Visual mismatch negativity is sensitive to symmetry as a perceptual category.

We investigated the sensitivity of visual mismatch negativity (vMMN) to an abstract and non-semantic category, vertical mirror symmetry. Event-related potentials (ERPs) elicited by random and symmetric square patterns, delivered in passive oddball paradigm (participants played a video game), were recorded. In one of the conditions, symmetric patterns were frequent (standard) stimuli and the random patterns were infrequent (deviant) stimuli; in the other condition, the probabilities were reversed. We compared the ERPs elicited by symmetric stimuli as deviants and as standards, and, similarly, the ERPs elicited by the random deviants and random standards. As the difference between the ERPs elicited by random deviant and random standard stimuli, a posterior negativity emerged in two latency ranges (112-120 and 284-292 ms). These negativities were considered to be vMMN components. We suggest that the two vMMN components are organised in cascade error signals. However, there was no significant difference between the ERPs elicited by symmetric deviants and those elicited by symmetric standards. The emergence of vMMN in response to the deviant random stimuli is considered to be a deviation of a perceptual category (in the symmetric standard sequence presented). Accordingly, random stimuli acquired no perceptual category; for this reason, the symmetric deviant (in the random standard sequence presented) elicited no vMMN. The results show that the memory system underlying vMMN is capable of coding perceptual categories such as bilateral symmetry, even if the stimulus patterns are unrelated to the ongoing behavior.