The world is a scary place, full of scary clowns, scary spiders, and scary things hiding inside that scary closet at night.
We all know fear. But so far, what happens inside the human brain during scary situations is still a big mystery.
Ajay Satpute, an assistant professor of psychology at Northeastern, is peering into people’s heads, probing their brains as they face their fears.
“Is there one brain circuit that controls fear across all the situations in which people feel fear and across different individuals?” says Satpute, who runs the Affective and Brain Sciences lab at Northeastern. “Or is it really idiosyncratic and unique and specific to each person?”
Using machine learning techniques to analyze brain scans, Satpute spots changes in blood flow inside the brains of people as they watch videos of spiders, precarious heights, and other things that evoke fear. Spotting these brain activation patterns can serve as the foundation for a model to trace the neural basis of fear, he says.
On Satpute’s computers, fear of spiders and heights both show brain activity that is distributed in similar brain regions. But, he says, these circuits show strikingly different patterns. The circuits that could trace what fear of spiders looks like in the brain are of little use for tracing fear of heights.
“There are thousands of brain circuits—millions and billions—where brain activations happen,” Satpute says. “Their combined pattern is what ends up creating this experience of fear in our studies. But it’s not like there is an area for fear, which would make it very easy to describe fear if that was true.”
Old studies of fear and anxiety in rodents led to the notion that a circuit center within the amygdala, an important group of nerve cells packed into two tiny almond shapes near the center of the brain, is consistently engaged across all types of fears and under different circumstances.
But applying those conclusions to the human brain isn’t always as straightforward, Satpute says. The human-rodent difference is particularly evident in people with amygdala damage.
“Some of them have a disruption to fear, but they also have disruption to a lot of other things,” Satpute says. “There’s a bunch of neuroimaging studies that show activation in the amygdala during fear, but many of them don’t.”
Some people like scary. They watch horror movies, tell ghost stories, and keep up with traditions that celebrate the supernatural. But it’s not that Satpute and his lab like scary things, per se.
Fear, he says, might be the best emotion to start relating what we think of as the mind with what we know of as the brain: first, because fear is a potent experience that has been induced in a robust number of controlled studies; second, because fear is triggered by different stimuli, such as predators and heights; and third, because not all fears are created equal. (Some people could be afraid of 2020, and others could fear that new hole on the iPhone.)
“Understanding the neural basis of fear in particular also addresses whether all people feel fear in the same way,” Satpute says. “Whether humans and non-human animals share homologous pathways for fear or not, and related translational questions for fear and anxiety disorders.”
The idea is that fear could also serve as a model for the neural processes behind sadness, happiness, and other emotions.
“If fear doesn’t have its own unique circuit, then we shouldn’t expect any emotion to,” Satpute says. “It really is the quintessential emotion for these questions.”
And to find answers to these questions, fear of heights might be particularly instrumental.
Conventional research has approached fear in the context of predator-and-prey situations. But heights involve no obvious predator. If there really were a common core pattern for brain activity in fear, brain activation patterns involving different types of fears would be markedly similar. But it just isn’t that way, Satpute says.
Same goes for fears that involve social interactions and no real predator, such as talking in front of a crowd. “One of the biggest fears people have is social evaluation,” he says. “Sometimes it ranks up there with fear of death.”
Why is it such a challenge to find fear in the human brain?
Cognitive neuroscientists like Satpute take subjective experiences (the mind) and relate them to processes inside their engine (the brain). People can feel fear and be sure it is real and powerful, but without a physical foundation, he says, emotions can’t be directly observed.
You could take the brain and look at all its parts, the blood flow, neural connections, and even model activation patterns to fire up other circuits in the network. All that, Satpute says, could tell us a lot about the brain as an organ, and practically nothing about the mind, which “is one of the things we really care about as humans.”
“On the one hand you have this physical thing that lives inside your skull,” Satpute says. “And on the other hand, you have this thing called the mind, which is only self-evident because we experience its phenomenology.”