Abstract
Purpose :
Current clinical subretinal implants yield lower spatial resolution than pixel spacing limit, and use only low-frequency (<7Hz) stimulation, resulting in discontinuous percepts. Here we assess the spatiotemporal characteristics of the network-mediated responses of retinal ganglion cells (RGCs) to subretinal photovoltaic stimulation. We measure responses of RGCs in the degenerated retina to photovoltaic spatiotemporal white noise and to full-field contrast steps. These responses are then compared with responses to corresponding visual stimuli in the healthy retina.
Methods :
Retinas were sandwiched between a photovoltaic array with 70um pixels and a 512-electrode multielectrode array (MEA), with RGC side facing the MEA. To stimulate the retinas electrically, 4-ms pulses of near-infrared light (880 nm) were applied at 20Hz. Continuous visible light (591 nm) was used for visual stimulation. Light was spatially modulated by an LCD screen, and focused onto the retina and photovoltaic array. High-frequency (20-30 Hz) spatiotemporal binary white noise was used to characterize responses in healthy (Long-Evans, LE) and degenerate (Royal College of Surgeons, RCS) retinas. Contrast sensitivity was assessed by applying full-field greyscale contrast steps of 500ms in duration with continuous illumination for LE and with 20Hz carrier frequency for RCS retinas.
Results :
In healthy retinas, ON RGCs showed photovoltaic OFF responses, and vice versa. Photoreceptor degeneration led to all RGCs having only ON photovoltaic responses. Degenerate retinas responded to network-mediated photovoltaic stimulation at naturalistic frequency, and exhibited spatially localized receptive fields with antagonistic center-surround organization. Most responsive RGCs in the degenerate retina exhibited transient and steady-state responses to full-field contrast steps. For these cells, contrast sensitivity of prosthetic vision was about 5 to 8 times worse than natural: +17%/-10% (Michelson) contrast elicited responses comparable to that of +/-2% contrast in healthy retinas, respectively.
Conclusions :
Complex spatio-temporal high-frequency subretinal stimulation induced fast and spatially localized responses in the degenerate retina with antagonistic center-surround organization. Contrast sensitivity was found to be much better than previously reported values. Both findings strengthen the possibility of providing functional prosthetic vision.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.