How scary does this spider seem to you? BC researchers explore how the brain signals varying responses to threats. (Image: Warren Matthews i Pixabay)
Lying awake in bed at night, you hear a strange sound somewhere in the house. Working in the yard, you suddenly happen upon a snake. Sitting at your desk, you glance to your right and see a spider crawling on the wall.
Your brain is all set to handle these kinds of rude surprises: The amygdala and prefrontal cortex estimate the size of the threat, and relay this information to the ventrolateral periaqueductal gray (vlPAG), which then triggers a fear response, such as freezing in place or bolting to a safe distance.
Michael McDannald (Peter Julian)
But the vlPAG may play a bigger role in how an individual responds to threats, according to Assistant Professor of Psychology Michael McDannald: Some studies suggest that, instead of simply generating a fear response, the vlPAG may also be involved in assessing the degree of threat. This expanded view of vlPAG neural activity could have significant implications for treating people who experience acute levels of fear and anxiety, he says—those who feel compelled to search their house for whatever it was that went bump in the night, for example, or who can’t bring themselves to go back in the room where the spider was.
“If the vlPAG helps to signal the size of a threat, rather than simply generating a fear response, this points to a broader neural network for threat estimation,” explained McDannald. “This is important because people with anxiety disorders tend to overestimate threats, and many treatments for anxiety target the brain regions involved in threat estimation.”
McDannald and doctorate candidate Kristina Wright explored this hypothesis about vlPAG neural activity in a recent experiment, results of which they published in the online journal eLife.
Kristina Wright
In the experiment, McDannald and Wright trained rats to associate three different 10-second tones with different probabilities of receiving a mild electric shock to the foot, and recorded vlPAG activity as the rats heard each of the tones. The recordings revealed two distinct groups of vlPAG neurons. Both groups responded most to the tone that signaled danger, less to the tone that signaled uncertainty, and least to the tone that signaled safety.
But McDannald notes that the groups of neurons responded at different times. One was most active at the start of the tone. The activity of this group depended upon the degree of threat, and not upon whether the rat showed a fear response. The second group of neurons increased its activity over the course of each tone. The activity of this group mainly reflected the degree of threat, but also represented the rat’s fear response to a lesser extent.
The amygdala is generally regarded as a key node of dysfunction in stress and anxiety disorders, McDannald says, but the findings from the experiment mark the vlPAG as another crucial site in what is known as “the fear circuit” in the neural network.
“There are still questions to answer,” said McDannald. “We are now looking at how the vlPAG work with the rest of the brain to signal threat. A complete understanding of the brain’s threat estimation circuit will lead to better therapies for maladaptive fear and anxiety.”
Sean Smith | University Communications | November 2019