WARNING: This episode contains discussion related to traumatic experiences that may be distressing for some listeners.
Janine excitedly shares details of her new pet dog Calli (#RescueGreyhound), who massively inspired this episode, since Calli displays a very strong “freeze response” in times of stress. Aleena and Janine take a look at some research into the freeze response, which is distinct from the fight or flight response, in both humans and other animals. Essentially, this is where the parasympathetic nervous system takes over during a stressful event, and there can be actual muscle paralysis. It has been documented when the brain assesses that fight and flight are not viable options. Janine also shares loads of cute and insightful stuff about her new rescue greyhound, and Aleena introduces a new segment to the podcast, without clearing it with Janine first…
Mastering of this episode, plus intro and outro music, by the ever-talented Dr Adrian Diery.
Can you trigger a human freeze response in the lab?
Aleena went right back to 2008 and to some of the earlier research into the freeze response in humans. In this somewhat creepy study, 404 participants went through a “CO2 challenge”: they wore a breathing apparatus and inhaled air with 20% CO2. Why? Well, breathing in air enriched with CO2 can lead to symptoms like breathlessness, rapid heartbeat, chest pain and dizziness – the kind of things that might make you feel a little panicked.
Before the CO2 challenge, few participants reported modest or greater desire to flee or feelings of immobility (i.e. the freeze response), and wanting to flee or freezing happened at about the same rate.
After the challenge, both the desire to flee and the freeze response more than doubled. But, these responses were still seen only in a minority of participants. The freeze response was significantly less common than the desire to flee. So, the CO2 challenge seemed to increase both the desire to flee and the freeze response, but it seemed to increase the desire to flee to a greater extent.
Since most participants reported NO strong desire to flee or freeze, it’s clear that there’s plenty of variability in how people response to threatening events. Many other psychological and situational factors are at play.
But just think, if the freeze response can be triggered in a controlled lab experiment where there’s no REAL threat to life, it’s probably underestimating of how frequent freezing is in real-world, life threatening situations…
Schmidt NB, Richey JA, Zvolensky MJ, Maner JK. (2008). Exploring human freeze responses to a threat stressor. Journal of behavior therapy and experimental psychiatry, 39(3): 292–304.
Heard that saying “deer in the headlights”? Turns out these deer are probably not displaying a freeze response at all. They’re just temporarily blinded by bright lights and in the process of adjusting their vision! More here.
How do sex differences affect the freeze response?
Interestingly, freezing seems to be the dominant stress response in many animals, including in rodents. Freezing has actually been used as the proxy for fear levels in rats in many studies. Janine outlines a study involving fear conditioning in rats.
Fear conditioning is a process where rats are subjected to electric shocks at the same time as a specific sound is played. After several rounds, the rats begin to associate the sound with pain. Eventually, playing the sound alone is sufficient to elicit a fear response. The fear conditioning phase is followed by an extinction phase where the sound continues to be played, without the electric shock, and eventually the fear response is extinguished.
The researchers measured the rats’ fear responses – did they freeze or did they dart? (Here, darting is analogous to “flight”.) They found significant differences between male and female rats. Females were four times more likely than males to display a darting response. This throws into question the previous assumption that freezing was a good way to measure stress levels in rats – this seems to be the case for males only.
Although darting was the dominant response in females, there was still a small group of females that did display a freeze response. The researchers looked into this a little further and found that females who darted recovered faster, than females who froze (in the extinction phase). This suggests that darting may be a better response overall and lead to better resilience in rats.
This study highlights the reality of both sex-based, and individual differences, in stress responses. It would be really cool to establish if a rat’s preference for darting versus freezing has a genetic basis, and is therefore heritable. This would suggest it may have been important in terms of rat evolutionary history, and that it would be a potential target of natural selection in the future. A cool companion commentary article in available here.
Although this study is in rats, the authors point out some of the broader implications, including that human females are two times more likely to suffer from Post-Traumatic Stress Disorder (PTSD) – a possible long-term response to a stressful and traumatic events. Understanding sex-based differences in fear responses is clearly very important. If we can understand how and why an individual responded to a traumatic event, this could greatly inform effective treatments.
Gruene TM, Flick K, Stefano A, Shea SD, Shansky RM. (2015). Sexually divergent expression of active and passive conditioned fear responses in rats. eLife, 4:e11352.
What brought out our inner square?
Aleena went rogue and hijacked her inner square segment to update us on her kitchen renovation dramas. You may have heard about this in our Muck Up Day episode?! The *unnamed random Swedish furniture store* is at it again. Warning: “How is Aleena’s kitchen renovation going?” may become its own ongoing segment.
Janine gushes more about her new rescue greyhound Calli. Much discussion around: pannus autoimmune eye condition; doggles (dog goggles; see Episode 6); chattering behaviour; existential grappling; plus more!
Image by Andrew Umansky on Unsplash
Enjoying Sister Doctor Squared?
Please rate and review us in your podcast player!
Sister Doctor Squared is now on Ko-fi. If you enjoy listening to the podcast, you might like to “buy us a coffee” to show your support! All donations go towards the costs of making the podcast and no contribution is too small.