Australian researchers have discovered a pathway in the brain that allows blind people to "detect and respond to visual stimuli they do not 'see'".
"If a person's primary visual cortex is damaged due to injury or a stroke, vision loss can occur because the brain can no longer receive input through this critical pathway," the University of Queensland team said on Friday.
"Nevertheless, some individuals with a damaged visual cortex experience a phenomenon called 'blindsight': they report an inability to see, yet are able to navigate and react to sudden movements or facial emotions correctly and above chance."
Dr Marta Garrido of the university's Queensland Brain Institute has been studying the phenomena for a decade. She hypothesised there could be a pathway between the eye straight to the amygdala, the part of the brain responsible for processing emotional information - bypassing the visual cortex altogether.
"This subcortical route had never been found in primates before when we started this project," said Dr Garrido.
But they'd found evidence for it in rodents, and now say they've found it in humans.
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Her team looked at MRI scans belonging to 622 people with working vision, and found neural connections between the eye and the amygdala.
"Amazingly, we were able to reconstruct this pathway in every person."
They then looked at MRI data recorded during a facial recognition study, and discovered that people who are better at recognising fear in a person's face have more white matter brain connections in the exact pathway between the eye and the amygdala.
"This paper settles the debate, but it also opens other questions about why the brain evolved to have alternative pathways that go parallel to each other," said Dr Garrido.
"One possibility I think is redundancy: it is useful to have redundancy mechanisms in the brain, so if one thing fails, for example in the case of stroke, then we still have an ability to process things that are really, really important, like danger and navigation."
The research was published in journal eLife.