Effects of intensity variation on human auditory evoked magnetic fields.

We recorded auditory evoked magnetic fields from 6 healthy subjects with a 122-channel whole-head neuromagnetometer. The stimuli were 200-ms 1-kHz tones delivered at 4 different intensities (40, 50, 60, and 65 dB HL). The tones were given once every second, binaurally in the first session, and monaurally to each ear in the second one. The four intensities were presented randomly and equiprobably within a single sequence. In both stimulus conditions, the 100-ms response (N100m) decreased in latency and increased in amplitude as a function of intensity in both hemispheres. No systematic dependence was found between stimulus intensity and the N100m source location in the auditory cortex. Our study illustrates a noninvasive method to examine the functional properties of human auditory cortex, allowing simultaneous comparison between signals arising from both hemispheres.