Analytical approaches for determining heat distributions and thermal criteria for infrared neural stimulation

Abstract

Infrared neural stimulation (INS) is becoming an important complementary tool to electrical stimulation. Since the mechanism of INS is photothermal, describing the laser-induced heat distribution is fundamental to determining the relationship between stimulation pulses and neural responses. This work developed both a framework describing the time evolution of the heat distribution induced by optical fluence and a new method to extract thermal criteria (e.g., temperature change and rate of change) for neural activation. To solve the general problem of describing the temperature distribution, a Green's function solution to the heat diffusion equation was determined and convolved with the optical fluence. This provided a solution in the form of a single integral over time, from which closed-form solutions can be determined for special cases. This work also yielded an expression for thermal relaxation time, which provides a rigorous description of thermal confinement for INS. The developed framework was then applied to experimental data from the cochlea to extract the minimum temperature increase and rate of that increase to stimulate the cochlear spiral ganglion. This result, and similar analyses applied to other neural systems, can then shed light on the fundamental mechanism for INS and aid the development of optical neuroprostheses.

Photons and neurons

“…In a recent paper by Norton et al (Norton et al, 2013), an analytical approach has been used to determine the heat distribution and thermal criteria for INS. A solution for the heat diffusion equation was combined with the optical fluences.…”

“…The developed framework could well be applied to existing experimental data. A key conclusion that has been drawn from the model is that the rate in temperature change and not the absolute temperature are essential for INS (Norton et al, 2013). …”

Pulsed infrared radiation excites cultured neonatal spiral and vestibular ganglion neurons by modulating mitochondrial calcium cycling

“…Lowering the radiant exposure to ϳ398 mJ/cm 2 reduced the number of neurons stimulated, but the IR response persisted in neurons near the center of the beam. This could be explained by the fact that the optical beam of a multimode fiber tends toward a Gaussian-shaped envelope (Norton et al 2013), and the radiant energy is highest at the center of the beam. Based on these results, it is unlikely that the [Ca 2ϩ ] i transients were a result of activation of mechanosensitive Ca 2ϩ influx led by an optoacoustic phenomenon (Teudt et al 2011).…”

Histological Assessment of Thermal Damage in the Brain Following Infrared Neural Stimulation

“…Given the rather large parameter space, the question is most easily addressed by numerical models, a number of which have been published. (16–19) However, these models often do not include a refined description of the tissue being stimulated, treating it as a homogeneous aqueous environment.…”