J. E. Zimmerman

Magnetic auditory evoked fields: Interhemispheric asymmetry

Magnetic auditory evoked fields (MAEFs) were recorded from the right (11 subjects) and the left (7 subjects) hemisphere following 128 click stimuli delivered to contralateral, ipsilateral and both (bilateral) ears. Right hemisphere MAEFs were of higher amplitude following contralateral, compared to ipsilateral, stimulation in 9 of 11 subjects; mean contralateral response amplitude was 135 +/- 33% (S.D) of ipsilateral response amplitude. Left hemisphere MAEFs were of higher amplitude following contralateral stimulation in 7 of 7 subjects; mean contralateral response amplitude was 145 +/- 44% of ipsilateral response amplitude. These observations are compatible with evidence that a majority of centripetal auditory input is crossed, and/or that contralateral auditory stimulation activates a larger area of cortex than does ipsilateral stimulation.

SQUID instruments and shielding for low‐level magnetic measurements

We have used a thick‐walled aluminum enclosure to provide low‐frequency shielding for biomagnetic studies with SQUID magnetic gradiometers. The shield provides an attenuation proportional to frequency above 1/3 Hz. A 24‐hole fractional‐turn SQUID gradiometer has been used in the enclosure to carry out studies of magnetoencephalography on humans. An ’’asymmetric’’ configuration for a second‐derivative gradiometer has been developed which provides a significant increase in sensitivity and resolution over more conventional symmetric arrays.

A comparison of three types of pulse tube refrigerators: new methods for reaching 60K

Pulse tube or thermoacoustic refrigerators require only one moving part—an oscillating piston or diaphragm at room temperature. Refrigeration occurs within a tube connected to the pressure wave generator when the thermal relaxation time between gas and tube is comparable to a half period. Three types have been discussed in the literature recently by Gifford, by Mikulin, and by Wheatley. A record low temperature of 60 K was achieved in our work using a single stage pulse tube similar to that of Mikulin. Previously 105 K was the lowest temperature achieved. Because of only one moving part, all three types have the potential for long life, but their efficiency and intrinsic limitations have never been investigated. This paper compares the three types with each other and with common refrigerators such as Joule-Thomson and Stirling refrigerators. An apparatus is described which can measure the intrinsic behavior of the different types from temperatures of about 30 K to 300 K. Overall cycle efficiency as well as sources of loss such as conduction and regenerator ineffectiveness are discussed and the advantages of various phase shifting techniques to increase refrigeration capacity are compared.

Human magnetic auditory evoked fields.

Magnetoencephalographic (MEG) averaged auditory evoked fields to click stimuli (N = 512) were recorded from four human subjects. The MEG was recorded with an asymmetric second derivative SQUID gradiometer located in an aluminum shielded room. Unlike conventional EEG auditory evoked potentials, which have a widespread distribution, evoked magnetic fields appear to be localized to the general area of the primary auditory cortex and diminish rapidly in amplitude as the gradiometer is moved away in any direction.

Generation of Harmonics and Subharmonics of the Josephson Oscillation

The observation of harmonics and subharmonics of the Josephson oscillation is shown to be in agreement with a rather simple model of the junction. The generation of harmonics provides an explanation of induced steps in the current‐voltage characteristic which occur at submultiples of the usual induced step voltages. The subharmonic oscillation is seen to be a relaxation‐like process which can be easily understood in terms of a mechanical analog.

The human magnetoencephalogram: some EEG and related correlations

Simultaneous magnetoencephalographic (MEG) and electroencephalographic (EEG) data were recorded from six normal adult subjects. MEG signal strength and EEG voltage level appear to be linearly correlated. Spectral analysis suggested that the MEG and EEG data were produced by similar but non-identical generator systems. A vertex region magnetic averaged evoked response to flash was recorded in one (of four) subjects, consisting of a waveform similar to but out of phase with the simultaneous EEG averaged evoked response, such that cortical negativity was correlated with a magnetic field directed into the scalp. Eye movement artifact, which can seriously compromise EEG recordings, does not appear to be a major problem in MEG recordings.

Source origin of a 50-msec latency auditory evoked field component in young schizophrenic men

We recorded auditory evoked magnetic fields in response to 128 15-msec duration 1-KHz tone pips from both hemispheres of 6 young schizophrenic men. Auditory evoked potentials were recorded conventionally from a vertex lead. The approximately 50-msec latency component was identified in both the magnetic (M50) and electroencephalographic (EEG) (P50) recordings. Isofield topographical contour maps were used to estimate M50 source location and depth. Magnetic resonance imaging was used to identify the neuroanatomical structure(s) present at the estimated source location. M50 sources appeared to reside in the planum temporale in both left and right hemispheres in all subjects. Normal inter-hemispheric asymmetry (with respect to external bony landmarks) of the M50 source was not found in this patient group. Additionally, left (but not right) hemisphere source anatomy differed in several respects from data previously reported in normals.

Auditory evoked magnetic fields: Response amplitude vs. stimulus intensity

Abstract Auditory evoked magnetic fields (AEFs) and EEG auditory evoked potentials (AEPs) were recorded from the right hemisphere of 24 normal subjects. MEG and EEG recordings were made from a point 1/4 of the distance from T4 to C4, in response to contralateral ear stimulation by irregularly spaced 100 msec long 1 kHz tone bursts at 40, 60, 80 and 100 dB sound pressure level (SPL). A figure-8 SQUID gradiometer was used for magnetic recordings. MEG and EEG, bandpassed between 2 and 40 Hz, were averaged for 500 msec following 128 stimuli. A mean response was computed for the group of 24 subjects at each stimulus intensity, and the integrated area under the curve of this mean response for the first 200 msec was obtained. The amplitudes of the largest responses occurring within the first 250 msec after stimulus delivery were also measured, as was the amplitude of a biological noise (no stimulus) control. The amplitude of AEFs increased rapidly to approximately 60% of maximum at 60 dB SPL, with the field amplitudes produced by 60, 80 and 100 dB stimuli tending to plateau or even decrease slightly. In contrast, the amplitude of the simultaneously recorded AEPs increased linearly with increasing stimulus intensity. It is suggested that the plateau effect or decrease of the AEF amplitude with increasing stimulus intensity may be caused by an inward movement of the current dipole with increasing stimulus intensity and/or may reflect primarily local intracellular currents associated with single unit activation patterns in auditory cortical regions, whereas AEPs may reflect more widespread extracellular currents.

MEG and EEG auditory responses to tone, click and white noise stimuli ☆

We report the results of two experiments comparing the amplitude of MEG auditory evoked fields (AEFs) and EEG auditory evoked potentials (AEPs) to tone, click and white noise stimuli. In the first experiment, AEFs and AEPs, bandpassed between 2.0 and 40 Hz, were averaged for 500 msec following 128 irregularly spaced alternating 92 dB clicks and 88 msec long 2 kHz tones, in 16 adult human subjects (8 male, 8 female). Recordings were made from both hemispheres, one side at a time, at a position of the way up from T3 to C3 and T4 to C4. The magnetic sensor was a figure-8 SQUID gradiometer with a 4 cm baseline oriented for maximum sensitivity to a current dipole(s) perpendicular to the sylvian fissure. The dependent variable was the maximum amplitude of the responses occurring within 250 msec of stimulus delivery. A 4-way (2b × 2w × 2w × 2w) ANOVA was used for statistical analysis of amplitude data examining differences between sex, laterality (contralateral vs. ipsilateral), auditory stimuli (click vs. tone) and hemisphere (left vs. right). AEFs in response to tone stimuli were of higher amplitide than those for click stimuli, for both left and right hemispheres, and for both contralateral and ipsilateral ear stimulation (F = 65.2, df = 1, 14, P < 0.001). The opposite relationships (click responses of greater amplitude than tone responses) were found with EEG AEPs (F = 10.4, df = 1, 14, P < 0.01). AEFs and EEG AEPs were both of higher amplitude in response to contralateral compared to ipsilateral ear stimulation, when averaged across both left and right hemispheres, both clicks and tones, and both sexes. The second experiment, using 8 subjects, was identical except white noise bursts were used as stimuli instead of clicks. We found significantly higher amplitude AEFs to tones compared to equal length and SPL white noise bursts, with a non-significant trend for the reverse to be true for AEPs. Overall, tones appeared to produce the highest amplitude AEFs, and clicks the lowest amplitude, with white noise stimuli being intermediate. AEF amplitude as a function of stimulus type parallels previously reported auditory cortex unit activation by similar stimuli, and thus AEFs may more closely reflect cortical unit activity patterns, whereas other influences appear operative in influencing AEP amplitude.

Magnetic auditory evoked fields: Dipole orientation

Magnetic auditory evoked fields (MAEFs) were recorded from the left hemisphere of 7 normal human subjects in response to 512 binaural click stimuli. The instrument was a figure-eight SQUID gradiometer that measured the transverse gradient of the field perpendicular to the scalp. Its response to a current dipole source inside the head was a cosine function of the angular orientation of the gradiometer about an axis perpendicular to the scalp. Measurements were made at various orientations from which were derived the orientation of the equivalent dipole source. Our findings suggest that when considering total magnetic energy, current dipoles in the vicinity of the transverse gyri of Heschl, whose primary orientation are approximately perpendicular to the sylvian fissure, appear to be the major contributors to the magnetic fields produced by click stimuli.