Electroencephalography effects to semantic and non-semantic mismatch in properties of visually presented single-characters: The N2b and the N400

Event-related potential (ERP) correlates of semantic and non-semantic mismatch in the 200-400 ms time interval were dissociated. Twelve subjects observed pairs of single letters and digits presented serially, and decided whether the colour (blue or red) or the category membership (letter or number) of the second item in a pair matched the first one. In both tasks mismatch elicited central N2b components (220 ms peak latency in the colour matching task, and 280 ms peak latency in the category matching task). Furthermore, in the category matching task a centro-parietal negativity (N400) emerged. The topography of the N2b effect was clearly distinct from that of the N400 effect. Analysis of event-related spectral perturbations confirmed that N400 time-range effects were specific to category matching. We conclude that while the N400 effect is a specific marker of semantic mismatch, the N2b effect is a general correlate of detecting mismatch between the representations of task-relevant features.