Pattern matters: Snakes exhibiting triangular and diamond-shaped skin patterns modulate electrophysiological activity in human visual cortex.

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.