Abstract
Abstract: :
Purpose: Measuring the physiological response of the retina to electrical stimulation non–invasively will facilitate long–term study of prosthesis performance. In a manner similar to the traditional electroretinogram (ERG), recorded in response to photic stimuli, the response to electrical stimulation (eERG) can also be measured non–invasively at the cornea. The eERG response to current pulses delivered via subretinal electrodes is a complex waveform, with some general similarity to the photic ERG. The present aim is to identify the neural origins of the eERG waveform components. Methods: Pigmented P23H rats (a transgenic model of ADRP), 14–15 wks of age, exhibit significantly reduced visual function, and reduced cell numbers in the outer nuclear layer (ONL), compared to age–matched wild–type animals. In these animals, an array of gold stimulating electrodes (500 µm dia.) was placed in the subretinal space. Stimuli were biphasic, symmetrical, anodic first, constant current, 100 µsec pulse duration. Responses were recorded via a corneal electrode. The single–flash ERG and single–pulse eERG were recorded before and after injection of aspartate into the vitreous (4 µL, 1.6 M); 15–60 minutes post injection the ERG demonstrated a block of post–receptor synapses. Results: The eERG response is consistent between animals and consists of an initial negative component (peak ∼40 ms), a period of irregular oscillations, and a slow positive component (peak ∼320 ms). Following aspartate injection, the initial negative component of the eERG response was eliminated or significantly reduced; oscillatory components were eliminated; the slow positive component was typically reduced but preserved (n = 4). Conclusions: Bipolar subretinal electrodes likely target remnant cells in the outer nuclear layer in these animals, however the outer retinal response is not responsible for the early negative component of the eERG. The origin of the eERG response, as recorded at the cornea, appears to be almost entirely inner retinal neurons; further pharmacological dissection will correlate cell types with response components.
Keywords: electroretinography: non-clinical • retinal degenerations: hereditary