Electrophysiology publications

Background Direct current (DC) stimulation can modulate neuronal activity in ways that differ from pulsatile stimulation, but its intracranial use has been limited by concerns about tissue injury at the electrode/tissue interface. Quantitative safety limits for DC delivered through metal electrodes directly to the brain remain poorly defined. Objective To estimate histological safety boundaries for DC stimulation delivered through metal electrodes in a large-brain gyrencephalic animal model. Methods Cathodal DC stimulation was applied to the exposed cortical surface of ten anesthetized sheep using platinum–iridium disc electrodes typically used in clinical applications (surface area ≤ 4.15 mm 2 ). Currents of 5 to 1000 µA were delivered for 10 to 15 minutes at 36 cortical sites. Stimulation dose was quantified as charge density. Brains were removed shortly after stimulation and examined histologically for tissue damage, including necrosis, inflammation, gliosis, and demyelination. Lesion volumes were quantified and related to charge density. Results No lesions were observed at sites where no current or a low charge density (0.7 mC/mm 2 ) was delivered. With stimulation, lesion probability and volume increased with charge density, although variability was substantial. Lesions occurred in 3 of 18 sites at lower charge densities (1.4 to 10 mC/mm 2 ) and in 5 of 9 sites at higher charge densities (14.4 to 144.4 mC/mm 2 ). Linear regression of lesion volume against charge density yielded an estimated zero-lesion intercept of 2.3 mC/mm 2 , whereas alternative nonlinear models predicted thresholds up to 8.7 mC/mm 2 . Conclusion These findings suggest that it may be possible to apply cathodal DC stimulation directly to the cortical surface through metal electrodes without detectable histological damage when current intensity, duration, and electrode size are appropriately constrained. These findings provide quantitative guidance for the safe application of DC directly to neural tissue in experimental and translational neuromodulation studies.

Neural interfaces for monitoring and modulating spinal nerve activity are increasingly being designed to be flexible and stretchable to enhance their biomechanical compatibility and integration. However, excessive flexibility introduces practical limitations such as difficulty in insertion into narrow spinal spaces and long-term electrical instability, hindering real-world applications. In this study, we developed a spinal nerve interface by incorporating a liquid-metal conductor and dynamic stiffness-based variable-compliance structure, which can address the challenges of current flexible neural interface technologies. During insertion, the dynamic stiffness enhancer minimizes unintended buckling and ensures minimally invasive implantation into the intended target. The proximity of the proposed device to the spinal cord increases as it flexes automatically and rapidly in a humid environment. The liquid-metal conductor maintained stable electrical properties in freely moving rats, ensuring reliable and sustained functionality. This study lays the foundation for practical, fully implantable spinal bioelectronics designed with a focus on ease of implantation and long-term functionality.

Objective. Intracortical microstimulation (ICMS) is a widely used tool for neuroprostheses, but its long-term efficacy is often limited by foreign body response and neuroinflammatory responses at the electrode-tissue interface. Microglia orchestrate neuroinflammation and regulate synaptic plasticity, and low-frequency stimulation has been shown to promote anti-inflammatory microglial phenotypes. Approach. We investigated how 10 Hz ICMS influences microglia–neuron interactions during the first three days post-implantation using in vivo two-photon imaging in Cx3cr1-GFP/jRGECO1a mice. Microglial motility, morphology, and process orientation were tracked relative to electrode placement and neuronal calcium activity (measured as change in fluorescence, Δ F / F ). Main results. A 1 h session of 10 Hz ICMS did not induce overt classical morphological activation of microglia but robustly increased process motility, with extensions dynamically tracking neurons showing early activation or subsequent functional suppression. By post-implantation Day 2, microglial processes were significantly more likely to engage neurons with high early calcium responses after stimulation onset (mean orientation angle: 74.3° ± 11.8°), but this engagement shifted during prolonged stimulation (117.0° ± 9.2°, p = 0.0017), indicating context-dependent interactions. Contact frequency scaled with neuronal adaptation profiles, and neurons exhibiting depressed activity received the most contacts immediately after implantation (1.2 ± 0.3 contacts, p = 0.046). Significance. These findings reveal stimulus-associated, neuron-dependent surveillance behaviors of microglia during early post-implantation ICMS and suggest that microglia actively participate in short-term modulation of stimulated cortical circuits.

Background: Electrical stimulation targeting the motor thalamus (Mthal) represents an emerging strategy for regulating motor cortical activity patterns, however the fundamental mechanisms and optimal stimulation configurations are not well established. Objective: To characterize how burst Mthal stimulation modulates cortical activity and to identify stimulation parameters that maximize cortical co-firing via thalamocortical rebound mechanisms. Methods: We conducted acute electrophysiological experiments in anesthetized rats using Mthal stimulation with systematically manipulated burst stimulation parameters (i.e., number of pulses and pulse width) in the low-frequency band (1-8 Hz) modulation cycle. Cortical responses across large areas covering primary and secondary motor cortex were recorded. Additionally, intracortical recordings were performed for detailed spike activity monitoring during stimulation. Cortical activity patterns in local field potentials and spike activity were analyzed to quantify the effects of Mthal stimulation on motor cortex. Computational thalamocortical circuit models were employed to explore the mechanistic basis of frequency-selective modulation effects. Results: Burst Mthal stimulation elicited tunable cortical excitation, with optimal modulation achieved at 3-4 Hz. This preferred frequency corresponded to the natural rebound timing of thalamocortical (TC) cells and generated maximal cortical co-firing and synchronization. Higher stimulation frequencies (e.g., 8 Hz) resulted in suppressed and diminished responses. Computational modeling further validated that the cellular mechanism of the TC cell rebound excitation leads to frequency-dependent cortical modulation. Conclusion: Burst Mthal stimulation demonstrates frequency-selective modulation of cortical network excitability, with 3-4 Hz protocols providing optimal enhancement of synchronized activity through exploitation of thalamocortical rebound dynamics. These results establish motor thalamic stimulation as a promising and efficient methodology for precise cortical network control and offer mechanistic foundations for developing therapeutic neuromodulation interventions aimed at motor system rehabilitation.

Haptic feedback elicits sensations of touch to communicate information to the user. Recent developments in virtual reality and sensory prostheses have demonstrated the need for systems that can provide more information than the simple vibrotactor in your phone. A novel multidimensional encoding approach allows for percepts of signal intensity and flutter frequency in a single distal location using transcutaneous neural stimulation. In this study we aimed to determine if these percepts are able to convey useful information to participants performing motor control tasks and if they perform better than a typical intensity-only modulation approach. Eleven participants performed three types of tasks to assess the efficacy of multidimensional stimulation. The first set included discrete matching tasks, where participants had to differentiate between thirteen combinations of flutter frequency and intensity. In the other two sets, participants had to differentiate between simultaneous changes in the percepts. Participants in this study could correctly identify all discrete percept combinations and follow the changes in the percepts better than chance. Performance in the discrete task using a multidimensional approach showed increased information transfer compared to the individual modulation of intensity or flutter frequency. These results suggest that multidimensional encoding is a promising approach for increasing information throughput in sensory feedback systems. Hence, such an approach might improve upon conventional methods of providing graded percepts, creating more informative tactile percepts for haptic feedback through peripheral nerve stimulation.

Sympathetic hyperactivity is a common feature observed in both primary and secondary hypertension, and neuromodulation-based therapies targeting this overactivity are under active investigation. The intermediolateral nucleus (IML) in the spinal cord plays a central role in sympathetic regulation, and its direct stimulation induces a transient, frequency-dependent reduction in blood pressure. However, this effect gradually diminishes as the baroreflex restores blood pressure to baseline. Therefore, this study aims to develop a closed-loop (CL) stimulation system that adjusts parameters in real-time based on neural activity recorded from the IML and to enhance and prolong the transient antihypertensive effect. This strategy was evaluated in normotensive and angiotensin II-induced hypertensive rat models and compared to open-loop (OL) stimulation of equal intensity. In both models, CL stimulation produced a more sustained blood pressure reduction than that of OL stimulation. These findings suggest that CL spinal cord stimulation offers a more effective and durable therapeutic option for treating hypertension.

Target engagement determines therapeutic efficacy. In spinal cord stimulation (SCS), efficacy depends on the attributes of the electrodes used to deliver stimulation, particularly their position, interelectrode distance, size, and current orientation. We tested the effects of these electrode parameters using cervical stimulation to elicit forelimb muscle responses in the rat. We implanted devices over the C6 dorsal root entry zone (DREZ) in eight rats and measured responses in six forelimb muscles. We used custom designed electrode arrays to systematically vary position, distance, and size with linear arrays, and current orientation with circular arrays. Stimulation consisted of biphasic and pseudomonophasic pulse shapes with bipolar or distant return, and a high-definition montage with four returns surrounding a central contact to increase local current density. Efficacy was calculated from the motor thresholds of recruitment curves, and parameters were compared using linear mixed models. For position, stimulation over the DREZ was most effective, reducing thresholds by 25.9% ( p = 0.0002) relative to the midline; efficacy decreased as the electrode was positioned medial or lateral to the DREZ. For distance, placing the electrodes farther apart significantly improved efficacy ( p = 0.0022). For size, large electrodes increased SCS efficacy, reducing thresholds by 21.5% ( p = 0.0026) relative to small electrodes. After accounting for position and distance, current orientation did not affect SCS efficacy. Lastly, the high-definition montage decreased efficacy, increasing thresholds by 16.7% ( p = 0.003) relative to monopolar stimulation with a distant return. In conclusion, electrode position, distance, and size had the greatest effect on SCS efficacy, with the optimal parameters combining large electrodes at the DREZ position with a distant return. These results have the potential to lower the current needed to engage cervical spinal circuitry and to inform the design of SCS systems to improve dexterity in people with neurological injury or disease.

Gastrointestinal (GI) dysmotility and associated conditions affect over 20% of population, yet pharmacological, behavioural, and surgical interventions offer limited therapeutic efficacy. Targeted electrical stimulation addressing underlying neuromuscular pathology stands to transform our ability to treat dysmotility. Here, we developed a closed-loop GI neuroprosthesis which activates or relaxes GI tract musculature through electrochemical stimulation in response to sensed food stimuli. We additionally describe a tool supporting minimally invasive endoscopically guided implantation that can penetrate the mucosa, accurately localize the submucosa, and safely deploy this device to directly interface with the enteric nervous system. The neuroprosthesis enables generation of coordinated peristaltic waves, significantly increasing the motility rate in a swine model of oesophageal and stomach dysmotility (p < 0.05, student's t-test). Further, by directly modulating the myenteric plexus and thus mimicking meal ingestion, we induce peristalsis in a fasted state and achieve a metabolic response commensurate with a fed or satiated state. This neuroprosthesis and implantation platform expand opportunities in fundamental studies and treatments of metabolic and neuromuscular pathologies affecting the GI tract.

Although control of movement involves many cortical association areas, bidirectional brain-machine interfaces (BMIs) typically decode movement intent from the motor cortex and deliver feedback information to the primary somatosensory cortex (S1). Compared to the S1, the parietal and premotor areas encode more complex information about object properties, hand pre-shaping, and reach trajectories. BMIs therefore might deliver richer information to those cortical association areas than to primary areas. Here, we investigated whether instructions for a center-out task could be delivered via intracortical microstimulation (ICMS) in the anterior intraparietal area (AIP), dorsal posterior parietal cortex (dPPC), or dorsal premotor cortex (PMd) as well as the ventral premotor cortex (PMv) and S1. Two monkeys successfully learned to use AIP, PMv, or S1-ICMS instructions, but neither learned to use dPPC- or PMd-ICMS instructions. The AIP, PMv, and S1 may thus be effective cortical territory for delivering information to the brain, whereas the dPPC or PMd may be comparatively ineffective.

Neurostimulation for cardiovascular control faces challenges due to the lack of predictive modeling for stimulus-driven dynamic responses, which is crucial for precise neuromodulation via quality feedback. We address this by employing a digital twin approach that leverages computational mechanisms underlying neuro-hemodynamic responses during neurostimulation. Our results emphasize the computational role of the nucleus tractus solitarius (NTS) in the brainstem in determining these responses. The intrinsic neural circuit within the NTS harbors collective dynamics residing in a low-dimensional latent space, which effectively captures stimulus-driven hemodynamic perturbations. Building on this, we developed a digital twin framework for individually optimized predictive modeling of neuromodulatory outcomes. This framework potentially enables the design of closed-loop neurostimulation systems for precise hemodynamic control. Consequently, our digital twin based on neural computation mechanisms marks an advancement in the artificial regulation of internal organs, paving the way for precise translational medicine to treat chronic diseases.

The value associated with reward is sensitive to external factors, such as the time between the choice and reward delivery as classically manipulated in temporal discounting tasks. Subjective preference for two reward options is dependent on objective variables of reward magnitude and reward delay. Single neuron correlates of reward value have been observed in regions, including ventral striatum, orbital, and medial prefrontal cortex. Brain imaging studies show cortico-striatal-limbic network activity related to subjective preferences. To explore how oscillatory dynamics represent reward processing across brain regions, we measured local field potentials of rats performing a temporal discounting task. Our goal was to use a data-driven approach to identify an electrophysiological marker that correlates with reward preference. We found that reward-locked oscillations at beta frequencies signaled the magnitude of reward and decayed with longer temporal delays. Electrodes in orbitofrontal/medial prefrontal cortex, anterior insula, ventral striatum, and amygdala individually increased power and were functionally connected at beta frequencies during reward outcome. Beta power during reward outcome correlated with subjective value as defined by a computational model fit to the discounting behavior. These data suggest that cortico-striatal beta oscillations are a reward signal correlated, which may represent subjective value and hold potential to serve as a biomarker and potential therapeutic target.

Effective neural stimulation requires adequate parametrization. Gaussian-process (GP)-based Bayesian optimization (BO) offers a framework to discover optimal stimulation parameters in real time. Here, we first provide a general protocol to deploy this framework in neurostimulation interventions and follow by exemplifying its use in detail. Specifically, we describe the steps to implant rats with multi-channel electrode arrays in the hindlimb motor cortex. We then detail how to utilize the GP-BO algorithm to maximize evoked target movements, measured as electromyographic responses. For complete details on the use and execution of this protocol, please refer to Bonizzato and colleagues (2023).1.

Compound nerve action potentials (CNAPs) were used as a metric to assess the stimulation performance of a novel high-density, transverse, intrafascicular electrode in rat models. We show characteristic CNAPs recorded from distally implanted cuff electrodes. Evaluation of the CNAPs as a function of stimulus current and calculation of recruitment plots were used to obtain a qualitative approximation of the neural interface’s placement and orientation inside the nerve. This method avoids elaborate surgeries required for the implantation of EMG electrodes and thus minimizes surgical complications and may accelerate the healing process of the implanted subject.

Background Cuff electrodes target various nerves throughout the body, providing neuromodulation therapies for motor, sensory, or autonomic disorders. However, when using standard, thick silicone cuffs, fabricated in discrete circular sizes, complications may arise, namely cuff displacement or nerve compression, due to a poor adaptability to variable nerve shapes and sizes encountered in vivo. Improvements in cuff design, materials, closing mechanism and surgical approach are necessary to overcome these issues. Methods In this work, we propose a microfabricated multi-channel silicone-based soft cuff electrode with a novel easy-to-implant and size-adaptable design and evaluate a number of essential features such as nerve-cuff contact, nerve compression, cuff locking stability, long-term integration and stimulation selectivity. We also compared performance to that of standard fixed-size cuffs. Results The belt-like cuff made of 150 μm thick silicone membranes provides a stable and pressure-free conformal contact, independently of nerve size variability, combined with a straightforward implantation procedure. The adaptable design and use of soft materials lead to limited scarring and demyelination after 6-week implantation. In addition, multi-contact designs, ranging from 6 to 16 electrodes, allow for selective stimulation in models of rat and pig sciatic nerve, achieving targeted activation of up to 5 hindlimb muscles. Conclusion These results suggest a promising alternative to classic fixed-diameter cuffs and may facilitate the adoption of soft, adaptable cuffs in clinical settings.

Cochlear implants are among the most successful neural prosthetic devices to date but exhibit poor frequency selectivity and the inability to consistently activate apical (low frequency) spiral ganglion neurons. These issues can limit hearing performance in many cochlear implant patients, especially for understanding speech in noisy environments and in perceiving or appreciating more complex inputs such as music and multiple talkers. For cochlear implants, electrical current must pass through the bony wall of the cochlea, leading to widespread activation of auditory nerve fibers. Cochlear implants also cannot be implanted in some individuals with an obstruction or severe malformations of the cochlea. Alternatively, intraneural stimulation delivered via an auditory nerve implant could provide direct contact with neural fibers and thus reduce unwanted current spread. More confined current during stimulation can increase selectivity of frequency fiber activation. Furthermore, devices such as the Utah Slanted Electrode Array can provide access to the full cross section of the auditory nerve, including low frequency fibers that are difficult to reach using a cochlear implant. However, further scientific and preclinical research of these Utah Slanted Electrode Array devices is limited by the lack of a chronic large animal model for the auditory nerve implant, especially one that leverages an appropriate surgical approach relevant for human translation. This paper presents a newly developed transbullar translabyrinthine surgical approach for implanting the auditory nerve implant into the cat auditory nerve. In our first of a series of studies, we demonstrate a surgical approach in non-recovery experiments that enables implantation of the auditory nerve implant into the auditory nerve, without damaging the device and enabling effective activation of the auditory nerve fibers, as measured by electrode impedances and electrically evoked auditory brainstem responses. These positive results motivate performing future chronic cat studies to assess the long-term stability and function of these auditory nerve implant devices, as well as development of novel stimulation strategies that can be translated to human patients.

Cochlear Implants (CI) are an effective neuroprosthesis for humans with profound hearing loss, enabling deaf adults to have phone calls without lipreading and babies to have successful language development. However, CIs have significant limitations in complex hearing situations, motivating the need for further research, including studies in animal models. Here, we demonstrate the usefulness of wide field Ca++ imaging in assessing different CI stimulation strategies. One major challenge in electrophysiology in CI animals lies in excluding the CI electric artifacts from the recording, since they are orders of magnitude larger than the amplitude of action potentials. Also, electrophysiology can rarely sample large areas of neuropil at high spatial resolution. To circumvent these problems, we have set up an imaging system allowing us to monitor neural activity in the auditory cortex (AC) of CI supplied rats using the Ca++ sensitive dye OGB. Here we describe an initial experiment with this setup, in which we recorded cortical responses to 4 different stimulation patterns which were delivered across 3 CI channels to the contralateral ear. We then investigated two parameters that have been shown to affect intelligibility in CI users: pulse rate and relative pulse timing across CI channels. While pulse rate had only a very modest effect on the discriminability of the neural responses, the stimulation mode had a major effect, with simultaneous pulse timing, perhaps surprisingly, allowing much better pattern discrimination than interleaved sampling. The result suggests that allowing collisions of pulses on neighboring channels may not always be detrimental, at least if partial overlaps of pulses, in which anodic and cathodic pulse phases might cancel, are avoided.

This protocol was used to collect data now published in the Journal of Neural Engineering, Spatially selective stimulation of the pig vagus nerve to modulate target effect versus side effect using the multi-contact ImThera stimulating device. https://doi.org/10.1088/1741-2552/acb3fd Dataset has been published to pennsieve.io and can be found at: https://doi.org/10.26275/efbj-8evl

Intracortical microstimulation (ICMS) is commonly used in many experimental and clinical paradigms; however, its effects on the activation of neurons are still not completely understood. To document the responses of cortical neurons in awake nonhuman primates to stimulation, we recorded single-unit activity while delivering single-pulse stimulation via Utah arrays implanted in primary motor cortex (M1) of three macaque monkeys. Stimuli between 5 and 50 μA delivered to single channels reliably evoked spikes in neurons recorded throughout the array with delays of up to 12 ms. ICMS pulses also induced a period of inhibition lasting up to 150 ms that typically followed the initial excitatory response. Higher current amplitudes led to a greater probability of evoking a spike and extended the duration of inhibition. The likelihood of evoking a spike in a neuron was dependent on the spontaneous firing rate as well as the delay between its most recent spike time and stimulus onset. Tonic repetitive stimulation between 2 and 20 Hz often modulated both the probability of evoking spikes and the duration of inhibition; high-frequency stimulation was more likely to change both responses. On a trial-by-trial basis, whether a stimulus evoked a spike did not affect the subsequent inhibitory response; however, their changes over time were often positively or negatively correlated. Our results document the complex dynamics of cortical neural responses to electrical stimulation that need to be considered when using ICMS for scientific and clinical applications.

In humans, the level of consciousness is assessed by quantifying the spatiotemporal complexity of cortical responses using Perturbational Complexity Index (PCI) and related PCIst (st, state transitions). Here we validate PCIst in freely moving rats and mice by showing that it is lower in NREM sleep and slow wave anesthesia than in wake or REM sleep, as in humans. We then show that (1) low PCIst is associated with the occurrence of an OFF period of neuronal silence; (2) stimulation of deep, but not superficial, cortical layers leads to reliable PCIst changes across sleep/wake and anesthesia; (3) consistent PCIst changes are independent of which single area is being stimulated or recorded, except for recordings in mouse prefrontal cortex. These experiments show that PCIst can reliably measure vigilance states in unresponsive animals and support the hypothesis that it is low when an OFF period disrupts causal interactions in cortical networks.

Electrically evoked compound action potentials (ECAPs) generated in the subthalamic nucleus (STN) contain features that may be useful for titrating deep brain stimulation (DBS) therapy for Parkinson’s disease. Delivering a strong therapeutic effect with DBS therapies, however, relies on selectively targeting neural pathways to avoid inducing side effects. In this study, we investigated the spatiotemporal features of ECAPs in and around the STN across parameter sweeps of stimulation current amplitude, pulse width, and electrode configuration, and used a linear classifier of ECAP responses to predict electrode location. Four non-human primates were implanted unilaterally with either a directional (n = 3) or non-directional (n = 1) DBS lead targeting the sensorimotor STN. ECAP responses were characterized by primary features (within 1.6 ms after a stimulus pulse) and secondary features (between 1.6 and 7.4 ms after a stimulus pulse). Using these features, a linear classifier was able to accurately differentiate electrodes within the STN versus dorsal to the STN in all four subjects. ECAP responses varied systematically with recording and stimulating electrode locations, which provides a subject-specific neuroanatomical basis for selecting electrode configurations in the treatment of Parkinson’s disease with DBS therapy.