In a nutshell: Researchers have found a brain pathway that quickly transmits visual information, helping to settle a debate about its existence in humans.

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To evade danger, we need to detect and respond to threats quickly. These actions involve several different areas of the brain, including the amygdala – a small, highly connected structure that is responsible for coordinating fear and emotional responses.

When rodents hear sounds of danger, auditory signals are transmitted along a brain pathway to the amygdala – even if the brain region normally responsible for processing sound has been damaged. Whether a similar pathway exists for humans, and for visual information, is a topic of longstanding debate.

The location of the amygdala, deep within the brain, makes it difficult to study. But such a pathway could explain the phenomenon of ‘blindsight’ – the ability of some blind people to react to sudden movements or facial expressions without being able to see them. Although people with blindsight have vision loss as a result of damage to the primary visual cortex (V1), certain visual information seems to reach their brain through an independent pathway.

Queensland Brain Institute researcher Jessica McFadyen and her colleagues, Brain Function CoE chief investigators Jason Mattingley and Marta Garrido, looked for evidence of this pathway in humans. Using 3D modelling based on detailed brain scans from more than 600 people with undamaged V1 regions, they mapped connections between cells across the brain. In every single case, the researchers were able to reconstruct a pathway from the brainstem (which controls the flow of information between the body and the brain) to the amygdala.

Having found the pathway in humans, the researchers wanted to determine if it was involved in behaviour. They examined behavioural data from experiments in which participants were shown images of human faces and tested on their ability to recognise different expressions – fear, anger, happiness, sadness or neutral. The participants’ brain activity was measured as they completed the task, and the researchers used these measurements to make computer models of the blood flow in their brains.

The researchers found that when the participants looked at images of fearful or angry faces, the blood flow along the pathway increased. The stronger the connections were along the pathway, the better the participants were at recognising fear – but not other negative emotions, such as sadness or anger.

The discovery of the alternate pathway in humans settles a longstanding debate. In addition to explaining blindsight, it could also have implications for conditions such as autism and anxiety, which often affect how people recognise fear.

Next steps:
The researchers are studying whether this pathway is involved in sending rapid signals in the brain when we encounter an unexpected threat. They are also using the same experimental approach to study a person with blindsight, to see how the pathway to the amygdala has changed over time.

McFadyen, J., Mattingley, J. B., & Garrido, M. I. (2019). An afferent white matter pathway from the pulvinar to the amygdala facilitates fear recognition. eLife, 8, e40766. doi: 10.7554/eLife.40766

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