![]() Recent studies using event-related potentials (ERPs) have also tested predictions derived from the Snake Detection Theory in humans 10, 11, 12. They interpret the early gamma oscillations for snakes as indicative of feedforward processes that facilitate processing in the visual cortex. In a third study, Le and colleagues 9 demonstrated that images of snakes elicited gamma (30–80 Hz) oscillations in macaque pulvinar neurons in an early time window (0–200 ms after stimulus onset), whereas monkey faces elicited gamma oscillation in a later time window (300–500 ms after stimulus onset). As the authors noted, the pulvinar is part of a fast visual information processing pathway also involving the retina and superior colliculus, allowing for rapid, automatic visual detection of fear-related stimuli 7, 8. Neurons in the pulvinar were also found to respond more strongly (but not more quickly) to images of snakes in striking postures than in resting postures 6. Their study revealed the existence of pulvinar neurons that responded selectively faster and stronger to images of snakes than to images of angry and neutral monkey faces, monkey hands, and simple geometrical shapes. ![]() Le and colleagues 5 measured neuronal responses in the medial and dorsolateral pulvinar of macaques ( Macaca fuscata) that likely had no exposure to snakes before the experiment. Predatory pressure from snakes is proposed to have contributed to primate visual modification and expansion by weeding out those individuals with poorer ability, and favoring those with better ability, to visually detect motionless snakes.Įlectrophysiological evidence in support of greater visual sensitivity to snakes than to other stimuli is growing both in humans and other primates. The “Snake Detection Theory” 3, 4 argues that snakes were ultimately responsible for the origin of primates by acting as a selective pressure in the modification and expansion of primate visual systems such that vision is now their predominant sensory interface with the environment. The results suggest that the EPN snake effect is partly driven by snake skin scale patterns which are otherwise rare in nature.īoth humans and other primates can detect snakes faster than other, less life-threatening stimuli 1, 2. Likewise, the EPN was larger for partially exposed snakes than for partially exposed lizards and birds. ![]() ![]() Consistent with previous studies, and with the Snake Detection Theory, the EPN was significantly larger for snake skin pictures than for lizard skin and bird plumage pictures, and for lizard skin pictures than for bird plumage pictures. The EPN was scored as the mean activity (225–300 ms after picture onset) at occipital and parieto-occipital electrodes. Participants watched a random rapid serial visual presentation of these pictures. In task 2, we employed pictures of partially exposed snakes, lizards, and birds. In Task 1, we employed pictures with close-ups of snake skins, lizard skins, and bird plumage. Here, we examined whether snake skin patterns and partially exposed snakes elicit a larger EPN in humans. Ethological research has recently shown that macaques and wild vervet monkeys respond strongly to partially exposed snake models and scale patterns on the snake skin. ![]() Studies of event-related potentials in humans have established larger early posterior negativity (EPN) in response to pictures depicting snakes than to pictures depicting other creatures. ![]()
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