Human auditory evoked gamma-band magnetic fields.

Abstract

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...

Gamma and beta neural activity evoked during a sensory gating paradigm: Effects of auditory, somatosensory and cross-modal stimulation

“…Note first that the gamma-band response contributes a large positive-going wave to ERP component Pa, which corresponds to activation of the primary auditory cortex (Kraus et al, 1982;Woods et al, 1987). This is consistent with the observation that the early stimulus-evoked gamma band activity arises from sensory cortical regions (Barth and MacDonald, 1996;Pantev et al, 1991). Note that the peak amplitude of this waveform receives relatively little contribution from beta band activity, which was shown in the present study to be particularly sensitive to ISI.…”

“…Further supporting evidence includes the finding that gamma oscillations are generally localized to specific sensory cortical regions (Barth and MacDonald, 1996;Pantev et al, 1991;Pantev, 1995) whereas beta activity exhibits more widespread expression across cortex and more robust synchronization between cortical regions including association areas (Roelfsema et al, 1997;von Stein et al, 1999). Nevertheless, applicability of this theory to a general neural model of stimulus-driven salience processing requires empirical demonstrations that other relevant properties, such as novelty and ISI, also appropriately modulate oscillatory neuron population activity, especially in the beta band.…”

Generation of human auditory steady-state responses (SSRs). II: Addition of responses to individual stimuli

“…The latency range of the observed additional activity raises the question of its relationship with the transient gamma band response (GBR) which has been recently discovered by electric (Makeig and Galambos, 1989) and magnetic (Pantev et al, 1991) encephalographic studies. It has been suggested that this response does not participate in the generation of the 40 Hz-SSR since these neural activities originate from separate cortical sources (Pantev et al, 1993).…”

Differences of temporal dynamics and signal complexity of gamma band oscillations in first-episode psychosis during a working memory task

“…Our findings of significant differences of gamma band activity in the lag time of 2-4 ms coincides directly with the thalamocortical conduction time (Albe-Fessard et al 1986;Allison et al 1991;Pantev et al 1991;Ribary et al 1991). We also demonstrated significant differences for gamma oscillations in the lag time of 15 ms, which refers to perceptual binding occurring in a time segment of &15 ms (Joliot et al 1994;Llinas and Ribary 2001) on the basis of synchronization (i.e.…”