Electrophysiology of the X-Ray Phosphene

Abstract

X-rays are known to evoke a sensation of light (phosphene) by a direct action, other than fluorescence, on the retina. A study was made of the discharge of nerve impulses at single retinal ganglion cells on stimulation of the frog retina with light and with X-rays. These stimuli evoked discharges indistinguishable in types and latencies. Stimulation with either raised the response thresholds to both. Both thresholds dropped during nonstimulation. X-rays, but not light, produced a reversible drop in the neuronal action potential, and reversible immediate and cumulative rises in the thresholds. The threshold response was dose-dependent to longer stimulus durations for light than for X-rays. These threshold responses to illumination are known to depend on the energization of visual purple by light. Both X-rays and light bleached visual purple, and with the same ratio of bleaching dose to threshold visibility dose. X-rays also denatured the proteins of the retina; light did not. The energy absorbed in visual purple for a threshold response was about 4 x 10-3 erg/cm2 of retina for X-rays, and 2-40 times smaller for light. These findings are consistent with but do not prove, the hypothesis that the X-ray phosphene in the dark-adapted retina is produced through a direct action of the rays on the visual purple of the retina.