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

Infrared neuromodulation-a review

Infrared (IR) neuromodulation (INM) is an emerging light-based neuromodulation approach that can reversibly control neuronal and muscular activities through the transient and localized deposition of pulsed IR light without requiring any chemical or genetic pre-treatment of the target cells. Though the efficacy and short-term safety of INM have been widely demonstrated in both peripheral and central nervous systems, the investigations of the detailed cellular and biological processes and the underlying biophysical mechanisms are still ongoing. In this review, we discuss the current research progress in the INM field with a focus on the more recently discovered IR nerve inhibition. Major biophysical mechanisms associated with IR nerve stimulation are summarized. As the INM effects are primarily attributed to the spatiotemporal thermal transients induced by water and tissue absorption of pulsed IR light, temperature monitoring techniques and simulation models adopted in INM studies are discussed. Potential translational applications, current limitations, and challenges of the field are elucidated to provide guidance for future INM research and advancement.

Modulation of neuronal dynamics by sustained and activity-dependent continuous-wave near-infrared laser stimulation

Significance

Near-infrared laser illumination is a non-invasive alternative/complement to classical stimulation methods in neuroscience but the mechanisms underlying its action on neuronal dynamics remain unclear. Most studies deal with high-frequency pulsed protocols and stationary characterizations disregarding the dynamic modulatory effect of sustained and activity-dependent stimulation. The understanding of such modulation and its widespread dissemination can help to develop specific interventions for research applications and treatments for neural disorders.

Aim

We quantified the effect of continuous-wave near-infrared (CW-NIR) laser illumination on single neuron dynamics using sustained stimulation and an open-source activity-dependent protocol to identify the biophysical mechanisms underlying this modulation and its time course.

Approach

We characterized the effect by simultaneously performing long intracellular recordings of membrane potential while delivering sustained and closed-loop CW-NIR laser stimulation. We used waveform metrics and conductance-based models to assess the role of specific biophysical candidates on the modulation.

Results

We show that CW-NIR sustained illumination asymmetrically accelerates action potential dynamics and the spiking rate on single neurons, while closed-loop stimulation unveils its action at different phases of the neuron dynamics. Our model study points out the action of CW-NIR on specific ionic-channels and the key role of temperature on channel properties to explain the modulatory effect.

Conclusions

Both sustained and activity-dependent CW-NIR stimulation effectively modulate neuronal dynamics by a combination of biophysical mechanisms. Our open-source protocols can help to disseminate this non-invasive optical stimulation in novel research and clinical applications.

Nanoparticle-based optical interfaces for retinal neuromodulation: a review

Degeneration of photoreceptors in the retina is a leading cause of blindness, but commonly leaves the retinal ganglion cells (RGCs) and/or bipolar cells extant. Consequently, these cells are an attractive target for the invasive electrical implants colloquially known as "bionic eyes." However, after more than two decades of concerted effort, interfaces based on conventional electrical stimulation approaches have delivered limited efficacy, primarily due to the current spread in retinal tissue, which precludes high-acuity vision. The ideal prosthetic solution would be less invasive, provide single-cell resolution and an ability to differentiate between different cell types. Nanoparticle-mediated approaches can address some of these requirements, with particular attention being directed at light-sensitive nanoparticles that can be accessed via the intrinsic optics of the eye. Here we survey the available known nanoparticle-based optical transduction mechanisms that can be exploited for neuromodulation. We review the rapid progress in the field, together with outstanding challenges that must be addressed to translate these techniques to clinical practice. In particular, successful translation will likely require efficient delivery of nanoparticles to stable and precisely defined locations in the retinal tissues. Therefore, we also emphasize the current literature relating to the pharmacokinetics of nanoparticles in the eye. While considerable challenges remain to be overcome, progress to date shows great potential for nanoparticle-based interfaces to revolutionize the field of visual prostheses.

Characterization and closed-loop control of infrared thalamocortical stimulation produces spatially constrained single-unit responses

Deep brain stimulation (DBS) is a powerful tool for the treatment of circuitopathy-related neurological and psychiatric diseases and disorders such as Parkinson's disease and obsessive-compulsive disorder, as well as a critical research tool for perturbing neural circuits and exploring neuroprostheses. Electrically mediated DBS, however, is limited by the spread of stimulus currents into tissue unrelated to disease course and treatment, potentially causing undesirable patient side effects. In this work, we utilize infrared neural stimulation (INS), an optical neuromodulation technique that uses near to midinfrared light to drive graded excitatory and inhibitory responses in nerves and neurons, to facilitate an optical and spatially constrained DBS paradigm. INS has been shown to provide spatially constrained responses in cortical neurons and, unlike other optical techniques, does not require genetic modification of the neural target. We show that INS produces graded, biophysically relevant single-unit responses with robust information transfer in rat thalamocortical circuits. Importantly, we show that cortical spread of activation from thalamic INS produces more spatially constrained response profiles than conventional electrical stimulation. Owing to observed spatial precision of INS, we used deep reinforcement learning (RL) for closed-loop control of thalamocortical circuits, creating real-time representations of stimulus-response dynamics while driving cortical neurons to precise firing patterns. Our data suggest that INS can serve as a targeted and dynamic stimulation paradigm for both open and closed-loop DBS.

Eliciting calcium transients with UV nanosecond laser stimulation in adult patient-derived glioblastoma brain cancer cellsin vitro

Objective.Glioblastoma (GBM) is the most common and lethal type of high-grade adult brain cancer. The World Health Organization have classed GBM as an incurable disease because standard treatments have yielded little improvement with life-expectancy being 6-15 months after diagnosis. Different approaches are now crucial to discover new knowledge about GBM communication/function in order to establish alternative therapies for such an aggressive adult brain cancer. Calcium (Ca2+) is a fundamental cell molecular messenger employed in GBM being involved in a wide dynamic range of cellular processes. Understanding how the movement of Ca2+behaves and modulates activity in GBM at the single-cell level is relatively unexplored but holds the potential to yield opportunities for new therapeutic strategies and approaches for cancer treatment.Approach.In this article we establish a spatially and temporally precise method for stimulating Ca2+transients in three patient-derived GBM cell-lines (FPW1, RN1, and RKI1) such that Ca2+communication can be studied from single-cell to larger network scales. We demonstrate that this is possible by administering a single optimized ultra-violet (UV) nanosecond laser pulse to trigger GBM Ca2+transients.Main results.We determine that 1.58µJµm-2is the optimal UV nanosecond laser pulse energy density necessary to elicit a single Ca2+transient in the GBM cell-lines whilst maintaining viability, functionality, the ability to be stimulated many times in an experiment, and to trigger further Ca2+communication in a larger network of GBM cells.Significance.Using adult patient-derived mesenchymal GBM brain cancer cell-lines, the most aggressive form of GBM cancer, this work is the first of its kind as it provides a new effective modality of which to stimulate GBM cells at the single-cell level in an accurate, repeatable, and reliable manner; and is a first step toward Ca2+communication in GBM brain cancer cells and their networks being more effectively studied.

Spatially specific, closed-loop infrared thalamocortical deep brain stimulation

Deep brain stimulation (DBS) is a powerful tool for the treatment of circuitopathy-related neurological and psychiatric diseases and disorders such as Parkinson's disease and obsessive-compulsive disorder, as well as a critical research tool for perturbing neural circuits and exploring neuroprostheses. Electrically-mediated DBS, however, is limited by the spread of stimulus currents into tissue unrelated to disease course and treatment, potentially causing undesirable patient side effects. In this work, we utilize infrared neural stimulation (INS), an optical neuromodulation technique that uses near to mid-infrared light to drive graded excitatory and inhibitory responses in nerves and neurons, to facilitate an optical and spatially constrained DBS paradigm. INS has been shown to provide spatially constrained responses in cortical neurons and, unlike other optical techniques, does not require genetic modification of the neural target. We show that INS produces graded, biophysically relevant single-unit responses with robust information transfer in thalamocortical circuits. Importantly, we show that cortical spread of activation from thalamic INS produces more spatially constrained response profiles than conventional electrical stimulation. Owing to observed spatial precision of INS, we used deep reinforcement learning for closed-loop control of thalamocortical circuits, creating real-time representations of stimulus-response dynamics while driving cortical neurons to precise firing patterns. Our data suggest that INS can serve as a targeted and dynamic stimulation paradigm for both open and closed-loop DBS.

Emerging trends in the development of flexible optrode arrays for electrophysiology

Optical-electrode (optrode) arrays use light to modulate excitable biological tissues and/or transduce bioelectrical signals into the optical domain. Light offers several advantages over electrical wiring, including the ability to encode multiple data channels within a single beam. This approach is at the forefront of innovation aimed at increasing spatial resolution and channel count in multichannel electrophysiology systems. This review presents an overview of devices and material systems that utilize light for electrophysiology recording and stimulation. The work focuses on the current and emerging methods and their applications, and provides a detailed discussion of the design and fabrication of flexible arrayed devices. Optrode arrays feature components non-existent in conventional multi-electrode arrays, such as waveguides, optical circuitry, light-emitting diodes, and optoelectronic and light-sensitive functional materials, packaged in planar, penetrating, or endoscopic forms. Often these are combined with dielectric and conductive structures and, less frequently, with multi-functional sensors. While creating flexible optrode arrays is feasible and necessary to minimize tissue-device mechanical mismatch, key factors must be considered for regulatory approval and clinical use. These include the biocompatibility of optical and photonic components. Additionally, material selection should match the operating wavelength of the specific electrophysiology application, minimizing light scattering and optical losses under physiologically induced stresses and strains. Flexible and soft variants of traditionally rigid photonic circuitry for passive optical multiplexing should be developed to advance the field. We evaluate fabrication techniques against these requirements. We foresee a future whereby established telecommunications techniques are engineered into flexible optrode arrays to enable unprecedented large-scale high-resolution electrophysiology systems.

Thermal effects on neurons during stimulation of the brain

All electric and magnetic stimulation of the brain deposits thermal energy in the brain. This occurs through either Joule heating of the conductors carrying current through electrodes and magnetic coils, or through dissipation of energy in the conductive brain.Objective.Although electrical interaction with brain tissue is inseparable from thermal effects when electrodes are used, magnetic induction enables us to separate Joule heating from induction effects by contrasting AC and DC driving of magnetic coils using the same energy deposition within the conductors. Since mammalian cortical neurons have no known sensitivity to static magnetic fields, and if there is no evidence of effect on spike timing to oscillating magnetic fields, we can presume that the induced electrical currents within the brain are below the molecular shot noise where any interaction with tissue is purely thermal.Approach.In this study, we examined a range of frequencies produced from micromagnetic coils operating below the molecular shot noise threshold for electrical interaction with single neurons.Main results.We found that small temperature increases and decreases of 1∘C caused consistent transient suppression and excitation of neurons during temperature change. Numerical modeling of the biophysics demonstrated that the Na-K pump, and to a lesser extent the Nernst potential, could account for these transient effects. Such effects are dependent upon compartmental ion fluxes and the rate of temperature change.Significance.A new bifurcation is described in the model dynamics that accounts for the transient suppression and excitation; in addition, we note the remarkable similarity of this bifurcation's rate dependency with other thermal rate-dependent tipping points in planetary warming dynamics. These experimental and theoretical findings demonstrate that stimulation of the brain must take into account small thermal effects that are ubiquitously present in electrical and magnetic stimulation. More sophisticated models of electrical current interaction with neurons combined with thermal effects will lead to more accurate modulation of neuronal activity.

Visualizing Lipid Dynamics Role in Infrared Neural Stimulation using Stimulated Raman Scattering

Infrared neural stimulation, or INS, uses pulsed infrared light to yield label-free neural stimulation with broad experimental and translational utility. Despite its robust demonstration, INS's mechanistic and biophysical underpinnings have been the subject of debate for more than a decade. The role of lipid membrane thermodynamics appears to play an important role in how fast IR-mediated heating nonspecifically drives action potential generation. Direct observation of lipid membrane dynamics during INS remains to be shown in a live neural model system. We used hyperspectral stimulated Raman scattering (hsSRS) microscopy to study biochemical signatures of high-speed vibrational dynamics underlying INS in a live neural cell culture model. Findings suggest that lipid bilayer structural changes are occurring during INS in vitro in NG108-15 neuroglioma cells. Lipid-specific signatures of cell SRS spectra varied with stimulation energy and radiant exposure. Spectroscopic observations agree with high-speed ratiometric fluorescence imaging of a conventional lipophilic membrane structure reporter, di-4-ANNEPS. Overall, the presented findings support the hypothesis that INS causes changes in the lipid membrane of neural cells by changing lipid membrane packing order. Furthermore, this work highlights the potential of hsSRS as a method to study biophysical and biochemical dynamics safely in live cells.

Single infrared light pulses induce excitatory and inhibitory neuromodulation

The excitatory and inhibitory effects of single and brief infrared (IR) light pulses (2 µm) with millisecond durations and various power levels are investigated with a custom-built fiber amplification system. Intracellular recordings from motor axons of the crayfish opener neuromuscular junction are performed ex vivo. Single IR light pulses induce a membrane depolarization during the light pulses, which is followed by a hyperpolarization that can last up to 100 ms. The depolarization amplitude is dependent on the optical pulse duration, total energy deposition and membrane potential, but is insensitive to tetrodotoxin. The hyperpolarization reverses its polarity near the potassium equilibrium potential and is barium-sensitive. The membrane depolarization activates an action potential (AP) when the axon is near firing threshold, while the hyperpolarization reversibly inhibits rhythmically firing APs. In summary, we demonstrate for the first time that single and brief IR light pulses can evoke initial depolarization followed by hyperpolarization on individual motor axons. The corresponding mechanisms and functional outcomes of the dual effects are investigated.

A Method for Recording the Bioelectrical Activity of Neural Axons upon Stimulation with Short Pulses of Infrared Laser Radiation

The aim of the study was to develop a method for long-term non-invasive recording of the bioelectrical activity induced in isolated neuronal axons irradiated with short infrared (IR) pulses and to study the effect of radiation on the occurrence of action potentials in axons of a neuron culture in vitro.

Materials and Methods

Hippocampal cells of mouse embryos (E18) were cultured in microfluidic chips made of polydimethylsiloxane and containing microchannels for axonal growth at a distance of up to 800 μm. We studied the electrophysiological activity of a neuronal culture induced by pulses of focused laser radiation in the IR range (1907 and 2095 nm). The electrophysiological activity of the neuronal culture was recorded using a multichannel recording system (Multi Channel Systems, Germany).

Results

The developed microfluidic chip and the optical stimulation system combined with the multichannel registration system made it possible to non-invasively record the action potentials caused by pulsed IR radiation in isolated neuronal axons in vitro. The propagation of action potentials in axons was detected using extracellular microelectrodes when the cells were irradiated with a laser at a wavelength of 1907 nm with a radiation power of 0.2-0.5 W for pulses with a duration of 6 ms and 0.5 W for pulses with a duration of 10 ms. It was shown that the radiation power positively correlated with the occurrence rate of axonal response. Moreover, the probability of a response evoked by optical stimulation increased at short optical pulses. In addition, we found that more responses could be evoked by irradiating the neuronal cell culture itself rather than the axon-containing microchannels.

Conclusion

The developed method makes it possible to isolate the axons growing from cultured neurons into a microfluidic chip, stimulate the neurons with infrared radiation, and non-invasively record the axonal spiking. The proposed approach allowed us to study the characteristics of neuronal responses in cell cultures over a long (weeks) period of time. The method can be used both in fundamental research into the brain signaling system and in the development of a non-invasive neuro-interface.

Opto-thermal technologies for microscopic analysis of cellular temperature-sensing systems

Could enzymatic activities and their cooperative functions act as cellular temperature-sensing systems? This review introduces recent opto-thermal technologies for microscopic analyses of various types of cellular temperature-sensing system. Optical microheating technologies have been developed for local and rapid temperature manipulations at the cellular level. Advanced luminescent thermometers visualize the dynamics of cellular local temperature in space and time during microheating. An optical heater and thermometer can be combined into one smart nanomaterial that demonstrates hybrid function. These technologies have revealed a variety of cellular responses to spatial and temporal changes in temperature. Spatial temperature gradients cause asymmetric deformations during mitosis and neurite outgrowth. Rapid changes in temperature causes imbalance of intracellular Ca2+ homeostasis and membrane potential. Among those responses, heat-induced muscle contractions are highlighted. It is also demonstrated that the short-term heating hyperactivates molecular motors to exceed their maximal activities at optimal temperatures. We discuss future prospects for opto-thermal manipulation of cellular functions and contributions to obtain a deeper understanding of the mechanisms of cellular temperature-sensing systems.

Near-infrared stimulation of the auditory nerve: A decade of progress toward an optical cochlear implant

OBJECTIVES

We provide an appraisal of recent research on stimulation of the auditory system with light. In particular, we discuss direct infrared stimulation and ongoing controversies regarding the feasibility of this modality. We also discuss advancements and barriers to the development of an optical cochlear implant.

METHODS

This is a review article that covers relevant animal studies.

RESULTS

The auditory system has been stimulated with infrared light, and in a much more spatially selective manner than with electrical stimulation. However, there are experiments from other labs that have not been able to reproduce these results. This has resulted in an ongoing controversy regarding the feasibility of infrared stimulation, and the reasons for these experimental differences still require explanation. The neural response characteristics also appear to be much different than with electrical stimulation. The electrical stimulation paradigms used for modern cochlear implants do not apply well to optical stimulation and new coding strategies are under development. Stimulation with infrared light brings the risk of heat accumulation in the tissue at high pulse repetition rates, so optimal pulse shapes and combined optical/electrical stimulation are being investigated to mitigate this. Optogenetics is another promising technique, which makes neurons more sensitive to light stimulation by inserting light sensitive ion channels via viral vectors. Challenges of optogenetics include the expression of light sensitive channels in sufficient density in the target neurons, and the risk of damaging neurons by the expression of a foreign protein.

CONCLUSION

Optical stimulation of the nervous system is a promising new field, and there has been progress toward the development of a cochlear implant that takes advantage of the benefits of optical stimulation. There are barriers, and controversies, but so far none that seem intractable.

LEVEL OF EVIDENCE

NA (animal studies and basic research).

Response of primary auditory neurons to stimulation with infrared light in vitro

OBJECTIVE

Infrared light can be used to modulate the activity of neuronal cells through thermally-evoked capacitive currents and thermosensitive ion channel modulation. The infrared power threshold for action potentials has previously been found to be far lower in the in vivo cochlea when compared with other neuronal targets, implicating spiral ganglion neurons (SGNs) as a potential target for infrared auditory prostheses. However, conflicting experimental evidence suggests that this low threshold may arise from an intermediary mechanism other than direct SGN stimulation, potentially involving residual hair cell activity.

APPROACH

Patch-clamp recordings from cultured SGNs were used to explicitly quantify the capacitive and ion channel currents in an environment devoid of hair cells. Neurons were irradiated by a 1870 nm laser with pulse durations of 0.2-5.0 ms and powers up to 1.5 W. A Hodgkin-Huxley-type model was established by first characterising the voltage dependent currents, and then incorporating laser-evoked currents separated into temperature-dependent and temperature-gradient-dependent components. This model was found to accurately simulate neuronal responses and allowed the results to be extrapolated to stimulation parameter spaces not accessible during this study.

MAIN RESULTS

The previously-reported low in vivo SGN stimulation threshold was not observed, and only subthreshold depolarisation was achieved, even at high light exposures. Extrapolating these results with our Hodgkin-Huxley-type model predicts an action potential threshold which does not deviate significantly from other neuronal types.

SIGNIFICANCE

This suggests that the low-threshold response that is commonly reported in vivo may arise from an alternative mechanism, and calls into question the potential usefulness of the effect for auditory prostheses. The step-wise approach to modelling optically-evoked currents described here may prove useful for analysing a wider range of cell types where capacitive currents and conductance modulation are dominant.

Shining light on cardiac electrophysiology: From detection to intervention, from basic research to translational applications

Cardiac arrhythmias are an important group of cardiovascular diseases, which can occur alone or in association with other cardiovascular diseases. The development of cardiac arrhythmias cannot be separated from changes in cardiac electrophysiology, and the investigation and clarification of cardiac electrophysiological changes are beneficial for the treatment of cardiac arrhythmias. However, electrical energy-based pacemakers and defibrillators, which are widely used to treat arrhythmias, still have certain disadvantages. Thereby, optics promises to be used for optical manipulation and its use in biomedicine is increasing. Since visible light is readily absorbed and scattered in living tissues and tissue penetration is shallow, optical modulation for cells and tissues requires conversion media that convert light energy into bioelectrical activity. In this regard, fluorescent dyes, light-sensitive ion channels, and optical nanomaterials can assume this role, the corresponding optical mapping technology, optogenetics technology, and optical systems based on luminescent nanomaterials have been introduced into the research in cardiovascular field and are expected to be new tools for the study and treatment of cardiac arrhythmias. In addition, infrared and near-infrared light has strong tissue penetration, which is one of the excellent options of external trigger for achieving optical modulation, and is also widely used in the study of optical modulation of biological activities. Here, the advantages of optical applications are summarized, the research progresses and emerging applications of optical-based technologies as detection and intervention tools for cardiac electrophysiological are highlighted. Moreover, the prospects for future applications of optics in clinical diagnosis and treatment are discussed.

Identifying optimal parameters for infrared neural stimulation in the peripheral nervous system

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 / cm 2 ) stimulation threshold than that of the Ho:YAG laser at 2120 nm ( ∼ 0.7    J / cm 2 ). 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 at different wavelengths and pulse shapes

Neural stimulation with infrared radiation has been explored for brain tissue, peripheral nerves, and cranial nerves including the auditory nerve. Initial experiments were conducted at wavelengths between λ = 1850 and λ = 2140 nm and the radiant energy was delivered with square pulses. Water absorption of the infrared radiation at λ = 1860 nm is similar to absorption at wavelengths between λ = 1310 and λ = 1600 nm, which are in the radiation wavelength range used for the communication industry. Technology for those wavelengths has already been developed and miniaturized and is readily available. The possibility of the infrared light to evoke compound action potentials (CAP) in the cochlea at λ = 1,375, λ = 1,460, and λ = 1550 nm was explored and compared to that of λ = 1860 nm in guinea pigs. Furthermore, rise and fall times of the 100 μs long pulses were changed and four basic pulse shapes (square, triangular, ramp-up, and ramp-down) were explored in their ability to evoke a CAP. In animals with pure tone threshold averages (PTAs) above 70 dB SPL, the results show that the favorable wavelength is λ = 1460 nm to reach threshold for stimulation and λ = 1375 nm or λ = 1460 nm for obtaining maximum amplitude. The most favorable pulse shape is either ramp-up or triangular.

Infrared inhibition impacts on locally initiated and propagating action potentials and the downstream synaptic transmission

Significance: Systematic studies of the physiological outputs induced by infrared (IR)-mediated inhibition of motor nerves can provide guidance for therapeutic applications and offer critical insights into IR light modulation of complex neural networks. Aim: We explore the IR-mediated inhibition of action potentials (APs) that either propagate along single axons or are initiated locally and their downstream synaptic transmission responses. Approach: APs were evoked locally by two-electrode current clamp or at a distance for propagating APs. The neuromuscular transmission was recorded with intracellular electrodes in muscle cells or macro-patch pipettes on terminal bouton clusters. Results: IR light pulses completely and reversibly terminate the locally initiated APs firing at low frequencies, which leads to blocking of the synaptic transmission. However, IR light pulses only suppress but do not block the amplitude and duration of propagating APs nor locally initiated APs firing at high frequencies. Such suppressed APs do not influence the postsynaptic responses at a distance. While the suppression of AP amplitude and duration is similar for propagating and locally evoked APs, only the former exhibits a 7% to 21% increase in the maximum time derivative of the AP rising phase. Conclusions: The suppressed APs of motor axons can resume their waveforms after passing the localized IR light illumination site, leaving the muscular and synaptic responses unchanged. IR-mediated modulation on propagating and locally evoked APs should be considered as two separate models for axonal and somatic modulations.

Short-wave Infrared Neural Stimulation Drives Graded Sciatic Nerve Activation Across A Continuum of Wavelengths

Infrared neural stimulation (INS) is an optical stimulation technique which uses coherent light to stimulate nerves and neurons and which shows increased spatial selectivity compared to electrical stimulation. This could improve deep brain, high channel count, or vagus nerve stimulation. In this study, we seek to understand the wavelength dependence of INS in the near-infrared optical window. Rat sciatic nerves were excised ex vivo and stimulated with wavelengths between 700 and 900 nm. Recorded compound nerve action potentials (CNAPs) showed that stimulation was maximized in the 700 nm window despite comparable laser power levels across wavelengths. Computational models demonstrated that wavelength-based activation dependencies were not a result of passive optical properties. This data demonstrates that INS is both wavelength and power level dependent, which inform stimulation systems to actively target neural microcircuits in humans.

Stimulation of water and calcium dynamics in astrocytes with pulsed infrared light

Astrocytes are non-neuronal cells that govern the homeostatic regulation of the brain through ions and water transport, and Ca2+ -mediated signaling. As they are tightly integrated into neural networks, label-free tools that can modulate cell function are needed to evaluate the role of astrocytes in brain physiology and dysfunction. Using live-cell fluorescence imaging, pharmacology, electrophysiology, and genetic manipulation, we show that pulsed infrared light can modulate astrocyte function through changes in intracellular Ca2+ and water dynamics, providing unique mechanistic insight into the effect of pulsed infrared laser light on astroglial cells. Water transport is activated and, IP3 R, TRPA1, TRPV4, and Aquaporin-4 are all involved in shaping the dynamics of infrared pulse-evoked intracellular calcium signal. These results demonstrate that astrocyte function can be modulated with infrared light. We expect that targeted control over calcium dynamics and water transport will help to study the crucial role of astrocytes in edema, ischemia, glioma progression, stroke, and epilepsy.

Infrared inhibition and waveform modulation of action potentials in the crayfish motor axon

The infrared (IR) inhibition of axonal activities in the crayfish neuromuscular preparation is studied using 2 µm IR light pulses with varying durations. The intracellular neuronal activities are monitored with two-electrode current clamp, while the IR-induced temperature changes are measured by the open patch technique simultaneously. It is demonstrated that the IR pulses can reversibly shape or block locally initiated action potentials. Suppression of the AP amplitude and duration and decrease in axonal excitability by IR pulses are quantitatively analyzed. While the AP amplitude and duration decrease similarly during IR illumination, it is discovered that the recovery of the AP duration after the IR pulses is slower than that of the AP amplitude. An IR-induced decrease in the input resistance (8.8%) is detected and discussed together with the temperature dependent changes in channel kinetics as contributing factors for the inhibition reported here.

Thermal block of action potentials is primarily due to voltage-dependent potassium currents: a modeling study

OBJECTIVE

Thermal block of action potential conduction using infrared lasers is a new modality for manipulating neural activity. It could be used for analysis of the nervous system and for therapeutic applications. We sought to understand the mechanisms of thermal block.

APPROACH

To analyze the mechanisms of thermal block, we studied both the original Hodgkin/Huxley model, and a version modified to more accurately match experimental data on thermal responses in the squid giant axon.

MAIN RESULTS

Both the original and modified models suggested that thermal block, especially at higher temperatures, is primarily due to a depolarization-activated hyperpolarization as increased temperature leads to faster activation of voltage-gated potassium ion channels. The minimum length needed to block an axon scaled with the square root of the axon's diameter.

SIGNIFICANCE

The results suggest that voltage-dependent potassium ion channels play a major role in thermal block, and that relatively short lengths of axon could be thermally manipulated to selectively block fine, unmyelinated axons, such as C fibers, that carry pain and other sensory information.

Multichannel optrodes for photonic stimulation

An emerging method in the field of neural stimulation is the use of photons to activate neurons. The possible advantage of optical stimulation over electrical is attributable to its spatially selective activation of small neuron populations, which is promising in generating superior spatial resolution in neural interfaces. Two principal methods are explored for cochlear prostheses: direct stimulation of nerves with infrared light and optogenetics. This paper discusses basic requirements for developing a light delivery system (LDS) for the cochlea and provides examples for building such devices. The proposed device relies on small optical sources, which are assembled in an array to be inserted into the cochlea. The mechanical properties, the biocompatibility, and the efficacy of optrodes have been tested in animal models. The force required to insert optrodes into a model of the human scala tympani was comparable to insertion forces obtained for contemporary cochlear implant electrodes. Side-emitting diodes are powerful enough to evoke auditory responses in guinea pigs. Chronic implantation of the LDS did not elevate auditory brainstem responses over 26 weeks.

Pulsed infrared releases Ca2+ from the endoplasmic reticulum of cultured spiral ganglion neurons

Inner ear spiral ganglion neurons were cultured from day 4 postnatal mice and loaded with a fluorescent Ca²⁺ indicator (fluo-4, -5F, or -5N). Pulses of infrared radiation (IR; 1,863 nm, 200 µs, 200-250 Hz for 2-5 s, delivered via an optical fiber) produced a rapid, transient temperature increase of 6-12°C (above a baseline of 24-30°C). These IR pulse trains evoked transient increases in both nuclear and cytosolic Ca²⁺ concentration ([Ca²⁺]) of 0.20-1.4 µM, with a simultaneous reduction of [Ca²⁺] in regions containing endoplasmic reticulum (ER). IR-induced increases in cytosolic [Ca²⁺] continued in medium containing no added Ca²⁺ (±Ca²⁺ buffers) and low [Na⁺], indicating that the [Ca²⁺] increase was mediated by release from intracellular stores. Consistent with this hypothesis, the IR-induced [Ca²⁺] response was prolonged and eventually blocked by inhibition of ER Ca²⁺-ATPase with cyclopiazonic acid, and was also inhibited by a high concentration of ryanodine and by inhibitors of inositol (1,4,5)-trisphosphate (IP₃)-mediated Ca²⁺ release (xestospongin C and 2-aminoethoxydiphenyl borate). The thermal sensitivity of the response suggested involvement of warmth-sensitive transient receptor potential (TRP) channels. The IR-induced [Ca²⁺] increase was inhibited by TRPV4 inhibitors (HC-067047 and GSK-2193874), and immunostaining of spiral ganglion cultures demonstrated the presence of TRPV4 and TRPM2 that colocalized with ER marker GRP78. These results suggest that the temperature sensitivity of IR-induced [Ca²⁺] elevations is conferred by TRP channels on ER membranes, which facilitate Ca²⁺ efflux into the cytosol and thereby contribute to Ca²⁺-induced Ca²⁺-release via IP₃ and ryanodine receptors. NEW & NOTEWORTHY Infrared radiation-induced photothermal effects release Ca²⁺ from the endoplasmic reticulum of primary spiral ganglion neurons. This Ca²⁺ release is mediated by activation of transient receptor potential (TRPV4) channels and involves amplification by Ca²⁺-induced Ca²⁺-release. The neurons immunostained for warmth-sensitive channels, TRPV4 and TRPM2, which colocalize with endoplasmic reticulum. Pulsed infrared radiation provides a novel experimental tool for releasing intracellular Ca²⁺, studying Ca²⁺ regulatory mechanisms, and influencing neuronal excitability.

Thermal Excitation of the Mechanotransduction Apparatus of Hair Cells

Although a hair bundle is normally deflected by mechanical stimuli, we found that irradiation of a hair cell from the bullfrog's sacculus with ultraviolet light causes rapid motion of the hair bundle toward its tall edge. This movement is associated with opening of mechanotransduction channels and disappears when tip links are disrupted. We localized the absorptive element responsible for the motion to the region directly below the hair bundle and measured an action spectrum similar to the absorption spectra of mitochondrial constituents. Temperature measurements revealed heating around the site of absorption; direct heating of the hair bundle confirmed that the response to light is mediated through heat. Although mechanical offsets of the hair bundle revealed that heat softens gating springs, it also acts directly to open transduction channels. This study identifies an unconventional method of hair-cell stimulation and clarifies the previously unexplained sensitivity of auditory organs to thermal stimulation.

A review of optical pacing with infrared light

Optical pacing (OP) uses pulsed infrared light to initiate heartbeats in electrically excitable cardiac tissues without employing exogenous agents. OP is an alternative approach to electrical pacing that may overcome some its disadvantages for some applications. In this review, we discuss the initial demonstrations, mechanisms, safety, advantages and applications of OP.

Auditory Neural Activity in Congenitally Deaf Mice Induced by Infrared Neural Stimulation

To determine whether responses during infrared neural stimulation (INS) result from the direct interaction with spiral ganglion neurons (SGNs), we tested three genetically modified deaf mouse models: Atoh1-cre; Atoh1^f/f (Atoh1 conditional knockout, CKO), Atoh1-cre; Atoh1^f/kiNeurog1 (Neurog1 knockin, KI), and the Vglut3 knockout (Vglut3^−/−) mice. All animals were exposed to tone bursts and clicks up to 107 dB (re 20 µPa) and to INS, delivered with a 200 µm optical fiber. The wavelength (λ) was 1860 nm, the radiant energy (Q) 0-800 µJ/pulse, and the pulse width (PW) 100–500 µs. No auditory responses to acoustic stimuli could be evoked in any of these animals. INS could not evoke auditory brainstem responses in Atoh1 CKO mice but could in Neurog1 KI and Vglut3^−/− mice. X-ray micro-computed tomography of the cochleae showed that responses correlated with the presence of SGNs and hair cells. Results in Neurog1 KI mice do not support a mechanical stimulation through the vibration of the basilar membrane, but cannot rule out the direct activation of the inner hair cells. Results in Vglut3^−/− mice, which have no synaptic transmission between inner hair cells and SGNs, suggested that hair cells are not required.

Neural Crest Stem Cells Can Differentiate to a Cardiomyogenic Lineage with an Ability to Contract in Response to Pulsed Infrared Stimulation

INTRODUCTION

Cellular cardiomyoplasty has rapidly risen to prominence in the clinic following a myocardial infarction; however, low engraftment of transplanted cells limits the therapeutic benefit to these procedures. Recently, lineage-specific stem cells differentiated into cardiomyocytes have gained much attention to assist in the repair of an injured heart tissue; however, questions regarding the ideal cell source remain. In the present study, we have identified a source that is easy to extract stem cells from and show that the cells present have a high plasticity toward the cardiomyogenic lineage. We focused on the recently discovered neural crest stem cells residing in the periodontal ligament that can be easily obtained through dental procedures.

MATERIALS AND METHODS

Neural crest stem cells were obtained from human excised third molars and differentiated in culture using a protocol for directed differentiation into cardiomyocytes. Differentiation of cells was assessed through gene expression and immunostaining studies. Optical stimulation using pulsed infrared radiation (IR) (λ = 1863 nm) was delivered to cell aggregates to study their contractile ability.

RESULTS

We show that neural crest stem cells can be differentiated to a cardiomyogenic lineage, which was verified through immunostaining and gene expression. We observed a significant increase in cardiomyocyte-specific markers, NK2 homeobox 5 (NKX2.5) and troponin T type 2 (TNNT2), with positive changes in tropomyosin I (TPM1), gap junction protein alpha 1/Cx43 (GJA1/Cx43), and myocyte enhancement factor 2C (MEF2C). Furthermore, we were able to elicit and maintain pulse-by-pulse contractile responses in the derived cells, including in cardiospheres, with pulsed IR delivered at various radiant energies. The contractility in responses to IR could be maintained at different frequencies (0.25-2 Hz) and up to 10-min durations. While these cells did not maintain their contractility following cessation of IR, these cells demonstrated responses to the optical stimuli that are consistent with previous reports. We also found no evidence for irreversible mitochondrial depolarization in these cells following the long duration of infrared stimulation, suggesting the robustness of these cells.

CONCLUSIONS

Overall, these results suggest the merit of neural crest-derived stem cells for cardiomyogenic applications and a potential cell source for repair that should contribute to efforts to translate cell-based strategies to the clinic.

Millisecond infrared laser pulses depolarize and elicit action potentials on in-vitro dorsal root ganglion neurons

This work focuses on the optical stimulation of dorsal root ganglion (DRG) neurons through infrared laser light stimulation. We show that a few millisecond laser pulse at 1875 nm induces a membrane depolarization, which was observed by the patch-clamp technique. This stimulation led to action potentials firing on a minority of neurons beyond an energy threshold. A depolarization without action potential was observed for the majority of DRG neurons, even beyond the action potential energy threshold. The use of ruthenium red, a thermal channel blocker, stops the action potential generation, but has no effects on membrane depolarization. Local temperature measurements reveal that the depolarization amplitude is sensitive to the amplitude of the temperature rise as well as to the time rate of change of temperature, but in a way which may not fully follow a photothermal capacitive mechanism, suggesting that more complex mechanisms are involved.

An infrared optical pacing system for screening cardiac electrophysiology in human cardiomyocytes

Human cardiac myocytes derived from pluripotent stem cells (hCM) have invigorated interest in genetic disease mechanisms and cardiac safety testing; however, the technology to fully assess electrophysiological function in an assay that is amenable to high throughput screening has lagged. We describe a fully contactless system using optical pacing with an infrared (IR) laser and multi-site high fidelity fluorescence imaging to assess multiple electrophysiological parameters from hCM monolayers in a standard 96-well plate. Simultaneous multi-site action potentials (FluoVolt) or Ca2+ transients (Fluo4-AM) were measured, from which high resolution maps of conduction velocity and action potential duration (APD) were obtained in a single well. Energy thresholds for optical pacing were determined for cell plating density, laser spot size, pulse width, and wavelength and found to be within ranges reported previously for reliable pacing. Action potentials measured using FluoVolt and a microelectrode exhibited the same morphology and rate of depolarization. Importantly, we show that this can be achieved accurately with minimal damage to hCM due to optical pacing or fluorescence excitation. Finally, using this assay we demonstrate that hCM exhibit reproducible changes in repolarization and impulse conduction velocity for Flecainide and Quinidine, two well described reference compounds. In conclusion, we demonstrate a high fidelity electrophysiological screening assay that incorporates optical pacing with IR light to control beating rate of hCM monolayers.

Selective inhibition of small-diameter axons using infrared light

Novel clinical treatments to target peripheral nerves are being developed which primarily use electrical current. Recently, infrared (IR) light was shown to inhibit peripheral nerves with high spatial and temporal specificity. Here, for the first time, we demonstrate that IR can selectively and reversibly inhibit small-diameter axons at lower radiant exposures than large-diameter axons. We provide a mathematical rationale, and then demonstrate it experimentally in individual axons of identified neurons in the marine mollusk Aplysia californica, and in axons within the vagus nerve of a mammal, the musk shrew Suncus murinus. The ability to selectively, rapidly, and reversibly control small-diameter sensory fibers may have many applications, both for the analysis of physiology, and for treating diseases of the peripheral nervous system, such as chronic nausea, vomiting, pain, and hypertension. Moreover, the mathematical analysis of how IR affects the nerve could apply to other techniques for controlling peripheral nerve signaling.

Ryanodine and IP3 receptor-mediated calcium signaling play a pivotal role in neurological infrared laser modulation

Pulsed infrared (IR) laser energy has been shown to modulate neurological activity through both stimulation and inhibition of action potentials. While the mechanism(s) behind this phenomenon is (are) not completely understood, certain hypotheses suggest that the rise in temperature from IR exposure could activate temperature- or pressure-sensitive ion channels or create pores in the cellular outer membrane, allowing an influx of typically plasma-membrane-impermeant ions. Studies using fluorescent intensity-based calcium ion ([Formula: see text]) sensitive dyes show changes in [Formula: see text] levels after various IR stimulation parameters, which suggests that [Formula: see text] may originate from the external solution. However, activation of intracellular signaling pathways has also been demonstrated, indicating a more complex mechanism of increasing intracellular [Formula: see text] concentration. We quantified the [Formula: see text] mobilization in terms of influx from the external solution and efflux from intracellular organelles using Fura-2 and a high-speed ratiometric imaging system that rapidly alternates the dye excitation wavelengths. Using nonexcitable Chinese hamster ovarian ([Formula: see text]) cells and neuroblastoma-glioma (NG108) cells, we demonstrate that intracellular [Formula: see text] receptors play an important role in the IR-induced [Formula: see text], with the [Formula: see text] response augmented by ryanodine receptors in excitable cells.

Bringing new dimensions to drug discovery screening: impact of cellular stimulation technologies

The current mandate for the drug discovery industry is to develop more efficient drugs faster while reducing the costs associated with their development. Incorporation of cell stimulation technologies during screening assays is expected to revolutionize the discovery of novel drugs as well as safety pharmacology. In this review, we highlight 'classical' and emerging cell stimulation technologies that provide the ability to evaluate the effects of drug candidates on cells in different functional states to assess clinically relevant phenotypes.

Challenges for the application of optical stimulation in the cochlea for the study and treatment of hearing loss

INTRODUCTION

Electrical stimulation has long been the most effective strategy for evoking neural activity from bionic devices and has been used with great success in the cochlear implant to allow deaf people to hear speech and sound. Despite its success, the spread of electrical current stimulates a broad region of neural tissue meaning that contemporary devices have limited precision. Optical stimulation as an alternative has attracted much recent interest for its capacity to provide highly focused stimuli, and therefore, potentially improved sensory perception. Given its specificity of activation, optical stimulation may also provide a useful tool in the study of fundamental neuroanatomy and neurophysiological processes. Areas covered: This review examines the advances in optical stimulation - infrared, nanoparticle-enhanced, and optogenetic-based - and its application in the inner ear for the restoration of auditory function following hearing loss. Expert opinion: Initial outcomes suggest that optogenetic-based approaches hold the greatest potential and viability amongst optical techniques for application in the cochlea. The future success of this approach will be governed by advances in the targeted delivery of opsins to auditory neurons, improvements in channel kinetics, development of optical arrays, and innovation of opsins that activate within the optimal near-infrared therapeutic window.

Pressure in the Cochlea During Infrared Irradiation

OBJECTIVE

The purpose of the study is to demonstrate laser-evoked pressure waves in small confined volumes such as the cochlea.

METHODS

Custom-fabricated pressure probes were used to determine the pressure in front of the optical fiber in a small dish and patch pipettes to measure temperature changes. Pressure probes were inserted into scala tympani (ST) or vestibuli during laser stimulation. With a sensitive microphone the pressure was measured in the outer ear canal.

RESULTS

Heating was spatially confined. The heat relaxation time was 35 ms. During laser stimulation in the cochlea at 17 μJ/pulse, the pressure in the outer ear canal (EC) was 43.5 dB (re 20 μPa). The corresponding intracochlear pressure was calculated to be about 78.5 dB (re 20 μPa) using the middle ear reverse transfer function of -35 dB. At 164 μJ/pulse, the pressure in the EC was on average 63 dB (re 20 μPa) and the intracochlear pressure was estimated to be 98 dB (re 20 μPa), which is similar to the value obtained with the pressure probe, 100 dB (re 20 μPa). Side-emitting optical fibers were used to steer the beam path. The pressure values were independent of the orientation of the beam path. Evoked compound action potentials of the auditory nerve were maximum when spiral ganglion neurons were in the beam path.

CONCLUSION

Pressure waves are generated during infrared laser stimulation. The intracochlear pressure was independent from the orientation of the beam path.

SIGNIFICANCE

Neural responses required the spiral ganglion neurons to be directly irradiated.

Infrared inhibition of embryonic hearts

Infrared control is a new technique that uses pulsed infrared lasers to thermally alter electrical activity. Originally developed for nerves, we have applied this technology to embryonic hearts using a quail model, previously demonstrating infrared stimulation and, here, infrared inhibition. Infrared inhibition enables repeatable and reversible block, stopping cardiac contractions for several seconds. Normal beating resumes after the laser is turned off. The block can be spatially specific, affecting propagation on the ventricle or initiation on the atrium. Optical mapping showed that the block affects action potentials and not just calcium or contraction. Increased resting intracellular calcium was observed after a 30-s exposure to the inhibition laser, which likely resulted in reduced mechanical function. Further optimization of the laser illumination should reduce potential damage. Stopping cardiac contractions by disrupting electrical activity with infrared inhibition has the potential to be a powerful tool for studying the developing heart.

Heat pulse excitability of vestibular hair cells and afferent neurons

In the present study we combined electrophysiology with optical heat pulse stimuli to examine thermodynamics of membrane electrical excitability in mammalian vestibular hair cells and afferent neurons. We recorded whole cell currents in mammalian type II vestibular hair cells using an excised preparation (mouse) and action potentials (APs) in afferent neurons in vivo (chinchilla) in response to optical heat pulses applied to the crista (ΔT ≈ 0.25°C per pulse). Afferent spike trains evoked by heat pulse stimuli were diverse and included asynchronous inhibition, asynchronous excitation, and/or phase-locked APs synchronized to each infrared heat pulse. Thermal responses of membrane currents responsible for APs in ganglion neurons were strictly excitatory, with Q10 ≈ 2. In contrast, hair cells responded with a mix of excitatory and inhibitory currents. Excitatory hair cell membrane currents included a thermoelectric capacitive current proportional to the rate of temperature rise (dT/dt) and an inward conduction current driven by ΔT An iberiotoxin-sensitive inhibitory conduction current was also evoked by ΔT, rising in <3 ms and decaying with a time constant of ∼24 ms. The inhibitory component dominated whole cell currents in 50% of hair cells at -68 mV and in 67% of hair cells at -60 mV. Responses were quantified and described on the basis of first principles of thermodynamics. Results identify key molecular targets underlying heat pulse excitability in vestibular sensory organs and provide quantitative methods for rational application of optical heat pulses to examine protein biophysics and manipulate cellular excitability.

Radiant energy required for infrared neural stimulation

Infrared neural stimulation (INS) has been proposed as an alternative method to electrical stimulation because of its spatial selective stimulation. Independent of the mechanism for INS, to translate the method into a device it is important to determine the energy for stimulation required at the target structure. Custom-designed, flat and angle polished fibers, were used to deliver the photons. By rotating the angle polished fibers, the orientation of the radiation beam in the cochlea could be changed. INS-evoked compound action potentials and single unit responses in the central nucleus of the inferior colliculus (ICC) were recorded. X-ray computed tomography was used to determine the orientation of the optical fiber. Maximum responses were observed when the radiation beam was directed towards the spiral ganglion neurons (SGNs), whereas little responses were seen when the beam was directed towards the basilar membrane. The radiant exposure required at the SGNs to evoke compound action potentials (CAPs) or ICC responses was on average 18.9 ± 12.2 or 10.3 ± 4.9 mJ/cm(2), respectively. For cochlear INS it has been debated whether the radiation directly stimulates the SGNs or evokes a photoacoustic effect. The results support the view that a direct interaction between neurons and radiation dominates the response to INS.

Target structures for cochlear infrared neural stimulation

Infrared neural stimulation (INS) is a method to depolarize neurons with infrared light. While consensus exists that heating of the target structure is essential, subsequent steps that result in the generation of an action potential are controversially discussed in the literature. The question of whether cochlear INS is an acoustic event has not been clarified. Results have been published that could be explained solely by an acoustic event. However, data exist that do not support an acoustical stimulus as the dominant factor in cochlear INS. We review the different findings that have been suggested for the mechanism of INS. Furthermore, we present the data that clarify the role of an acoustical event in cochlear INS. Masking experiments have been performed in hearing, hearing impaired, and severely hearing impaired animals. In normal hearing animals, the laser response could be masked by the acoustic stimulus. Once thresholds to acoustic stimuli were elevated, the ability to acoustically mask the INS response gradually disappeared. Thresholds for acoustic stimuli were significantly elevated in animals with compromised cochlear function, while the thresholds for optical stimulation remained largely unchanged. The results suggest that the direct interaction between the radiation and the target structure dominates cochlear INS.

Infrared neural stimulation fails to evoke neural activity in the deaf guinea pig cochlea

At present there is some debate as to the processes by which infrared neural stimulation (INS) activates neurons in the cochlea, as the lasers used for INS can potentially generate a range of secondary stimuli e.g. an acoustic stimulus is produced when the light is absorbed by water. To clarify whether INS in the cochlea requires functioning hair cells and to explore the potential relevance to cochlear implants, experiments using INS were performed in the cochleae of both normal hearing and profoundly deaf guinea pigs. A response to laser stimulation was readily evoked in normal hearing cochlea. However, no response was evoked in any profoundly deaf cochleae, for either acute or chronic deafening, contrary to previous work where a response was observed after acute deafening with ototoxic drugs. A neural response to electrical stimulation was readily evoked in all cochleae after deafening. The absence of a response from optical stimuli in profoundly deaf cochleae suggests that the response from INS in the cochlea is hair cell mediated.