Asynchronous axonal firing patterns evoked via continuous subthreshold kilohertz stimulation

The comfort of temporal interference stimulation on the forearm: computational and psychophysical evaluation

Objective.Transcutaneous electrical stimulation aims to restore sensation and function in individuals with sensory or motor deficits. However, limited selectivity and unintended nerve recruitment often result in discomfort. Temporal interference (TI) stimulation has been proposed as a novel approach to non-invasive nerve stimulation, hypothesising that low-frequency modulation of kilohertz carriers reduces activation thresholds. Prior studies have produced conflicting results regarding comfort in kilohertz-frequency stimulation, and the practical applicability of TI remains unclear. This study addresses these gaps by systematically analysing the role of depth of modulation in activation thresholds and comfort, focusing on peripheral nerves and clinically relevant stimulation levels.Approach.This study uses a dual-method approach combining computational and psychophysical experiments targeting the median nerve. Computational modelling involved nine magnetic resonance imaging-informed finite element models to account for anatomical variability and biophysical neural activation predictions using NEURON., test Psychophysical experiments with 19 participants determined stimulation thresholds and comfort levels. Statistical analysis using the Friedman test and Bonferroni correction assessed the impact of carrier and beat frequencies, and depth of modulation on activation thresholds and comfort.Main results.The results showed that the activation thresholds did not vary with the depth of modulation, challenging the core assumption underlying temporal interference stimulation. Despite that, comfort significantly increased with carrier frequencies as low as 500 Hz, with no further significant changes at higher frequencies. Computational modelling results showed an association between increased comfort and asynchronous nerve activation patterns, providing a possible explanation for the observed improvement in comfort.Significance.By challenging a core assumption of TI stimulation, this study shifts the focus from threshold modulation to optimising comfort in peripheral nerve stimulation. These findings establish a foundation for developing kilohertz-frequency stimulation protocols prioritising user comfort, particularly in applications such as functional electrical stimulation for rehabilitation or sensory feedback for prostheses.

Response of dorsal horn neurons in mice to high-frequency (kHz) biphasic stimulation is not sensitive to local temperature rise

Clinically accepted for treatment of chronic pain 10 kHz-frequency electric spinal cord stimulation (10 kHz-SCS) releases more power in tissue compared to conventional low-frequency (<100 Hz) stimulation due to increased duty cycle. This is equivalent to the release of more heat in a surrounding tissue, which may change the functional state of affected neural elements. In the case of SCS, plausible candidates to be affected by thermal a component of kHz-frequency electric field stimulation (kHz-FS) are dorsal column axons and neurons of the superficial layers of the dorsal horn. In this study, we tested the hypothesis that joule heat produced by kHz-FS modulates neuronal excitability. In slices of the mouse spinal cord, we monitored membrane potential and membrane input resistance in neurons of lamina II during exposure to kHz-FS. Surprisingly, we found no correlation between temperature rise and changes of membrane parameters. Furthermore, the depolarizing effect of kHz-FS was always immediate and remained persistent throughout stimulation, whereas rise of temperature was delayed for 1-2 s and reached its saturation level within the following few seconds. Thus, we concluded that the thermal component has an insignificant role in the mechanism of kHz-FS action.

Selective intrafascicular stimulation of myelinated and unmyelinated nerve fibers through a longitudinal electrode: A computational study

Carbon nanotube (CNT) fiber electrodes have demonstrated exceptional spatial selectivity and sustained reliability in the context of intrafascicular electrical stimulation, as evidenced through rigorous animal experimentation. A significant presence of unmyelinated C fibers, known to induce uncomfortable somatosensory experiences, exists within peripheral nerves. This presence poses a considerable challenge to the excitation of myelinated Aβ fibers, which are crucial for tactile sensation. To achieve nuanced tactile sensory feedback utilizing CNT fiber electrodes, the selective stimulation of Aβ sensory afferents emerges as a critical factor. In confronting this challenge, the present investigation sought to refine and apply a rat sciatic-nerve model leveraging the capabilities of the COMSOL-NEURON framework. This approach enables a systematic evaluation of the influence exerted by stimulation parameters and electrode geometry on the activation dynamics of both myelinated Aβ and unmyelinated C fibers. The findings advocate for the utilization of current pulses featuring a pulse width of 600 μs, alongside the deployment of CNT fibers characterized by a diminutive diameter of 10 μm, with an exclusively exposed cross-sectional area, to facilitate reduced activation current thresholds. Comparative analysis under monopolar and bipolar electrical stimulation conditions revealed proximate activation thresholds, albeit with bipolar stimulation exhibiting superior fiber selectivity relative to its monopolar counterpart. Concerning pulse waveform characteristics, the adoption of an anodic-first biphasic stimulation modality is favored, taking into account the dual criteria of activation threshold and fiber selectivity optimization. Consequently, this investigation furnishes an efficacious stimulation paradigm for the selective activation of touch-related nerve fibers, alongside provisioning a comprehensive theoretical foundation for the realization of natural tactile feedback within the domain of prosthetic hand applications.

High-frequency amplitude-modulated sinusoidal stimulation induces desynchronized yet controllable neural firing

Regaining sensory feedback is pivotal for people living with limb amputation. Electrical stimulation of sensory fibers in peripheral nerves has been shown to restore focal percepts in the missing limb. However, conventional rectangular current pulses induce sensations often described as unnatural. This is likely due to the synchronous and periodic nature of activity evoked by these pulses. Here we introduce a fast-oscillating amplitude-modulated sinusoidal (FAMS) stimulation waveform that desynchronizes evoked neural activity. We used a computational model to show that sinusoidal waveforms evoke asynchronous and irregular firing and that firing patterns are frequency dependent. We designed the FAMS waveform to leverage both low- and high-frequency effects and found that membrane non-linearities enhance neuron-specific differences when exposed to FAMS. We implemented this waveform in a feline model of peripheral nerve stimulation and demonstrated that FAMS-evoked activity is more asynchronous than activity evoked by rectangular pulses, while being easily controllable with simple stimulation parameters. These results represent an important step towards biomimetic stimulation strategies useful for clinical applications to restore sensory feedback.