Oculomotor behavior of blind patients seeing with a subretinal visual implant

Electronic implants are able to restore some visual function in blind patients with hereditary retinal degenerations. Subretinal visual implants, such as the CE-approved Retina Implant Alpha IMS (Retina Implant AG, Reutlingen, Germany), sense light through the eye's optics and subsequently stimulate retinal bipolar cells via ∼1500 independent pixels to project visual signals to the brain. Because these devices are directly implanted beneath the fovea, they potentially harness the full benefit of eye movements to scan scenes and fixate objects. However, so far, the oculomotor behavior of patients using subretinal implants has not been characterized. Here, we tracked eye movements in two blind patients seeing with a subretinal implant, and we compared them to those of three healthy controls. We presented bright geometric shapes on a dark background, and we asked the patients to report seeing them or not. We found that once the patients visually localized the shapes, they fixated well and exhibited classic oculomotor fixational patterns, including the generation of microsaccades and ocular drifts. Further, we found that a reduced frequency of saccades and microsaccades was correlated with loss of visibility. Last, but not least, gaze location corresponded to the location of the stimulus, and shape and size aspects of the viewed stimulus were reflected by the direction and size of saccades. Our results pave the way for future use of eye tracking in subretinal implant patients, not only to understand their oculomotor behavior, but also to design oculomotor training strategies that can help improve their quality of life.