Significance: Infrared neural stimulation (INS) utilizes pulsed infrared light to selectively elicit neural activity without exogenous compounds. Despite its versatility in a broad range of biomedical applications, no comprehensive comparison of factors pertaining to the efficacy and safety of INS such as wavelength, radiant exposure, and optical spot size exists in the literature.
Aim: Here, we evaluate these parameters using three of the wavelengths commonly used for INS, 1450 nm, 1875 nm, and 2120 nm.
Approach: In an in vivo rat sciatic nerve preparation, the stimulation threshold and transition rate to 100% activation probability were used to compare the effects of each parameter.
Results: The pulsed diode lasers at 1450 nm and 1875 nm had a consistently higher (∼1.0 J / cm2) stimulation threshold than that of the Ho:YAG laser at 2120 nm (∼0.7 J / cm2). In addition, the Ho:YAG produced a faster transition rate to 100% activation probability compared to the diode lasers. Our data suggest that the superior performance of the Ho:YAG is a result of the high-intensity microsecond spike at the onset of the pulse. Acute histological evaluation of diode irradiated nerves revealed a safe range of radiant exposures for stimulation.
Conclusion: Together, our results identify measures to improve the safety, efficacy, and accessibility of INS technology for research and clinical applications.
Infrared neural stimulation (INS) has the potential to serve as a spatially precise tool for neuroscientists and clinicians. Because of difficulties comparing various INS techniques in the current literature, we present a study comparing the stimulation threshold and transition rate to 100% activation probability for diode and Ho:YAG lasers. The stimulation threshold was lower for the Ho:YAG than the diode lasers across all examined spot sizes. The Ho:YAG also exhibited a nearly binary transition to 100% activation that can be mimicked with diode lasers by decreasing the pulse width. These results will help maximize the performance of laser diodes for INS.
Infrared neural stimulation (INS) is a promising neuromodulation technique capable of exciting neural tissue without the need for exogeneous agents or genetic modification. Due to its high spatial specificity, INS could improve upon traditional methods of selective neural stimulation in both the laboratory and the clinic. As of yet, no study has compared the efficacy and safety of using different INS parameters such as spot size and wavelength. Moreover, differences in the methods of determining energy deposition and laser spot size make it difficult to compare stimulation parameters used in the current literature. Here, we present results comparing INS efficacy using 1450nm and 1875nm light over a range of spot sizes and radiant exposures. Stimulation thresholds were determined using recorded compound muscle action potentials (CMAPs) and visible muscle contractions in an in vivo rat sciatic nerve model. Overall, 1450nm light required lower radiant exposures to achieve threshold activation as compared to 1875nm. While radiant exposures remained relatively constant across different spot sizes when using 1450nm, the threshold radiant exposures for 1875nm exposures increased with spot size suggesting deeper nerves fibers tend to be activated. Moreover, exposures using a flat-top beam profile yielded less variability in the stimulation threshold than those using a Gaussian profile. As in previous studies, histology confirmed that damaging radiant exposures are several times greater than the stimulation threshold for both 1450nm and 1875nm. Our results provide valuable insight for future studies involving INS and for further developing INS as both a research and clinical tool.
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