Evaluation of Spatial Resolution of Suprachoroidal–transretinal Stimulation by Single–unit Recording

Abstract

Abstract: : Purpose: We developed a new electrical stimulation method for retinal prosthesis, Suprachoroidal–Transretinal Stimulation (STS, Kanda et al., IOVS 2004). A STS based on retinal prosthesis has a great advantage to minimize retinal insult due to stimulus electrodes having no direct contact to the retina, but such configuration might lead to poor resolution. To evaluate the spatial resolution, we investigated whether it can make focal stimulation, with in vivo single–unit recording. Methods: A stimulating electrode array, which has nine platinum electrodes with 0.1 mm of diameter each, was placed on sclera after lamellar resection, whereas a reference electrode of stainless steel wire (diameter: 0.2 mm) was inserted into the vitreous. A single biphasic pulse of 50–500 µA in amplitude and 0.5 ms each in duration was applied to one of the stimulating electrode on the array. 63 single–unit activities were recorded from relay cell in Lateral Geniculate Nucleus (LGN) of adult cats (n=8). Spike discharges evoked by STS of various current intensities were recorded for each unit. We analyzed the relationship between a threshold of each unit by STS and the distance from the center of its receptive field (RF) to the stimulating electrode. Results: Responses to STS consisted of early discharges (latency: 5–15 ms) and late discharges (after 30 ms). We focused on the early discharges, because they appeared when the RF located near stimulating electrode. As the distance decreased, the threshold tended to become lower. For example, RFs which have the threshold lower than 100 µA existed within 2 degrees from stimulating electrode. Conclusions: The activation of retinal ganglion cells can be localized within radius of 2 degrees in cat’s visual field by low strength STS, hence a retinal prosthesis by STS can restore visual acuity better than counting fingers.

Keywords: electrophysiology: non-clinical • retina: proximal (bipolar, amacrine, and ganglion cells)