Evoked brain potentials: how far have we come since 1875?

Abstract

Richard Caton’s original publication, from which it all started, is remarkable alike for its brevity and high information content¹. In one succinct paragraph, he gives a clear account of the spontaneous electrical activity of the brain, of motor potentials and of sensory evoked responses. Even a modern abstracting service would be hard put to it to do better! His experiments, on the brain of the rabbit or monkey, were carried out using a galvanometer originally invented by William Thompson, Lord Kelvin, an instrument whose response time necessarily limited it to the recording of what would now be called ‘slow potentials’. He notes the invariable presence of spontaneous electrical activity: ‘in every brain hitherto examined, the galvanometer has indicated the existence of electric currents’; and the occurrence of negative waves associated with localized function: ‘When any part of the grey matter is in a state of functional activity, its electric current usually exhibits negative variation’. He also described clearly both motor potentials, related to head-turning and mastication, and visually evoked potentials: ‘On the areas shown by Dr Ferrier to be related to rotation of the head and to mastication, negative variation of the current was observed to occur whenever those two acts respectively were performed. Impressions to the senses were found to influence the currents of certain areas; e.g. currents of that part of the rabbit’s brain which Dr Ferrier has shown to be related to movements of the eyelids, were found to be markedly influenced by stimulation of the opposite retina by light.’ Few of us can boast of covering anything like as much new ground even in our longest monographs!