Researchers have found a short cut in the brain that allows some blind people to navigate the world and react to sudden movements – as if they can actually see what’s in front of them – and to recognise facial expressions. This is a condition known as blindsight.
The pathway also explains how sighted people unconsciously, reflexively respond to a threat they haven’t seen, at least consciously.
This, then, may explain experiences that people ascribe to a “sixth sense” – feelings they can’t otherwise account for.
How the blind can see
Blindsight occurs when a person’s primary visual cortex is damaged by an injury or a stroke, but the eyes are still functional.
The vision loss occurs because the visual cortex can no longer process or receive input from the eyes.
However, the information taken in through the retina is somehow processed elsewhere in the brain. The person can’t see or consciously respond to what’s happening in the world – but the body responds as if the person can see what is in front of them.
For more than 10 years, Dr Marta Garrido, Affiliated Research Fellow at the Queensland Brain Institute, has been looking for an explanation as to how the brain accomplishes this kind of magical thinking.
Dr Garrido’s theory was that an independent pathway exists in the brain that bypasses the visual cortex and instead carries information from the eyes to the thalamus, and then directly to the amygdala, a primitive region in the brain that processes emotional information.
This neural short cut would allow “rapid threat detection independent of visual perception,” she said.
The primal brain, at work before you know it
From an evolutionary point of view, this would make sense in terms of surviving imminent danger.
“If you were threatened by an animal in the jungle, the threat would take a longer time to process in the visual cortex: Working out what kind of animal it is, when the more important information isn’t in that level of detail” Dr Garrido told The New Daily.
“So this pathway would serve as a short cut to convey the information more quickly.”
Such a short cut has been demonstrated in rodents, but has rarely been shown in the human brain – and the theory remained controversial.
Last week, Dr Garrido and colleagues published a study that found the proposed pathway exists in humans with normal vision, linking the retina through two subcortical regions to the amygdala, hence bypassing the cortex.
They found the short cut by looking at the MRI brain scans of 622 people with functioning visual cortices and analysing diffusion imaging data, which allows neural connections to be examined three-dimensionally. They were able to reconstruct the pathway in every person.
The first region is called the superior colliculus. “It’s a very ancient region of the brain, conserved across vertebrates, with direct inputs from the retina,” Dr Garrida said.
“It responds to visual stimuli and directs eye movements.”
The other region is called pulvinar, which is part of the thalamus that is thought to be a relay station of sensory information to the rest of the brain.
Having established the existence of the pathway existence of the structure, the researchers then investigated if the short cut had a direct impact on behaviour.
Dr Garrido and her PhD student Jessica McFadyen then looked at functional magnetic resonating imaging 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.
They found 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”.
In other words, people with a stronger short cut to the amygdala were better at recognising fear.
Where that weird feeling comes from
The research has important implications for settling a long-standing scientific debate about how the brain processes visual information and, by extension, explaining biological phenomena such as blindsight.
“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,” Dr Garrido said.
The researchers are now planning experiments with a subject who became blind after two strokes that left him without the left and the right visual cortex.
“We want to examine the brain circuitry underpinning certain visual residual abilities, such as being able to recognise emotion in people’s faces,” Dr Garrido said.
“We expect that brain plasticity has occurred in this subcortical pathway as a consequence of the cortical route being disrupted.”