Development of an extraocular retinal prosthesis : evaluation of stimulation parameters in the cat

Electrical stimulation of the retina with extraocular electrodes is a new approach to developing a retinal prosthesis for blind patients. We have evaluated stimulus and electrode configurations for an extraocular retinal prosthesis (ERP). In anesthetized cats, ERP disc electrodes of 1 mm, 2 mm and 3 mm diameter were sutured to the sclera over the lateral globe. Electrically evoked potentials (EEPs) were recorded over the ipsilateral visual cortex, which resulted from the retinal stimulation of the ERP electrodes with a return electrode placed at the medial canthus. Square pulses, triangular pulses and the effects of dark adaptation and electrode size on the amplitude and thresholds for a cortical response were investigated. Square pulses were more effective than triangular pulses for stimulating the retina. Dark adaptation leads to a large increase in the threshold for retinal stimulation. There was no difference in the threshold for stimulation between electrodes of 1 mm and 3 mm diameter. Stimulation of the retina with extraocular electrodes elicits an EEP that is similar to that generated by retinal stimulation with intraocular electrodes. The use of square pulses is preferred to triangular pulses to minimize the peak current density at the electrode-tissue interface. As there is little difference in the threshold current for retinal stimulation with 1 mm or 3 mm electrodes, 3 mm electrodes are preferred as this will decrease the charge density at the active surface of the electrode.