Asymmetry of P3 amplitude during oddball tasks reflects the unnaturalness of visual stimuli

This study examined the relationship between the 'naturalness' of a visual stimulus and the event-related potentials measured during an oddball task. The study focused on asymmetry of the P3 amplitude during an oddball task or P3 asymmetry. Participants performed two visual oddball tasks using a pair of stimuli (A and B): one in which A was the target stimulus and B was the standard stimulus and vice versa. The stimuli consisted of natural-unnatural pairs of visual stimuli (e.g. upright-inverted faces, possible-impossible human poses). As a result of comparing the amplitudes of the target stimuli, P3 asymmetry was found in natural-unnatural pairs; that is, their naturalness differentiated the target P3 amplitude: larger P3 to the unnatural target than to the natural one. This study showed that P3 asymmetry reflected unnaturalness and unfamiliarity of visual stimuli.