Published in Research

Why do patients with congenital blindness show visual processing activity in the brain?

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A joint Brazilian study evaluated thalamocortical connections that cause the cross-modal plasticity found in congenitally blind (CB) individuals.(via)

Give me some background first.

Analyzing the brains of people with congenital blindness allowed investigators to understand how the brain built and maintained its structural organization and hierarchy of processing without one of the major sensory input sources.

Many brain alterations can occur in response to a lack of visual input. Those with CB often experience atrophy of the optic chiasm and reduced microstructural integrity of optic radiations and the geniculocalcarine tract.

Talk about the study.

Researchers looked to investigate the potential for alternative neural connections by using magnetic resonance imaging (MRIs) to analyze the thalamic remapping of cortical connections in people with congenital blindness.

To organize the data, six cortical masks—prefrontal, precentral, postcentral, posterior parietal, occipital, and temporal areas—were predefined using the Harvard-Oxford Atlas. Probabilistic tracking was conducted with a PROBTRACKX tool.

Researchers calculated the number of samples reaching each target mask as a proportion of the total number of samples reaching any target mask. They then performed hard segmentation of each thalamus based on its connectivity to each of the six ipsilateral cortical areas, and then a voxel-wise analysis of the thalamocortical connectivity to compare the results between the two groups. (via)

Who were the participants?

A total of 20 participants were split into two groups: CB group included six males and four females (all who could read Braille) with a mean age of 31.8 ; the control group had a mean age of 32.2 and the same ratio of males and females.

All participants were right-handed, had no history of neurologic or psychiatric diseases, and were not taking brain-active medication.

What were the findings?

Researchers noted structural connectivity changes in the thalamus. Further, the thalamus area that connects with vision was found to be weaker/smaller in CB participants, giving space to connections with hearing (which were strengthened in comparison to the non-visually impaired participants.

Increased thalamo-temporal connectivity was observed, specifically in the medial geniculate nucleus (MGN), lateral geniculate nucleus (LGN), and the pulvinar bilaterally. Additionally, decreased thalamo-occipital connectivity was noted, particularly in the left pulvinar and lateral posterior nucleus.

Differences in white matter microstructure were recorded between the control and CB groups. A whole-brain analysis showed decreased fractional anisotropy (FA) in the major white matter structures connecting the occipital cortex in the CB group. Further analysis restricted to the whole thalamus also revealed a diffuse decrease in FA in the CB group.

Significance?

This is the first time a study noted an alternative mapping in connectivity of the thalamus with the occipital and temporal cortices in humans. This plastic reorganization may be a mechanism that explains how nonvisual stimuli reach and activate the visual cortex in congenitally blind people.

The study authors noted that neuroimaging studies such as this allow researchers to navigate the brain's structure and better understand the diversity of brain plasticity, which can also pave the way for new visual rehabilitation initiatives. (via)