Source location of a 50 msec latency auditory evoked field component

We recorded auditory evoked magnetic fields in response to 128 15 msec duration 1 kHz tone pips from both hemispheres of 6 normal adult males. Auditory evoked potentials were recorded conventionally from a vertex lead. An approximately 50 msec latency component was identified in both the magnetic (called the M50) and EEG (called the P50) recordings. Isofield topographical contour maps were used to estimate M50 source location and depth. With respect to extracranial bony landmarks, M50 source locations were significantly higher and tended to be more posterior, over the left hemisphere. M50 and P50 latencies were not significantly different in 5 of 6 subjects; in one, M50 latencies were significantly longer than P50 latencies over the left hemisphere. Magnetic resonance images in 5 subjects were used to identify the neuroanatomical structure(s) present at the estimated source location. M50 sources appeared to reside in the cortex of the planum temporale in both left and right hemispheres in all 5 subjects.