This paper presents data concerning auditory evoked responses in the middle latency range (wave Pam/ Pa) and slow latency range (wave Nlm/Nl) recorded from 12 subjects. It is the first group study to report multi-channel data of both MEG and EEG recordings from the human auditory cortex. The experimental procedure involved potential and current density topographical brain mapping as well as magnetic and electric source analysis. Responses were compared for the following 3 stimulus frequencies: 500, 1000 and 4000 Hz. It was found that two areas of the auditory cortex showed mirrored tonotopic organization; one area, the source of Nlm/Nl wave, exhibited higher frequencies at progressively deeper locations, while the second area, the source of the Pam/Pa wave, exhibited higher frequencies at progressively more superficial locations. The Pa tonotopic map was located in the primary auditory cortex anterior to the Nlm/Nl mirror map. It is likely that Nlm/Nl results from activation of secondary auditory areas. The location of the Pa map in AI, and its NI mirror image in secondary auditory areas is in agreement with observations from animal studies.

We have discovered a ca. 40 (22,23) have identified a transient electric GBR locked in time to the onset of auditory stimuli that increases in amplitude as the stimulus rate is decreased below one per second. We report here measurements of a comparable magnetic GBR, using a new-generation large-array biomagnetometer. Large-array magnetic measurements allow accurate and reliable localization of the source of the magnetic GBR in relation to the better studied M100 and M200 slowwave (low-frequency; see below) components of the field evoked by auditory stimuli (24). METHODSThe auditory stimulus-evoked magnetic brain activity generated in the left temporal cortex of 20 right-handed adult subjects was recorded in a magnetically shielded room with a 37-channel biomagnetometer (Biomagnetic Technologies, San Diego, CA). The response of one subject was recorded in 30 blocks of 128 stimuli. Three to six blocks were recorded from each of the other subjects. During the measurements subjects lay on their right side with eyes open and were asked to remain alert. Stimuli were 1000-Hz tone bursts of 60 dB relative to normal hearing level (nHL) (80-dB nHL in one session) with 500-ms duration and 10-ms rise and fall times, presented to the right ear at interstimulus intervals of 4 s. Blocks of 128 stimulus-related epochs of 1000 ms were recorded with a 200-ms prestimulus baseline. Electroencephalographic epochs were recorded simultaneously from one derivation (C,-earlobe).The neuromagnetic field pattern was recorded over a circular area 144 mm in diameter above the auditory cortex through 37 axially symmetric first-order gradiometer pickup coils (diameter, 20 mm; baseline, 50 mm). Each coil was connected to a superconducting quantum interference device (SQUID) sensor that produced a voltage proportional to the magnetic field radial to the coil. The intrinsic noise level was <10 ff/Hzl/2 in all but one of the channels. A sensor position indicator system determined the spatial locations of the sensors relative to the head and insured that no head movements occurred during the measurements. In successive sessions, the magnetometer was carefully repositioned to allow response comparison and grand averaging. Blocks of 128 stimulus-related epochs of 1000 ms were recorded with a 200-ms prestimulus baseline in the passband of0.1-95 Hz and a sampling rate of 215 Hz. The wide-band responses were first averaged with artifact rejection and then digitally filtered in two passbands, 1-20 Hz and 28-48 Hz, to separate the evoked gamma-band and slow-wave field components.For each subject a spherical model was fit to the digitized head shape, and the location, orientation, and amplitude ofan equivalent current dipole tangential to the surface of the model sphere (25, 26) were estimated in each passband for each point in time. The origin of the coordinate system used is the midpoint between the preauricular points; the x-axis joins the origin to the nasion; the y-axis passes between the preauricular points; the z-axis is perpendicular to th...

MAGNETIC source imaging revealed that the topographic representation in the somatosensory cortex of the face area in upper extremity amputees was shifted an average of t.5 cm toward the area that would normally receive input from the now absent nerves supplying the hand and fingers. Observed alterations provide evidence for extensive plastic reorganization in the adult human cortex following nervous system injury, but they are not a sufficient cause of the phantom phenomenon termed 'facial remapping'.