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Seung Woo Lee, Giorgio Bonmassar, Shelley I. Fried; Activation of Retinal Ganglion Cells By Microcoil-Induced Magnetic Stimulation. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5530.
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Magnetic stimulation has gained attention recently as a potential method to activate neurons, i.e. as a neural prosthetic. Use of this approach has several potential advantages over conventional electrical stimulation, e.g. possible reduction in inflammation at the electrode-tissue interface and/or improved compatibility with magnetic resonance imaging (MRI). Unfortunately, existing TMS systems are too large to be viable for use as a neural prosthetic. Here we demonstrate that the magnetic fields arising from coils as small as 500 µm strongly activate retinal ganglion cells and explore how the parameters of stimulation influence activation.
Cell-attached patch clamping was used to record spikes from rabbit retinal ganglion cells in the isolated rabbit retina. Magnetic stimulation was delivered epiretinally via a thin-film based micro-coil (1,000 (L) X 500 (W) X 350 (H) μm); the coil had a resistance of 4 Ω and an inductance of 100 nH. Driving voltage input for the micro-coil was generated by a function generator and amplified by an audio amplifier. Pseudo-monophasic stimulus waveforms with both positive and negative phases were delivered at a frequency of 1 Hz with an inter-phase-interval of 1 second.
Magnetic stimulation successfully elicited spiking from RGCs. While single spikes with short latencies of 1-2 ms appeared at stimulus levels of 2-3 V, bursts of spikes with latencies of >5 ms were elicited by stimulus levels above 3 V. Responses were sensitive to the distance between the coil and the targeted ganglion cell as well as to the orientation of the coil relative to the cell.
Our results indicate that magnetic stimulation via micro-coils can activate neurons. Both short and long latency spikes were observed suggesting that magnetic stimulation is capable of activating ganglion cells directly as well as activating neurons presynaptic to ganglion cells. The orientation sensitivity we observed suggests that it may be possible to selectively restrict activity to specific regions of neural tissue relative to the coil.
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