PITCH CHANGE OF A CONTINUOUS TONE ACTIVATES 2 DISTINCT PROCESSES IN HUMAN AUDITORY-CORTEX - A STUDY WITH WHOLE-HEAD MAGNETOMETER

Previous studies have shown that a frequency change in a continuous to ne elicits an NI type of ERP (event-related potential) component. It r emained unclear, however, whether this response is a ''genuine'' N1 (o nset detector response) or the mismatch negativity (MMN), a change-det ector type of ERP response, elicited in previous studies by an infrequ ent change in a sequence of homogeneous stimuli. A further possibility is a nearly perfect overlap of the two types of ERP components. The a dvent of modern, high-resolution magnetometers has opened a new, power ful way to tackle such component-overlap problems. Subjects were prese nted with a continuous tone of 988 Hz which was occasionally increased to 1108 Hz for a period of 100 msec. The magnetic responses to this c hange consisted of two partially overlapping components with peaks sep arated by 30 msec. The earlier component was probably generated by neu ronal populations of tile auditory cortex corresponding to the suprate mporal N1. whereas the later one, generated anteriorly and inferiorly to the first, probably reflects a mismatch process causing the magneti c equivalent of the electrical MMN.