Threshold analysis of a quasimonopolar stimulation paradigm in visual prosthesis

The complexity of surgical implantation has always been a significant obstacle in the development of visual prosthetics. Implanting in the epi and sub-retinal spaces allows the prosthesis direct access to the retina, resulting in lower stimulation thresholds, potentially at the expense of robust mechanical stability and interface longevity. Implanting the stimulating electrode in the supra-choroidal space greatly simplifies surgery and improves mechanical stability. This is achieved at the cost of a higher activation threshold and reduced focus of the electric field at the target site of stimulation, given the increased distance between the stimulating electrodes and the target tissue. In order to contain the spread of the stimulating field, the authors proposed a hexagonal arrangement of return electrodes, at a further cost to the stimulation threshold over that of a monopolar stimulation paradigm. This study analyses the effect on activation thresholds of activating simultaneously the hexpolar guard electrodes and the distant monopolar return in what we have termed a quasimonopolar configuration. Results show that introducing a small element of monopolar stimulation significantly lowers the activation threshold otherwise required by a pure hexpolar return.