THE PRESENT WORK is part of a more general investigation into neuro-physiological activities of the cerebral cortex which may be involved in the elaboration or mediation of conscious sensation. The primary concern in these studies is the dynamics of the processes involved, rather than their topographical representation.

Production of threshold levels of conscious sensation by electrical stimulation of human somatosensory cortex

Most problems in neurophysiology can be attacked more fruitfully in animals other thar, man, for the obvious reasons of controllability of conditions and of our moral restraints on the experimental procedures which are tolerable for human studies. But if one wants to investigate cerebral mechanisms underlying subjective experience (of sensation, in the present context), it should also be obvious that recourse must be had to human subjects for primary validation of the subjective phenomenon under study. Direct approaches to the brain of waking subjects are of course limited by compatibility with therapeutic procedures and by the patient’s condition and informed consent. Electrical stimulation of (and, more recently, recording from) the cerebral cortex and deeper structures has provided one approach which, when suitably utilized, makes possible informative studies with no irreversible effects on the subject. The problems susceptible to investigation can be much broader than the initial classical one of the topographical relations of cortical sites to the body sites of the subjectively referred sensations, and some of these problems will be considered in this article.

Electrical Stimulation of Cortex in Human Subjects, and Conscious Sensory Aspects

Itch Evoked by Electrical Stimulation of the Skin

INVESTIGATIONS of the sensory threshold to stimulation of the skin have become increasingly intensive during the last sixty years.lg2 Electrical stimuli were selected in this investigation to determine the pain threshold because of the following advantages of this method: 11 It is a simple way in which an exactly measurable and, therefore, reproducible stimulus can be applied over various regions of the body and can easily be expressed in physical terms (milliamperes). 21 It has the least chance of damaging the tissues. 31 It produces an easily recognizable and easily definable “pricking pain” sensation. The clinician needs to measure the pain sense quantitatively instead of relying on his subjective impression obtained with the help of his so-called “neurologic pin.” In this study, we define pain threshold in the same way as Bonica,3 that is, as the lowest electrical current-expressed in milliampereswhich at a fixed frequency and impulse duration first evokes a sensation of (pricking) pain. The purpose of this study was to develop a practical and reliable method for clinically determining pain thresholds of the skin, particularly in patients with disturbances of pain sensation. Specifically, this study attempted to establish those characteristics of a reproducible electrical stimulus necessary to measure and standardize the pain threshold of the skin. The following problems also were investigated: [l] the relationship and characteristics of an electrical stimulus required for its being exactly reproducible as pain threshold stimulus, [2] factors which might influence the pain threshold, [3] possible variations of the pain threshold in the course of time (in the same individual), [4] the pain threshold of different parts of the body surface, [5] possible variation of the pain threshold in different individuals, and [6] the influence of diseases of the nervous system on the pain threshold. The electrical method with square wave impulses has many advocates as noted by BjOrt~,~ Sige1,S Chemnitius and associates,6-H and Blake and associate^.^ Harris and Blockuslo described this method as being perhaps one of the most suitable for future use because it is very simply applied and constant reproducibility of the stimulus is possible. A survey of the literature regarding electrical stimulation of the skin indicates that control of voltage alone is quite unsatisfactory. In this study, it is stressed that in electrical skin stimulation, the applied voltage does not

Measurement of the pain threshold determined by electrical stimulation and its clinical application.i. method and factors possibly influencing the pain threshold

When electrical current is passed through the body by means of electrodes applied to the skin, sensory nerves can be stimulated. This results in sensations that vary from barely perceivable to highly unpleasant. Such electrocutaneous stimulation of sensory nerves has been studied by engineers, neurologists, physiologists and psychologists, who have investigated the interrelationship between the physical parameters of the electrical stimulus and the psychological effect that it evokes, have used electrical stimulation to study the nervous system and its pathologies and have tried to use cutaneous stimulation as a communication medium. Despite a large number of publications reporting the results of this research, certain basic questions still remain unanswered.

Harry Sigel

Papers

Prick threshold stimulation with square-wave current; a new measure of skin sensibility.