A measurable neurophysiological factor of psychiatric significance

Abstract

1. 1. The sedation threshold is a clinical neurophysiological test, which determines the amount of intravenous amobarbital sodium required to produce certain EEG changes, accompanied by slurred speech. Previous investigations have shown the threshold to be related to several significant psychiatric variables, such as degree of manifest anxiety. The purpose of the present study was to further the aim of defining the neurophysiological factor, measured by the sedation threshold, by testing the hypothesis that the threshold is a function of the rate of depressant action of amobarbital on brain activity. 2. 2. An acceptable index of rate of depressant action was required to test the hypothesis. Analysis of data from 399 psychiatric patients and 45 nonpatient subjects showed that the amplitude of frontal fast frequency activity, produced by amobarbital, could provide such an index. The mean amplitude at the threshold, which represents a particular level of depressant action, was approximately constant for groups of subjects with different thresholds. The rate of increase of mean amplitude, which could be taken as the index of depressant action, was amenable to quantitative expression as the slope of a rectilinear function, when the logarithm of amplitude was used in calculation. In confirmation of the hypothesis, there was a highly significant inverse correlation between the sedation threshold and this index of rate of depressant action. 3. 3. In contrast to the sedation threshold, fast frequency amplitude as an absolute value, either before the injection or at threshold, was not significantly related to psychiatric diagnosis. Amplitude at threshold was correlated with amplitude before injection, but neither value was significantly correlated with the sedation threshold. 4. 4. From the conclusion that the sedation threshold is a function of rate of depressant action of amobarbital, it was suggested that it measures a time characteristics of neuronal activity, which is probably an important factor influencing cerebral excitability. The extent to which the sedation threshold findings agree with neurophysiological theories of psychiatric disorder derived from behavioral observation was considered.