Do the Brains of Blind Persons "Rewire" or Adjust to Significantly Enhance the Other Senses? New Research Says Yes
by Maureen Duffy
An age-old question that surfaces regularly in my work is this one: "Is it true that blind people develop super senses, like extra-sensitive hearing or touch, to compensate for not being able to see?"
A variation of the "super senses" question asks this: "Are the other senses truly enhanced, or do people without the sense of sight – and the input it provides – learn to pay closer attention to information received through the other senses?"
Indeed, researchers, scholars, and philosophers have addressed this elusive question for many years:
- In 1749, Denis Diderot, the Enlightenment philosopher and encyclopedist, presciently described blind persons, and blindness itself, as worthy of scientific and philosophical inquiry in his 1749 Letter on the Blind for the Use of Those Who See.
- Much scientific inquiry has focused on the process of echolocation: Could it one day become a "complete sensory replacement" for sight? Echolocation is used by some blind persons to navigate independently within a variety of environments by actively creating sounds (snapping the fingers, making clicking sounds with the mouth and tongue) and then interpreting the sound waves as they are reflected from nearby objects.
- Cognitive scientists are investigating visual-to-auditory sensory substitution devices that could someday substitute for vision.
Most recently, neuroscience researchers from Harvard Medical School and Boston University have revealed that the brains of people who are born blind or became blind in early childhood are wired differently from the brains of people who were born with sight. Their study describes, for the first time, the combined structural, functional, and anatomical changes in the brain that are evident in people born with blindness that are not present in the brains of sighted persons. As a result of their research, they conclude that the brain is able to "rewire" itself when visual information is not available.
The Brain "Rewiring" Study
This new blindness and the brain study, titled Multimodal magnetic resonance-imaging [MRI] reveals large-scale structural and functional connectivity changes in profound early blindness, has been published online in the March 22, 2017 edition of PLoS ONE, an international, peer-reviewed, open-access online journal, published monthly by the Public Library of Science (PLoS). The PLoS is a non-profit organization of scientists and physicians who are committed to making the world's scientific and medical literature a freely available public resource.
The authors are Corinna M. Bauer; Gabriella V. Hirsch; Lauren Zajac; Bang-Bon Koo; Olivier Collignon; and Lotfi B. Merabet, from the Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, MA; Boston University School of Medicine; and the University of Trento, Italy.
About the Brain Plasticity Research
Excerpted from Brain 'rewires' itself to enhance other senses in blind people, via EurekAlert!, the Global Source for Science News:
The researchers used MRI multimodal brain imaging techniques to reveal these changes in a group of 12 subjects with early blindness (those born with or who have acquired profound blindness prior to the age of three), and they compared the scans to a group of 16 normally sighted subjects (all subjects were of the same age range).
[Editor's note: Magnetic resonance imaging (MRI) uses a strong magnetic field and radio waves to create detailed images of the organs and tissues within the body, including the brain.]
On the scans of those with early blindness, the team observed structural and functional connectivity changes, including evidence of enhanced connections, sending information back and forth between areas of the brain that they did not observe in the normally sighted group.
These connections that appear to be unique in those with profound blindness suggest that the brain "rewires" itself in the absence of visual information to boost other senses. This is possible through the process of neuroplasticity, or the ability of our brains to naturally adapt to our experiences.
The researchers hope that increased understanding of these connections will lead to more effective rehabilitation efforts that will enable blind individuals to better compensate for the absence of visual information.
"Our results demonstrate that the structural and functional neuroplastic brain changes occurring as a result of early ocular blindness may be more widespread than initially thought," said lead author Corinna M. Bauer, Ph.D. "We observed significant changes not only in the occipital cortex (where vision is processed), but also areas implicated in memory, language processing, and sensory motor functions."
"Even in the case of being profoundly blind, the brain rewires itself in a manner to use the information at its disposal so that it can interact with the environment in a more effective manner," said senior author Lotfi Merabet, O.D., Ph.D. "If the brain can rewire itself - perhaps through training and enhancing the use of other modalities like hearing, and touch and language tasks such as braille reading - there is tremendous potential for the brain to adapt."
More about the Study from PLoS
Excerpted from Multimodal MR-imaging reveals large-scale structural and functional connectivity changes in profound early blindness, with full text, graphs, and figures available online as an open-source article:
Ocular blindness has served as an important model in helping to understand the consequences of sensory deprivation on brain development. Extensive work in animal models has provided compelling anatomical and behavioral evidence regarding the dramatic neuroplastic changes that result from altering visual experience.
In humans, there has been considerable interest in relating neuroplastic changes with compensatory behaviors observed in individuals living with profound blindness. Indeed, there is mounting support that blind individuals (particularly, when blind from birth or very early in life) demonstrate comparable, and in some cases even superior, behavioral skills as compared to their sighted counterparts.
[Editor's note: Neuroplasticity refers to the ability of our brains to adapt naturally to our experiences and specific life needs.]
Taken together, a contemporary view suggests that these compensatory behaviors may be intimately related to underlying changes in the overall structural and functional organization of the brain. This reorganization implicates areas responsible for the processing of intact senses such as touch, hearing, and smell. At the same time, there is also evidence of cross-modal reorganization within occipital cortex; that is to say, the area of the brain normally ascribed to processing visual information.
Specifically, numerous neuroimaging studies have demonstrated that blind individuals show robust activation within occipital cortical areas while performing a variety of nonvisual tasks (e.g. braille reading, sound localization, and odor perception), as well as higher order cognitive tasks including language processing and verbal memory recall.
The Study Participants
A total of 28 subjects were recruited for the study and separated into two groups comprised of 12 early blind (6 females, mean age 33.58 years) and 16 normally sighted controls (8 females, mean age 30.44 years). Comparing demographic factors between both groups revealed no statistically significant differences in terms of age or gender. For the purposes of this study, we defined "early blind" as documented residual vision no greater than light perception and/or hand motion acquired prior to the age of three (i.e. prior to the recall of visual memories and the development of high level language function).
While the majority of participants had diagnoses that could be considered as a "congenital" cause, we relied on documented clinical evidence of profound blindness based on a structured and functional assessment. The etiologies of blindness were varied and included retinal dystrophies as well as ocular malformations. However, no single diagnosis was represented in more than three subjects.
All blind participants were highly independent travelers, employed, college educated, and experienced braille readers. They were predominately right handed (based on self-report), but most used two hands for the purposes of reading braille text. Sighted controls had normal, or corrected-to-normal, visual acuity. Apart from blindness, the participants had no documented history of neurological abnormalities.
Interpretation of the Study Findings
As mentioned earlier, there has been considerable interest in relating compensatory behaviors in blind individuals within the context of observed structural and functional neuroplastic changes in the brain. While compensatory abilities in the blind have been reported across a wide variety of behavioral tasks and implicating different sensory modalities, it is important to note that these abilities are not universally evident across the blind population. Indeed, while there is evidence of enhanced sensory and cognitive task performance, there are also other reports suggesting that the blind are equal or even impaired on certain tasks compared to the their sighted counterparts.
This suggests that the absence of visual experience can induce either sensory compensation or the absence of calibration depending of the task-cognitive domain at play. Specifically, comparable (or even superior) perceptual and cognitive processing abilities in the blind (through the use of intact sensory modalities) would be in line with a "compensatory" hypothesis of neuroplasticity.
Recent work has also highlighted important differences in functional connectivity derived from resting state sequences with that of functional connectivity that characterizes task-specific activity. More specifically, it has been suggested in a number of reports that differences in connectivity patterns between populations are task dependent and that task related and resting state functional connectivity in fact do not coincide.
This provides further caution against inferring differences in functional integration between brain regions solely based on resting-state data. In this direction, it has been shown that whole-brain functional connectivity networks can fundamentally change in different task contexts and thus are not constrained by networks identified solely by resting-state analyses.
… To help resolve these inconsistencies, further studies should compare metrics of brain structure, white matter structural connectivity, along with measures of effective connectivity as well as individual parametric measures of behavioral performance. Again, by leveraging the advantages of multi-modal imaging, we are more likely to better understand how structure, connectivity, and behavior are reciprocally linked.
What Does the Future Hold?
According to a review of the research at Gizmodo, the researchers did not perform any behavioral or sensory tests that would determine if their subjects truly had heightened senses of touch, hearing, smell, but admit it's an area ripe for future examination:
Looking ahead, [the research team] would like to use this study as a model for studying brain changes in a different population of individuals with visual impairments, specifically those with visual dysfunctions not because of damage to the [eye], but because of early developmental brain damage to areas of the brain responsible for visual processing. This condition, known as cortical or cerebral visual impairment (CVI), is the leading cause of pediatric visual dysfunction in developed countries around the world.
VisionAware will continue to report on the progress of this research as results become available.
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