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Andrew C. Weitz, Mark S. Humayun, Robert H. Chow, James D. Weiland; Electrical Stimulation with 20-Hz Sinusoids Activates Focal Regions of Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5524. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate 20-Hz sinusoidal electrical stimulation as a means for producing focal activation of the retina.
We used calcium imaging to report retinal ganglion cell (RGC) responses to electrical stimulation. Long Evans rats were given an intravitreal injection of AAV2-CAG-GCaMP5G. This led to widespread retinal transduction within 2 weeks, which was largely limited to RGCs. Infected retinas were dissected out, placed ganglion cell-side-down on a transparent microelectrode array, and maintained in heated, oxygenated saline. 20-Hz sine waves of gradually increasing amplitude were delivered through a 200-µm-diameter electrode while recording epifluorescence image series. Each stimulus lasted 200 ms (4 sine periods). Images were postprocessed to obtain the threshold of every activated RGC. Kynurenic acid (1 mM) was applied in some experiments to pharmacologically isolate RGCs from bipolar cell input (Massey & Miller, J. Physiol., 1988).
As shown in the figure, stimulation with 20-Hz sinusoids activated focal regions of ganglion cell bodies without activating axon bundles. The left image shows background-subtracted GCaMP5G responses recorded during suprathreshold stimulation (blue circle represents the electrode perimeter). The right image shows a threshold map generated from 3 retinas (686 somata in total). Cells are binned in a grid according to their location relative to the stimulating electrode and optic disc, which lies to the left of the image. Delivering cathodic- versus anodic-first stimuli did not significantly affect thresholds (n = 39, p > 0.05). Application of kynurenic acid raised thresholds by 33.0±29.8% (n = 51, p < 0.001), suggesting that responses are mediated in part by bipolar cell stimulation.
20-Hz sinusoids activate focal regions of RGCs. This may have implications for epiretinal prostheses, as recent reports suggest pulsatile stimulation can activate RGC axons, leading to elongated perceptions.
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