Study finds early profound deafness reshapes peripheral vision in the brain

The human brain is creatively adaptable, producing astonishing works such as The Starry Night (twice) and the Large Hadron Collider, which has exposed other aspects of reality, including neon-20’s bowling-pin-shaped atomic nucleus.

It is also physically adaptable, with plasticity capable of altering neuronal connections, cell types and signalling pathways.

Such alterations may stem from neurological disorders, the still-unavoidable effects of ageing, or even from eating tomato paste for three months.

Sensory experience can likewise drive brain plasticity. A broad body of scientific research, for instance, has documented changes in the brain and enhanced visual abilities among D/deaf people.

A recent brain-imaging study indicates that adults who have experienced profound deafness since early childhood assign a comparatively greater share of their early visual system to the outermost parts of their visual field.

In a paper published in PNAS, UK psychologists and neuroscientists found that this ‘reorganisation’ seems to begin remarkably early in the visual pathway, before signals reach the brain’s principal visual centre.

This effect may matter for recognising possible dangers.

"Without sound cues, deaf people use vision as an 'early warning system' for peripheral events," the researchers say.

"Indeed, D/deaf adults exhibit superior performance in peripheral tasks involving detection or attention to transient or moving targets."

Brain plasticity in D/deaf people

The team enrolled 16 adults with early, profound deafness and 16 age-matched hearing participants as a control group.

They used structural magnetic resonance imaging (MRI) to examine neural anatomy, alongside functional MRI for retinotopic mapping. This approach identifies the brain areas that respond to different positions within an individual’s visual field.

Their attention centred on two important regions. One was the lateral geniculate nucleus (LGN) in the thalamus, which acts rather like a relay point for visual information travelling from the retina through the optic nerve.

The LGN sends projections to the primary visual cortex (V1), the main region for processing visual information at the rear of the brain.

The researchers aimed to match brain activity with participants’ real visual fields, then assess how the two groups differed.

Mapping peripheral vision in the LGN and V1

To do this, they calculated the proportion of active voxels - three-dimensional MRI units representing small amounts of brain tissue - across each participant’s visual field. They divided that field into eccentricity ‘bins’: concentric portions of vision extending outwards from central focus.

They then connected brain activity with these visual-field sections to examine the relationship between vision and brain tissue, specifically determining which visual regions of the brain responded most strongly to each part of a person’s sight.

To ensure their findings were not influenced by the way they set brain boundaries, the researchers also compared their manually drawn retinotopic V1 boundaries with two standardised brain atlases, or maps, one of which came from the Human Connectome Project.

The results indicate that, compared with hearing people, D/deaf people have LGN activity that is relatively more weighted towards peripheral vision. This suggests that visual reorganisation in the brain begins at least as early as the LGN.

Interestingly, however, there was no significant difference in LGN volume between the groups.

The neural preference for peripheral vision remained evident when the team looked further along the brain’s visual-processing systems and examined visual-field representations in V1.

Once again, total V1 volume did not differ between the two groups.

Its distribution, however, was different. D/deaf participants showed neural preferences for peripheral vision, whereas hearing participants displayed neural preferences for more central vision.

Importantly, these differences in volume seemed to result from changes in cortical surface area, rather than cortical thickness.

A peripheral-vision trade-off

Overall, the greater lifelong dependence on vision among people with early, profound deafness appears to be associated with compensatory plasticity in the LGN and V1, in response to auditory brain systems being used less.

The research does not demonstrate that D/deaf people have better vision in every circumstance.

Instead, the researchers say there is a visual trade-off, describing "a redistribution of neural resources in early D/deaf individuals, with a larger cortical surface representation of the periphery, at a cost of smaller representations of the central visual field."

In an exploratory analysis that was too small for valid statistical testing, the greatest rise in preference for peripheral vision - together with the corresponding reduction in central vision - was seen in D/deaf participants who learnt British Sign Language (BSL) as their first language.

Drawing on earlier evidence, the researchers note that neural differences in D/deaf individuals may originate even further upstream, in the retina. This light-sensitive, neuron-rich tissue at the back of the eye converts light into electrical signals that travel to the brain through the optic nerve.

Lastly, the researchers point out that some D/deaf participants in the study had genetic deafness, while the cause of deafness was unknown for others. This could indicate the role of an unidentified genetic precursor.

Future research could investigate the connection between genetic deafness, visual enhancement and particular brain-plasticity processes in more detail.

This research was published in the Proceedings of the National Academy of Sciences.

This article was fact-checked and edited by Rebecca Dyer. Although we take pride in our process, we are only human. If you notice an error, please let us know.