Visually Evoked Activity From a Subretinally Placed Artificial Silicon Retina

Abstract

Abstract: : Purpose: Assess whether a subretinally placed artificial silicon retina (ASR) microchip can be stimulated in situ and produce a stimulus–evoked response in a central visual structure, the superior colliculus (SC). Methods:We implanted an ASR microchip subretinally in Royal College of Surgeons (RCS) rats between 28 and 35 days of age, when their photoreceptors are degenerating. We recorded extracellular responses across the dorsal surface of the SC between 90 and 365 days of age (n = 16), allowing us to correlate responses on the retinotopic map with retinal placement of the ASR. For comparison, we recorded from age–matched rats in one of several control conditions: unoperated RCS (n = 17); unoperated normal (n = 10); sham operated RCS (n = 9); RCS with inactive ASRs (n = 6). In addition, we recorded from normal rats 1 week after implantation (n = 4). In all rats, we used a full–field light stimulus delivered to the eye. Because the ASR is responsive to both infrared (IR) and visible light, IR and white or blue light stimuli were used to distinguish between ASR– and residual photoreceptor evoked activity, respectively. Responses to these stimuli at comparable sites in the SC of all control groups were compared with those in the experimental groups (normal and RCS rats with active ASR implants). Results:Using a white or blue stimulus, activity was evoked in the SC of the majority of RCS control rats. The distribution of this activity was patchy across the topographic map and did not correspond well with inactive implant placement or surgical site. Young, unoperated RCS rats also showed a patchy pattern of evoked activity. In contrast, the IR stimulus failed to evoked responses in any of the controls. Whereas, the IR stimulus evoked a response in all normal rats with acute (1 wk), active implants and in 30% of RCS rats with chronic (3 – 12 mos) active implants. Further, this IR–evoked activity was confined to a region of the SC that corresponded topographically to implant placement and was proportional to the intensity of the IR stimulus. Conclusions:Subretinally placed ASR devices can be stimulated in situ and can generate central visual activity that is topographically correlated with ASR placement and proportional to stimulus intensity. Whether this activity is sufficient to provide functional vision remains an open question that will be tested.

Keywords: retina • electrophysiology: non-clinical • retinal degenerations: hereditary