Change-N1 of event-related potentials. A, the sound stimulus with an abrupt change used in this study. B, brain potentials recorded at Fz and mastoids produced by the Standard stimulus with a 500-ms standard sound (800 Hz, 70 dB SPL) and Deviant stimulus with a 250-ms standard followed by a 250-ms deviant (75 dB). C, Magnetic fields evoked by auditory deviations. Left, superimposed waveforms obtained from 102 magnetometers (Vectorview; ELEKTA Neuromag, Heksinki, Finland) elicited by three deviant stimuli. The Standard stimulus was a 500-ms standard sound (800 Hz at 70 dB). The Deviant stimulus was a 250-ms standard sound followed without a blank by a 250-ms deviant sound (840 Hz, ITD 0.4 ms or 75 dB). Right, magnetic field distribution at the peak of change-N1. Deviant stimuli evoked a magnetic field response consistent with symmetric dipoles with an intracellular current directed to the mastoid. These dipoles in both hemispheres are expected to create a positivity at both mastoids and a larger negativity (summation) at Fz. D, a similar change-N1 was elicited when the subject ignored (blue) and attended (pink) the sound. E, four consecutive recordings of 200 trials when the subject watched a movie and ignored the stimulus. F, effects of the abrupt change on change-N1. When a 500-ms sound at 70 dB and a 500-ms sound at 75 dB were presented at an even probability (b), change-N1 was not elicited, which clearly contrasts with the upper trace (a) when the abruptly changing deviant was used. Arrowheads indicate the change onset.