An electrophysiological index of stimulus unfamiliarity

This study investigated the functional significance of the N2 response to n ovel stimuli. In one condition, background, target, and deviant stimuli wer e simple geometric figures. In a second condition, all stimulus types were unfamiliar/unusual figures. In a third condition, background and target sti muli were unusual figures and deviant stimuli were simple shapes. Unusual f igures, whether they were deviant, target, or background stimuli, evoked la rger N2 responses than their simple, familiar counterparts. N2 elicited by an unusual background stimulus was larger than that evoked by simple, devia nt stimuli, a pattern opposite that exhibited by the subsequent P3. Devianc e from immediate context had limited influence over N2 amplitude. The resul ts suggest that novelty N2 and novelty P3 reflect the processing of differe nt aspects of "novel" visual stimuli. The novelty P3 is particularly sensit ive to deviation from immediate context. In contrast, the novelty N2 is sen sitive to deviation from long-term context that readers a stimulus unfamili ar and difficult to encode.