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JF Rizzo, SR Montezuma, OR Ziv; Effect of Epiretinal vs Transretinal In Vivo Electrical Stimulation of Rabbit Retina on the Amplitude of Evoked Cortical Potentials . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4484.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Determine the extent to which the amplitude of electrically evoked cortical potentials (EECPs) are influenced by the orientation of the stimulating electrical field near the retina. Method: A micromanipulator was used to hold a 125 µm inner diameter, bipolar platinum-iridium electrode on the retinal surface near the visual streak of three Dutch-belted rabbits that received isoflurane inhalation anesthesia. A second return electrode was placed in the retrobulbar space. Extradural electrodes were used to record EECPs, obtained by performing 50 consecutive stimulations that were computer averaged. Biphasic impulses (100-400 µA; 2-4 msec each phase) were delivered to the retina with a current source. Sequential epiretinal vs transretinal stimulation using identical stimulus parameters was performed. The effect upon the EECPs was also assessed after NBQX (28µM), D-AP7 (300 µM) and AP4 (75 µM) were delivered to the center of the vitreous cavity to selectively block specific neuronal pathways. Results: On average, pre-drug transretinal vs epiretinal EECP amplitudes were 82 vs 35 µV, respectively (paired t test: p=0.04). EECPs were still obtained (with both electrical field orientations) following administration of drugs. Quantitative results regarding the effect of these drugs are still preliminary. Conclusion: Transretinal electrical field stimulation produces larger EECP amplitudes than stimulus-matched epiretinal activation. This finding implies that thresholds are lower with transretinal stimulation, which is consistent with our parallel in vitro study that found a difference in thresholds (see RJ Jensen). This finding has relevance for a retinal prosthesis in that lower thresholds would lessen the system power demand and possibly improve the safety of chronic electrical stimulation. It is noteworthy that we recorded EECPs even after administration of drugs that blocked all synaptic input to the ganglion cells, which indicates that epiretinal stimulation directly drives ganglion cells or their axons even with charge levels not far above threshold.
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