Abstract
Abstract: :
Purpose: Currently, large electrodes (up to 520 µm in diameter) are being used to electrically stimulate the retinas of blind patients in an effort to partially restore vision. The purpose of our study is to determine how much current is needed to activate individual retinal ganglion cells with such electrodes. Methods: Single–cell recordings were made from the axons of ganglion cells with an in vitro rabbit retinal preparation. The ganglion cells were stimulated electrically (monophasic, cathodal pulses) with either a 125 µm or 500 µm diameter electrode positioned on the inner retinal surface within their optical receptive fields. A second, return electrode was placed on the sclera. Threshold currents, given below, are medians. Results: The following data were obtained from 23 OFF–center brisk–transient ganglion cells that were located in the inferior, mid–peripheral retina. With either electrode size, ganglion cells could be activated both directly and indirectly (i.e., via presynaptic neurons). With the 500 µm electrode, threshold currents for direct activation were 6.9 µA for 0.1–msec pulses and 2 µA for 1–msec and 10–msec pulses, while current thresholds for indirect activation were 54 µA for 0.1–msec pulses, 8.9 µA for 1–msec pulses and 2.5 µA for 10–msec pulses. With the 125 um electrode, threshold currents for direct activation were 0.75 µA for 0.1–msec pulses, 0.38 µA for 1–msec pulses and 0.39 µA for 10–msec pulses, while current thresholds for indirect activation were 15 µA for 0.1–msec pulses, 3 µA for 1–msec pulses and 0.68 µA for 10–msec pulses. Conclusions: Threshold currents obtained with the 125 µm electrode were 3 to 9 times lower (depending upon pulse duration) than those obtained with the 500 µm electrode. For both electrodes, short (0.1 msec) current pulses preferentially activated ganglion cells directly, while long (10 msec) current pulses tended to activate ganglion cells both directly and indirectly. As such, even relatively long stimulus pulses will activate ganglion cells directly. The variation of activation depth and combinations of activated neurons induced by electrical stimulation will presumably influence perceptual quality with a prosthesis.
Keywords: ganglion cells • retina • retina: proximal (bipolar, amacrine, and ganglion cells)