Physically deviant auditory stimuli occurring among frequent ("standard") stimuli (e.g., tones or phonetic stimuli) elicit the mismatch negativity (MMN) of the auditory event-related potential (ERP). The MMN is presumably generated by a mismatch process between the sensory input from a deviant stimulus and a neural sensory-memory trace representing the physical features of the standard stimulus. This process, as well as sensory analysis of auditory input and its encoding into the memory trace, appear to be automatic since the MMN is elicited even by changes in unattended auditory stimuli. Therefore the MMN indirectly provides a unique, objective measure of the central representation of a sound. This opens new possibilities for basic research as well as clinical and other applications.