The effect of inhaling dilute nitrous oxide upon recent memory and time estimation.

Abstract

THE STAGE OF ANALGESIA with amnesia which occurs with the inhalation of anaesthetic agents was recognized very soon after the il}troduction of ether. John Snow (1) described analgesia as the second degree of narcotism in which the pain of surgical operations was usually not felt. If there were responses to painful stimuli, they were either not remembered or recollected as having occurred in a dream. It is also of interest to rlote that he did not.-~hink it was necessary to mainsain deeper anaesthes{a for the greater part of protracted operations. The recent work of Summerfield and Steinberg (2) has made it clear that relatively small quantities of nitrous oxide in the inspired air can interfere with ~he establishment of new memories. By testing the ability of human subjects to learn nonsense syllables, they have demonstrated that inhalation of a mixture of 30 per cent nitrous oxide in oxygen retards the learning process; at the same time, the presence of the anaesthetic appears to lock previous memory traces into the nervous svstem so that information acquired prior to tile administratiot~ of ~itrous oxide is forgotten less rapidly that it is by stibjects breathing air. We undertook our own ex~)eriments upon the effects of light nitrous oxide anaesthesia for two reasons, the one academic and the other practical. In the first place, it seemed that an investigation of any controllable procedure that interfered with learning might ultimately reveal something of the nature of ]earning mechanisms at the cellular level. Secondly, the current practice of combining light general anaesthesia with muscular relaxants has raised the question--how light should tile anaesthetic be? it se,ems clear that the only requirement from the patient's point of view is, that anaesthesia should be su~Ecient to produce amnesia for the period of operation. Artusio (3) has described the stage of amnesia produced by the inhalation of diethvl ether and its use in major cardiac surgery. Our first attempts to il~vestigate the influence of N,,O on the learning process were made in 1950. \Ve were, therefore, tln;_iware of the work of Summerfield and Stc.inberg (2) which was tmblished in 1957. In this publication the) showed clearly (hat the effect of N._,() on subjects learning nollsense syllables was to decrease, the rate and to increase the retention of learning. These observations were entirely consistent with our own preliminary findi~lgs and we therefore turned our attentio~l to other effects of Net) inhalation. Thus the majority of the experimental results reported below describe a disturbance induced by nitrous oxide in estimates of the passage of time by human subjects and cats. Nevertheless those few ~Krom the \Vellcome Reaeart h [)epartment of Anaesthesia and 1he Department of Physiology, McGill I'niversity, Montreal.