THE AIM OF THIS SERIES OF experiments was to determine whether consonant and dissonant chords elicit similar or different electrophysiological effects out of a musical context and whether these effects are similar or different for musicians and nonmusicians. To this end, w e recorded t he e vent-related b rain potentials (ERPs) elicited by the different intervals of the chromatic scale that were classified into three categories: perfect consonances, imperfect consonances, and dissonances. Participants were to decide, on a six-point scale, whether the intervals evoked pleasant or unpleasant feelings. To test the hypothesis that the perception of dissonance results from the superposition of the partials of close frequencies (Helmholtz, 1877), two notes were either played together (harmonic intervals) or successively (melodic intervals). Since, in this latter case, the two notes are played at different points in time, the perception of roughness, if any, should be weaker than for harmonic intervals. In line with Helmholtz's hypothesis, results showed larger differences for harmonic than for melodic intervals, which were mainly found on the N1-P2 complex for musicians, on the N2 component for nonmusicians, and on a later negative component for both musicians and nonmusicians. However, these results also point to the influence of expertise and cultural factors, since different results were obtained when ERPs were averaged as a function of music theory and according to the participants' responses.