Simple and complex acoustic regularities are encoded at different levels of the auditory hierarchy.

The detection of auditory stimuli that deviate from a simple or complex auditory regularity is reflected by the mismatch negativity component of the human auditory evoked potential. Moreover, simple deviants of an oddball paradigm modulate the preceding middle-latency response of the auditory evoked potential. For the frequency oddball paradigms it has been shown that the Nb wave, at approximately 40 ms from stimulus onset, is enhanced in response to deviant compared with standard stimuli. In this study we tested whether the detection of auditory deviants in a (frequency-location) feature-conjunction paradigm is reflected by modulations of the Na, Pa or Nb wave of healthy human participants. In addition, a frequency oddball paradigm was applied to directly contrast the results of a simple and a complex invariance. Feature-conjunction deviants did not elicit any modulations of the tested middle-latency waves. Deviants of the frequency oddball paradigm, by contrast, elicited an enhancement of the Nb wave, confirming the outcome of precedent studies. In both conditions a significant mismatch negativity component was elicited, which showed larger amplitudes and shorter latencies in the oddball condition than in the feature-conjunction condition. These findings corroborate the idea that simple auditory regularities are encoded upstream of those of more complex auditory features and are in line with the idea of a hierarchically working auditory novelty system.