August 02

Snake Eyes: The Fear That Built Your Brain

Indiana Jones is a quintessential American hero, his fedora, satchel, and whip instantly recognizable around the world. He lives a double life, a scholarly professor of anthropology in public and a globetrotting treasure hunter in private. He defeats the Nazis and always gets the girl, displaying daring and fearlessness, with one notable exception. He has a debilitating fear of snakes; they are the only thing that ever scared him over the course of his adventures, and even elicited one of his most famous catchphrases, “Why did it have to be snakes?!”

Indiana Jones’s fear of snakes humanizes his larger-than-life figure, and many viewers likely related to that aspect of his personality more than any other. Around a third of people fear snakes to some degree, and in five percent of people, these fears are strong enough to become a clinical phobia (known as ophidiophobia, literally “snake fear” in Greek), where the intensity of their terror meaningfully interferes with life [1]. Ophidiophobia is the most common phobia in humans all over the world, regardless of race or gender.

Despite their common contours, a key distinction still exists between the ophidiophobia of Indiana Jones and most other people. In the opening to Indiana Jones and the Last Crusade, a young Indiana Jones falls into a box of snakes while evading his pursuers in a boxcar on a circus train, traumatizing him. Most cases of ophidiophobia do not result from any traumatic experience, but instead arise in infancy. Babies under a year old will both generally associate snakes with fear and experience a surge of adrenaline upon seeing them [2-3]. In other words, infants do not really need much experience with snakes to begin to fear them but are instead predisposed. This phenomenon is especially interesting because it is specific to snakes: we do not seem to have the same innate preparedness to fear to any other creature, except  spiders, another very common target of phobias [1, 3-4].

Interestingly, many anthropologists already believe that snakes have exerted a profound evolutionary effect on primates as our ancestors’ first and most dangerous predator. Dr. Lynne Isbell, an anthropologist at UC Davis, spent a significant portion of her career outlining “Snake Detection Theory,” a formal model defining these anthropologists’ rougher intuition [5]. Snake Detection Theory states that primates evolved our distinctive visual system, with three-color vision, strong depth perception, and expanded visual pathways in order to better detect snakes before they strike. These extensive specializations only took hold because of our uniquely long, violent history with snakes; primates in Madagascar and South America (where there are no venomous snakes) generally lack the specialized vision of most other primates.

animal-cute-wildlife-reptile-scale-fauna-lizard-snake-vertebrate-natter-slim-species-animal-world-serpent-macro-photography-young-animal-agamidae-non-hazardous-garter-snake-boa-constrict

Pictured: Mankind’s most terrifying mortal enemy. Source: Pixabay.

Snake Detection Theory originally predicted the existence of a defined visual module in primates to identify snakes, pay attention to them, and induce fear, at a time when no such experiments had shown either one. Only later did phobia research validate this theory, showing the mere presence of snakes can scare babies prior to any individual experience. These findings provided some of the first experimental evidence supporting the theory, showing a link to fear and strongly implying experience-independent recognition.

However, this implied early recognition of snakes still needed proving. Recent experiments performed by Dr. Vanessa LoBue (now at Rutgers) and Dr. Judy DeLoache at the University of Virginia went a long way towards bridging this gap. LoBue and DeLoache would show people of any age, from infants to adults, images of snakes interspersed with other natural objects (i.e. flowers), measuring the amount of time until they looked at the snake, as well as how long they spent looking at it. Their findings upheld Isbell’s predictions: at all ages, people would both preferentially look at snakes before anything else, and they would do so much faster than for other objects [6-7].

Even though Snake Detection Theory had correctly predicted enhanced visual recognition and attention towards snakes, as well as a corresponding preparedness for fear, the fear module’s composition and identity remained unknown. Isbell had an idea, though. She thought this fear module likely centered on the pulvinar, a small subregion of the thalamus that forwards information from the optic nerve to the visual cortex, for a number of reasons [5]. First, only primates possess it, to the exclusion of all other animals. Second, the pulvinar projects to the amygdala, a part of the brain that controls fear [8]. Third, the pulvinar facilitates gaze direction, as well as visual spatial attention and information filtering [9]. Fourth and finally, destruction of the pulvinar significantly impairs visual threat detection [8].

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The pulvinar, in all its glory. Source: Wikimedia Commons.

Now, all of these details do strongly suggest a role for the pulvinar in snake detection. However, none of them actually prove it. She needed experiments and electrical measurements to put her ideas to the test, none of which she could do on her own, so she partnered with Dr. Hisao Nishijo, a neuroscientist at the University of Toyama. Together, the two inserted electrodes into the pulvinar of macaques, recording the activity of individual neurons when shown images of snakes and other objects.

Their findings made an instant impact. By far, the most pulvinar neurons responded to images of snakes, with a much shorter delay and a far greater magnitude compared to their response to all other images, including faces, which primates instinctually pay attention to [10]. Further, their responses to snake images displayed an activity pattern associated with fast processing and dense, synergistic information transfer absent from their responses to other images [11]. Pulvinar neurons would even respond more strongly to snakes in more threatening postures [12]. Just as predicted, the pulvinar appears to contain a fear module specialized for snake detection.

We can call few things in society ubiquitous, but our fear of snakes just might qualify, affecting babies and Indiana Jones alike. Snakes have acquired an overwhelming association with evil, but these may follow from our fear’s origin, instead of serving as its source. Rather, ophidiophobia’s roots reach down into the most basic, primordial levels of our psyche, echoing our ancient terrors buried deep below. Perhaps this fear is not a simple disease, but something essential to being human.

References:

  1.     Fredrikson, M., Annas, P., Fischer, H., Wik, G. (1996). Gender and age differences in the prevalence of specific fears and phobias. Behav. Res. Ther. 34, 33-39.
  2.     DeLoache, J.S., LoBue, V. (2009). The narrow fellow in the grass: human infants associate snakes and fear. Dev. Sci. 12, 201-207.
  3.     Hoehl, S., Hellmer, K., Johansson, M., Gredebaeck, G. (2017). Itsy bitsy spider…: infants react with increased arousal to spiders and snakes. Front. Psychol. 8, 1710.
  4.     Rakison, D.H., Derringer, J. (2008). Do infants possess an evolved spider-detection mechanism? Cognition. 107, 381-393.
  5.     Isbell, L.A. (2006). Snakes as agents of evolutionary change in primate brains. J. Hum. Evol. 51, 1-35.
  6.     LoBue, V., and DeLoache, J.S. (2008). Detecting the snake in the grass: Attention to fear-relevant stimuli by adults and young children. Psychol. Sci. 19, 284-289.
  7.     LoBue, V., and DeLoache, J.S. (2010). Superior detection of threat-relevant stimuli in infancy. Dev. Sci. 13, 221-228.
  8.     Ward, R., Danziger, S., Bamford, S. (2005). Response to visual threat following damage to the pulvinar. Curr. Biol. 15, 571-573.
  9.     Robinson, D.L., Petersen, S.E. (1992). The pulvinar and visual salience. Trends Neurosci. 15, 127-132.
  10.  Le, Q.V., Isbell, L.A., Matsumoto, J., Nguyen, M., Hori, E., Maior, R.S., Tomaz, C., Tran, A.H., Ono, T., Nishijo, H. (2013). Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes. Proc. Natl. Acad. Sci. U.S.A. 110, 19000-19005.
  11.  Le, Q.V., Isbell, L.A., Matsumoto, J., Le, V.Q., Nishimaru, H., Hori, E., Maior, R.S., Tomaz, C., Ono, T., Nishijo, H. (2016). Snakes elicit earlier, and monkey faces, later, gamma oscillations in macaque pulvinar neurons. Sci. Rep. 6, 20595.
  12.  Le, Q.V., Isbell, L.A., Matsumoto, J., Le, V.Q., Hori, E., Tran, A.H., Maior, R.S., Tomaz, C., Ono, T., Nishijo, H. (2014). Monkey pulvinar neurons fire differentially to snake postures. PLoS ONE. 9, e114258.
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