Effects of auricular vagus nerve stimulation and electrical earlobe stimulation on motor-evoked potential changes induced by paired associative stimulation.

Paired associative stimulation (PAS) is a combination of transcranial magnetic stimulation (TMS) and peripheral nerve stimulation (PNS). PAS can induce long-term potentiation (LTP)-like plasticity in humans, manifested as motor-evoked potential (MEP) enhancement. We have developed a variant of PAS ("high-PAS"), which consists of high-frequency PNS and high-intensity TMS and targets spinal plasticity and promotes rehabilitation after spinal cord injury (SCI). Vagus nerve stimulation (VNS) promotes LTP-like plasticity and enhances recovery in SCI and stroke in humans and animals when combined with repetitive motor training. We combined high-PAS with simultaneous noninvasive transcutaneous auricular VNS (aVNS) to determine if aVNS enhances the extent of PAS-induced MEP amplitude increase. Sixteen healthy participants were stimulated for 20 min in four different sessions (PAS, PAS + aVNS, PAS + shamVNS, and aVNS) in a randomized single-blind setup. MEPs were measured before, immediately after, and at 30, 60, and 90 min post-stimulation. Stimulation protocols with PAS significantly potentiated MEPs (p = 0.005) when compared with aVNS (p = 0.642). Although not significant, MEP enhancement observed after PAS (43.5%) is further increased by aVNS (49.7%) and electrical earlobe stimulation (63.9%). Our aVNS setup failed to significantly enhance the effect of PAS, but sham VNS revealed a trend towards enhanced plasticity. Optimization of auricular VNS stimulation setup is required for possible tests of patients with SCI.

Restoration of Somatosensory Function by Pairing Vagus Nerve Stimulation with Tactile Rehabilitation.

OBJECTIVE: Sensory dysfunction is a common consequence of many forms of neurological injury, including stroke and nerve damage. Rehabilitative paradigms that incorporate sensory retraining can provide modest benefits, but the majority of patients are left with lasting sensory loss. We have developed a novel strategy that uses closed-loop vagus nerve stimulation (VNS) paired with tactile rehabilitation to enhance synaptic plasticity and facilitate recovery of sensory function. METHODS: A clinical case report provides initial evidence that a similar implementation of closed-loop VNS paired with a tactile rehabilitation regimen could improve recovery of somatosensory function. Here, we sought to build on these promising initial clinical data and rigorously evaluate the ability of VNS paired with tactile rehabilitation to improve recovery in an animal model of chronic sensory loss. The study design, including planned sample size, assessments, and statistical comparisons, was preregistered prior to beginning data collection (https://osf.io/xsnj5/). RESULTS: VNS paired with tactile rehabilitation resulted in a significant and nearly complete recovery of mechanosensory withdrawal thresholds. Equivalent tactile rehabilitation without VNS failed to improve sensory function. This VNS-dependent restoration of sensory thresholds was maintained for several months after the cessation of stimulation, illustrating long-term benefits. Moreover, VNS paired with tactile rehabilitation resulted in significant generalized improvements in other measures of sensorimotor forepaw function. INTERPRETATION: Given the safety and tolerability of VNS therapy, these findings suggest that incorporating VNS paired with sensory retraining into rehabilitative regimens may represent a fundamentally new method to increase recovery of sensory function after neurological injury. ANN NEUROL 2020;87:194-205.

Vagus nerve stimulation promotes extinction generalization across sensory modalities.

Traumatic experiences involve complex sensory information, and individuals with trauma-related psychological disorders, such as posttraumatic stress disorder (PTSD), can exhibit abnormal fear to numerous different stimuli that remind them of the trauma. Vagus nerve stimulation (VNS) enhances extinction of auditory fear conditioning in rat models for PTSD. We recently found that VNS-paired extinction can also promote extinction generalization across different auditory cues. Here we tested whether VNS can enhance extinction of olfactory fear and promote extinction generalization across auditory and olfactory sensory modalities. Male Sprague Dawley rats were implanted with a stimulating cuff on the cervical vagus nerve. Rats then received two days of fear conditioning where olfactory (amyl acetate odor) and auditory (9 kHz tones) stimuli were concomitantly paired with footshock. Twenty-four hours later, rats were given three days of sham or VNS-paired extinction (5 stimulations, 30-sec trains at 0.4 mA) overlapping with presentation of either the olfactory or the auditory stimulus. Two days later, rats were given an extinction retention test where avoidance of the olfactory stimulus or freezing to the auditory stimulus were measured. VNS-paired with exposure to the olfactory stimulus during extinction reduced avoidance of the odor in the retention test. VNS-paired with exposure to the auditory stimulus during extinction also decreased avoidance of the olfactory cue, and VNS paired with exposure to the olfactory stimulus during extinction reduced freezing when the auditory stimulus was presented in the retention test. These results indicate that VNS enhances extinction of olfactory fear and promotes extinction generalization across different sensory modalities. Extinction generalization induced by VNS may therefore improve outcomes of exposure-based therapies.

Correction to: A suite of automated tools to quantify hand and wrist motor function after cervical spinal cord injury.

The original article [1] contains several errors which the authors would like to rectify.

Pairing vagus nerve stimulation with tones drives plasticity across the auditory pathway.

Previous studies have demonstrated that pairing vagus nerve stimulation (VNS) with sounds can enhance the primary auditory cortex (A1) response to the paired sound. The neural response to sounds following VNS-sound pairing in other subcortical and cortical auditory fields has not been documented. We predicted that VNS-tone pairing would increase neural responses to the paired tone frequency across the auditory pathway. In this study, we paired VNS with the presentation of a 9-kHz tone 300 times a day for 20 days. We recorded neural responses to tones from 2,950 sites in the inferior colliculus (IC), A1, anterior auditory field (AAF), and posterior auditory field (PAF) 24 h after the last pairing session in anesthetized rats. We found that VNS-tone pairing increased the percentage of IC, A1, AAF, and PAF that responds to the paired tone frequency. Across all tested auditory fields, the response strength to tones was strengthened in VNS-tone paired rats compared with control rats. VNS-tone pairing reduced spontaneous activity, frequency selectivity, and response threshold across the auditory pathway. This is the first study to document both cortical and subcortical plasticity following VNS-sound pairing. Our findings suggest that VNS paired with sound presentation is an effective method to enhance auditory processing.NEW & NOTEWORTHY Previous studies have reported primary auditory cortex plasticity following vagus nerve stimulation (VNS) paired with a sound. This study extends previous findings by documenting that fields across the auditory pathway are altered by VNS-tone pairing. VNS-tone pairing increases the percentage of each field that responds to the paired tone frequency. This is the first study to document both cortical and subcortical plasticity following VNS-sound pairing.

Vagus nerve stimulation reverses the extinction impairments in a model of PTSD with prolonged and repeated trauma.

We have shown that vagus nerve stimulation (VNS) enhances extinction of conditioned fear and reduces anxiety in rat models of PTSD using moderate stress. However, it is still unclear if VNS can be effective in enhancing extinction of severe fear after prolonged and repeated trauma. Severe fear was induced in adult male rats by combining single prolonged stress (SPS) and protracted aversive conditioning (PAC). After SPS and PAC procedures, rats were implanted with stimulating cuff electrodes, exposed to five days of extinction training with or without VNS, and then tested for extinction retention, return of fear in a new context and reinstatement. The elevated plus maze, open field and startle were used to test anxiety. Sham rats showed no reduction of fear during extensive extinction training. VNS-paired with extinction training reduced freezing at the last extinction session by 70% compared to sham rats. VNS rats exhibited half as much fear as shams, as well as less fear renewal. Sham rats exhibited significantly more anxiety than naive controls, whereas VNS rats did not. These results demonstrate that VNS enhances extinction and reduces anxiety in a severe model of PTSD that combined SPS and a conditioning procedure that is 30 times more intense than the conditioning procedures in previous VNS studies. The broad utility of VNS in enhancing extinction learning in rats and the strong clinical safety record of VNS suggest that VNS holds promise as an adjuvant to exposure-based therapy in people with PTSD and other complex forms of this condition.

A suite of automated tools to quantify hand and wrist motor function after cervical spinal cord injury.

BACKGROUND: Cervical spinal cord injury (cSCI) often causes chronic upper extremity disability. Reliable measurement of arm function is critical for development of therapies to improve recovery after cSCI. In this study, we report a suite of automated rehabilitative tools to allow simple, quantitative assessment of hand and wrist motor function. METHODS: We measured range of motion and force production using these devices in cSCI participants with a range of upper limb disability and in neurologically intact participants at two time points separated by approximately 4 months. Additionally, we determined whether measures collected with the rehabilitative tools correlated with standard upper limb assessments, including the Graded Redefined Assessment of Strength, Sensibility, and Prehension (GRASSP) and the Jebsen Hand Function Test (JHFT). RESULTS: We find that the rehabilitative devices are useful to provide assessment of upper limb function in physical units over time in SCI participants and are well-correlated with standard assessments. CONCLUSIONS: These results indicate that these tools represent a reliable system for longitudinal evaluation of upper extremity function after cSCI and may provide a framework to assess the efficacy of strategies aimed at improving recovery of upper limb function.

Vagus Nerve Stimulation Enhances Stable Plasticity and Generalization of Stroke Recovery.

BACKGROUND AND PURPOSE: Chronic impairment of the arm and hand is a common consequence of stroke. Animal and human studies indicate that brief bursts of vagus nerve stimulation (VNS) in conjunction with rehabilitative training improve recovery of motor function after stroke. In this study, we tested whether VNS could promote generalization, long-lasting recovery, and structural plasticity in motor networks. METHODS: Rats were trained on a fully automated, quantitative task that measures forelimb supination. On task proficiency, unilateral cortical and subcortical ischemic lesions were administered. One week after ischemic lesion, rats were randomly assigned to receive 6 weeks of rehabilitative training on the supination task with or without VNS. Rats then underwent 4 weeks of testing on a task assessing forelimb strength to test generalization of recovery. Finally, the durability of VNS benefits was tested on the supination task 2 months after the cessation of VNS. After the conclusion of behavioral testing, viral tracing was performed to assess synaptic connectivity in motor networks. RESULTS: VNS enhances plasticity in corticospinal motor networks to increase synaptic connectivity to musculature of the rehabilitated forelimb. Adding VNS more than doubled the benefit of rehabilitative training, and the improvements lasted months after the end of VNS. Pairing VNS with supination training also significantly improved performance on a similar, but untrained task that emphasized volitional forelimb strength, suggesting generalization of forelimb recovery. CONCLUSIONS: This study provides the first evidence that VNS paired with rehabilitative training after stroke (1) doubles long-lasting recovery on a complex task involving forelimb supination, (2) doubles recovery on a simple motor task that was not paired with VNS, and (3) enhances structural plasticity in motor networks.

Knockdown of Dyslexia-Gene Dcdc2 Interferes with Speech Sound Discrimination in Continuous Streams.

UNLABELLED: Dyslexia is the most common developmental language disorder and is marked by deficits in reading and phonological awareness. One theory of dyslexia suggests that the phonological awareness deficit is due to abnormal auditory processing of speech sounds. Variants in DCDC2 and several other neural migration genes are associated with dyslexia and may contribute to auditory processing deficits. In the current study, we tested the hypothesis that RNAi suppression of Dcdc2 in rats causes abnormal cortical responses to sound and impaired speech sound discrimination. In the current study, rats were subjected in utero to RNA interference targeting of the gene Dcdc2 or a scrambled sequence. Primary auditory cortex (A1) responses were acquired from 11 rats (5 with Dcdc2 RNAi; DC-) before any behavioral training. A separate group of 8 rats (3 DC-) were trained on a variety of speech sound discrimination tasks, and auditory cortex responses were acquired following training. Dcdc2 RNAi nearly eliminated the ability of rats to identify specific speech sounds from a continuous train of speech sounds but did not impair performance during discrimination of isolated speech sounds. The neural responses to speech sounds in A1 were not degraded as a function of presentation rate before training. These results suggest that A1 is not directly involved in the impaired speech discrimination caused by Dcdc2 RNAi. This result contrasts earlier results using Kiaa0319 RNAi and suggests that different dyslexia genes may cause different deficits in the speech processing circuitry, which may explain differential responses to therapy. SIGNIFICANCE STATEMENT: Although dyslexia is diagnosed through reading difficulty, there is a great deal of variation in the phenotypes of these individuals. The underlying neural and genetic mechanisms causing these differences are still widely debated. In the current study, we demonstrate that suppression of a candidate-dyslexia gene causes deficits on tasks of rapid stimulus processing. These animals also exhibited abnormal neural plasticity after training, which may be a mechanism for why some children with dyslexia do not respond to intervention. These results are in stark contrast to our previous work with a different candidate gene, which caused a different set of deficits. Our results shed some light on possible neural and genetic mechanisms causing heterogeneity in the dyslexic population.

Cerebrolysin prevents deficits in social behavior, repetitive conduct, and synaptic inhibition in a rat model of autism.

Autism spectrum disorder (ASD) is a syndrome of diverse neuropsychiatric diseases of growing incidence characterized by repetitive conduct and impaired social behavior and communication for which effective pharmacological treatment is still unavailable. While the mechanisms and etiology of ASD are still unknown, a consensus is emerging about the synaptic nature of the syndrome, suggesting a possible avenue for pharmacological treatment with synaptogenic compounds. The peptidic mixture cerebrolysin (CBL) has been successfully used during the last three decades in the treatment of stroke and neurodegenerative disease. Animal experiments indicate that at least one possible mechanism of action of CBL is through neuroprotection and/or synaptogenesis. In the present study, we tested the effect of CBL treatment (daily injection of 2.5 mL/Kg i.p. during 15 days) on a rat model of ASD. This was based on the offspring (43 male and 51 female pups) of a pregnant female rat injected with valproic acid (VPA, 600 mg/Kg) at the embryonic day 12.5, which previous work has shown to display extensive behavioral, as well as synaptic impairment. Comparison between saline vs. CBL-injected VPA animals shows that CBL treatment improves behavioral as well as synaptic impairments, measured by behavioral performance (social interaction, Y-maze, plus-maze), maximal response of inhibitory γ-amino butyric acid type A receptor (GABAA R)-mediated synaptic currents, as well as their kinetic properties and adrenergic and muscarinic modulation. We speculate that CBL might be a viable and effective candidate for pharmacological treatment or co-treatment of ASD patients. © 2017 Wiley Periodicals, Inc.

Median and ulnar nerve injuries reduce volitional forelimb strength in rats.

INTRODUCTION: Peripheral nerve injuries (PNI) are among the leading causes of physical disability in the United States. The majority of injuries occur in the upper extremities, and functional recovery is often limited. Robust animal models are critical first steps for developing effective therapies to restore function after PNI. METHODS: We developed an automated behavioral assay that provides quantitative measurements of volitional forelimb strength in rats. Multiple forelimb PNI models involving the median and ulnar nerves were used to assess forelimb function for up to 13 weeks postinjury. RESULTS: Despite multiple weeks of task-oriented training following injury, rats exhibit significant reductions in multiple quantitative parameters of forelimb function, including maximal pull force and speed of force generation. DISCUSSION: This study demonstrates that the isometric pull task is an effective method of evaluating forelimb function following PNI and may aid in development of therapeutic interventions to restore function. Muscle Nerve 56: 1149-1154, 2017.

Reversing pathological neural activity using targeted plasticity.

Brain changes in response to nerve damage or cochlear trauma can generate pathological neural activity that is believed to be responsible for many types of chronic pain and tinnitus. Several studies have reported that the severity of chronic pain and tinnitus is correlated with the degree of map reorganization in somatosensory and auditory cortex, respectively. Direct electrical or transcranial magnetic stimulation of sensory cortex can temporarily disrupt these phantom sensations. However, there is as yet no direct evidence for a causal role of plasticity in the generation of pain or tinnitus. Here we report evidence that reversing the brain changes responsible can eliminate the perceptual impairment in an animal model of noise-induced tinnitus. Exposure to intense noise degrades the frequency tuning of auditory cortex neurons and increases cortical synchronization. Repeatedly pairing tones with brief pulses of vagus nerve stimulation completely eliminated the physiological and behavioural correlates of tinnitus in noise-exposed rats. These improvements persisted for weeks after the end of therapy. This method for restoring neural activity to normal may be applicable to a variety of neurological disorders.

Safety, Feasibility, and Efficacy of Vagus Nerve Stimulation Paired With Upper-Limb Rehabilitation After Ischemic Stroke.

BACKGROUND AND PURPOSE: Recent animal studies demonstrate that vagus nerve stimulation (VNS) paired with movement induces movement-specific plasticity in motor cortex and improves forelimb function after stroke. We conducted a randomized controlled clinical pilot study of VNS paired with rehabilitation on upper-limb function after ischemic stroke. METHODS: Twenty-one participants with ischemic stroke >6 months before and moderate to severe upper-limb impairment were randomized to VNS plus rehabilitation or rehabilitation alone. Rehabilitation consisted of three 2-hour sessions per week for 6 weeks, each involving >400 movement trials. In the VNS group, movements were paired with 0.5-second VNS. The primary objective was to assess safety and feasibility. Secondary end points included change in upper-limb measures (including the Fugl-Meyer Assessment-Upper Extremity). RESULTS: Nine participants were randomized to VNS plus rehabilitation and 11 to rehabilitation alone. There were no serious adverse device effects. One patient had transient vocal cord palsy and dysphagia after implantation. Five had minor adverse device effects including nausea and taste disturbance on the evening of therapy. In the intention-to-treat analysis, the change in Fugl-Meyer Assessment-Upper Extremity scores was not significantly different (between-group difference, 5.7 points; 95% confidence interval, -0.4 to 11.8). In the per-protocol analysis, there was a significant difference in change in Fugl-Meyer Assessment-Upper Extremity score (between-group difference, 6.5 points; 95% confidence interval, 0.4 to 12.6). CONCLUSIONS: This study suggests that VNS paired with rehabilitation is feasible and has not raised safety concerns. Additional studies of VNS in adults with chronic stroke will now be performed. CLINICAL TRIAL REGISTRATION: URL: https://www.clinicaltrials.gov. Unique identifier: NCT01669161.

Environmental acoustic enrichment promotes recovery from developmentally degraded auditory cortical processing.

It has previously been shown that environmental enrichment can enhance structural plasticity in the brain and thereby improve cognitive and behavioral function. In this study, we reared developmentally noise-exposed rats in an acoustic-enriched environment for ∼4 weeks to investigate whether or not enrichment could restore developmentally degraded behavioral and neuronal processing of sound frequency. We found that noise-exposed rats had significantly elevated sound frequency discrimination thresholds compared with age-matched naive rats. Environmental acoustic enrichment nearly restored to normal the behavioral deficit resulting from early disrupted acoustic inputs. Signs of both degraded frequency selectivity of neurons as measured by the bandwidth of frequency tuning curves and decreased long-term potentiation of field potentials recorded in the primary auditory cortex of these noise-exposed rats also were reversed partially. The observed behavioral and physiological effects induced by enrichment were accompanied by recovery of cortical expressions of certain NMDA and GABAA receptor subunits and brain-derived neurotrophic factor. These studies in a rodent model show that environmental acoustic enrichment promotes recovery from early noise-induced auditory cortical dysfunction and indicate a therapeutic potential of this noninvasive approach for normalizing neurological function from pathologies that cause hearing and associated language impairments in older children and adults.

Degraded neural and behavioral processing of speech sounds in a rat model of Rett syndrome.

Individuals with Rett syndrome have greatly impaired speech and language abilities. Auditory brainstem responses to sounds are normal, but cortical responses are highly abnormal. In this study, we used the novel rat Mecp2 knockout model of Rett syndrome to document the neural and behavioral processing of speech sounds. We hypothesized that both speech discrimination ability and the neural response to speech sounds would be impaired in Mecp2 rats. We expected that extensive speech training would improve speech discrimination ability and the cortical response to speech sounds. Our results reveal that speech responses across all four auditory cortex fields of Mecp2 rats were hyperexcitable, responded slower, and were less able to follow rapidly presented sounds. While Mecp2 rats could accurately perform consonant and vowel discrimination tasks in quiet, they were significantly impaired at speech sound discrimination in background noise. Extensive speech training improved discrimination ability. Training shifted cortical responses in both Mecp2 and control rats to favor the onset of speech sounds. While training increased the response to low frequency sounds in control rats, the opposite occurred in Mecp2 rats. Although neural coding and plasticity are abnormal in the rat model of Rett syndrome, extensive therapy appears to be effective. These findings may help to explain some aspects of communication deficits in Rett syndrome and suggest that extensive rehabilitation therapy might prove beneficial.

Activation of the anti-inflammatory reflex blocks lipopolysaccharide-induced decrease in synaptic inhibition in the temporal cortex of the rat.

Stress is a potential trigger for a number of neuropsychiatric conditions, including anxiety syndromes and schizophrenic psychoses. The temporal neocortex is a stress-sensitive area involved in the development of such conditions. We have recently shown that aseptic inflammation and mild electric shock shift the balance between synaptic excitation and synaptic inhibition in favor of the former in this brain area (Garcia-Oscos et al., 2012), as well as in the prefrontal cortex (Garcia-Oscos et al., 2014). Given the potential clinical importance of this phenomenon in the etiology of hyperexcitable neuropsychiatric illness, this study investigates whether inactivation of the peripheral immune system by the "anti-inflammatory reflex" would reduce the central response to aseptic inflammation. For a model of aseptic inflammation, this study used i.p. injections of the bacterial toxin lipopolysaccharide (LPS; 5 µM) and activated the anti-inflammatory reflex either pharmacologically by i.p. injections of the nicotinic α7 receptor agonist PHA543613 or physiologically through electrical stimulation of the left vagal nerve (VNS). Patch-clamp recording was used to monitor synaptic function. Recordings from LPS-injected Sprague Dawley rats show that activation of the anti-inflammatory reflex either pharmacologically or by VNS blocks or greatly reduces the LPS-induced decrease of the synaptic inhibitory-to-excitatory ratio and the saturation level of inhibitory current input-output curves. Given the ample variety of pharmacologically available α7 nicotinic receptor agonists as well as the relative safety of clinical VNS already approved by the FDA for the treatment of epilepsy and depression, our findings suggest a new therapeutic avenue in the treatment of stress-induced hyperexcitable conditions mediated by a decrease in synaptic inhibition in the temporal cortex.

Vagal nerve stimulation blocks interleukin 6-dependent synaptic hyperexcitability induced by lipopolysaccharide-induced acute stress in the rodent prefrontal cortex.

The ratio between synaptic inhibition and excitation (sI/E) is a critical factor in the pathophysiology of neuropsychiatric disease. We recently described a stress-induced interleukin-6 dependent mechanism leading to a decrease in sI/E in the rodent temporal cortex. The aim of the present study was to determine whether a similar mechanism takes place in the prefrontal cortex, and to elaborate strategies to prevent or attenuate it. We used aseptic inflammation (single acute injections of lipopolysaccharide, LPS, 10mg/kg) as stress model, and patch-clamp recording on a prefrontal cortical slice preparation from wild-type rat and mice, as well as from transgenic mice in which the inhibitor of IL-6 trans-signaling sgp130Fc was produced in a brain-specific fashion (sgp130Fc mice). The anti-inflammatory reflex was activated either by vagal nerve stimulation or peripheral administration of the nicotinic α7 receptor agonist PHA543613. We found that the IL-6-dependent reduction in prefrontal cortex synaptic inhibition was blocked in sgp130Fc mice, or - in wild-type animals - upon application sgp130Fc. Similar results were obtained by activating the "anti-inflammatory reflex" - a neural circuit regulating peripheral immune response - by stimulation of the vagal nerve or through peripheral administration of the α7 nicotinic receptor agonist PHA543613. Our results indicate that the prefrontal cortex is an important potential target of IL-6 mediated trans-signaling, and suggest a potential new avenue in the treatment of a large class of hyperexcitable neuropsychiatric conditions, including epilepsy, schizophrenic psychoses, anxiety disorders, autism spectrum disorders, and depression.

Vagus nerve stimulation during rehabilitative training improves functional recovery after intracerebral hemorrhage.

BACKGROUND AND PURPOSE: Vagus nerve stimulation (VNS) delivered during rehabilitative training enhances neuroplasticity and improves recovery in models of cortical ischemic stroke. However, VNS therapy has not been applied in a model of subcortical intracerebral hemorrhage (ICH). We hypothesized that VNS paired with rehabilitative training after ICH would enhance recovery of forelimb motor function beyond rehabilitative training alone. METHODS: Rats were trained to perform an automated, quantitative measure of forelimb function. Once proficient, rats received an intrastriatal injection of bacterial collagenase to induce ICH. Rats then underwent VNS paired with rehabilitative training (VNS+Rehab; n=14) or rehabilitative training without VNS (Rehab; n=12). Rehabilitative training began ≥9 days after ICH and continued for 6 weeks. RESULTS: VNS paired with rehabilitative training significantly improved recovery of forelimb function when compared with rehabilitative training without VNS. The VNS+Rehab group displayed a 77% recovery of function, whereas the Rehab group only exhibited 29% recovery. Recovery was sustained after cessation of stimulation. Both groups performed similar amounts of trials during rehabilitative, and lesion size was not different between groups. CONCLUSIONS: VNS paired with rehabilitative training confers significantly improved forelimb recovery after ICH compared to rehabilitative training without VNS.

Behavioral and neural discrimination of speech sounds after moderate or intense noise exposure in rats.

OBJECTIVES: Hearing loss is a commonly experienced disability in a variety of populations including veterans and the elderly and can often cause significant impairment in the ability to understand spoken language. In this study, we tested the hypothesis that neural and behavioral responses to speech will be differentially impaired in an animal model after two forms of hearing loss. DESIGN: Sixteen female Sprague-Dawley rats were exposed to one of two types of broadband noise which was either moderate or intense. In nine of these rats, auditory cortex recordings were taken 4 weeks after noise exposure (NE). The other seven were pretrained on a speech sound discrimination task prior to NE and were then tested on the same task after hearing loss. RESULTS: Following intense NE, rats had few neural responses to speech stimuli. These rats were able to detect speech sounds but were no longer able to discriminate between speech sounds. Following moderate NE, rats had reorganized cortical maps and altered neural responses to speech stimuli but were still able to accurately discriminate between similar speech sounds during behavioral testing. CONCLUSIONS: These results suggest that rats are able to adjust to the neural changes after moderate NE and discriminate speech sounds, but they are not able to recover behavioral abilities after intense NE. Animal models could help clarify the adaptive and pathological neural changes that contribute to speech processing in hearing-impaired populations and could be used to test potential behavioral and pharmacological therapies.

Different timescales for the neural coding of consonant and vowel sounds.

Psychophysical, clinical, and imaging evidence suggests that consonant and vowel sounds have distinct neural representations. This study tests the hypothesis that consonant and vowel sounds are represented on different timescales within the same population of neurons by comparing behavioral discrimination with neural discrimination based on activity recorded in rat inferior colliculus and primary auditory cortex. Performance on 9 vowel discrimination tasks was highly correlated with neural discrimination based on spike count and was not correlated when spike timing was preserved. In contrast, performance on 11 consonant discrimination tasks was highly correlated with neural discrimination when spike timing was preserved and not when spike timing was eliminated. These results suggest that in the early stages of auditory processing, spike count encodes vowel sounds and spike timing encodes consonant sounds. These distinct coding strategies likely contribute to the robust nature of speech sound representations and may help explain some aspects of developmental and acquired speech processing disorders.