Snake detection theory
{{short description|Evolutionary theory regarding primate vision}}
File:Vipera_aspis_aspis.jpgs, were crucial for the evolution of primates' visual systems.]]
The snake detection theory (SDT),{{cite journal|last1=Isbell|first1=Lynne A.|authorlink=Lynne Isbell|title=Snakes as agents of evolutionary change in primate brains|journal=Journal of Human Evolution|date=1 July 2006|volume=51|issue=1|pages=1–35|doi=10.1016/j.jhevol.2005.12.012|pmid=16545427|bibcode=2006JHumE..51....1I |citeseerx=10.1.1.458.2574}}{{cite book|last1=Isbell|first1=Lynne A.|title=The Fruit, the Tree, and the Serpent|date=2009|url=https://books.google.com/books?id=yKzIuw4YBCoC&q=The+Fruit%2C+the+Tree%2C+and+the+Serpent+isbell&pg=PR7|publisher=Harvard University Press|language=en|isbn=9780674033016}}{{Cite web |last1=Allman Updyke |first1=Erin |last2=Welsh |first2=Erin |name-list-style=and |year=2022 |orig-date=31/05/2022 |title=Episode 97 Snake Venoms: Collateral Damage |url=https://thispodcastwillkillyou.com/2022/05/31/episode-97-snake-venoms-collateral-damage/ |website=This Podcast Will Kill You |publisher=Exactly Right Network}} also sometimes called the snake detection hypothesis, suggests that snakes contributed to the evolution of visual systems in primates.
According to the theory, predatory pressure on early primate populations from snakes selected individuals who were best able to
recognize them, improving their survival chances and therefore transferring such skill to their offspring. From this point of view, snakes were responsible for the modification and expansion of primate visual systems which made vision the most developed sensory interface with the external environment for modern primates.
In her book The Fruit, the Tree, and the Serpent (2009),{{cite book|last1=Isbell|first1=Lynne A.|title=The Fruit, the Tree, and the Serpent|date=2009|url=https://books.google.com/books?id=yKzIuw4YBCoC&q=The+Fruit%2C+the+Tree%2C+and+the+Serpent+isbell&pg=PR7|publisher=Harvard University Press|language=en|isbn=9780674033016}} anthropologist Lynne Isbell writes that snakes evolved to be difficult to detect and mortally dangerous. Surviving the peril of snakes for millions of years required selective pressure favoring primates' specialized visual systems. Compared to that of other mammals, the pulvinar region of the brain – which helps to visually detect relevant objects – is disproportionately large and effective in the brains of primates (including humans).
The concept of snakes being a special threat to humans has been confirmed by population-based studies. Ophidiophobia (phobia of snakes) is one of the most common and intense phobias among the general population. Furthermore, a study reported that around 50% of people experience dreams about snakes.
Empirical studies
Many empirical studies have found evidence for the theory. Primates, including humans, are able to quickly detect snakes.{{cite journal|last1=Ohman|first1=A.|last2=Flykt|first2=A.|last3=Esteves|first3=F.|title=Emotion drives attention: detecting the snake in the grass|journal=Journal of Experimental Psychology. General|date=2001|volume=130|issue=3|pages=466–478|pmid=11561921|issn=0096-3445|doi=10.1037/0096-3445.130.3.466|citeseerx=10.1.1.640.3659}}{{cite journal|last1=Shibasaki|first1=Masahiro|last2=Kawai|first2=Nobuyuki|title=Rapid detection of snakes by Japanese monkeys (Macaca fuscata): an evolutionarily predisposed visual system|journal=Journal of Comparative Psychology|date=2009|volume=123|issue=2|pages=131–135|doi=10.1037/a0015095|pmid=19450020|issn=0735-7036}}
Some studies have found that humans can detect snake images before subjective visual perception.{{cite journal|last1=Ohman|first1=A.|last2=Soares|first2=J. J.|title=On the automatic nature of phobic fear: conditioned electrodermal responses to masked fear-relevant stimuli|journal=Journal of Abnormal Psychology|date=1993|volume=102|issue=1|pages=121–132|pmid=8436688|issn=0021-843X|doi=10.1037/0021-843X.102.1.121}} However, the pre-conscious detection of snake stimuli is still under debate by the scientific community.{{cite journal|last1=Grassini|first1=Simone|last2=Holm|first2=Suvi K.|last3=Railo|first3=Henry|last4=Koivisto|first4=Mika|title=Who is afraid of the invisible snake? Subjective visual awareness modulates posterior brain activity for evolutionarily threatening stimuli|journal=Biological Psychology|date=1 December 2016|volume=121|issue=Part A|pages=53–61|doi=10.1016/j.biopsycho.2016.10.007|pmid=27760371|s2cid=9516831}}
Regardless, snakes images have been experimentally demonstrated to be detected more rapidly compared to other fear-relevant stimuli: empirical evidences have shown that snakes are more rapidly detected compared to spiders; according to the snake detection theory this is because the arachnids were, historically, a less relevant threat to primates.{{cite journal|last1=Öhman|first1=Arne|last2=Soares|first2=Sandra C.|last3=Juth|first3=Pernilla|last4=Lindström|first4=Björn|last5=Esteves|first5=Francisco|title=Evolutionary derived modulations of attention to two common fear stimuli: Serpents and hostile humans|journal=Journal of Cognitive Psychology|date=1 February 2012|volume=24|issue=1|pages=17–32|doi=10.1080/20445911.2011.629603|s2cid=146142767|issn=2044-5911}}
Snake stimuli are particularly distracting during perceptual tasks, suggesting that the brain preferentially processes snake stimuli, even when attentional processes are demanded by other targets.{{cite journal|last1=Soares|first1=Sandra C.|title=The lurking snake in the grass: interference of snake stimuli in visually taxing conditions|journal=Evolutionary Psychology|date=15 April 2012|volume=10|issue=2|pages=187–197|doi=10.1177/147470491201000202|pmid=22947633|s2cid=34814663|issn=1474-7049|doi-access=free|hdl=10316/102719|hdl-access=free}}
Enhanced snake detection has also been found in young children.{{cite journal|last1=LoBue|first1=Vanessa|last2=DeLoache|first2=Judy S.|title=Detecting the Snake in the Grass: Attention to Fear-Relevant Stimuli by Adults and Young Children|journal=Psychological Science|date=1 March 2008|volume=19|issue=3|pages=284–289|doi=10.1111/j.1467-9280.2008.02081.x|pmid=18315802|s2cid=12776572|language=en|issn=0956-7976}}
Brain imaging investigations have found further evidence for the theory. Support for the idea of a high visual sensitivity to snakes has been proven in primate neural activity in response to snake threats.{{cite journal|last1=Van Le|first1=Quan|last2=Isbell|first2=Lynne A.|last3=Matsumoto|first3=Jumpei|last4=Nguyen|first4=Minh|last5=Hori|first5=Etsuro|last6=Maior|first6=Rafael S.|last7=Tomaz|first7=Carlos|last8=Tran|first8=Anh Hai|last9=Ono|first9=Taketoshi|last10=Nishijo|first10=Hisao|title=Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes|journal=Proceedings of the National Academy of Sciences of the United States of America|date=19 November 2013|volume=110|issue=47|pages=19000–19005|doi=10.1073/pnas.1312648110|pmid=24167268|issn=1091-6490|pmc=3839741|bibcode=2013PNAS..11019000V|doi-access=free}} Non-invasive electroencephalogram (EEG) studies have found an enhanced visual brain activity in response to images of snakes in humans.{{cite journal|last1=Van Strien|first1=J. W.|last2=Eijlers|first2=R.|last3=Franken|first3=I. H. A.|last4=Huijding|first4=J.|title=Snake pictures draw more early attention than spider pictures in non-phobic women: evidence from event-related brain potentials|journal=Biological Psychology|date=2014|volume=96|pages=150–157|doi=10.1016/j.biopsycho.2013.12.014|pmid=24374241|issn=1873-6246|url=http://repub.eur.nl/pub/51065|hdl=1765/51065|s2cid=16106346|hdl-access=free}}{{cite journal|last1=Van Strien|first1=Jan W.|last2=Franken|first2=Ingmar H. A.|last3=Huijding|first3=Jorg|title=Testing the snake-detection hypothesis: larger early posterior negativity in humans to pictures of snakes than to pictures of other reptiles, spiders and slugs|journal=Frontiers in Human Neuroscience|date=2014|volume=8|pages=691|doi=10.3389/fnhum.2014.00691|pmid=25237303|pmc=4154444|issn=1662-5161|doi-access=free}}{{cite journal|last1=Van Strien|first1=Jan W.|last2=Christiaans|first2=Gerwin|last3=Franken|first3=Ingmar H. A.|last4=Huijding|first4=Jorg|title=Curvilinear shapes and the snake detection hypothesis: An ERP study|journal=Psychophysiology|date=2016|volume=53|issue=2|pages=252–257|doi=10.1111/psyp.12564|pmid=26481589|hdl=1874/322508 |issn=1540-5958|hdl-access=free}}{{cite journal|last1=He|first1=Hongshen|last2=Kubo|first2=Kenta|last3=Kawai|first3=Nobuyuki|title=Spiders do not evoke greater early posterior negativity in the event-related potential as snakes|journal=NeuroReport|date=10 September 2014|volume=25|issue=13|pages=1049–1053|doi=10.1097/WNR.0000000000000227|pmid=25026534|s2cid=27839800|issn=1473-558X}}{{cite journal|last1=Grassini|first1=Simone|last2=Holm|first2=Suvi K.|last3=Railo|first3=Henry|last4=Koivisto|first4=Mika|title=Who is afraid of the invisible snake? Subjective visual awareness modulates posterior brain activity for evolutionarily threatening stimuli|journal=Biological Psychology|date=2016|volume=121|issue=Pt A|pages=53–61|doi=10.1016/j.biopsycho.2016.10.007|pmid=27760371|s2cid=9516831|issn=1873-6246}}