Abstract
Using 25 msec single current pulse or repetitive square waves of varying frequencies, the electrical sensitivity of the eye (the reciprocal of the eye's current threshold) was determined under states of a complete dark-adaptation and extrafoveal illuminations of reduced intensities with the least perceptible electrical phosphene as the index. The increase in electrical sensitivity caused by illumination was quantitatively expressed by K, which was defined by the formula K=100 (E-E0)/E0. In this case, E represents electrical sensitivity of the light-adapted eye to the stimulating current with given parameters, and E0, the electrical sensitivity of the dark-adapted eye to the current with the same parameters.
1. The K-value for 25 msec single current pulse exhibited a maximum at a level of 10-3 to 10-2 mL, whereas K-values for the repetitive square waves of 20 cps were nearly zero or slightly negative for all the levels of illumination. The findings indicate that the K for 25 msec single shock reflects more sensitively the rod response to suprathreshold light stimuli than K for the current at 20 cps. This justifies our proposed view that pulse width plays a more fundamental role in selective stimulation of the rod elements than frequency of repetition.
2. The K-values were determined for varying durations of the stimulating current consisting of repetitive square pulses. A maximal K was obtained by a current of 25 msec in duration, irrespective of the number of pulses and the pulse width.
3. The K was measured with double shocks with varying intervals. The K took the maximal value when the pulse interval was about 25 msec. In addition, a close similarity in configuration was demonstrated between the scotopic visibility curve of HECHT and WILLIAMS and the spectral distribution curve of K for the twin shocks separated by about a 25 msec interval. Thus, YONEMURA and NANGO'S finding that 25 msec single current pulse can exert a selective stimulating effect upon the rod elements has been reproduced to a certain extent by using twin shocks with about a 25 msec interval.
