Ramped pulse shapes are more efficient for cochlear implant stimulation in an animal model

Optimal transcorneal electrical stimulation parameters for preserving photoreceptors in a mouse model of retinitis pigmentosa

JOURNAL/nrgr/04.03/01300535-202419110-00034/figure1/v/2024-03-08T184507Z/r/image-tiff Retinitis pigmentosa is a hereditary retinal disease that affects rod and cone photoreceptors, leading to progressive photoreceptor loss. Previous research supports the beneficial effect of electrical stimulation on photoreceptor survival. This study aims to identify the most effective electrical stimulation parameters and functional advantages of transcorneal electrical stimulation (tcES) in mice affected by inherited retinal degeneration. Additionally, the study seeked to analyze the electric field that reaches the retina in both eyes in mice and post-mortem humans. In this study, we recorded waveforms and voltages directed to the retina during transcorneal electrical stimulation in C57BL/6J mice using an intraocular needle probe with rectangular, sine, and ramp waveforms. To investigate the functional effects of electrical stimulation on photoreceptors, we used human retinal explant cultures and rhodopsin knockout (Rho-/-) mice, demonstrating progressive photoreceptor degeneration with age. Human retinal explants isolated from the donors' eyes were then subjected to electrical stimulation and cultured for 48 hours to simulate the neurodegenerative environment in vitro. Photoreceptor density was evaluated by rhodopsin immunolabeling. In vivo Rho-/- mice were subjected to two 5-day series of daily transcorneal electrical stimulation using rectangular and ramp waveforms. Retinal function and visual perception of mice were evaluated by electroretinography and optomotor response (OMR), respectively. Immunolabeling was used to assess the morphological and biochemical changes of the photoreceptor and bipolar cells in mouse retinas. Oscilloscope recordings indicated effective delivery of rectangular, sine, and ramp waveforms to the retina by transcorneal electrical stimulation, of which the ramp waveform required the lowest voltage. Evaluation of the total conductive resistance of the post-mortem human compared to the mouse eyes indicated higher cornea-to-retina resistance in human eyes. The temperature recordings during and after electrical stimulation indicated no significant temperature change in vivo and only a subtle temperature increase in vitro (~0.5-1.5°C). Electrical stimulation increased photoreceptor survival in human retinal explant cultures, particularly at the ramp waveform. Transcorneal electrical stimulation (rectangular + ramp) waveforms significantly improved the survival and function of S and M-cones and enhanced visual acuity based on the optomotor response results. Histology and immunolabeling demonstrated increased photoreceptor survival, improved outer nuclear layer thickness, and increased bipolar cell sprouting in Rho-/- mice. These results indicate that transcorneal electrical stimulation effectively delivers the electrical field to the retina, improves photoreceptor survival in both human and mouse retinas, and increases visual function in Rho-/- mice. Combined rectangular and ramp waveform stimulation can promote photoreceptor survival in a minimally invasive fashion.

Tripolar configuration and pulse shape in cochlear implants reduce channel interactions in the temporal domain

The present study investigates effects of current focusing and pulse shape on threshold, dynamic range, spread of excitation and channel interaction in the time domain using cochlear implant stimulation. The study was performed on 20 adult guinea pigs using a 6-channel animal cochlear implant, recording was performed in the auditory midbrain using a multielectrode array. After determining the best frequencies for individual recording contacts with acoustic stimulation, the ear was deafened and a cochlear implant was inserted into the cochlea. The position of the implant was controlled by x-ray. Stimulation with biphasic, pseudomonophasic and monophasic stimuli was performed with monopolar, monopolar with common ground, bipolar and tripolar configuration in two sets of experiments, allowing comparison of the effects of the different stimulation strategies on threshold, dynamic range, spread of excitation and channel interaction. Channel interaction was studied in the temporal domain, where two electrodes were activated with pulse trains and phase locking to these pulse trains in the midbrain was quantified. The results documented multifactorial influences on the response properties, with significant interaction between factors. Thresholds increased with increasing current focusing, but decreased with pseudomonophasic and monophasic pulse shapes. The results documented that current focusing, particularly tripolar configuration, effectively reduces channel interaction, but that also pseudomonophasic and monophasic stimulation and phase duration intensity coding reduce channel interactions.

Electrical promontory stimulation test using a portable peripheral nerve stimulator with an ear canal electrode

OBJECTIVE

An intact cochlear nerve is necessary for successful cochlear implantation (CI). Although the promontory stimulation test (PST) using a promontory stimulator (PS) and a transtympanic needle electrode is invasive, it is still commonly used to verify cochlear nerve function. PSs are currently unavailable because they are no longer manufactured; however, considering that PST continues to be beneficial in certain situations, alternative equipment is needed. The PNS-7000® (PNS) was developed as a neurologic instrument for stimulating the peripheral nerves. This study investigated the usefulness of the ear canal stimulation test (ECST) using PNS with a silver ball ear canal electrode, which is a new noninvasive alternative technique to the PST.

METHODS

ECST was performed from November 2013 to December 2018 using PS and PNS for patients with severe to profound sensorineural hearing loss. The electrical threshold, most comfortable loudness level, uncomfortable loudness level, dynamic range, and gap detection were measured in the ECST. The results of these measured PNS items were compared with PS.

RESULTS

ECST was performed in 61 ears of 35 patients (age, 59.9 ± 20.1 years) using PS and PNS. The sound sensation was elicited in 51 (83.6%) and 52 (85.2%) ears with PS and PNS, respectively. All items excluding GAP were measured in 46 (75%) and 43 (70%) ears at 50 and 100 Hz, respectively. GAP was measured in 33 ears by the ascending and descending methods using PS and PNS. Spearman's rank-order correlation coefficient revealed a significant positive linear correlation between the PS and PNS results in all measurements. No significant difference was found between the PS and PNS thresholds in all measured items.

CONCLUSIONS

PNS is a useful instrument for performing ECST as a new alternative to PS. ECST using a silver ball electrode is a less invasive and easier test than PST.

A Virtual Inner Ear Model Selects Ramped Pulse Shapes for Vestibular Afferent Stimulation

Bilateral vestibular deficiency (BVD) results in chronic dizziness, blurry vision when moving the head, and postural instability. Vestibular prostheses (VPs) show promise as a treatment, but the VP-restored vestibulo-ocular reflex (VOR) gain in human trials falls short of expectations. We hypothesize that the slope of the rising ramp in stimulation pulses plays an important role in the recruitment of vestibular afferent units. To test this hypothesis, we utilized customized programming to generate ramped pulses with different slopes, testing their efficacy in inducing electrically evoked compound action potentials (eCAPs) and current spread via bench tests and simulations in a virtual inner model created in this study. The results confirmed that the slope of the ramping pulses influenced the recruitment of vestibular afferent units. Subsequently, an optimized stimulation pulse train was identified using model simulations, exhibiting improved modulation of vestibular afferent activity. This optimized slope not only reduced the excitation spread within the semicircular canals (SCCs) but also expanded the neural dynamic range. While the model simulations exhibited promising results, in vitro and in vivo experiments are warranted to validate the findings of this study in future investigations.

A review of neurophysiological effects and efficiency of waveform parameters in deep brain stimulation

Neurostimulation has diverse clinical applications and potential as a treatment for medically refractory movement disorders, epilepsy, and other neurological disorders. However, the parameters used to program electrodes-polarity, pulse width, amplitude, and frequency-and how they are adjusted have remained largely untouched since the 1970 s. This review summarizes the state-of-the-art in Deep Brain Stimulation (DBS) and highlights the need for further research to uncover the physiological mechanisms of neurostimulation. We focus on studies that reveal the potential for clinicians to use waveform parameters to selectively stimulate neural tissue for therapeutic benefit, while avoiding activating tissue associated with adverse effects. DBS uses cathodic monophasic rectangular pulses with passive recharging in clinical practice to treat neurological conditions such as Parkinson's Disease. However, research has shown that stimulation efficiency can be improved, and side effects reduced, through modulating parameters and adding novel waveform properties. These developments can prolong implantable pulse generator lifespan, reducing costs and surgery-associated risks. Waveform parameters can stimulate neurons based on axon orientation and intrinsic structural properties, providing clinicians with more precise targeting of neural pathways. These findings could expand the spectrum of diseases treatable with neuromodulation and improve patient outcomes.

Anodic Polarity Minimizes Facial Nerve Stimulation as a Side Effect of Cochlear Implantation

One severe side effect of the use of cochlear implants (CI) is coincidental facial nerve stimulation (FNS). Clinical methods to alleviate FNS range from the reprogramming of processor settings to revision surgery. We systematically assessed different changes in CI stimulation modes that have been discussed in the literature as "rescue factors" from FNS: electrode configuration (broad to focused), pulse shape (symmetric biphasic to pseudo-monophasic), and pulse polarity (cathodic to anodic). An FNS was assessed, based on electrophysiological thresholds, in 204 electrically evoked compound action potential (eCAP) input/output functions recorded from 33 ears of 26 guinea pigs. The stimulation level difference between auditory nerve eCAP threshold and FNS threshold was expressed as the eCAP-to-FNS offset. Coincidental FNS occurred in all animals and in 45% of all recordings. A change from monopolar to focused (bipolar, tripolar) configurations minimized FNS. The Euclidean distance between the CI contacts and the facial nerve explained no more than 33% of the variance in FNS thresholds. For both the FNS threshold and the eCAP-to-FNS offset, the change from cathodic to anodic pulse polarity significantly reduced FNS and permitted a gain of 14-71% of the dynamic range of the eCAP response. This "anodic rescue effect" was stronger for pseudo-monophasic pulses as compared to the symmetric biphasic pulse shape. These results provide possible mechanisms underlying recent clinical interventions to alleviate FNS. The "anodic-rescue effect" may offer a non-invasive therapeutic option for FNS in human CI users that should be tested clinically, preferably in combination with current-focusing methods.

What Is the Benefit of Ramped Pulse Shapes for Activating Auditory Cortex Neurons? An Electrophysiological Study in an Animal Model of Cochlear Implant

In all commercial cochlear implant (CI) devices, the activation of auditory nerve fibers is performed with rectangular pulses that have two phases of opposite polarity. Recently, several papers proposed that ramped pulse shapes could be an alternative shape for efficiently activating auditory nerve fibers. Here, we investigate whether ramped pulse shapes can activate auditory cortex (ACx) neurons in a more efficient way than the classical rectangular pulses. Guinea pigs were implanted with CI devices and responses of ACx neurons were tested with rectangular pulses and with four ramped pulse shapes, with a first-phase being either cathodic or anodic. The thresholds, i.e., the charge level necessary for obtaining significant cortical responses, were almost systematically lower with ramped pulses than with rectangular pulses. The maximal firing rate (FR) elicited by the ramped pulses was higher than with rectangular pulses. As the maximal FR occurred with lower charge levels, the dynamic range (between threshold and the maximal FR) was not modified. These effects were obtained with cathodic and anodic ramped pulses. By reducing the charge levels required to activate ACx neurons, the ramped pulse shapes should reduce charge consumption and should contribute to more battery-efficient CI devices in the future.

Comparison of response properties of the electrically stimulated auditory nerve reported in human listeners and in animal models

Cochlear implants (CIs) provide acoustic information to implanted patients by electrically stimulating nearby auditory nerve fibers (ANFs) which then transmit the information to higher-level neural structures for further processing and interpretation. Computational models that simulate ANF responses to CI stimuli enable the exploration of the mechanisms underlying CI performance beyond the capacity of in vivo experimentation alone. However, all ANF models developed to date utilize to some extent anatomical/morphometric data, biophysical properties and/or physiological data measured in non-human animal models. This review compares response properties of the electrically stimulated auditory nerve (AN) in human listeners and different mammalian models. Properties of AN responses to single pulse stimulation, paired-pulse stimulation, and pulse-train stimulation are presented. While some AN response properties are similar between human listeners and animal models (e.g., increased AN sensitivity to single pulse stimuli with long interphase gaps), there are some significant differences. For example, the AN of most animal models is typically more sensitive to cathodic stimulation while the AN of human listeners is generally more sensitive to anodic stimulation. Additionally, there are substantial differences in the speed of recovery from neural adaptation between animal models and human listeners. Therefore, results from animal models cannot be simply translated to human listeners. Recognizing the differences in responses of the AN to electrical stimulation between humans and other mammals is an important step for creating ANF models that are more applicable to various human CI patient populations.

Direct modulation of microglial function by electrical field

Non-invasive electric stimulation (ES) employing a low-intensity electric current presents a potential therapeutic modality that can be applied for treating retinal and brain neurodegenerative disorders. As neurons are known to respond directly to ES, the effects of ES on glia cells are poorly studied. A key question is if ES directly mediates microglial function or modulates their activity merely via neuron-glial signaling. Here, we demonstrated the direct effects of ES on microglia in the BV-2 cells—an immortalized murine microglial cell line. The low current ES in a biphasic ramp waveform, but not that of rectangular or sine waveforms, significantly suppressed the motility and migration of BV-2 microglia in culture without causing cytotoxicity. This was associated with diminished cytoskeleton reorganization and microvilli formation in BV-2 cultures, as demonstrated by immunostaining of cytoskeletal proteins, F-actin and β-tubulin, and scanning electron microscopy. Moreover, ES of a ramp waveform reduced microglial phagocytosis of fluorescent zymosan particles and suppressed lipopolysaccharide (LPS)-induced pro-inflammatory cytokine expression in BV-2 cells as shown by Proteome Profiler Mouse Cytokine Array. The results of quantitative PCR and immunostaining for cyclooxygenase-2, Interleukin 6, and Tumor Necrosis Factor-α corroborated the direct suppression of LPS-induced microglial responses by a ramp ES. Transcriptome profiling further demonstrated that ramp ES effectively suppressed nearly half of the LPS-induced genes, primarily relating to cellular motility, energy metabolism, and calcium signaling. Our results reveal a direct modulatory effect of ES on previously thought electrically “non-responsive” microglia and suggest a new avenue of employing ES for anti-inflammatory therapy.

A fully implantable wireless bidirectional neuromodulation system for mice

Novel research in the field of bioelectronic medicine requires neuromodulation systems that pair high-performance neurostimulation and bio-signal acquisition hardware with advanced signal processing and control algorithms. Although mice are the most commonly used animal in medical research, the size, weight, and power requirements of such bioelectronic systems either preclude use in mice or impose significant constraints on experimental design. Here, a fully-implantable recording and stimulation neuromodulation system suitable for use in mice is presented, measuring 2.2 cm3 and weighing 2.8 g. The bidirectional wireless interface allows simultaneous readout of multiple physiological signals and complete control over stimulation parameters, and a wirelessly rechargeable battery provides a lifetime of up to 5 days on a single charge. The device was implanted to deliver vagus nerve stimulation (n = 12 animals) and a functional neural interface (capable of inducing acute bradycardia) was demonstrated with lifetimes exceeding three weeks. The design utilizes only commercially-available electrical components and 3D-printed packaging, with the goal of facilitating widespread adoption and accelerating discovery and translation of future bioelectronic therapeutics.

Estimating health of the implanted cochlea using psychophysical strength-duration functions and electrode configuration

It is generally believed that the efficacy of cochlear implants is partly dependent on the condition of the stimulated neural population. Cochlear pathology is likely to affect the manner in which neurons respond to electrical stimulation, potentially resulting in differences in perception of electrical stimuli across cochlear implant recipients and across the electrode array in individual cochlear implant users. Several psychophysical and electrophysiological measures have been shown to predict cochlear health in animals and were used to assess conditions near individual stimulation sites in humans. In this study, we examined the relationship between psychophysical strength-duration functions and spiral ganglion neuron density in two groups of guinea pigs with cochlear implants who had minimally-overlapping cochlear health profiles. One group was implanted in a hearing ear (N = 10) and the other group was deafened by cochlear perfusion of neomycin, inoculated with an adeno-associated viral vector with an Ntf3-gene insert (AAV.Ntf3) and implanted (N = 14). Psychophysically measured strength-duration functions for both monopolar and tripolar electrode configurations were then compared for the two treatment groups. Results were also compared to their histological outcomes. Overall, there were considerable differences between the two treatment groups in terms of their psychophysical performance as well as the relation between their functional performance and histological data. Animals in the neomycin-deafened, neurotrophin-treated, and implanted group (NNI) exhibited steeper strength-duration function slopes; slopes were positively correlated with SGN density (steeper slopes in animals that had higher SGN densities). In comparison, the implanted hearing (IH) group had shallower slopes and there was no relation between slopes and spiral ganglion density. Across all animals, slopes were negatively correlated with ensemble spontaneous activity levels (shallower slopes with higher ensemble spontaneous activity levels). We hypothesize that differences in strength-duration function slopes between the two treatment groups were related to the condition of the inner hair cells, which generate spontaneous activity that could affect the across-fiber synchrony and/or the size of the population of neural elements responding to electrical stimulation. In addition, it is likely that spiral ganglion neuron peripheral processes were present in the IH group, which could affect membrane properties of the stimulated neurons. Results suggest that the two treatment groups exhibited distinct patterns of variation in conditions near the stimulating electrodes that altered detection thresholds. Overall, the results of this study suggest a complex relationship between psychophysical detection thresholds for cochlear implant stimulation and nerve survival in the implanted cochlea. This relationship seems to depend on the characteristics of the electrical stimulus, the electrode configuration, and other biological features of the implanted cochlea such as the condition of the inner hair cells and the peripheral processes.

The Perception of Ramped Pulse Shapes in Cochlear Implant Users

The electric stimulation provided by current cochlear implants (CI) is not power efficient. One underlying problem is the poor efficiency by which information from electric pulses is transformed into auditory nerve responses. A novel stimulation paradigm using ramped pulse shapes has recently been proposed to remedy this inefficiency. The primary motivation is a better biophysical fit to spiral ganglion neurons with ramped pulses compared to the rectangular pulses used in most contemporary CIs. Here, we tested the hypotheses that ramped pulses provide more efficient stimulation compared to rectangular pulses and that a rising ramp is more efficient than a declining ramp. Rectangular, rising ramped and declining ramped pulse shapes were compared in terms of charge efficiency and discriminability, and threshold variability in seven CI listeners. The tasks included single-channel threshold detection, loudness-balancing, discrimination of pulse shapes, and threshold measurement across the electrode array. Results showed that reduced charge, but increased peak current amplitudes, was required at threshold and most comfortable levels with ramped pulses relative to rectangular pulses. Furthermore, only one subject could reliably discriminate between equally-loud ramped and rectangular pulses, suggesting variations in neural activation patterns between pulse shapes in that participant. No significant difference was found between rising and declining ramped pulses across all tests. In summary, the present findings show some benefits of charge efficiency with ramped pulses relative to rectangular pulses, that the direction of a ramped slope is of less importance, and that most participants could not perceive a difference between pulse shapes.

Effect of the Relative Timing between Same-Polarity Pulses on Thresholds and Loudness in Cochlear Implant Users

The effect of the relative timing between pairs of same-polarity monophasic pulses has been studied extensively in single-neuron animal studies and has revealed fundamental properties of the neurons. For human cochlear implant listeners, the requirement to use charge-balanced stimulation and the typical use of symmetric, biphasic pulses limits such measures, because currents of opposite polarities interact at the level of the neural membrane. Here, we propose a paradigm to study same-polarity summation of currents while keeping the stimulation charge-balanced within a short time window. We used pairs of mirrored pseudo-monophasic pulses (a long-low phase followed by a short-high phase for the first pulse and a short-high phase followed by a long-low phase for the second pulse). We assumed that most of the excitation would stem from the two adjacent short-high phases, which had the same polarity. The inter-pulse interval between the short-high phases was varied from 0 to 345 μs. The inter-pulse interval had a significant effect on the perceived loudness, and this effect was consistent with both passive (membrane-related) and active (ion-channel-related) neuronal mechanisms contributing to facilitation. Furthermore, the effect of interval interacted with the polarity of the pulse pairs. At threshold, there was an effect of polarity, but, surprisingly, no effect of interval nor an interaction between the two factors. We discuss possible peripheral origins of these results.