Mind & Body

Seeing and Reacting to a Threat Doesn't Happen in the Order You Think

Imagine that you're on a hike in the woods when all of a sudden, you see that you're a few feet away from a snarling mountain lion. Your heart starts to pound, your hands start to sweat, and you can feel that familiar fight-or-flight reaction wash over you. Here's a seemingly foolish question: Which happened first? Was it your visual system processing the image of the threat, or your emotion center kicking into high gear? While it seems obvious that your visual system processed the mountain lion first — how else would your emotion center know? — that's not actually the case. As far as your brain is concerned, threats are far too important to trust them to your visual cortex.

The Eye Zone's Connected to the Fear Zone

In what was probably a great "WTF" moment in science history, a pair of researchers in the early 1990s discovered that rats that had damage to the auditory cortex — the brain's sound-processing center — could still be startled by a sudden loud noise. The conclusion was intriguing: The sense of hearing must take two paths through the brain, one through the auditory cortex, and one somewhere else. That led scientists to wonder if the visual system had a secondary path, too.

Since then, some fascinating research has found all sorts of ways that human and animal brains trigger an emotional reaction without conscious input from the auditory or visual cortex. For example, one prominent study from 1998 showed people faces with neutral expressions interspersed with quick 30-millisecond flashes of fearful expressions. The researchers found that even though people weren't even aware that they'd seen a fearful face, brain scans told a different story: The amygdala, which is the center of emotion processing, lit up at the sight of a fearful facial expression.

So it may not be all that surprising to learn that people with cortical blindness, or blindness caused by damage to the visual cortex, can still react to sudden movements or emotional facial expressions that they shouldn't be able to see. Many can even still navigate their surroundings. One explanation for this so-called "blindsight" is that, just like that second auditory pathway, there's a second visual pathway in the human brain that bypasses the visual cortex and goes from the eyes to the brain relay station known as the thalamus, and directly to the emotion-centric amygdala. There hasn't been much evidence for this controversial theory, however — until now. Last month, researchers from the University of Queensland announced that they'd found evidence for this secondary pathway.

Taking a Shortcut

In the paper, Dr. Marta Garrido and her team point out that scientists have technically captured images of this pathway already — it's just that an image isn't proof of a brain structure's function. To get that proof, the team collected MRI brain scans of 622 people from the Human Connectome Project, all of whom had a functioning visual cortex. For that project, people had undergone brain scans while performing various tasks, one of which involved looking at faces with angry or fearful expressions, then gave consent for those scans to be used by researchers.

Garrido's team began their study by imaging this secondary pathway from the participants' eyes to the thalamus and on to the amygdala, then estimating the density of its connections. Next, they plugged brain data from the faces task into a computer model to figure out the direction information flowed in response to the facial expressions and see if that secondary pathway played a role. It did; when people saw the threatening facial expressions, information flowed from the thalamus directly to the amygdala without stopping by the visual cortex.

To make an even stronger case, the researchers also determined that the denser the connections in this pathway, the better people were at recognizing that faces were fearful.

"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," Dr. Garrido said in a statement. "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."

This gives a solid explanation for why a sighted person's heart might race before they can process the fact that there's a mountain lion nearby and why a blind person might flinch when a basketball whizzes past.

"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,'" Garrido said. "And understanding where that feeling comes from is fascinating."

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Written by Ashley Hamer February 13, 2019

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