Concerning the Interrelation between Absolute Sensitivity and Strength of the Nervous System

Abstract

Summary ^*

This paper is concerned with the relation between absolute sensitivity and the “strength” of the C.N.S., in the sense in which the term was used By Pavlov. The authors advance the hypothesis that “weakness” of the nervous system is correlated with high reactivity, and hence with high sensitivity to peripheral stimulation; conversely, “strength” of the nervous system is correlated with low reactivity; and hence with low sensitivity to peripheral stimulation.

Two sets of experiments were carried out in order to test this hypothesis, one concerned with visual and the other with auditory sensitivity. In the visual experiments, absolute sensitivity to light in the periphery of the dark adapted eye was measured on 26 subjects by means of an adaptometer. Measurements were taken at two minute intervals over a period of about half an hour, the mean threshold value for each subject being calculated from the results obtained at a number of experimental sessions distributed over 3 to 5 days. In the auditory experiments, absolute threshold for sound intensity was determined in 22 subjects. A pure tone of 1,000 c.p.s. was fed to the subject binaurally through headphones and the sound pressure level systematically varied by the experimenter. Threshold measurements were made at 1 min. intervals over a 15 to 20 min. period. Sensitivity is calculated as a value inversely proportional to that of the threshold.

In order to determine the “strength” of nerve cells, the authors made use of a method involving a conditioned decrease of sensitivity to light or sound. The rationale of the method is as follows: The strength of a conditioned reflex is normally found to increase with increase in intensity (or duration) of the conditioned stimulus until a “physiological limit” is reached. Above this limit, further increase in stimulus intensity no longer produces an increase in response, which may indeed diminish as a consequence of “trans-marginal inhibition”. Hence a determination of the stimulus intensity above which there is no increase in response strength provides a method of defining the physiological limit, or “strength”, of the central nervous processes involved.

The actual method used here consisted in exposure for 10 to 15 sec. of an illuminated panel as the unconditioned stimulus, the resulting reduction in peripheral visual sensitivity being the unconditioned response. A pure tone of 1,000 c.p.s. and a red light of low intensity were used as conditioned stimuli. The conditioned stimulus was applied 5 sec. before the onset of the unconditioned stimulus, the duration of their combined application being 10 sec. After the conditioned reflex was well established the tests of “strength” were begun. These consisted in the application of the conditioned stimulus (always reinforced) ten times in succession at intervals of 2 min. The conditioned stimulus was also presented twice without reinforcement, once at the beginning and again at the end of the series. If the strength of the conditioned reflex showed no decrement on this second trial as compared with the first, it was concluded that the physiological limit or “strength” of the corresponding central nervous structures was high; if, however, there was decrement, this was taken to mean a low physiological limit or “weakness”. The criterion of decrement was a decrease in strength of the conditioned reflex of 10 per cent. or more.

These experiments were also repeated after the administration of 0.2 gr. of caffeine, which is thought to raise the excitability, and hence diminish the “strength”, of central nervous processes.

The results appear to demonstrate a negative correlation between sensitivity, visual or auditory, and the “strength” of nervous processes. In general, subjects with “weak” nervous systems gave low absolute threshold values, indicating high sensitivity, whereas those with “strong” nervous systems gave high threshold values, indicating low sensitivity, There were, however, some exceptions to this rule.

In discussion of these results, the authors lay stress on functional changes which are believed to occur in neurones as a consequence of sustained excitation. In general, they argue that both sensitivity and “strength” of nervous processes can be related to a more basic dimension of neural “reactivity”.