Electrical Stimulation publications

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.

Reduction in neuronal firing from wake to sleep, i.e., sleep firing rate homeostasis (sFRH), is essential for restorative sleep. Sleep dysfunction is common in Parkinson's disease (PD), but sFRH has not been investigated. Here, using a within-subject design in the nonhuman primate model of PD, we report that thalamocortical sFRH is disrupted in parkinsonism. These findings can inform therapeutic approaches tailored to normalize sFRH and reestablish restorative sleep in PD.

Background: Otitis media (OM) is an inflammatory disease of the middle ear characterized by mucosal remodeling, effusion, and conductive hearing loss. Although antibiotics and surgical procedures remain standard treatments, their efficacy is often limited by recurrence, antibiotic resistance, and chronic progression. Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive neuromodulatory technique that activates the cholinergic anti-inflammatory pathway through α7 nicotinic acetylcholine receptor (α7nAChR) signaling. This study aimed to evaluate the anti-inflammatory effects of taVNS in a lipopolysaccharide (LPS)-induced mouse model of acute otitis media (AOM) and to assess whether these effects may involve α7nAChR-dependent mechanisms. Methods: AOM was induced by transtympanic injection of LPS in BALB/c mice, followed by taVNS applied to the auricular concha using biphasic square pulses (0.3 mA, 20 Hz, 200 μs). Methyllycaconitine, a selective α7nAChR antagonist, was administered to assess receptor involvement. Auditory function, tympanic membrane morphology, and mucosal changes were assessed through auditory brainstem response testing, otoscopic imaging, and histological analysis. Cytokine levels in middle ear effusion and serum were quantified by ELISA, while inflammatory gene and protein expression were analyzed using qPCR and Western blotting. Statistical analyses were performed using one-way ANOVA or Kruskal-Wallis tests with post hoc comparisons. Results: taVNS significantly improved hearing thresholds and reduced mucosal thickening and goblet cell hyperplasia in AOM mice. It markedly decreased tumor necrosis factor-α, interleukin (IL)-1β, and IL-6 levels in middle ear effusion, and mechanistically, this anti-inflammatory effect was associated with suppression of NF-κB activation without altering TLR4 or MYD88 expression. These effects were abolished by methyllycaconitine pretreatment. taVNS also reduced spleen enlargement and systemic cytokine concentrations, indicating modulation of both local and systemic inflammation. Conclusions: taVNS was associated with attenuation of LPS-induced acute otitis media and reduced NF-κB activation and downstream cytokine expression in a manner consistent with the involvement of α7nAChR-related signaling. By attenuating excessive inflammatory responses, taVNS was associated with improved auditory function and reduced middle ear injury, suggesting its potential as a non-invasive therapeutic strategy for LPS-induced acute otitis media.

Adeno-associated virus (AAV) unequivocally emerges as one of the most powerful and promising delivery vectors for gene therapy targeting hereditary hearing loss. Following AAV transduction in the inner ear, varying degrees of natural immune responses are triggered, primarily characterized by macrophage activation and the secretion of pro-inflammatory factors. Additionally, the production of neutralizing antibodies may affect the efficacy of gene therapy. To evaluate immune dynamics, we injected AAV1 and AAV-ie (capsids with distinct transfection efficiencies) into murine cochleae and analyzed temporal transcriptomic profiles. Our results demonstrate that both capsids induce immune activity but with critical temporal and intensity differences that AAV1 elicits significantly later and milder immune reactions compared to AAV-ie. These findings establish that dynamic cochlear gene expression profiles directly inform the selection of immunologically optimized AAV vectors to minimize adverse responses in future hereditary hearing loss gene therapies.

Objective Electrical stimulation of the baroreceptors pathways at the carotid sinus bulb – known as baroreflex activation therapy (BAT) – is intended to change autonomic tone and ultimately reduce blood pressure and heart rate. BAT is pre-market approved by the United States Food and Drug Administration (FDA) for the treatment of heart failure and received an FDA humanitarian device exemption for drug resistant hypertension. However, responder rates are limited by side-effects including numbness in the head and neck, altered speech, respiratory constriction, dry cough, vomiting, and altered sensory and motor function of the tongue. We hypothesized that these side-effects are driven by activation of other nearby nerve fibers of similar or lower threshold than the carotid sinus nerve. In this study, we sought to identify the neural sources responsible for off-target muscle activation contributing to these side-effects. These sources would inform strategies mitigating off-target activation in BAT therapy. Methods Domestic swine were used in this work as the diameter and thickness of the swine carotid artery are closer to human than those of canine models. A monopolar disk electrode mimicking the clinical CVRx® Neo electrode was surgically placed proximal to the carotid bifurcation with the position optimized for stimulation dose responsive changes in blood pressure. Evoked responses were recorded during dose response testing from multiple neck muscles, and the corresponding off-target nerve pathways were identified by sequential transection of nearby nerves. Results The following activated off-target muscle groups and their corresponding nerve pathways were verified which included 1) the cricoarytenoid via recurrent laryngeal nerve, 2) cricothyroid via superior laryngeal nerve, 3) sternocleidomastoid via accessory nerve, and 4) the tongue via hypoglossal nerve. The constrictor muscle group was also activated through a more complex neural pathway. Conclusion We identified multiple sources of therapy-limiting side-effects of BAT in the swine model. These results will help guide the design of improved stimulation electrodes, surgical placement, and parameter programming to reduce BAT-associated side-effects while increasing on-target activation in patients.

Objective: The purpose of this paper is to present a chronic, preclinical, pilot study of a multi-electrode auditory nerve implant. To date, the most successful auditory neuroprosthesis is the cochlear implant, but these devices have known limitations. Intraneural stimulation of the auditory nerve is a promising alternative, but preclinical investigations of multi-electrode devices have been limited to acute or semi-chronic studies. Methods: Six cats were chronically implanted in the right auditory nerve with a 15-channel Utah Slant ArrayTM. The array's position in the internal auditory canal was confirmed using computed tomography. Electrode impedance and evoked responses were recorded regularly during the indwelling period to evaluate performance. Results: Two of the six implants met endpoint criteria at 6 months, with stable electrode impedances and robust evoked responses on at least 50% of their channels. Another two implants exhibited stable electrode impedances, but activation thresholds increased over three months. Two implants failed within the first two weeks due to device migration caused by connector-related issues. Conclusion: This study reports the first successful chronic deployment of an multi-electrode auditory nerve implant in a preclinical model. Two implanted cats exhibited stable chronic performance, marked by a consistent set of electrodes capable of eliciting evoked responses. The failure modes of the remaining four cats are discussed and recommendations are made to stabilize implant placement. The study lays a solid foundation for future chronic safety and efficacy investigations as part of an effort to translate this new neuroprosthetic technology into the clinic.

Introduction: Learning-induced cortical reorganization is facilitated by neuromodulator release. However, the precise temporal relationship between neuromodulator release and neural activity required to induce plasticity is not well understood. In single synapses, eligibility traces regulate plasticity such that neuromodulator arrival must occur within seconds of activity. Cortical plasticity can also be induced by pairing neuromodulator release with neural activity, but it is unknown whether plasticity in populations of neurons is regulated by the same temporal constraints. Methods: To determine whether the synaptic eligibility trace regulates cortical plasticity, we used vagus nerve stimulation (VNS) to drive neuromodulator release 3 s before, 1 s before, concurrent with, or 3 s after tone-evoked activity in auditory cortex. After 20 days of pairing, we used in vivo electrophysiology to quantify the proportion of auditory cortex responding to the paired tone frequency. Results: We found that VNS 1 s before or concurrent with tone onset significantly expanded the representation of the paired tone frequency (-1s, p = 0.04; 0s, p = 0.02), whereas 3 s before or after did not (-3s, p = 0.87; +3s, p = 0.53). Confirming the role of neuromodulators in this eligibility trace, we found that depletion of norepinephrine prevented VNS-tone pairing-induced plasticity (0s + DSP4, p = 0.23). Discussion: Our study demonstrates that cortical plasticity is regulated by time constraints similar to the synaptic eligibility trace and highlights the importance of considering the timing of neuromodulator release when employing methods to enhance plasticity.

The clinical use of alternating current stimulation (ACS) has been confounded by heterogeneity in patient responses. Optimization of this treatment requires understanding the dependence of neuronal responses on ACS parameters. We sought to characterize neuronal responses across varying ACS frequencies and amplitudes and assess the extent to which changes in neuronal activity persist beyond the cessation of the stimulation, thereby informing ACS optimization pathways and evaluating the potential for sustained neuromodulation. We delivered subcutaneous ACS while recording intracortical electrophysiological activity in adult rats. We applied ACS for 1 or 20 minutes and analyzed ensuing spiking activity, power spectra, and low-frequency to high-frequency phase–amplitude coupling (LF-HF PAC). Both durations of ACS elicited a modulation of firing patterns (either in rate or entrainment) in most neurons, in a stimulation frequency- and amplitude-dependent manner. Changes in firing rate were more frequent, albeit modest, whereas changes in entrainment were rarer, but pronounced. Increasing ACS amplitude or frequency increased the degree of entrainment. Further, entrainment changes persisted beyond the 20-minute ACS, at half their online level. Neuronal power and LF-HF PAC increased proximal to excited, but decreased near suppressed neurons. Neuronal entrainment was observed alongside decreased power and LF-HF PAC. The prominent entrainment response to prolonged high-frequency ACS observed here supports the potential of ACS for facilitating signal integration and promoting functional reorganization via spike timing-dependent plasticity. In turn, we found broader temporal windows of excitation under low ACS frequencies engendered greater variability in spike timing, which may be utilized to reduce hypersynchronous or task-irrelevant activity.

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.

In adults with acquired hearing loss, cortical reorganization and cross‐modal plasticity critically influence the timing and outcomes of cochlear implantation. This study examined the effects of 2 weeks of bilateral auditory deprivation—induced by intense broadband noise—on the structure and gene expression in the primary auditory (A1) and visual (V1) cortices of adult rats. The results revealed that the total neuronal counts in both the A1 and V1 remained unchanged compared to controls. Morphological analysis showed no significant change in mature dendritic spine density in either region, but immature spine density was significantly reduced in the V1 cortex. Ultrastructural examination demonstrated notable synaptic alterations in the A1 cortex, including significant reductions in postsynaptic density (PSD) length, thickness, area, and synaptic vesicle number—changes that were not observed in the V1 cortex. Transcriptomic profiling indicated a region‐specific response, with 197 differentially expressed genes (DEGs) in the A1 cortex, compared to a broader alteration of 545 DEGs in the V1 cortex, which were significantly involved in neuronal signaling and synaptic transmission. These findings demonstrate that auditory deprivation induces distinct molecular and synaptic remodeling in the A1 and V1 cortices, identifying neuronal plasticity and synaptic regulation as central mechanisms for cross‐modal reorganization in the adult brain.

Bilateral cochlear implant (BiCI) users do not perform as well as typical hearing listeners when discriminating the direction of sound motion. This is likely due to the lack of synchronization between two independently operating sound processors. The use of bilaterally linked processors can improve the timing of electrical stimulation across the ears. However, these improvements are potentially counteracted by spectral peak-picking in sound coding strategies that may activate different electrodes across the ears, thereby reducing the fidelity of the binaural cues that BiCI users rely on for sound localization. To improve interaural synchrony, a bilateral peak-picking strategy that guarantees coordinated stimulation across the ears was developed and tested using the CCi-MOBILE, a bilaterally linked research processor. The auditory motion tracking abilities of nine BiCI users were measured using the bilateral peak-picking strategy and compared to performance with commercially available, unsynchronized processors. Results showed no effect of bilateral peak-picking. However, a small benefit was observed when tracking the range of motion of a sound when listening with bilateral synchronization which appears to be due to reduced interaural level difference changes with auditory motion. These findings suggest that dynamic auditory perception with BiCI may likely need bilaterally-synchronized hardware.

It is widely known that users of cochlear implants (CIs) often show deficits in binaural hearing. In particular, interaural time difference (ITD) discrimination tends to be substantially worse in CI users. However, only very few studies have investigated how CI stimulation affects the “precedence effect” (PE), and how this might be affected by abnormal hearing experience during development. We performed behavioral experiments to measure temporal weighting functions (TWFs) to quantify the PE in three different cohorts of rats: normally hearing (NH) acoustically stimulated, neonatally deafened (ND) CI-stimulated, and adult deafened (AD) CI-stimulated animals. Lateralization responses were recorded to bursts of eight stimulus pulses in which the ITD of each pulse varied independently, and probit analysis revealed the perceptual weight of each pulse. The NH and ND-CI cohorts were tested at 50, 300, and 900 pps; the AD-CI animals were tested only at 300 pps. The NH animals show a pronounced “onset dominance” at higher pulse rates, that is, the weighting of the first pulse was much larger than for subsequent pulses, exactly as one would expect given previous studies on human listeners. In contrast, the TWFs for the ND-CI and AD-CI animals were much flatter, with a reduction of onset dominance by ∼75%, especially for higher pulse rates. Since neither of our CI cohorts had any experience of reverberation in their electric hearing, we interpret these results to indicate that the PE may require domain-specific experience to develop.

Selective activation of small versus large diameter nerve fibers represents a broad challenge in the electrical modulation of neural activity for clinical applications. For example, off-target activation of motor nerve fibers was implicated in several failed vagus nerve stimulation (VNS) clinical trials by effectively limiting maximum applicable current amplitudes, thereby preventing robust activation of smaller diameter fibers responsible for therapeutic effects. Kilohertz high-frequency (HF) block is one of several strategies proposed to overcome this gap, but has yet to be thoroughly investigated in large, multifascicular, mixed nerves comparable in scale to humans. Here, we evaluated effects of HF block on the neural, muscular, and cardiac responses evoked by traditional low-frequency (LF) VNS in a large animal (swine) model. We found that HF selectively blocks neural fibers, and their associated physiological responses, in a size dependent manner with a high degree of specificity and establish initial thresholds for achieving block in large multifascicular nerves. Critically, we demonstrate that appropriately titrating HF amplitude can eliminate off-target LF-evoked motor responses while preserving activation of smaller diameter fibers targeted by clinical VNS. Beyond VNS, these data establish HF as a potentially viable strategy in any neuromodulation therapies where off-target activation of large diameter fibers limits therapeutic efficacy.

Primate societies rely on the production and interpretation of social signals, in particular those displayed by the face. Facial movements are controlled, according to the dominant neuropsychological schema, by two separate circuits, one originating in medial frontal cortex controlling emotional expressions, and a second one originating in lateral motor and premotor areas controlling voluntary facial movements. Despite this functional dichotomy, cortical anatomy suggests that medial and lateral areas are directly connected and may thus operate as a single network. Here, we test these contrasting hypotheses through structural and functional MRI (fMRI) guided electrical stimulation and simultaneous multichannel recordings from key facial motor areas in the macaque monkey brain. These areas include medial facial motor area M3 (located in the anterior cingulate cortex); two lateral face-related motor areas: M1 (primary motor) and PMv (ventrolateral premotor); and S1 (primary somatosensory cortex). Cortical responses evoked by intracortical stimulation revealed that medial and lateral areas can exert significant functional impact on each other. Simultaneous recordings of local field potentials in all facial motor areas further confirm that during facial expressions, medial and lateral facial motor areas significantly interact, primarily in the alpha and beta frequency ranges, whereas during voluntary chewing, coupling occurs at lower frequencies. These functional interactions varied across facial movement types. Thus, at the cortical level, the control of facial movements is not mediated through independent (medial/lateral) functional streams, but results from an interacting sensorimotor network.

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.

The cochlear implant is a widely available hearing restoration technology that can provide speech understanding in quiet environments. This technology struggles however, in noisy settings or situations involving multiple speakers. The primary cause of these performance limitations is a poor neural interface, in which the bony wall of the cochlea separates the electrode surface from the auditory nerve fibers, thus causing unwanted current spread and non-specific frequency activation. This study utilizes an alternative auditory prosthetic technology (auditory nerve implant, ANI) that enables direct auditory nerve stimulation, which provides a potentially superior neural interface and enables more precise targeting of auditory nerve fibers than traditional cochlear implants. As auditory nerve implants progress towards clinical translation, new implant designs and stimulation strategies will be created. Animal models to efficiently test and iterate through these new designs will be useful for the continued development of ANI technology. We present a viable surgical approach in the non-human primate (rhesus macaque) along with electrophysiological results that demonstrate robust activation of the auditory system at low current levels via intraneural stimulation. Our findings indicate that the rhesus macaque, which possesses an inner ear anatomy more similar to the human compared to other animal models used in the hearing field (e.g., rodents, felines and ferrets), has strong potential as a useful preclinical testbed involving an upright head model for future ANI prototypes and stimulation strategy development.

The hypothalamus is critical for regulating behaviors essential for survival and locomotion, but how it integrates internal needs and transmits locomotion commands to the spinal cord (SC) remains unclear. We found that glutamatergic neurons in lateral hypothalamic area (LHA) are essential for regulating motivated locomotor activity. Using single-neuron projectome analysis, trans-synaptic tracing, and optogenetic manipulation, we showed that LHA facilitates motivated locomotion during food seeking via pontine reticular nucleus, oral part (PnO) projection neurons, rather than direct SC projections or indirect stress signaling via medial septum and diagonal band. Activating PnO-SC projection neurons also initiated locomotion. Importantly, LHA-PnO projection neurons were crucial for regulating locomotor recovery following mouse spinal cord injury (SCI). Motor cortex signals gated LHA deep brain stimulation treatment markedly promoted long-term restoration of hindlimb motor functions after severe SCI. Thus, we have identified a hypothalamic-pontine-spinal pathway and the stimulation paradigm for potential therapeutic intervention after SCI.

Objective. Infrared neural stimulation (INS) has emerged as a potent neuromodulation technology, offering safe and focal stimulation with superior spatial recruitment profiles compared to conventional electrical methods. However, the neural dynamics induced by INS stimulation remain poorly understood. Elucidating these dynamics will help develop new INS stimulation paradigms and advance its clinical application. Approach. In this study, we assessed the local network dynamics of INS entrainment in the auditory thalamocortical circuit using the chronically implanted rat model. Our approach focused on measuring INS energy-based local field potential (LFP) recruitment induced by focal thalamocortical stimulation. We further characterized linear and nonlinear oscillatory LFP activity in response to single-pulse and periodic INS and performed spectral decomposition to uncover specific LFP band entrainment to INS. Finally, we examined spike-field transformations across the thalamocortical synapse using spike-LFP coherence coupling measures. Main results. We found that INS significantly increases LFP amplitude as a log-linear function of INS energy per pulse, primarily entraining to LFP β and γ bands with synchrony extending to 200 Hz in some cases. A subset of neurons demonstrated nonlinear, chaotic oscillations linked to information transfer across cortical circuits. Finally, we utilized spike-field coherences to correlate spike coupling to LFP frequency band activity and suggest an energy-dependent model of network activation resulting from INS stimulation. Significance. We show that INS reliably drives robust network activity and can potently modulate cortical field potentials across a wide range of frequencies in a stimulus parameter-dependent manner. Based on these results, we propose design principles for developing full coverage, all-optical thalamocortical auditory neuroprostheses.

Objectives: Following deafness, a cochlear implant (CI) can be used for the restoration of hearing. CI effectiveness relies on the condition of the auditory nerve, which typically degenerates after deafness. The nerve’s condition can be assessed with the electrically evoked compound action potential (eCAP), the whole-nerve response to an electric pulse. Changes in the eCAP following an increase in the inter-phase gap (IPG) of a biphasic pulse have been reported to be informative of neural survival. This IPG effect can be explained by the temporal separation of the hyperpolarizing phase from the depolarizing phase. We hypothesize that increasing the phase duration (PD) has a similar effect. Design: We investigated the PD effect in normal hearing and in ototoxically deafened guinea pigs (total N = 40) with various conditions of the auditory nerve by recording eCAPs to biphasic current pulses with alternating polarity and with varying PD and IPG. The eCAP data were obtained from both chronically and acutely implanted guinea pigs by using CI stimulation paradigms with a fixed charge and varying PD (30, 50, or 100 µs) and IPG (2.1 or 30 µs). We evaluated six eCAP measures: five derived from the amplitude growth function and the N 1 latency. We examined the relationships of PD and IPG effects with the survival of the spiral ganglion cells. Results: The PD effects were stronger for latency than IPG effects, but weaker for the other five evaluated eCAP measures. The PD effect did not correlate as well with neural survival as the IPG effect. The IPG effect decreased with increasing PD, and accordingly, the stronger correlations between the IPG effect and neural survival were found for a short PD. Notably, the latency increase with increasing PD was greater than 1, which indicates the second phase of the pulse significantly contributes to the eCAP. This second-phase contribution was larger for lower neural survival. Conclusions: The PD effect of latency has predictive power in assessing neural survival. When using other eCAP measures than latency, the best approach to assess neural survival is using the IPG effect with a short PD (around 30 µs).

The thalamocortical pathways exhibit neuroplasticity not only during the critical period but also in adulthood. In this study, we investigated how cholecystokinin (CCK) modulates age-dependent thalamocortical plasticity. Our findings demonstrated that CCK is expressed in thalamocortical neurons and that high-frequency stimulation (HFS) of the thalamocortical pathway triggers the release of CCK in auditory cortex (ACx), as detected by a CCK sensor. HFS of the medial geniculate body (MGB) induced thalamocortical long-term potentiation (LTP) in wild-type young adult mice. However, knockdown of CCK expression in MGB neurons or blockade of the CCK-B receptor (CCKBR) in the ACx abolished HFS-induced LTP. Interestingly, this LTP could not be elicited in juvenile (3-week-old) or aged mice (over 18-month-old) due to distinct mechanisms: the absence of CCKBR in juveniles and the inability to release CCK in aged mice. Notably, exogenous administration of CCK into the ACx rescued LTP in aged mice and significantly improved frequency discrimination. These findings highlight the potential of CCK as a therapeutic intervention for ameliorating neuroplasticity deficits associated with thalamocortical connectivity.