behavioral improvements and the anatomical changes observed in the brain.

Benefits of treatment

According to the study, the researchers discovered that younger children who had cataracts removed showed greater and faster improvements in face-perception ability than older children. Nonetheless, all of the children demonstrated some degree of improvement in this skill, as well as changes in the structure of the white matter.

Sinha suggests that the findings provide evidence that older children can benefit from this type of surgery and support the need for it to be made available to them.

According to Sinha, the results indicate that children of all ages can benefit from this surgery, and thus, it highlights the importance of offering them medical treatment regardless of their age. He also adds that since the brain has exceptional capabilities to reorganize and alter its structure, it is crucial to take advantage of its plasticity by providing early intervention to children with visual impairments.

Sinha's lab is currently analyzing more imaging data from patients in Project Prakash. They are conducting a study to determine whether there are any changes in gray matter thickness in the sensory processing areas of the brain after treatment. They are also using functional MRI to locate visual functions such as face perception and compare them to the areas of the brain where they appear in people born with normal vision.

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The research was funded by the National Eye Institute.