Perceptual biases toward snakes – which precede the development of fearful behaviors – would thus act as a catalyst for fear learning 30, 31, 32. Primates would rather be prepared to learn to fear snakes, so that fear of snakes (and of other evolutionary recurrent threats) is acquired more easily than fear of other non-recurrent threats 29. This is clear evidence that fear of snakes is not innate in humans, just as extensive work has previously demonstrated in monkeys (see, e.g. Importantly, these physiological studies, together with other research examining infants’ and toddlers’ approach/avoidance behaviors toward snakes 20, 25, 27 reported no fear reaction or signs of distress in these participants. Infants’ sympathetic responses to videos, pictures and hissings of snakes likewise support the observation that snakes capture their attention 24, 25, 26. This visual bias further impacts infants’ processing of subsequent stimuli 19, 21. When presented with pictures in the visual periphery, infants indeed shift attention faster towards snake pictures than towards threat-irrelevant pictures 19, 20, 21, 22, 23. Remarkably, infants under one year of age also show rapid detection of snake pictures and preferential orienting toward these ancestrally threat-relevant stimuli. The coiled body shape of snakes would be a critical feature in attracting participants’ attention 12, 14, 15. When presented with an array of pictures, human adults 5, 6, 7, 8, 9 and children 10, 11, 12, 13, 14, 15, but also lab-reared monkeys 16, 17, 18, are faster at detecting a snake among threat-irrelevant pictures than vice versa. Yet, it is unclear whether such detection mechanism is implemented in the naive, immature brain.īehaviorally, both human and non-human primates are remarkable snake detectors. Primates would therefore be evolutionarily tuned to swiftly detect and process ancestrally threat-relevant stimuli based on their visual features. Evidence for the existence of such a neurobiological substrate for efficient detection of snakes in primates stems essentially from the identification of thalamic neurons in the macaque brain that selectively respond to images of snakes 4. The evolutionary pressure exerted by snakes would also have led to the development of a “fear module” in the primate brain - a structure that is selectively sensitive to and automatically activated by evolutionary threat-relevant stimuli, allowing their rapid detection 3. The vital need to spot snakes rapidly would have shaped primates’ brain such that they developed keen perceptual abilities, and, in particular, the ability to rapidly detect and process visual cues suggestive of snakes 1, 2. This hypothesis is at the core of the Snake Detection Theory, which posits that the ancient predator-prey relationship between snakes and primates played a substantial role in the evolution and expansion of the latter’s visual system. Over evolution and across species, natural selection may have therefore selected individuals equipped with perceptual systems tuned to detect predators quickly, so enabling better defensive behavior. They argue for the existence in humans of an inborn, brain-anchored mechanism to swiftly detect snakes based on their characteristic visual features.ĭetecting predators is essential for survival. These results therefore demonstrate that a single fixation at snakes is sufficient to generate a prompt and large selective response in the infant brain. The temporal dynamics of these neural responses support that infants devote increased attention to snakes than to non-snake stimuli. We showed that glancing at snakes generates specific neural responses in the infant brain, that are higher in amplitude than those generated by frogs or caterpillars, especially in the occipital region of the brain. All animals were presented in their natural background. To do so, we recorded scalp electrical brain activity in 7- to 10-month-old infants watching sequences of flickering animal pictures. Here, we provide electrophysiological evidence for a brain-anchored evolved predisposition to rapidly detect snakes in humans, which does not depend on previous exposure or knowledge about snakes.
Given that snakes are the first of primates’ major predators, natural selection may have fostered efficient snake detection mechanisms to allow for optimal defensive behavior. Detecting predators is essential for survival.