Local Tissue Lesions as a Means of Assessing the Local or Remote Origin of Evoked Potentials within the Brain

Abstract

EVOKED potentials are often recorded ‘monopolarly’, that is, by means of an exploratory electrode referred to a distant ‘indifferent’ lead. Although this procedure makes it possible to determine the polarity of the observed deflexions and, in favourable conditions, to graphically reproduce the potential fields set up by the stimulus, its indiscriminate use is not without dangers, especially when large probes are used to explore some regions deep in the brain, the anatomical arrangement of which in ‘open’ nuclei¹ (that is, with long dendrites running in various directions far from the apparent limits of the nuclei) results in what has been defined as ‘open’ field of action currents²—a concept indicating that potential changes can be recorded monopolarly near the active pool as well as at distant points.