On the Relation Between Somatic Evoked Potentials and Fields

Abstract

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 brain's 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 is to 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 the 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.