Enhanced early visual processing in response to snake and trypophobic stimuli

Abstract

Background: Trypophobia refers to aversion to clusters of holes. We investigated whether trypophobic stimuli evoke augmented early posterior negativity (EPN).

Methods: Twenty-four participants filled out a trypophobia questionnaire and watched the random rapid serial presentation of 450 trypophobic pictures, 450 pictures of poisonous animals, 450 pictures of snakes, and 450 pictures of small birds (1800 pictures in total, at a rate of 3 pictures/s). The EPN was scored as the mean activity at occipital electrodes (PO3, O1, Oz, PO4, O2) in the 225-300 ms time window after picture onset.

Results: The EPN was significantly larger for snake pictures than for the other categories, and significantly larger for trypophobic pictures and poisonous animal pictures than for bird pictures. Remarkably, the scores on the trypophobia questionnaire were correlated with the EPN amplitudes for trypophobic pictures at the occipital cluster (r = -.46, p = .025).

Conclusions: The outcome for the EPN indicates that snakes, and to a somewhat lesser extent trypophobic stimuli and poisonous animals, trigger early automatic visual attention. This supports the notion that the aversion that is induced by trypophobic stimuli reflects ancestral threat and has survival value. The possible influence of the spectral composition of snake and trypophobic stimuli on the EPN is discussed.

Keywords: EEG/ERP, early posterior negativity (EPN); Evolution; Phylogenetic fear; Snake detection; Trypophobia.

Fig. 1

Illustrative examples of snake, trypophobic, poisonous animal, and small bird stimuli. The depicted photographs are public domain (pixabay.com); they are similar to the stimuli used in the present research

Fig. 2

Diagram of the EEG electrodes included in the statistical analysis

Fig. 3

a The early posterior negativity (EPN) in response to snake (red line), trypophobic (blue line), poisonous animal (green line) and bird pictures (black line) across the five occipital electrodes (O1/2, Oz, PO3/4). The depicted waveform for each condition is the grand average of 24 participants with approximately 450 epochs per participant. Negativity is up. b Topographic maps of the differences in EPN mean amplitudes (225–300 ms) between snake vs. bird pictures (left), trypophobic vs. bird pictures (middle), and poisonous animal vs. bird pictures (right)

Fig. 4

Mean energy for each frequency band as a function of picture category. Error bars depict standard error of means. Frequency bands are expressed in cycles per degree of visual angle. High spatial frequencies are on top