Rare stimuli or rare changes: what really matters for the brain?

Previous research has demonstrated that the electric brain response to changes consists of a negative deflection with a latency of 200 ms post-stimulus. Two different hypotheses, namely the mismatch and the rareness accounts, have been invoked to explain this electrocortical response. In the present study, this negative component emerged only for visual changes caused by the presentation of rare stimuli, documenting a precise brain response to rareness, and not to change per se. Crucially, no specific electrophysiological marker was evident when the change was rare, but consisted of stimuli frequently seen. Hence, we suggest that changes are preferentially processed by the brain only when they involve the occurrence of new and rare stimuli.