Retinal Phosphenes Induced by Transorbital Electrical Stimulation: Influence of Light Adaptation, Electrode Montage, and View Direction

In this study, the perception of phosphenes was used as a surrogate identifier for stimulation sites for use in retinal-degenerative diseases. We aimed to investigate the influence of adaptation, electrode montage, and direction of view on electrically induced phosphenes. We developed a practical methodology to assess non-invasive ocular electrical stimulation, addressing specific areas in the retina. Phosphene thresholds were identified under light and dark adaptation for non-invasive transorbital electrical stimulation. The location and extent characterized the morphology of electrically induced retinal phosphenes for five directions of view and for seven electrode montages in 62 participants. Our results indicated the lowest phosphene thresholds under light adaptation. Cumulative charts of phosphenes visualized the location of phosphene hot spots and their focality for the investigated directions of view and electrode montages. Under consistent light adaptation, we found changes in electrode montage generated stronger changes in the phosphenes' morphology and distribution. Adjusting the electrodes in the orbital vicinity was more effective than changing the direction of view to shift the phosphene hot spot location to a pathological region to induce neuronal activity there. In this study, we established the first practical methodology to adapt non-invasive transorbital electrical stimulation to address specific areas in the retina.