ELECTROSTIMULATION OF THE HUMAN EYE ADAPTED TO MONOCHROMATIC LIGHT, BY USING EXPONENTIALLY INCREASING CURRENT

Abstract

Taking, as a criterion, the electrical phosphene evoked by electrostimulation of the human eye with use of the exponentially increasing currents of varying time-constant (RC), a study was made on the relation between the electric threshold (Vf) of the eye and the radiance (N) of monochromatic rays of differ ent wavelengths used as the adapting light.(1) In any cases of adapting light of different wavelengths excepting the red light, there were two different types of Vf-log N curve depending upon whether the time-constant was shorter than 25 msec. or longer than 75 msec. This result was the same as that obtained with the white adapting light.(2) In the case of time-constant longer than 75 msec., the effect of the red adapting light upon Vf-log N curve was quite different from that of the other adapting light; the curve showed a form of single nature, whereas in the case of time-constant shorter than 25 msec. the same red light had not such specific effect as in the former case.(3) Taking, as a criterion, the reciprocal of the radiance corresponding to the crest on I>Vf-log N N curve for 0 msec in RC (or the left edge of the horizontal part on the curve for 100 msec. in RC), a spectral sensitivity curve was obtained. This curve was proved to be in conformity with scotopic luminosity curve.(4) It was concluded that the electric threshold, as determined by the exponentially increasing current of time-constant longer than 75 msec. was equivalent to the differential threshold of brightness discrimination.(5) The experiments supported Mita's view that there might be in the retina, at least near the threshold excitation, two sorts of the neural elements which were stimulated separately depending upon the time-constant of the stimulating current. The one for longer time-constant may be the photoreceptor cells, while the other for time-constant the bipolar cells.