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Yukari Nakano, Yasuo Terasawa, Atsushi Tanaka, Tomomitsu Miyoshi, Hajime Sawai; The effects of transcranial direct current stimulation (tDCS) to response decay of electrical train stimulation to the retina. Invest. Ophthalmol. Vis. Sci. 2020;61(7):2210.
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© ARVO (1962-2015); The Authors (2016-present)
Because transcranial direct current stimulation (tDCS) can enhance visually evoked cortical responses [H. Monai, et al. Nat. Commun. 2016], it may be beneficial for retinal prostheses, in which continuous stimulation causes a decrease in electrical evoked potentials (EEPs) and sensation of phosphenes. To test these hypotheses, we evaluated cortical field responses to a train of retinal electrical stimulation before and after tDCS.
In rats (Long-Evans, n = 3) anesthetized with urethane (1.75 g/kg), the dorsal surface of the right visual cortex (VC) was exposed and covered with conductive gel. For the tDCS, an anodal electrical current (0.1 mA) was applied for 10 min through the VC via the conductive gel. A train of 100 biphasic electrical pulses (0.5 ms, 20 Hz) was applied to the left retina before and after the application of tDCS, and the EEPs from the right VC were recorded. The average signal was 40 times of train stimulations at each time point. In this study, the mean amplitude before electrical train stimulation was set as the baseline, and the difference between the baseline and the first negative response peak (N1), approximately 15 ms after each electrical stimulation, was defined as the EEP amplitude.
The average EEP in response to the first pulse of 100 stimulations showed the greatest N1 amplitude, while the second pulse induced only a small EEP. After a dozen of stimulation pulses, the N1 response gradually recovered, as shown in Fig. 1. The recovery time (time taken by the second and subsequent EEPs to reach 20% of the first EEP) was compared before and after tDCS application. The recovery time occurred approximately 300 ms earlier after tDCS than before it; however, there was no significant difference (Fig. 2).
This study shows that VC responses to retinal stimulation quickly diminish and then slowly recover, which may be accelerated by tDCS. Therefore, tDCS may be beneficial for phosphene sensation maintenance against rapid reduction during repetitive retinal electrical stimulation. Further evaluations required using animal models of diseases targeted by retinal prostheses.
This is a 2020 ARVO Annual Meeting abstract.
Fig. 1. Example of response waveform by electrical train stimulation
Fig. 2. The time took for the second and subsequent EEPs of the 100 train stimulations to reach 20% of the first EEP
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