Cochlear implants artificially restore hearing to people with hearing loss through electrical stimulation of the auditory nerve, but hearing outcomes are limited by the broad spread of current throughout the cochlea fluids. Optogenetic stimulation can improve spatial precision within the cochlea, but cannot achieve the high stimulation rates used in contemporary cochlear implants. Hybrid (optogenetic and electrical) stimulation offers a means of achieving both high spatial precision whilst maintaining high stimulation rates. We recorded auditory nerve responses to three modes of stimulation – light/optogenetic, electrical, and hybrid – and compared the activation thresholds and temporal precision across the modalities.
Infrared light can be used to modulate the activity of neuronal cells with broad generality and without any need for exogenous materials. The action potential response has been shown to be associated with heating due to the absorption of light by water in and around the illuminated tissues, which gives rise to at least two distinct processes: namely, the temperature pulses cause depolarizing capacitive currents due to an intramembrane thermo-mechanical effect, and in addition, temperature-sensitive TRPV ion channels (and likely, voltage-gated channels) drive additional membrane depolarization. However, substantial differences between the activation threshold of primary auditory neurons (<20 mJ/cm^2) and other neuronal types (>300 mJ/cm^2) in vivo have generated some controversy in the field. A temperature-dependent Hodgkin-Huxley type model, which combines capacitive currents and the experimentally-derived characteristics of voltage-gated potassium and sodium ion channels in primary auditory neurons, was used to accurately explain the in vitro response to 1870 nm infrared illumination. TRPV channels do not make a significant contribution in this case, suggesting that the detailed mechanism of the neuronal response to infrared light is dependent on the specific cell type. Furthermore, based on this detailed understanding of the cell behaviour, it is shown that action potentials cannot be generated at safe laser power levels. This suggests that the previously reported response of the auditory system to infrared stimulation in vivo might arise from a different mechanism, and calls into question the potential usefulness of the effect for auditory prostheses.
A model of infrared neural stimulation (INS) has been developed to allow the temporal characteristics of different stimulation parameters and geometries to be better understood. The model uses a finite element approach to solve the heat equation and allow detailed analysis of heat during INS with both microsecond and millisecond laser pulses. When compared with experimental data, the model provides insight into the mechanisms behind INS. In particular, the analysis suggests that there may be two broad regimes of INS: the process tends to be limited by the total pulse energy for pulse lengths below 100 μs, while the temperature gradient with respect to time becomes more important above 100 μs.
Infrared neural stimulation (INS) is a novel technique for stimulating neurons with infrared light, rather than the traditional electrical means. There has been significant discussion in the literature on the mechanisms behind INS, while recent work has shown that infrared light stimulates neurons by causing a reversible change in their membrane capacitance. Nevertheless, the effect of different laser parameters on neuronal responses is still not well understood. To better understand this and to assist in designing light delivery systems, modelling of spatial and temporal characteristics of light delivery during INS has been performed. Monte Carlo modelling of photon transport in tissue allows the spatial characteristics of light to be determined during INS and allows comparisons of varying geometries and fibre designs. Finite element analysis of heat conduction can then be used to reveal the behavior of different pulse durations and the resulting temperature decay. The combination of the two methods allows for further insights into the mechanisms of INS and assists in understanding different mechanisms which promote INS. The model suggests there may be two regimes of INS, namely temperature limited for pulses under 100 μs and temperature gradient limited for longer pulses. this is compatible with previously published data, but requires further experimentation for confirmation. The model also provides a tool for optimising the design of emitters and implants.
A Monte Carlo model has been developed to simulate light transport and absorption in neural tissue during infrared neural stimulation (INS). A range of fiber core sizes and numerical apertures are compared illustrating the advantages of using simulations when designing a light delivery system. A range of wavelengths, commonly used for INS, are also compared for stimulation of nerves in the cochlea, in terms of both the energy absorbed and the change in temperature due to a laser pulse. Modeling suggests that a fiber with core diameter of 200 μm and NA = 0.22 is optimal for optical stimulation in the geometry used and that temperature rises in the spiral ganglion neurons are as low as 0.1°C. The results show a need for more careful experimentation to allow different proposed mechanisms of INS to be distinguished.
Sensing and telecommunication applications requiring the bending of optical fibers to small diameters are on the
increase. Recent work has shown that the centre wavelength of fiber Bragg gratings has a bend dependence the
magnitude of which varies with the type of fiber in which the grating is written. In this work the basis of the centre
wavelength shift is investigated by modeling the effects of several potential causes for standard and depressed cladding
fiber designs. The majority of the expected affects, including bend induced stress and mode field deformation, were
found to result in small wavelength shifts in the opposite direction to those observed experimentally. However, a new
account of the shift, based on simplistic geometrical optics, does show wavelength changes in the observed direction, of
up to -0.15 nm, which is in the range of the experimentally measured shifts.
Optical fibers are finding increased usage in biomedical applications, during which the fibers are often subjected to
bending. Bending optical fibers can affect transmission properties and increase the likelihood of failure. In this paper the
bending of fibers is discussed in relation to biomedical and sensing applications.
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