Harvard University researchers and colleagues have found that the neural architecture in the auditory cortex—the part of the brain that processes sound—is virtually identical in profoundly deaf and hearing people. The study raises a host of new questions about the role of experience in processing sensory information, but could point the way toward potential new avenues for intervention in deafness.
An article published in the July 18, 2016 edition of Scientific Reports, was written by co-authors Ella Striem-Amit, a postdoctoral researcher in Dr Alfonso Caramazza’s Cognitive Neuropsychology Laboratory at Harvard and the lead author on the paper; Mario Belledonne, MD, PA, Harvard; Jorge Almeida, PhD, University of Coimbra; and Quanjing Chen, Yuxing Fang, Zaizhu Han, and Yanchao Bi, PhD, Beijing Normal University.
In an article by Peter Reuell for the Harvard Gazette, the researchers explained that their study’s aim, in part, was to better understand what causes the brain to organize the way it does. They wanted to find out how important each person’s experience is for their brain development. In audition, a lot is known about brain organization in hearing people and in animals, but it is not known if the same organization is retained in congenitally deaf people. The similarities between deaf and hearing brain architecture, noted lead author Striem-Amit, suggest that the organization of the auditory cortex doesn’t critically depend on experience, but is likely based on innate factors. So in a person who is born deaf, the brain is still organized in the same manner.
The similarities between deaf and hearing brains do not necessarily suggest that experience plays no role in processing sensory information. Evidence from other studies has shown that cochlear implants are far more successful when implanted in toddlers and young children than when implanted in older patients who have spent more time without auditory input, suggesting that without sensory input during key periods of brain plasticity in early life, the brain may not process information as effectively.
Mapping the Auditory Cortex
To understand the organization of the auditory cortex, Striem-Amit and her collaborators obtained what are called “tonotopic” maps showing how the auditory cortex responds to various tones. The researchers placed study volunteers in an MRI scanner and played different tones — some high frequency, some low frequency — and tracked which regions in the auditory cortex were activated. The team also asked both hearing and deaf study participants to simply relax in the scanner, and tracked their brain activity over several minutes. This allowed the researchers to map which areas were functionally connected or showed correlated patterns of activation to each other. The research team used the areas showing frequency preference in the tonotopic maps to study the functional connectivity profiles related to tone preference in the hearing and congenitally deaf groups and found them to be virtually identical.
“There is a balance between change and typical organization in the auditory cortex of the deaf,” said Bi, the senior researcher. “But even when the auditory cortex shows plasticity to processing vision, its typical auditory organization can still be found.”
The researchers acknowledge that the study raises a host of questions that have yet to be answered. They have established that the architecture is in place, but haven’t yet determined if it serves a function. They know, for example, that the auditory cortex of the deaf is also active when they view sign language and other visual information. The question is: What do these regions do in the deaf? Are they actually processing something similar to what they process in hearing people, only through vision?
The researchers report that in addition to their previous studies of deaf animals, their previous studies of people born blind suggest clues to the puzzle. In the blind, the topographical architecture of the visual cortex (the visual parallel of the tonotopic map, called the “retinotopic”) is like that in the sighted. Beyond topographic organization, regions of the visual cortex show specialization in processing certain categories of objects in sighted individuals; the same specialization in the congenitally blind when stimulated through other senses. For example, the blind reading Braille, or letters delivered through sound, process that information in the same area used by sighted subjects in processing visual letters.
“The principle that much of the brain’s organization develops largely regardless of experience is established in blindness,” Striem-Amit said. “Perhaps the same principle applies also to deafness.”
Source: Harvard Gazette, Harvard University; Nature, Scientific Reports
Image credits: Scientific Reports, Harvard University, © Leszek Glasner | Dreamstime.com