| The aim of this study is to provide guidelines for the design of a retinal prosthesis with specific emphasis on spatial resolution. Optical recordings from retinal ganglion cells (RGCs) using novel methods provide the first clear picture of how the retina sees electrical stimuli. Analysis of the spatial properties of the response show that the site of action potential initiation is not in the soma, but in the initial segment of the axon, ∼ 40 mum from the soma. Furthermore, 60-mus pulses directly stimulate RGCs, while longer pulses target inner retinal neurons. Short pulse widths have a more focal response area than long pulses. The smallest response area was about 100 mum, even for electrodes as small as 30 mum.;Separation of the electrode array from the retina by a saline gel increases the area of the response and raises stimulation threshold. Spacers with thickness less than 25% of the electrode diameter have little effect on the response size. Therefore, attachment of the array in vivo should have a gap less than 25% of the electrode diameter in order to preserve resolution.;Axonal stimulation remains a problem that threatens to severely limit the perception of pattern stimulation. Recordings at 400-mus and 1000-mus pulse widths from distant peripheral somata indicate that axons are stimulated at roughly 160% of the threshold for local RGCs, but 60-mus pulses avoid axons to at least 200% of the local RGC threshold. Data suggest a means to selectively stimulate RGCs proximal to the electrode at short pulse widths. Bipolar configurations, directing electric field lines transverse to axon bundles, avoided stimulation of those bundles, but could not avoid axons that pass over the cathodic electrode, except for the smallest (10-mum) electrodes. Pulse width appears to be the best means of control at the present time. The optimal array configurations are either a grid of electrodes spaced by 100 mum, or a conformal array of 10-mum bipolar pairs oriented with field lines transverse to the axon bundles. We can expect a resolution of 0.4 degrees of visual field corresponding to Snellen acuity of 20/480. |
