The functional nature of the pre-attentive automatic auditory feature analysis was investigated using the mismatch negativity (MMN) component of the auditory event-related brain potential. MMNs to ignored sounds deviating in one, two or three dimensions (duration (D), frequency (F), intensity (I)) were recorded. When the corresponding MMN processes elicited by stimuli simultaneously deviating in multiple features are independent from each other, then the empirically measured MMN elicited by multiple deviants equals the sum of the MMINs elicited by the corresponding single deviants. Indeed, MMNs to double DF- and to double DI-deviants showed additivity for frontocentral as well as for subtemporal sites, whereas MMN to double FI-deviants and MMN to triple DFI-deviants were additive only at subtemporal sites indicating that frontal and temporal MMN generators reveal differential degree of additivity. This finding demonstrates that not all combinations of stimulus dimensions are processed independently from each other.