Purpose: Optical stimulation methods in development aim to provide high spatial selectivity of target tissue, overcoming a critical limitation of contemporary neural prostheses. The purpose of this study is to determine if tapered fibers are capable of delivering the minimum necessary power density 1W/mm2 within a 0.050mm spot size to induce focused infrared neural stimulation (INS). Materials: A numerical simulation program based on equations derived from Snell’s law was developed in MATLAB to predict the energy emitted from a tapered fiber coupled to a Capella laser (λ=1863nm, Lockheed Martin Aculight). Energy predictions were compared to emittance from a tapered fiber (core diameter = 200µm, tapered output face = 50µm, NA=0.22) to determine its accuracy. Energy measurements were made at 17.8, 41.6, 65.4, 89.3, and 113.1µJ output energy and at distances between 0-2 mm from the fiber-tip with a Coherent FieldMax Energy meter coupled to a detector with a 2.1 mm aperture. Results: Mean difference between the predicted and measured energy ranged from 4.3±1.9µJ (17.8µJ) to 16.3±11.3 µJ (113.1µJ). Minimum required power density within a 0.05 mm spot size was predicted to be achieved at 0 mm for all energies, at 2 mm for 41.6µJ, and at distances ≥ 1.0 mm for 17.8 µJ. Conclusion: A numerical simulation program was developed that accurately predicts within minimal error the emittance from a tapered fiber. The predicted results indicate feasibility of tapered optical fibers to provide a more efficient and selective means of delivering the minimum power density necessary to achieve INS.
Infrared neural stimulation (INS) has been used in the past to evoke neural activity from hearing and partially deaf
animals. All the responses were excitatory. In Aplysia californica, Duke and coworkers demonstrated that INS also
inhibits neural responses , which similar observations were made in the vestibular system [2, 3]. In deaf white cats
that have cochleae with largely reduced spiral ganglion neuron counts and a significant degeneration of the organ of
Corti, no cochlear compound action potentials could be observed during INS alone. However, the combined electrical
and optical stimulation demonstrated inhibitory responses during irradiation with infrared light.
Among neural prostheses cochlear implants (CIs) are considered the most successful devices. They restore some hearing to ~210,000 severe-to-profound hearing impaired people. Despite the devices’ success, the performance of the implanted individuals in noisy environments is poor and music perception is rudimentary. It has been argued that increasing the number of independent channels for stimulation can improve the performance of a CI user in challenging hearing environments. An optical method, stimulating neurons with infrared radiation, has been suggested as a novel approach to increase the number of independent channels. Infrared neural stimulation (INS) works through the deposition of heat into the tissue. Thermal damage is therefore a potential risk, particularly for longterm exposure. To verify the efficacy and safety of INS, cats were implanted for about 4 weeks and were continuously stimulated daily for 6-8 hours. Cochlear function did not change during the stimulation, and histology did not reveal signs of damage. Tissue growth following the implantation was largely localized at the cochleostomy.
Infrared neural stimulation (INS) describes a method, by which an infrared laser is used to stimulate neurons. The major benefit of INS over stimulating neurons with electrical current is its spatial selectivity. To translate the technique into a clinical application it is important to know the energy required to stimulate the neural structure. With this study we provide measurements of the radiant exposure, at the target structure that is required to stimulate the auditory neurons. Flat polished fibers were inserted into scala tympani so that the spiral ganglion was in front of the optical fiber. Angle polished fibers were inserted along scala tympani, and rotating the beveled surface of the fiber allowed the radiation beam to be directed perpendicular to the spiral ganglion. The radiant exposure for stimulation at the modiolus for flat and angle polished fibers averaged 6.78±2.15 mJ/cm2. With the angle polished fibers, a 90º change in the orientation of the optical beam from an orientation that resulted in an INS-evoked maximum response, resulted in a 50% drop in the response amplitude. When the orientation of the beam was changed by 180º, such that it was directed opposite to the orientation with the maxima, minimum response amplitude was observed.
The application of photonics to manipulate and stimulate neurons and to study neural networks has gained momentum over the last decade. Two general methods have been used: the genetic expression of light or temperature sensitive ion channels in the plasma membrane of neurons (Optogenetics and Thermogenetics) and the direct stimulation of neurons using infrared radiation (Infrared Neural Stimulation, INS). Both approaches have their strengths and challenges, which are well understood with a profound understanding of the light tissue interaction(s). This paper compares the opportunities of the methods for the use in cochlear prostheses. Ample data are already available on the stimulation of the cochlea with INS. The data show that the stimulation is selective, feasible at rates that would be sufficient to encode acoustic information and may be beneficial over conventional pulsed electrical stimulation. A third approach, using lasers in stress confinement to generate pressure waves and to stimulate the functional cochlea mechanically will also be discussed.
It has been demonstrated previously that infrared neural stimulation (INS) can be used to stimulate spiral ganglion cells
in the cochlea. With INS, neural stimulation can be achieved without direct contact of the radiation source and the tissue
and is spatially well resolved. The presence of fluids or bone between the target structure and the radiation source may
lead to absorption or scattering of the radiation and limit the efficacy of INS. To develop INS based cochlear implants, it
is critical to determine the beam path of the radiation in the cochlea. In the present study, we utilized noninvasive X-ray
microtomography (microCT) to visualize the orientation and location of the optical fiber within the guinea pig and cat
cochlea. Overall, the results indicated that the optical fiber was directed towards the spiral ganglion cells in the cochlea
and not the nerve fibers in the center of the modiolus. The fiber was approximately 300 μm away from the target
structures. In future studies, results from the microCT will be correlated with physiology obtained from recordings in
It has been demonstrated that spiral ganglion cells in the cochlea can be stimulated with infrared radiation (IR). The
potential benefits of infrared neural stimulation (INS) include the possibility of stimulating the neurons without direct
physical contact between the stimulation source and the neural tissue and the improved spatial selectivity. In order to
determine how INS can be best incorporated in both research and in neural interfaces, it is critical to identify the optimal
stimulation parameters for the laser. This study focuses on direct comparison of amplitudes of neural responses evoked
by various IR pulse shapes and durations. With the present experiments, the results indicate that the peak power is an
important variable for stimulating auditory neurons. While the radiant energy has little effect on amplitudes of
compound potentials (CAP) evoked by infrared pulses shorter than 70 μs, it impacts the amplitudes for pulses 80 μs and
longer. In addition, we show that the shape of the infrared pulse is important and varying the shape may allow an
expansion of response dynamic range while stimulating neurons. The results indicate that square pulses were the most
effective pulse shape to evoke CAPs.