Combined Optical and Electrical Stimulation of Neural Tissue In Vivo

Abstract

The recent development of low-intensity, pulsed infrared light for neural activation has provided a new nerve stimulation modality that avoids the limitations of traditional electrical methods such as the necessity of contact, presence of a stimulation artifact and poor spatial precision. Infrared neural stimulation is, however, limited by a 2:1 ratio of damaging radiant exposures to stimulation threshold radiant exposures. For infrared neural stimulation to become more applicable and eventually suitable for implantation, the range of safe and effective radiant exposures as indicated by this ratio must be increased. In this study, we have shown that this ratio is increased to as much as 7:1 by combining the infrared pulse with a subthreshold depolarizing electrical stimulus. Our results indicate a nonlinear relationship between the subthreshold depolarizing electrical stimulus (expressed as percentage of electrical stimulation threshold) and the additional optical energy required to reach stimulation threshold (expressed as percentage of optical stimulation threshold). The results also show that the change in optical threshold decreases linearly as the delay between the electrical and optical pulses is increased. The primary benefit of infrared neural stimulation is spatial selectivity and we have shown that precision is maintained for this combined stimulation modality. Our findings are evaluated in the context of latent addition and "superexcitability" according to previously published results. The results of this study are expected to facilitate the development of applications for infrared neural stimulation, as well as target the efforts to uncover the mechanism by which infrared light activates neural tissue.