Electrodeless, Non-Invasive Stimulation of Retinal Neurons Using Time-Varying Magnetic Fields

Conventional approaches for stimulating retinal neurons use microelectrode arrays and electronic devices, parts of which must be implanted in the eye requiring surgical procedures. For non-invasive neuronal stimulations, many researches have investigated the behavioral and neurophysiological effects of using repetitive transcranial magnetic stimulation methods. However, the magnetic stimulation approaches so far have focused only on stimulating the bulk areas of the cortical region in the brain, and there has been no prior report on magnetically stimulating localized, thin layers of neurons from a sufficient distance that does not require a surgical implant. This paper proposes a surgically non-invasive retinal stimulation method by using time-varying magnetic fields for the first time. Retinal stimulations are achieved by inducing eddy currents on retinal ganglion cells with time-varying magnetic fields. The stimulator is developed using a voltage source, a voltage booster, a trigger circuit, a driver circuit, a storage capacitor bank, and a stimulating coil. In in vitro experiments, retinal responses are successfully evoked from a distance of up to 5 cm. The average number of the first spikes per pulse is 0.31 for the control, 0.38 for 270 V, 0.60 for 450 V, and 0.69 for 700 V. These results clearly indicate that the time-varying magnetic fields can elicit neural responses, and that more neural responses are evoked as the charging voltage is increased during short and medium latencies. The magnetic stimulation method allows the possibilities of a non-invasive retinal stimulation without using any surgically implanted devices.