Strength Duration Curves for Electrical Stimulation of the Human Eye

Abstract

Baumgardt's surprising report of a minimum at about 30 milliseconds in the strength-duration curve for electrical stimulation of the human eye is confirmed. Arguments are presented against his explanation of the phenomenon. An alternative hypothesis is developed: that the complex strength-duration curve is due to the interaction of on and off processes, which are separately excited by the make and break of the current: that these processes summate when separated by 30 milliseconds but are mutually inhibitory at 70 milliseconds: that these processes are the same as those known to be aroused by light. Two new experiments are designed to test this hypothesis, and the predicted result has been obtained from each. Experiments are devised which show that the site of stimulation is the retina. The accuracy of the method is discussed. Over the range 10 to 100 milliseconds distortion is found to be small. For shorter pulses distortion is to be expected in all cases, and it is suggested that the chronaxie is an expression merely of the condenser effect of the tissues. The use of this term should be discontinued in connection with stimulation through the skin. Some parallels between these results and brightness comparison experiments with light are pointed out. The nature and location of the interaction is discussed.