This study takes part in the analysis of phosphene perception thresholds as obtained by electrical stimulation through a spiral cuff electrode previously implanted around the right optic nerve of a blind retinitis pigmentosa patient. The enhanced efficiency of higher frequency or multiple pulse stimulation trains indicate temporal integration. Similarly, spatial summation takes place with increasing intensity. Considering a uniform population of typical axons, a rough model of phosphene perception thresholds can be constructed. Therefore, the strength-duration equation is combined with an S shaped axon recruitment curve. A simplified EPSP integrating synaptic mechanism takes temporal and spatial summation into account. Despite the crude approximations, a stable and well fitting model is identified, providing expected values against which all experimental values can be compared, showing obvious fluctuations. The observed changes demonstrate a rather negligible long term drop in the average threshold. The implanted cuff electrode thus appears stable in this human application.

PURPOSE:To explore electrically induced phosphenes in blind patients with retinitis pigmentosa (RP) in comparison with healthy subjects and to develop a screening test for candidates for an optic nerve visual prosthesis implantation.METHODS:Phosphenes are obtained by charge balanced biphasic pulse stimulations through a surface cathode over the closed eyelids and an anode near the opposite ear. The resulting strength-duration relationship for somatosensory, phosphene, and pain threshold has been recorded in five RP patients as well as in 10 healthy volunteers.RESULTS:In sighted subjects, the average rheobase and chronaxy for phosphene perception are 0.28 mA and 3.07 msec, respectively. For pulse durations longer than 2 msec, phosphenes are usually obtained at current strengths below the level giving rise to any other electrically generated sensation. In RP patients, however, phosphenes are not so easily obtained. One in five had no visual response at all. Another patient reported a flash perception for the longest pulse durations only. Spontaneous phosphenes interfered heavily with the stimulation in a third person. Finally, despite the higher threshold, two patients displayed normally shaped strength-duration curves.CONCLUSIONS:The surface stimulation has proven harmless, adequate, and very helpful to ascertain that the optic nerve can be electrically activated in completely blind individuals. Long-duration stimulation pulses yield very low phosphene thresholds in healthy subjects. Anterior visual pathways activation requires higher currents in RP patients.

The aim of the study is to determine which of the existing myelinated mammalian nerve fibre models better fits experimental data resulting from electrical stimulation of the human optic nerve and from propagation velocity measured on primates. The macroscopic electric potential is computed in a 3D, inhomogeneous and anisotropic nerve model. The Chiu-Sweeney (CS) and the Schwarz-Wesselink (SW) membrane descriptions are then considered. Variations in parameters that are not well established (encapsulation-tissue thickness, nerve-fascicle conductivity, geometric and electrochemical fibre cable parameters) are taken into account. Results demonstrate that the SW model predictions are in better agreement with the experimental data than those of the CS model, although thresholds are still too high. When channel densities are varied, the SW model turns out to be more robust than the CS model. For a suitable leakage channel density value (about 8% of the original one), the SW model predicts a conduction velocity of 11.4ms−1 and an excitation threshold of 0.055 mA (for 0.1 ms pulse duration), which is in very good agreement with experimental values (11 ms−1 and 0.055 mA). Potassium current in the SW model is necessary for stability. Introduction of a potassium-like current can restore stability in the CS system.

An original numerical method is developed to compute the 3D electric potential generated by a dot-contact cuff electrode implanted around an axisymmetrical, inhomogeneous, anisotropic nerve. The technique is based on a 2D finite-element approach coupled with a semi-analytical Fourier spectral decomposition to approximate the solution behaviour in the azymuthal direction. The method only requires a 2D FEM mesh and allows an accurate electrode description, with any number of contacts at different angular positions. Results show that the convergence of the Fourier series is very fast: typically, the relative error due to series truncation (estimated by the norm of the difference between the solution computed with M modes and the one computed with M— 1 modes, normalised by the norm of the solution computed with M modes) reaches the order of 10−3 with six spectral modes (M=6). As a consequence, the whole algorithm has the complexity of a 2D approach.

A blind volunteer affected with retinitis pigmentosa was intracranially implanted with a selfsizing cuff electrode around her right optic nerve. The nerve cuff electrode included 4 monopolar contacts. Its leads were brought through the skin where they ended in an external connector12. After recovery from surgery, electrical activation of the implanted optic nerve was undertaken. A specially dedicated Computer Based Stimulator was used. Stimulation consisted in charge balanced biphasic rectangular pulses. The stimulation resulted consistently and exclusively in visual sensations. The thresholds have remained stable for 14 months, so far. Phosphenes were broadly distributed throughout the visual field. They were either solid surface, or organized in rows, arrays, or clusters of dots. A topological organization was observed between phosphene location and the stimulating contact. Increasing the stimulation frequency decreased the current threshold for perception. The results of this experiment are consistent with the hypothesis that a visual prosthesis, based on electrical stimulation of the optic nerve, can be developed for human subjects with retinitis pigmentosa.

A blind volunteer with retinitis pigmentosa was chronically implanted with a self-sizing spiral cuff electrode around an optic nerve. Electrical stimuli applied to the nerve produced localized visual sensations that were broadly distributed throughout the visual field and could be varied by changing the stimulating conditions. These results demonstrate the potential for constructing a visual prosthesis, based on electrical stimulation of the optic nerve, for blind subjects who have intact retinal ganglion cells.