Auditory nerve responses to combined optogenetic and electrical stimulation in chronically deaf mice.

. Optogenetic stimulation of the auditory nerve offers the ability to overcome the limitations of cochlear implants through spatially precise stimulation, but cannot achieve the temporal precision nor temporal fidelity required for good hearing outcomes. Auditory midbrain recordings have indicated a combined (hybrid) stimulation approach may permit improvements in the temporal precision without sacrificing spatial precision by facilitating electrical activation thresholds.

Provision of interaural time difference information in chronic intracochlear electrical stimulation enhances neural sensitivity to these differences in neonatally deafened cats.

Although performance with bilateral cochlear implants is superior to that with a unilateral implant, bilateral implantees have poor performance in sound localisation and in speech discrimination in noise compared to normal hearing subjects. Studies of the neural processing of interaural time differences (ITDs) in the inferior colliculus (IC) of long-term deaf animals, show substantial degradation compared to that in normal hearing animals. It is not known whether this degradation can be ameliorated by chronic cochlear electrical stimulation, but such amelioration is unlikely to be achieved using current clinical speech processors and cochlear implants, which do not provide good ITD cues.

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.

Optical stimulation of neural tissue.

Electrical stimulation has been used for decades in devices such as pacemakers, cochlear implants and more recently for deep brain and retinal stimulation and electroceutical treatment of disease. However, current spread from the electrodes limits the precision of neural activation, leading to a low quality therapeutic outcome or undesired side-effects. Alternative methods of neural stimulation such as optical stimulation offer the potential to deliver higher spatial resolution of neural activation.

Thermal damage threshold of neurons during infrared stimulation.

In infrared neural stimulation (INS), laser-evoked thermal transients are used to generate small depolarising currents in neurons. The laser exposure poses a moderate risk of thermal damage to the target neuron. Indeed, exogenous methods of neural stimulation often place the target neurons under stressful non-physiological conditions, which can hinder ordinary neuronal function and hasten cell death.

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

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.

Optical Stimulation of Neurons.

Our capacity to interface with the nervous system remains overwhelmingly reliant on electrical stimulation devices, such as electrode arrays and cuff electrodes that can stimulate both central and peripheral nervous systems. However, electrical stimulation has to deal with multiple challenges, including selectivity, spatial resolution, mechanical stability, implant-induced injury and the subsequent inflammatory response. Optical stimulation techniques may avoid some of these challenges by providing more selective stimulation, higher spatial resolution and reduced invasiveness of the device, while also avoiding the electrical artefacts that complicate recordings of electrically stimulated neuronal activity.

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.

Nanoparticle-enhanced infrared neural stimulation.

Objective: Recent research has demonstrated that nerves can be stimulated by transient heating associated with the absorption of infrared light by water in the tissue. There is a great deal of interest in using this technique in neural prostheses, due to the potential for increased localization of the stimulus and minimization of contact with the tissue. However, thermal modelling suggests that the full benefits of increased localization may be reduced by cumulative heating effects when multiple stimulus sites and/or high repetition rates are used.

Infrared Neural Stimulation: Influence of Stimulation Site Spacing and Repetition Rates on Heating.

A model to simulate heating as a result of pulse repetitions during infrared neural stimulation (INS), with both single- and multiple-emitters is presented. This model allows the temperature increases from pulse trains rather than single pulses to be considered. The model predicts that using a stimulation rate of 250 Hz with typical laser parameters at a single stimulation site results in a temperature increase of 2.

Modeling of the temporal effects of heating during infrared neural stimulation.

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.

Modeling of light absorption in tissue during infrared neural stimulation.

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.