‘Blindsight’ explained: there’s a neural shortcut
Research confirms existence of an alternative brain pathway. Nick Carne reports.
Australian researchers have confirmed the existence of a pathway in the brain that enables some blind people to detect and respond to visual stimuli they can’t 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 by the normal route. However, some people who report an inability to see are still able to navigate and react to sudden movements or facial emotions correctly and above chance.
It’s a phenomenon called “blindsight”, but has been something of a mystery.
A much-debated possible explanation has been that an independent pathway in the brain bypasses the visual cortex, carrying information from the eyes to the thalamus and from there straight to the amygdala, a brain region responsible for processing emotional information.
Such a “neural shortcut” would allow rapid threat detection independent of visual perception, but to date the only direct evidence of it has been in rodents.
Now, Marta Garrido and colleagues at the Queensland Brain Institute have confirmed that it exists in humans.
They did so by looking at the magnetic resonance imaging (MRI) brain scans of 622 people with functioning visual cortices and analysing diffusion imaging data, which allows neural connections to be examined in three dimensions.
“Amazingly, we were able to reconstruct this pathway in every person,” Garrido says.
While the imaging demonstrated the existence of the structure, the researchers were still not certain it had a direct impact on behaviour, so they looked at functional MRI (fMRI) data from a facial recognition task to see if there was a link.
The task measured the brain’s response to viewing faces with emotions such as anger, fear, happiness, neutrality, and sadness.
“We saw that people who had greater connectivity in this pathway were better at recognising fear in people’s faces, and that this was specific to fear, but not other negative emotions like sadness or anger,” Garrido says
“The denser this pathway to the amygdala was, the better people were recognising fear. So that was very exciting; we showed that it exists, and has a bearing on behaviour.”
Garrido says the finding settles the debate about how the brain processes visual information and, by extension, explains biological phenomena such as blindsight; but it also raises questions about why the brain evolved to have alternative pathways that go parallel to each other.
“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,” she says.
“You ask someone with blindsight how they know where to navigate to, and they will tell you, I don’t know, I just had a feeling. And understanding where that feeling comes from is fascinating.”
The findings are published in a paper in the journal eLife.