THE AUDITORY-EVOKED SUSTAINED FIELD - ORIGIN AND FREQUENCY-DEPENDENCE

A sound lasting for several seconds is known to elicit a basline shift in electrical and magnetic records. We have studied the dependence of the magnetic field distribution of this ''per-stimulatory'' sustained field (SF) on tone frequency. Tone bursts of 2 sec duration and 60 dB nHL intensity were presented to 11 subjects at varying interstimulus intervals between 5 and 7 sec. The carrier frequencies of 250, 1000 an d 4000 Hz varied randomly from trial to trial. The field distributions obtained are consistent with the view that the auditory evoked sustai ned field activity originates in the supratemporal cortex. Differences in the locations of equivalent current dipoles of the SF from those o f the M100 wave of the slow auditory evoked field are consistent acros s subjects. The SF source locations corresponding to stimulus frequenc ies over an extended frequency range are arranged in a tonotopic manne r and support the idea that the sources of the M100 and the SF are cur rent dipole sheets located on the superior surface of the primary audi tory cortex.