Abstract
Abstract: :
Purpose: To test the hypothesis that epi–retinal electrical pulse durations >0.5 ms preferentially stimulate retinal cells deep to the ganglion cell layer. This work relates to the development of a retinal prosthesis. Methods: A micromanipulator was used to position a 500 µm inner diameter bipolar electrode near the visual streak on the epi–retinal surface of Dutch–belted rabbits. Extradural electrodes were used to record electrical evoked cortical potentials (EECPs) over the occipital cortex by performing 50 consecutive computer–averaged stimulations. Symmetric biphasic impulses (7 – 1600 µA; 0.25–2 ms per phase) were delivered to the retina with a current source. The effect on the EECPs of sequential, epi–retinal (return electrode on retinal surface) vs. trans–retinal (i.e. return electrode behind sclera) stimulation was compared. The effect upon the EECPs was then assessed after NBQX (56µM), D–AP7 (600 µM) and APB (150 µM) were delivered to the vitreous cavity to selectively block neuronal input to ganglion cells. Median values are reported. Results: There was no significant difference in threshold charge and EECP amplitude between epi–retinal and trans–retinal stimulation. Threshold response using a 0.25 ms pulse required a significantly lower charge than a 2 ms pulse (p<0.001). Pharmacological blockade of input to the ganglion cells did not significantly alter the threshold charge or EECP amplitude. EECP amplitude increased linearly with increasing charge using both 0.25 ms and 2 ms pulses, even after synaptic blockade of deeper layers. Conclusions: Epi–retinal vs. trans–retinal orientation of the simulating electrical field did not significantly alter cortical responses. Shorter stimulus durations were more charge efficient for both electrical field orientations. The lack of degradation of cortical amplitudes post drug instillation reveals activation of ganglion cells with epi–retinal stimulation, even up to 2 ms. These results do not support a previously published hypothesis that pulse durations >0.5 ms preferentially stimulate deeper retinal neurons.
Keywords: retina • electrophysiology: non–clinical • retinal connections, networks, circuitry