Recently, hybrid neural stimulation combining electrical and optical techniques was demonstrated. By applying a subthreshold
electrical stimulus with infrared neural stimulation (INS), hybrid stimulation was shown to reduce INS
thresholds as much as 3-fold while maintaining spatial selectivity, thus overcoming the risk of thermally-induced tissue
damage associated with INS and the fundamental lack of spatial specificity associated with electrical stimulation. While
the potential of hybrid stimulation is evident, a better fundamental understanding of the interaction between tissue, light,
thermal gradients and current is necessary before this new stimulation paradigm can be further refined and optimized for
clinical implementation. A key element of this understanding is the spatial superposition of the electrical and optical
stimuli. A successful hybrid stimulation paradigm requires accurate recruitment of the same neurons by each modality. If
the same neurons are not targeted by both electrical and optical stimulation, then hybrid stimulation will suffer from lack
of repeatability and consistency. Here we present evidence as to how light and current interact spatially within neural
tissue. There exists a finite spatial region that is excitable by hybrid stimulation. This region is shown to change in size
and location by altering the optical and electrical components of the hybrid stimulus. By taking advantage of these
results, we are now able to achieve greater control of hybrid stimulation and can better apply this promising technology.
Low-intensity, pulsed infrared light provides a novel nerve stimulation modality that avoids the limitations of traditional electrical methods such as necessity of contact, presence of a stimulation artifact, and relatively poor spatial precision. Infrared neural stimulation (INS) is, however, limited by a 2:1 ratio of threshold radiant exposures for damage to that for stimulation. We have shown that this ratio is increased to nearly 6:1 by combining the infrared pulse with a subthreshold electrical stimulus. Our results indicate a nonlinear relationship between the subthreshold depolarizing electrical stimulus and additional optical energy required to reach stimulation threshold. The change in optical threshold decreases linearly as the delay between the electrical and optical pulses is increased. We have shown that the high spatial precision of INS is maintained for this combined stimulation modality. Results of this study will 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.