The present study aimed to investigate whether the implicit click here system underlying vMMN was capable of registering vertical mirror symmetry as a perceptual category. Several behavioral studies have shown that the visual system is particularly sensitive to various forms of symmetry (for a review, see Treder (2010)). According to Carmody et al. (1977) and Tyler et al. (1995), stimulus duration in the 40–80-ms range is long enough for the recognition of symmetric patterns. Other behavioral studies have shown that symmetry can be detected automatically (Baylis & Driver, 1994; Wagemans, 1995; Huang

et al., 2004; Machilsen et al., 2009). Vertical mirror symmetry is a salient feature of living objects, and has obvious biological significance (Tyler & Hardage, 1996). However, so far, no ERP study has analysed the level of processing that is sensitive to symmetry and the automaticity of sensitivity to symmetry. Few studies have investigated the processing of symmetric stimuli on the basis of event-related brain activity. Jacobsen & Höfel (2003) and Höfel & Jacobsen (2007) reported a posterior negative wave elicited by symmetric patterns. In these studies, symmetry as such was task-irrelevant; participants made aesthetic judgements, performed a detection task, or contemplated the beauty of the stimuli. The negativity emerged

in the 380–890-ms poststimulus latency range, so this effect may not be a correlate of elementary perceptual processes. However, in a sequence of alternatively presented random and symmetric dot-patterns, the symmetric patterns elicited a sustained posterior negativity with ~ 220-ms Trichostatin A onset, whereas random patterns elicited positivity with earlier onset (~ 130 ms) (Norcia et al., 2002). Such activities Uroporphyrinogen III synthase were considered to be correlates of the appearance of global forms, i.e. an activity more general than a specific symmetry effect. In the present study, we tested

whether the system underlying vMMN is sensitive to symmetry as a perceptual category. If this is so, the regular presentation of stimuli belonging to the same perceptual category (symmetry) will establish a mental representation containing the sequential rule of the stimulation. Irregular stimuli (which do not belong to this category) will violate the prediction that derives from mental representation, and therefore elicit the vMMN component. For this reason, we infrequently embedded symmetric patterned stimuli (deviants) in a series of random patterned stimuli (standards), whereas, in another condition, random deviants appeared in the context of symmetric standards. Thus, we could compare the ERPs elicited by categorically identical standard and deviant stimuli. We expect an ERP difference between the deviant and standard random pattern; we hypothesise that the ERP difference is a vMMN, i.e. a posterior negativity within the 100–300-ms latency range.