An Australian boy missing the visual processing centre of his brain has baffled doctors by seeming to have near-normal sight.
The 7-year-old, known as “BI”, lost his primary visual cortex shortly after he was born due to a rare metabolic disorder called medium-chain acyl-Co-A dehydrogenase deficiency.
Normally, the primary visual cortex is crucial for sight because it processes electrical signals relayed from the eyes. People with damage to this area are said to have “cortical blindness”.
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However, BI has remarkably well-preserved vision, says at Monash University in Melbourne, Australia. “You wouldn’t think he is blind,” he says. “He navigates his way around without any problems and plays soccer and video games,” he says.
In a series of tests run by Mundiñano and his colleagues, BI scored perfectly when asked to name objects, identify colours, and discriminate between different people’s faces displayed on a screen. He could also identify whether a face was happy, fearful or neutral, and reach out and grasp different-sized blocks placed in front of him.
Blindsight
The only major flaw in BI’s vision was strong short-sightedness. He was only able to read the top letter on an eye chart when standing 3 metres away or closer.
People with damaged visual cortices have previously been found to sometimes have a degree of unconscious visual awareness, known as “blindsight”. For example, some are able to navigate around an obstacle course even though they don’t consciously feel like they can see.
BI’s case is astonishing because he is the first person with no visual cortex to be identified as having nearly normal conscious vision, says Mundiñano, who presented the case at the of the Australasian Neuroscience Society in Sydney this week.
This is probably because he lost his visual cortex in the first 2 weeks of his life, when his brain was still highly malleable and adaptable, says Mundiñano.
Flexible brain
Using MRI brain scanning, the team found that another area of the boy’s brain appears to have taken over the role of seeing. Compared to other boys his age, he had more neural fibres running between two centres near the visual cortex at the back of his brain, called the pulvinar and middle temporal area.
The pulvinar is part of the thalamus, which is normally involved in relaying sensory signals, while the middle temporal area helps detect motion.
Previous experiments have found that monkeys also retain many of their visual abilities if their primary visual cortex is damaged when they are in early infancy. Similarly to BI, this preservation is associated with between the pulvinar and middle temporal area.
Together, these findings suggest that newborn brains can learn to reroute visual information via other centres if the primary visual cortex is missing, says Mundiñano. “Younger brains just tend to recover and adapt much better.”
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