Study Uncovers Why Congenitally Blind People Show Activity in Visual-Processing Areas of the Brain

Study Uncovers Why Congenitally Blind People Show Activity in Visual-Processing Areas of the Brain

January 23, 2023
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A Brazilian study recently published in the journal Human Brain Mapping identified for the first time the reorganization of anatomical areas in the brains of patients with congenital blindness. The study was jointly conducted by the D'Or Institute of Research and Education (IDOR), the Federal University of Rio de Janeiro (UFRJ), and the Center for Specialized Ophthalmology in Brazil.

Several decades ago, scientific investigations reported the intriguing discovery that people born blind may engage the vision-processing areas of the brain, the occipital cortex, when engaging in a non-visual activity, such as Braille reading (a tactile language system). These findings provided more evidence of brain plasticity, or the brain's ability to reorganize its connections in response to adversity. This process may entail a variety of structural changes, such as the formation of new brain pathways or the reorganization of existing connections.

“Soon after we are born, we are exposed to stimuli captured by our senses, which are fundamental to determine the brain’s circuitry. It is also a time in which our brain is in great transformation. Technically we could think that the occipital cortex would be functionless in people who were born blind, but we know that this is not the case. It is activated. What we lacked to understand was the structural process behind it,” explained Dr Fernanda Tovar-Moll, corresponding author of the current study and president of IDOR.

Magnetic resonance imaging techniques were utilized in the study to examine structural connectivity in the human brain and the possibility of alternative neural connections. The brain images of ten people with congenital blindness and Braille readers were compared to those of ten people with normal vision.

The scientists observed structural changes in connectivity in the thalamus, a structure located in the diencephalon, the central region of the brain that receives, processes, and distributes information captured by the main human senses - such as vision, hearing, and touch - to the different brain regions after detailed analysis.

Credit: All rights belong to the D'Or Institute for Research and Education (IDOR)

The yellow box (left) depicts thalamic areas that exhibited increased connectivity with the temporal cortex, including MGN, LGN and pulvinar bilaterally. The blue box (right) depicts thalamic territory that exhibited decreased connectivity with the occipital cortex in congenitally blind individuals, namely the left pulvinar/lateral posterior nucleus. The white box (middle) shows thalamic territories obtained from an atlas based on the Colin27 Average Brain58 and depicts the location of LGN (green), MGN (dark pink), pulvinar (red), medial dorsal (yellow), ventral anterior (orange), anterior (purple), and lateral posterior (light pink) nuclei. A graphical overlay (dark blue) of thalamic areas that exhibited both increased connectivity to the temporal cortex and decreased connectivity to the occipital cortex (p<0.05, FWE-corrected) in CB individuals is shown (white box, bottom). L = left; R = right; A = anterior; P = posterior. The coordinates are given according to the MNI space and plotted on the MNI standard brain. Color bars represent the t-value. Credit: All rights belong to the D'Or Institute for Research and Education (IDOR)

Unlocking the Brain's Secret to Recognizing and Overcoming Adversity

“Plasticity has been the research focus of our group for many years now, and in this case of cross-modal plasticity in congenitally blind people, in which distant areas of the brain present this communication, we suspected that the phenomenon would be originating in the thalamus, as it is the brain structure responsible for connecting several cortical regions, and it could be an area that with little change in the axonal circuitry [part of the neuron responsible for conducting electrical impulses] would be able to connect cortices that were distant from one another,” commented the neuroscientist.

The study also discovered that in blind people, the area of the thalamus dedicated to connecting with the occipital cortex (vision) was smaller and weaker, making room for connections with the temporal cortex (hearing), which were shown to be strengthened when compared to those observed in people without visual impairment. This means that, in addition to being active, the visual cortex is infiltrated by connections that sharpen other senses like hearing and touch.

Revolutionary Discovery: Alternative Mapping of Thalamus Connectivity with Occipital and Temporal Cortices

It was the first time that a study described an alternative mapping in the connectivity of the thalamus with the occipital and temporal cortices in humans, and these plastic reorganizations may be a mechanism capable of explaining how non-visual stimuli reach and activate the visual cortex in people who are born blind.

“Neuroimaging studies allow us to navigate the structure of the brain and better understand the diversity of brain plasticity, which can also pave the way for discoveries such as new visual rehabilitation initiatives”, added Dr Tovar-Moll, informing that her research group is still involved in other studies with congenitally blind people in which they investigate, in addition to the structure, the functional adaptations of brain plasticity in this population.