Traditionally, neuroscientists studying the human fear response have proposed a twinned mental process to describe how we deal with scary stuff. This model, the "low road/high road" established in the late 1990s, posits that fearful data takes two simultaneous routes in the brain. The first path – the "low road" – is a quick and dirty assessment system that takes in data quickly (it actually operates fast than vision, so it process fragments of sensory data) and kicks in a protective response. Think of it as your startle response. There are three steps in the low road process:
1. The thalamus – this part of the brain sorts and distributes sensory data
2. The amygdala – this part of the brain decodes emotions, assesses threat levels, and stores fearful memories
3. The hypothalamus – this part of the brain kicks in the "fight of flight" response
The "high road" process adds two steps to the fear response, adding to the overall length of the process and slowing the reaction speed considerably. The "high road" starts at the thalamus, but then goes to the hippocampus, a section of the brain responsible for storing conscious memory and assisting in the analysis of new data. After this, the data loops through the sensory cortex as the hippocampus "checks" its assumptions and conclusions against streaming sensory data. Once the hippocampus has reached a conclusion, it transmits data through the amygdala to the hypothalamus, either confirming the need for a "flight or fight" response or sending an all-clear signal.
For nearly a decade, the dominant theories of how fear works in the human brain have focused on this twinned process. The model has been popularized in the press – forming the basis for the bestseller The Gift of Fear and getting mentions in such pop psych books as Blink - and has even showed up in the pro-am blog circuit, notably in well-intentioned arguments regarding whether or not this has any implications for horror literature and film.
Unfortunately, this relatively simple and elegant model (which I was personally a huge fan of) is about to get more complicated. Researchers led by neuroscientist Larry Swanson of the University of Southern California have established the existence of a fear pathway that does not involve the amygdala. From the report in Science Daily:
The study adds to evidence that primal fear responses do not depend on the amygdala – long a favored region of fear researchers – but on an obscure corner of the primeval brain.
A group of neuroscientists led by Larry Swanson of the University of Southern California studied the brain activity of rats and mice exposed to cats, or to rival rodents defending their territory.
Both experiences activated neurons in the dorsal premammillary nucleus, part of an ancient brain region called the hypothalamus.
Swanson's group then made tiny lesions in the same area. Those rodents behaved far differently.
"These animals are not afraid of a predator," Swanson said. "It's almost like they go up and shake hands with a predator."
Lost fear of cats in rodents with such lesions has been observed before. More important for studies of social interaction, the study replicated the finding for male rats that wandered into another male's territory.
Instead of adopting the usual passive pose, the intruder frequently stood upright and boxed with the resident male, avoided exposing his neck and back, and came back for more even when losing.
"It's amazing that these lesions appear to abolish innate fear responses," said Swanson, who added: "The same basic circuitry is found in primates and people that we find in rats and mice."
What's the take away?
"This is a new perspective on what part of the brain controls fear," he said.
He explained that most amygdala studies have focused on a different type of fear, which might more accurately be called caution or risk aversion.
In those studies, animals receive an electric shock to their feet. When placed in the same environment a few days later, they display caution and increased activity of the amygdala.
But the emotion experienced in that case may differ from the response to a physical attack.
"We're not just dealing with one system that controls all fear," Swanson said.