Event-related potentials reveal how non-attended complex sound patterns are represented by the human brain

Event-related potentials (ERPs) to complex auditory sound patterns consisting of eight 50-ms segments differing in frequency were recorded from the human scalp while the subject was performing a visual search task. Randomly occurring frequency changes of single segments elicited a 'mismatch negativity' although the subjects were not attending to the auditory stimuli. This negative deflection in the ERP waveform elicited by the changed pattern compared with the ERP waveform elicited by the standard pattern is consistent with the hypothesis that the spectro-temporal features of complex sound patterns are automatically and precisely represented in passive auditory sensory memory. The finding that an easily discriminable change, that is, a change with a high discrimination performance measured in a separate condition, elicited a larger mismatch negativity than a poorly discriminable change suggests that discrimination is based on these representations.