Perceptual thresholds and electrode impedance in three retinal prosthesis subjects

Abstract

Three test subjects blind from retinitis pigmentosa were implanted with retinal prostheses as part of a FDA-approved clinical trial. The implant consisted of an extraocular unit that contained electronics for wireless data, power, and generation of stimulus current, and an intraocular unit that consisted of 16 platinum stimulating electrodes arranged in a 4 x 4 pattern within a silicone rubber substrate. The array was held to the retina by a small tack. The stimulator was connected to the array by a multiwire cable and was controlled by a computer based external system that allowed precise control over each electrode. Perception thresholds and electrode impedance were obtained on each electrode from the subjects over several months of testing. The electrode distance from the retina was determined from optical coherence tomography imaging of the array and retina. Across all subjects, average thresholds ranged from 24-702 microA (1-ms pulse). The data show that proximity to the retina played a role in determining the threshold and impedance, but only for electrodes that were greater than 0.5 mm from the retina.

A Treatise of the Physical Aspects of Phosphenes and Single-Cell Selectivity in Retinal Stimulation

“…The prosthetic devices developed by Mahadevappa et al, [10], Rizzo et al, [12], Klauke et al, [13] Eger et al, [14] and Humayun et al, [16] used big-sized electrode diametersof 500, 200 and 100 µm, respectively. The electric current field from large electrodes indiscriminately drives local retinal activity in an abnormal way, leading to complex retinal responses [29].…”

“…A phosphene is defined as any visual sensation represented as a spot of light in the visual field caused by methods other than stimulation of the visual system by light. Phosphenes can be elicited by mechanical forces [2], magnetic stimulation [3][4][5] or electrical stimulation [6][7][8][9][10][11][12][13][14][15].…”

“…In recent years, epi-and subretinal implants have been developed [6][7][8][9][10][11][12][13][14][15]. Humayun et al, [6][7][8][9], Mahadevappa et al, [10], Rizzo III et al, [11,12], Klauke et al, [13], Eger et al, [14] and Keserüet al, [15] and recent clinical trials [16,17] have reported that blind patients perceived sensations of spots of light while the retina was electrically stimulated.…”

“…Humayun et al, [6][7][8][9], Mahadevappa et al, [10], Rizzo III et al, [11,12], Klauke et al, [13], Eger et al, [14] and Keserüet al, [15] and recent clinical trials [16,17] have reported that blind patients perceived sensations of spots of light while the retina was electrically stimulated. These experiments clearly validated the viability of producing perception of light patterns in blind patients by electrically activating nearby retinal ganglion cells (RGCs).…”

“…The foremost concern is that advanced retinal implants induce atypical patterns of retinal activity [16,17]. Relatively large electrode diameter in implanted devices [10,12,14,16,17] likely activates hundreds or thousands of cells over their area of stimulation. Not only does this coarse stimulation of cells restrict detailed perception, but also the activity generated by stimulation remains dissimilar to the healthy retina [29].…”