Influence of dark adaptation on the perceptual threshold of electrically evoked phosphenes

Abstract

Aims/Purpose: Phosphenes are light phenomena in the eye that have been studied since the 18th century. They can be evoked mechanically or by electrostimulation, for example. The perception of phosphenes and their threshold, the limit of phosphene perception, has been the subject of numerous studies. In this work, the dynamics of the threshold shift of electrically evoked phosphenes during dark adaptation is recorded. Statistically it was tested whether the phosphene thresholds differ before and after dark adaptation.Methods: A study was conducted on 16 healthy subjects (7 w, 9 m, 28.5 ± 6.8 yr). Each subject sat approximately 30 cm in front of a black cloth‐covered semi‐circular screen placed on a table. On the table was a signal generator (33250A, Agilent Technologies Inc, Santa Clara, CA, USA) with a rotary control for the subject's own setting of the current intensity for the current stimulation (sinusoidal, 20 Hz) using a transcranial electrical stimulator (DC‐Stimulator PLUS, neuroConn GmbH, Ilmenau, Germany). The electrodes were a ring electrode at one eye and a rectangular electrode at the back of the head. After the subject had been instructed, and the electrodes had been attached, the subject had to set the phosphene threshold in room light conditions (125 cd/sqm) 10 times in succession (baseline B1, bright adapted). Lightproof glasses were then worn. For 40 min, each subject adjusted the threshold at intervals of 2 min. After 40 min, 10 more threshold values were recorded (baseline B2, dark adapted). Means were calculated for each subject for both baselines B1 and B2 and tested for discrimination using the Wilcoxon signed‐rank test.Results: All subjects were able to successfully complete the test. The global thresholds before and after dark adaptation were 132.3 ± 13.8 μA and 329.5 ± 38.0 μA respectively. The difference between the conditions B1 and B2 is significant (p = 0.0002). All measurement curves of the threshold progression show an increasing behaviour with a pronounced plateau after approximately 15 min.Conclusions: We successfully performed a study on the generation of phosphenes during current stimulation. The dynamics of the threshold progression from light to dark adaptation could be recorded. A significant threshold shift is shown for all subjects.