The eye was stimulated by subthreshold conditioning electric stimuli of various wave-forms, and then the changes in the electrical excitability of the eye were investigated by applying a single constant current pulse of 0.1 second in duration. Percentage increases in electrical excitability following the conditioning stimulus were denoted by ζ and plotted as ordinates against the time after the conditioning stimulus.
1) In experiments carried out with condenser discharge (C. D.) as conditioning stimulus, the ζ-time curve had a crest at I second for the time constants ranging from 0.0002 to 0.04 seconds, at 1.5 seconds for those from 0.06 to 0.12 seconds and at 2 seconds for those from 0.16 to 0.68 seconds.
2) When exponentially increasing currents (E. I. C.) were used as conditioning stimuli, the crest of the ζ-time curve was found at 2 seconds for the shortest time constant. The crest was shifted toward 3 seconds as the time constant was increased, and there appeared two crests at 3 and 4.5 seconds for the largest time constant.
3) Rectangular pulses from 0.0015 to 0.5 seconds in duration showed effects similar to those of C. D.
4) The effects stated in (1), (2) and (3) are closely similar to the effects of colored lights used instead of the conditioning electric stimuli. From comparison of the effects of both kinds of stimuli, it was concluded that the red and yellow receptors are selectively stimulated by C. D., while the blue and rod receptors are excited by E. I. C. Rectangular pulses seemed to have an action intermediate between C. D. and E. I. C.
5) The effect of the intensity of the conditioning electric stimulus was studied upon ζ values, and it was found that the threshold strength was optimal. This is a striking contrast to the effect of photic stimuli which always increases with increasing intensities.
I wish to express my thanks to Prof. K. Motokawa for suggesting this in-vestigation as well as for constant guidance in the course of the work.
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