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Wolfgang Fink, Erich Schmid; Electric Field Shaping Via Separatrices For Focused Electric Retinal Stimulation Via Retinal Implants. Invest. Ophthalmol. Vis. Sci. 2013;54(15):350.
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© ARVO (1962-2015); The Authors (2016-present)
To introduce a method for shaping electric fields for focused stimulation of the retina via retinal implants. Phosphene-based visual perception through electrical stimulation tacitly assumes that such stimulation is confined to prescribed target volumes. However, electric currents cannot be focused like a laser. State-of-the-art retinal implants use remote return electrodes, or adjacent return electrodes with an additional remote return electrode. Such configurations, especially with high electrode densities, are prone to crosstalk.
We are using multi-poles of electrodes and optimization of separatrices (i.e., mathematical surfaces between distinct bundles of field lines) to avoid crosstalk and to guide the stimulation field into the target volumes. As an example, in the electrode configuration of Fig. 1a, four dipoles (blue electrodes) form the guiding field, and the red electrodes the stimulation field. The stimulation field cannot penetrate into the guiding field because of the non-crossing rule of field lines (Fig. 1b). The separatrix between the guiding field and stimulation field forms a ridge with peaks and saddle points. At the saddle points the stimulation field line density is maximal. Optimization routines are used for shifting the saddle points into the target volumes.
In a hexagonal array of electrodes a ring of dipoles (blue electrodes) builds up a separatrix that has the shape of a crater (Fig. 2a). The stimulation current emitted from the center electrode rises like a narrow fountain and spreads out above the crater rim (Fig. 2b). The time profiles of guiding and stimulation fields are chosen such that the stimulation field has a lower stimulation threshold than the guiding field (Schmid & Fink, arXiv:1210.5348v1); the guiding field is not shown in Fig. 2b.
Separatrices can focus electric stimulation currents into prescribed target volumes. The space and time profiles of the resulting electric currents are entirely determined by the spatial arrangement of the electrodes and by their respective activation potentials. The caveat is that one image-giving pixel is formed by a multi-pole. However, image resolution can be restored by switching the center of the pixel from electrode to electrode for subsequent stimulation events (i.e., local area scanning; arXiv:1210.5348v1).
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