D. Brenner

Somatically evoked magnetic fields of the human brain.

The human brain is found to produce a magnetic field near the scalp which varies in synchrony with periodic electrical stimulation applied to a finger. Use of a highly sensitive superconducting quantum interference device as a magnetic field detector reveals that the brains field is sharply localized over the primary projection area of the sensory cortex contralateral to the digit being stimulated. The phase of the response at the stimulus frequency varies monotonically with the repetition rate and at intermediate frequencies yields a latency of approximately 70 milli-seconds for cortical response.

Visually evoked magnetic fields of the human brain

Magnetic field variations from the human brain produced by visual stimulation have been observed in a normal laboratory setting with a superconducting quantum interference device and no magnetic shielding of the subject. Previously unknown temporal and spatial features of the field near the scalp are reported.

Characterization of the human auditory cortex by the neuromagnetic method

SummaryNeuromagnetic studies show that the location of cortical activity evoked by modulated tones and by click stimuli in the steady state paradigm can be determined non-invasively with a precision of a few millimeters. The progression of locations for tones of increasing frequency establish an orderly tonotopic map in which the distance along the cortex varies as the logarithm of the frequency. The active region responding to clicks lies at a position that is consistent with this map if the stimulus is characterized by the frequency of the peak of its power spectrum. A latency of about 50 ms observed for the response to clicks is in close correspondance with a strong component of the transient response to an isolated click reported in the literature. Monaural stimulation of the ear contralateral to the hemisphere being monitored produces a latency which is about 8 ms shorter than stimulation of the ipsilateral ear, in agreement with previous studies of transient responses. The amplitudes of the responses for binaurally presented clicks for sleeping subjects is substantially diminished for repetition rates above 20 Hz but is enhanced for lower rates.

On the relation between somatic evoked potentials and fields.

Recently Okada et al. (1981) reported that stimulation of the median nerve with a brief electrical impulse at the wrist evoked a transient change in the brains magnetic field. This somatic evoked field (SEF) is similar in its temporal waveform to the response to the same stimulus reported for the electrical potential recorded on the pial surface of the exposed brain. Moreover, both measures differ substantially from the somatic evoked potential (SEP) recorded at the scalp. The present paper describes a more detailed account of the SEF as well as an analysis of its relation to the SEP and to the somatic pial response (SPR). Its purpose of the use the three measures in clarifying our understanding of the nature and locations of sources of the SEF. This paper is divided into three sections. The first is a background section which reviews basic principles and models that are widely used in deducing the locations of sources of evoked potentials and fields. It indicates the types of currents which may give rise to the SEF, and distinguishes between them and the current which is associated with the SEP. It concludes with a rationale for the experiments described in the next section. The experiments described in the second section determined how the SEF varies with the position from which it is recorded at the scalp. These variations turn out to be essential to our understanding of the nature and location of the sources of the SEF. The third section summarizes the results of the experiments and makes clear how they affect theories of the origin of the SEF. The findings also have implications for our understanding of the SEP and SPR. The most salient findings are: (1) The SEF recorded normal to the head provides essentially the same information as that provided by reported potential recordings from the exposed surface of the brain (the SPR). (2) The SEF originates in the cortex of the cerebrum in the vicinity of the central sulcus. (3) The currents that account for identifiable components of the SEF are opposite in direction to those that account for corresponding components of the SPR. This result is consistent with models that ascribe the detected field normal to the scalp to intracellular currents, whereas the VEP is associated with extracellular currents flowing in the opposite direction.

Modulation transfer functions of the human visual system revealed by magnetic field measurements

Properties of a neural source of magnetic field localized in the occipital lobe was measured in a steady-state paradigm using contrast reversing gratings. Comparisons with scalp potentials provided evidence that the evoked field was associated with intracellular currents. Its modulation transfer functions were similar to the analogous functions for the scalp potential and the detection of a grating. Moreover, the amplitude of the evoked field was linearly related to the potential amplitude and their phases were nearly identical. An analysis of the results in terms of theoretical relations between evoked field and potential led us to conclude that these two measures may yield a similar characterization of the source when one dipolar source predominantly gives rise to both measures, but they may yield complementary information when multiple sources contribute to the measures.

Evoked neuromagnetic fields of the human brain

Detectable magnetic fields are associated with electrical activity in the brain that may be evoked by sensory stimuli. Neuromagnetic fields as weak as 20 femtotesla can be studied with modern SQUID detectors without recourse to magnetic shielding. Studies in our laboratory and elsewhere reveal that the field patterns may be sharply localized near the appropriate area of the cortex for visual, somatosensory, or auditory stimuli. The temporal features of the magnetic response to visual stimuli reveal aspects of the organization of the visual system of the brain.

Application of a SQUID for monitoring magnetic response of the human brain

Noise characteristics are reported for a SQUID system which is sufficiently sensitive to detect visually evoked magnetic fields of the human brain without shielding. Discrimination against the much larger ambient background fields is obtained through use of a flux transporter with detection coils in the form of a second order gradiometer. The continuous spectrum of noise, coherent noise at isolated frequencies, and transient noise features are described. Examples of the spatial variation of the evoked neuromagnetic field are given with emphasis on those aspects which are important considerations in the design of effective detection systems.

Biomedical applications of SQUIDs

SQUID magnetic field detectors are currently used in nearly a dozen laboratories in the United States and Europe to study biomagnetic fields produced by various organs of the human body. Some of the more promising findings of clinical and fundamental interest are briefly reviewed. Emphasis is placed on which technological improvements in the performance of SQUID systems may prove beneficial in future biomedical applicaions.