Brain reorganization cluttered after sense restored


Alexander Aghayere

Brain reorganization cluttered after sense restored

By Sports & Health Editor

When people are born, their brains are primed to receive instructions on how to wire themselves based on the kinds of sensory input information they receive. However, for individuals born with sensory impairments, the neural blueprint is fleshed out differently—the brain plays to the strengths of the available sensory input.

In a study published Dec. 17 in The Journal of Neurophysiology, researchers from the University of Montreal and University of Trento in Italy followed a severely visually impaired patient as she underwent sight restoration surgery. Before undergoing surgery  to have a pros-      thesis implanted in her right eye, the patient’s occipital cortex—the part of the brain that is generally presumed to be responsible for processing visual input—was registering sound in order to compensate for the lack of input from her sense of sight, although she still registered some residual vision.

According to Matthew Dye, assistant professor of speech and hearing science at the University of Illinois at Urbana-Champaign, crossmodal plasticity—the interchange of brain regions receiving signals from senses they normally would not—is also commonly observed in deaf individuals, where visual information processing routes through the temporal lobes, which are thought to be geared toward picking up sound signals.

“Where [crossmodal plasticity] is most commonly seen is you get sensory deprivation of some kind, so in deaf individuals or blind individuals,” Dye said. “What we know about the brain is while your genes kind of program a lot of brain development, there’s also this big effect of experience in sensory input.”

The study participant, who lived with very low vision since birth, underwent functional magnetic resonance imaging three weeks before the sight-restoration surgery as well as twice after, at six weeks and again after seven months. According to the study, while the crossmodal auditory responses still overlapped with the patient’s visual cortex months after the procedure, there was some change in how the brain responded to sound. 

“When we compared the brain activity maps between sessions, post- and pre-surgery, we [saw] some regions in the so-called normal visual cortex showed a decreased response to these auditory stimuli with time,” said Giulia Dormal, University of Montreal researcher and lead author of the study. “Seven months after surgery, we still found there were strong auditory-driven responses—the brain responses to these auditory stimuli in the occipital cortex.”

According to Dormal, the team found that although the visual cortex did maintain a certain ability to tune itself based on the sensory input it receives, the fMRI from seven months after the surgery reflected that the post-implant reorganization may only be able to harmonize with the new information to a limited extent.

The researchers aimed to test as many aspects of visual perception as possible before and after the procedure, running evaluations of behavior, visual acuity, basic tasks and contrast sensitivity. Dormal said the team also tested higher level skills, such as motor perception and face recognition, because they are regulated by regions of the brain further up in the visual hierarchy.

“Still seven months after surgery she is below normal [vision] except for very raw spatial information,” Dormal said. “Telling us she indeed has better vision, but especially for raw visual information, whereas she has much more trouble with details.”

Depriving the occipital cortex of sensory input changes both its function and structure, according to Abhishek Banerjee, a postdoctoral fellow at the Massachusetts Institute of Technology’s Sur Laboratory.

“There is this window of plasticity, a window of opportunity when you manipulate sensory input, you can still see changes inside the brain,” Banerjee said. “It’s called the critical window.” 

Banerjee said this window of plasticity operates on a kind of bell-shaped curve, as it is controlled by a number of internal and external factors, such as cell type or the kind of sensory input received, which influence how malleable the cortex is and for what length of time. These changes can take place in a short amount of time, especially if they occur during this critical period.

“If you deprive sensory input, there are some very quick but very robust changes that take place,” Banerjee said. “[When] crossmodal interaction is taking place, we understand very little at the cell-type level, in a more reduced manner than fMRI and studies where it’s easy to look at changes like that. But this is an emerging area, and in the next 10 years people will understand more about how different modalities are processed together but at a very high resolution, at the cellular level.”

According to Dormal, the many cognitive shifts that occur in the brain when sensory deprivation takes place are more readily measurable. Behavioral changes become apparent along with sensory compensations that patients make when their senses are impaired. 

“What this study tells us is it gives some interesting perspective, that even in an adult brain there are still changes occurring and the brain is plastic, much less in adulthood than in childhood, the brain is capable of [reorganizing itself] even in adulthood,” Dormal said. “It would be interesting to run group studies in order to measure the impact of this crossmodal plasticity, how this auditory-driven reorganization that occurs in the blind brain impacts visual recovery. [Its ability to] impair visual recovery is only one side of the story. It could also help as well.”