The neural and perceptual mechanisms that support the efficient visual
detection of snakes in humans are still not fully understood. According
to the Snake Detection Theory, selection pressures posed by snakes on
early primates have shaped the development of the visual system.
Previous studies in humans have investigated early visual
electrophysiological activity in response to snake images vs. various
alternative dangerous or non-dangerous stimuli. These studies have shown
that the Early Posterior Negativity (EPN) component is selectively
elicited by snake or snake-like images. Recent findings yielded the
complementary/alternative hypothesis that early humans (and possibly
other primates) evolved an aversion especially for potentially harmful
triangular shapes, such as teeth, claws or spikes. In the present study
we investigated the effect of triangular and diamond-shaped patterns in
snake skins on the ERP correlates of visual processing in humans. In the
first experiment, we employed pictures of snakes displaying either
triangular/diamond-shaped patterns or no particular pattern on their
skins, and pictures of frogs as control. Participants observed a random
visual presentation of these pictures. Consistent with previous studies,
snakes elicited an enhanced negativity between 225 and 300 ms (EPN)
compared to frogs. However, snakes featuring triangular/diamond-shaped
patterns on their skin produced an enhanced EPN compared to the snakes
that did not display such patterns. In a second experiment we used
pictures displaying only skin patterns of snakes and frogs. Results from
the second experiment confirmed the results of the first experiment,
suggesting that triangular snake-skin patterns modulate the activity in
human visual cortex. Taken together, our results constitute an important
contribution to the snake detection theory.