Sequences of identical acoustic stimuli were presented to normal subjects reading a book while event-related brain potentials (ERP) elicited by these stimuli were recorded. Occasional irrelevant decreases and increases in stimulus duration elicited an ERP component called the mismatch negativity (MMN). This component was larger over the right hemisphere irrespective of the ear stimulated. These data implicate memory representations which develop automatically and represent the physical features of the repetitive stimulus accurately. Further, when an input does not match with such a trace the MMN is generated. The memory traces involved appear to be those of the acoustic sensory memory, the 'echoic' memory.