Yoshio C. Okada

THE HIPPOCAMPAL FORMATION AS A SOURCE OF THE SLOW ENDOGENOUS POTENTIALS

Magnetic fields were detected with a SQUID sensor at the temporal and occipital areas of the head in response to a frequent and an infrequent attended visual stimulus. The time-course of the magnetic field for the infrequent stimulus correlated highly with the simultaneously measured electrical potential that showed the commonly observed N2-P3 complex. Analysis of the pattern of the magnetic field showed that the sources of N2 and P3 lay deep in the brain within the hippocampal formation.

Somatotopic organization of the human somatosensory cortex revealed by neuromagnetic measurements

SummaryThe primary projection areas in the human somatosensory cortex activated by electrical stimulation of the digits of the hand and the ankle were localized by measuring the magnetic field outside the head contralateral to the side of stimulation. Most of the spatial variation in the amplitude of the field component normal to the scalp could be accounted for by representing each source as a single current dipole in a spherical conducting medium with solely concentric variations in electrical conductivity, although the fit of this model to the data showed some statistically significant deviations. Based on the best-fitting parameter values of the model, we found that the projection areas of the thumb, the index finger, the little finger and the ankle were located at successively more medial positions along the primary somatosensory cortex, at an average depth of 2.2 cm from the scalp surface.

Quisqualate, kainate and NMDA can initiate spreading depression in the turtle cerebellum

This study evaluated the role of excitatory amino acid (EAA) receptor activation in spreading depression (SD), using the in vitro turtle cerebellum as a model system. SD was triggered by electrical stimulation or by elevated K+ after the cerebellum had been conditioned for at least 30 min with physiological saline in which most of the chloride had been replaced by propionate. SD was recognized as a transient (1-3 min) negative shift of extracellular potential accompanied by depression of evoked potentials (15-30 min) and an increase of extracellular K+ up to 60 mM, which spread across the cerebellum at rates of 1-7 mm/min. SD usually commenced in the granular layer, which apparently contains the 3 major EAA receptor subtypes, quisqualate, kainate and N-methyl-D-aspartate (NMDA), then subsequently spread to the molecular layer, which is largely free of NMDA receptors. Glutamate, aspartate, NMDA, kainate and quisqualate all triggered SD. Kynurenic acid and 2-aminophosphonovaleric acid (APV) inhibited SD under certain conditions further suggesting involvement of EAA receptors. The initiation of SD was blocked by high Mg2+ and facilitated in low extracellular Mg2+, which also eliminated the delay in molecular layer SD onset. Our data suggest that no one EAA receptor subtype is singly responsible for SD.

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.

Magnetic field associated with spreading depression: a model for the detection of migraine

Slow variations of the magnetic field were recorded in real time during spreading depression (SD) in the isolated turtle cerebellum. The magnetic signal lasted for 2-10 min with the largest amplitude in the first minute. The field strength was of sufficient magnitude to be measured unaveraged at 2-4 cm from the tissue. The directions and time course of the magnetic signal indicated that cerebellar SD is accompanied by current normal to the cerebellar surface. The observations reported here are of clinical interest due to the potential involvement of SD in various neurological disorders, notably head trauma and migraine.

Magnetic field of the human sensorimotor cortex

A magnetic field associated with voluntary finger flexion was found to be confined over a well-defined region of the scalp overlying the sensorimotor cortex contralateral to the finger. The magnetic field had opposite directions over two regions of the scalp superior and inferior to the classical finger area of the cortex, implying that the field was generated by a source or sources in this area. The field whose onset begins approximately 50 ms prior to muscle activation was associated with a cortical source lying about 2.8 cm beneath the scalp and the active area shifted posteriorly 3-6 mm into the somatosensory area during the finger movement. Consistent with this shift, the early component of the magnetic field prior to flexion of a finger was absent when it was passively moved by the finger of the opposite hand, but the later components were present.

Distortion of magnetic evoked fields and surface potentials by conductivity differences at boundaries in brain tissue

We investigated the conditions under which inhomogeneity in electrical conductivity may significantly modify the magnetic evoked field (MEF) due to primary currents (i.e., neuronal currents) in the brain. In the case of an isolated turtle cerebellum immersed in a large bath of physiological saline, our theoretical analysis showed the cerebellar surface to significantly enhance the MEF due to a primary current, by a factor of as much as two, for experimentally determined values of the conductivities of the cerebellar tissue and saline. A further parametric investigation of the conductivity effect revealed that conductivity boundaries may significantly modify the MEF due to neuronal currents located within 1 mm of a conductivity boundary, as would be the case for active neurons near an edema, an anoxic fringe such as might occur during stroke, or a ventricle in the human head. For a stationary neural source, conductivity boundaries may modify the magnitude of its MEF without affecting its temporal waveform. However, this boundary effect was found to be small for a model geometry locally approximating cortical sources in a sulcus or a fissure, where the boundary effects from adjacent sulcal walls tend to cancel each other.

Magnetic evoked field associated with transcortical currents in turtle cerebellum.

The neural basis of magnetic evoked fields of the brain was studied with an isolated turtle cerebellum as a model preparation. The turtle cerebellum is a nearly flat tissue with neural processes arranged along three orthogonal axes of symmetry. According to theoretical results, this geometry should enable us to selectively measure the magnetic field due to a subpopulation of nerve cells whose longitudinal axes are perpendicular to the cerebellar surface, by simply placing the cerebellum vertically in the bath so that these cells are horizontal and by measuring the field along the rostrocaudal axis perpendicular to the longitudinal axis of these nerve cells. To elicit neural activity in these cells the dorsal midline was electrically stimulated with a bipolar electrode. Consistent with our expectations, the one-dimensional profile of the field normal to bath surface (Bz) was antisymmetrical along the rostrocaudal axis, implying that the underlying currents were transcortical. Also, the Bz field at a field extremum varied as a cosine of the orientation of the cerebellum when it was rotated about its rostrocaudal axis with the amplitude being zero when the cerebellum was horizontal. The Bz field was dipolar as judged by statistically excellent fits of the dipolar field to the one-dimensional field profile and to the distance function relating the field magnitude at an extremum to measuring distance. This was the case even for the initial component thought to be due to antidromic action currents invading the soma and dendrites of Purkinje cells. We also showed that the dipolar term of the source could be localized within 1 mm of the actual source location in most cases.

MEG source models and physiology.

We report in vitro experiments on the source(s) of the magnetic fields produced by the brain. Theoretical arguments suggest that the dominant sources should be dipolar and oriented parallel to the scalp. Using an isolated turtle cerebellum as a model, we find that the fields produced following dorsal stimulation are attributable to current flow perpendicular to the cerebellum surface, suggesting Purkinje cell sources. We also discuss observations of longer lasting fields associated with spreading depression induced in the cerebellum.

Retinotopic map on the visual cortex for eccentrically placed patterns: First noninvasive measurement

SummaryThe observed cortical magnetic field evoked by a stimulus presented at various eccentricities in the visual field was interpreted as arising from current dipoles along the longitudinal fissure. The depth of the source increased as the eccentricity was increased, in agreement with the classical retinotopic map.RiassuntoIl campo magnetico corticale osservato, suscitato da uno stimolo presentato a varie eccentricità nel campo visivo, è stato interpretato come derivante da dipoli di corrente lungo la scissura longitudinale. La profondità della sorgente aumenta con l’aumento dell’eccentricità, in accordo con la classica mappatura retinotopica.