Electrodeless, non-invasive stimulation of retinal neurons using time-varying magnetic fields

Conventional approaches for stimulating retinal neurons use microelectrode arrays and electronic devices which must be invasively implanted in the eye. For non-invasive neuronal stimulations, many researches have made efforts of investigating behavioral and neurophysiologic effects using transcranial magnetic stimulation (TMS) methods. However, this approach focuses only on stimulating bulk areas of the cortical region in the brain, and there has been no prior report of magnetically stimulating localized, thin layers of retinal neurons from a sufficient distance that does not require a surgical implant. This paper proposes a completely non-invasive retinal stimulation using time-varying magnetic fields for the first time. Retinal stimulations are achieved by inducing eddy currents on retinal ganglion cells (RGCs) with time-varying magnetic fields. The stimulator is composed of a voltage source, a voltage booster, a trigger circuit, a driver circuit, a storage capacitor bank, and a stimulating coil. The maximum voltage rating and the capacitance of the storage capacitor bank are 1 kV and 1.3 mF, respectably. Through in vitro experiments, retinal responses are successfully evoked from a distance of up to 5 cm. The RGC responses with latency in 0–70 ms are determined. Compared to the control data, the total number of the first spikes per pulse increases from 0.31 to 0.69. The magnetic stimulation method allows the possibilities of a non-invasive retinal stimulation without using any surgically implanted devices.