Dissecting the Effect of Certain and Uncertain Changes in Stimulus Modality

■ The repetition of nociceptive stimuli of identical modality, intensity, and location at short and constant interstimulus inter-vals (ISIs) determines a strong habituation of the correspond-ing EEG responses, without affecting the subjective perception of pain. To understand what determines this response habitua-tion, we (i) examined the effect of introducing a change in the modality of the repeated stimulus, and (ii) dissected the relative contribution of bottom–up, stimulus-driven changes in modality and top–down, cognitive expectations of such a change, on both laser-evoked and auditory-evoked EEG responses. Multichannel EEG was recorded while participants received trains of three stim-uli (S1–S2–S3, a triplet) delivered to the hand dorsum at 1-sec ISI. S3 belonged either to the same modality as S1 and S2 or to the other modality. In addition, participants were either explic