“Fearless” climbers: how the amygdala mediates fear
Rock climbing, both in a specialized gym and outdoors on natural rock formations, is a very popular hobby here in Southern California. Some people find it exhilarating and enjoyable, while others are simply terrified. Like with any athletic venture, climbers assess risk before embarking on new routes, and may quell their fears with the knowledge that they are working safely and efficiently. One professional climber in particular may come to mind – Alex Honnold, featured in the film Free Solo. Honnold has gained fame for his climbing prowess both on and off the rope, but he is a true standout in the discipline of free soloing. Free Solo followed Honnold’s journey of ascending El Capitan’s Freerider, a 3,000-foot-tall sport climbing route in Yosemite National Park, without a rope or any other protective measures3. Honnold scales mountain faces with confidence and strength, barely emoting as he mechanically moves along his route. He is spoken about as someone who is “fearless,” and this has prompted many to question if there are differences in his brain that may allow him to achieve extraordinary feats without the obstacle of fear.
If the thought of free soloing a 3,000-foot mountain makes your palms sweat and your heart beat faster, do not feel bad – this fear may well be mediated by a part of your amygdala known as the basolateral amygdala, or BLA (Fig. 1). The amygdala is classically thought of as the brain’s center of fear and stress. It is situated within a deep group of structures known as the limbic system, which is attributed to much of humans’ emotional processing capabilities, as well as maintaining homeostasis (the body’s way of sustaining stable conditions internally), and even encoding memories1. The BLA specifically is responsible for the processing of fearful responses to frightening stimuli, including but not limited to the fear of falling from a very tall rock wall.
The basolateral amygdala’s role in fear
While the BLA is not exclusively responsible for mediating one’s fear of heights, recent literature has highlighted it as an important brain area for response to height-related fear. One common method of characterizing if an area of the brain is associated with specific experiences is to monitor its activity when a human or animal is presented with controlled versions of such stimuli or experiences. Scientists may do this by measuring the activity of specific neurons during behavioral tasks. This approach was used to demonstrate that the BLA specifically represents fear of heights in a study of fear in mice2.
In this study, Dr. Jun Liu and colleagues worked to identify subgroups of BLA neurons which were activated specifically when the mice were in high places. The group of BLA neurons focused on in this study proved to be distinct from neighboring BLA neurons that showed activation following exposure to other fearful stimuli, such as odors from predators, startling auditory tones, and looming shadows designed to simulate approaching predators. The fact that the neurons they focused on didn’t become active during those other fearful situations importantly helped to prove that there are specific neurons in the BLA that function to encode a fear of heights. When recording from these ‘high place fear’ associated neurons, their results showed that mice who were placed in high-up environments displayed a significant increase in BLA neuron activity2. In addition to increased neural activity, mice also displayed increased freezing behavior and increased heart rates, two common indicators of fear and stress. Interestingly, the BLA was strongly activated when the mice were placed on an open platform, but not when they were placed in a transparent, enclosed platform, indicating that the increase in fear perception was limited to platforms upon which they were less secure (Fig. 2). Much like humans who may feel more comfortable looking out a window from the top of the Empire State Building, but not walking to the edge, the fear of heights seems to be associated with the perception of the likelihood of falling.
Is fear processed differently among different people?
While a peek into the brains of mice sheds some light on how instinctual fear is mediated by the BLA, particularly in cases of frightening heights, the question persists – are the brains of people who seek out potentially dangerous activities, such as free soloing, operating differently than others?
Since it is known that the BLA is strongly activated while overlooking a high place, triggering feelings of apprehension and fear in humans and mice alike, this may lead you to wonder what Honnold’s BLA is doing as he reaches heights from which any fall would likely be lethal. Leading up to the release of Free Solo, there had been much chatter about the way Alex Honnold’s brain worked. A neuroimaging lab at the Medical University of South Carolina got into contact with Honnold and began to probe this very question. Before discussing the results of Honnold’s brain scans, it is interesting to examine why this group was perfectly suited to work with him. Dr. Jane Joseph has long studied and published about neural activity in “thrill-seekers,” people who enjoy engaging in tasks the average person would find intimidating. In 2009, Dr. Joseph published a paper examining sensation-seeking behaviors in humans. This study utilized functional magnetic resonance imaging (fMRI) to monitor neural activity in humans. fMRI measures changes in blood flow that occur alongside brain activity to provide a readout of neural activity across the brain through the duration of the scan4. In this study, researchers classified volunteers into high sensation seeking or low sensation seeking groups based on self-reporting in a questionnaire. fMRI was conducted while volunteers were shown photographs with a high or low arousal value. Simply put, the expected emotional response varied greatly between high and low arousal stimuli. Low arousal images included “ordinary scenes”, people, food, and objects. Alternatively, high arousal photographs were much less blasé, depicting a plethora of images including extreme sports and bodily harm (Fig. 3)5. When high sensation seeking volunteers were shown high arousal stimuli, regions of the brain associated with arousal, addiction, and reinforcement displayed an increase in activity5. Conversely, in low sensation seeking volunteers, these same images led to heightened neural activity in brain regions which are broadly associated with modulation and regulation of emotions5. This begs the question, do some people experience fear differently than others? Do those who seek out more extreme experiences process them differently than those who do not?
To shed light on this debate, Dr. Joseph’s research group performed fMRI on Honnold (Fig. 4). It will not surprise you that one area of extreme interest in this imaging experiment was the amygdala. Before performing the brain scan, they administered a questionnaire to Honnold similar to the one used in Dr. Joseph’s 2009 study, aiming to assign a value to his degree of sensation-seeking behavior. Results of their questions revealed that he scored twenty percent higher than the typical HSS volunteer analyzed in their work, as well as ranking twice as highly in sensation seeking activity compared to the average person6. Remarkably, they found that Honnold’s amygdala was not activated to the same degree as a typical volunteer’s would be when shown images with high arousal values – it requires a much higher input of fear stimuli to elicit a response. In short, this means that Honnold can endure much scarier situations before the brain regions responsible for mediating fear become active.
A recent interview with Honnold sheds light on his perspective of his own brain’s inner workings following Free Solo’s success. When chatting with the Harvard Business Review, he lamented the assumptions people made about him after catching a glimpse of his fMRI results. Essentially, he underscores the importance of regarding his amygdala as one that he has been training in the craft of high level climbing for decades. He remarked, “I’ve spent 25 years conditioning myself to work in extreme conditions, so of course my brain is different—just as the brain of a monk who has spent years meditating or a taxi driver who has memorized all the streets of a city would be different”7. Much like any other professional athlete, Honnold has conditioned his body (and brain) to excel at his craft. In a sport like rock climbing with such a vital mental component, especially a discipline as risky as free soloing, it is unsurprising that he has trained his amygdala to the same degree he has trained his muscles.
While much work needs to be done before we can disentangle the effects of innate fear levels and the effects of diligent training, Honnold’s remarks echo the oldest question in all of biology – nature versus nurture. While his baseline fear may well have differed from the average person, there is no doubt his brain activity has been affected over time by his robust experience and dedication to his craft. Next time you find yourself on the precipice of a mountain after a hike, or doing some rock climbing yourself, perhaps you will reflect on the role of your BLA if you begin to feel nervous. Like the mice we discussed earlier showed us, it is perfectly normal to feel fear when you are on a ledge and your fear response causes your heart rate to creep up. However, if you train yourself for 25 years, perhaps you’ll feel a bit less fearful.
1Moini, J., & Piran, P. (2020). Limbic, olfactory, and gustatory systems. In Functional and clinical neuroanatomy: A guide for health care professionals. essay, Academic Press.
2 Liu, J., Lin, L., & Wang, D. V. (2021). Representation of fear of heights by basolateral amygdala neurons. The Journal of Neuroscience, 41(5), 1080–1091. https://doi.org/10.1523/jneurosci.0483-20.2020
3 Vasarhelyi, E. C., & Chin, J. (2018). Free Solo. National Geographic Documentary Films.
4 Glover, G. H. (2011). Overview of functional magnetic resonance imaging. Neurosurgery Clinics of North America, 22(2), 133–139. https://doi.org/10.1016/j.nec.2010.11.001
5 Joseph, J. E., Liu, X., Jiang, Y., Lynam, D., & Kelly, T. H. (2009). Neural correlates of emotional reactivity in Sensation seeking. Psychological Science, 20(2), 215–223. https://doi.org/10.1111/j.1467-9280.2009.02283.x
6 Donovan, B. (2019, March 18). The Science of Risk: How a neuroscientist and professional climber learned from one another. MUSC. Retrieved March 29, 2022, from https://web.musc.edu/about/news-center/2019/03/18/how-a-neuroscientist-and-professional-climber-learned-from-one-another
7Harrell, E. (2021). Life’s Work: An Interview with Alex Honnold. Harvard Business Review.
featured photo by Jimmy Chin
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