Impaired Discrimination of Electrocutaneous Stimulation in the Paretic Hand of Stroke Survivors.

Artificial sensory feedback via electrocutaneous stimulation can be used to assist or rehabilitate stroke survivors with sensory deficits. Conveying the magnitude of tactile stimuli is an important aspect of artificial sensory feedback. Here, we explore how stroke-related sensory deficits impact the ability of electrocutaneous stimulation to convey the magnitude of tactile stimuli. Using classical psychophysical methods, we quantified the threshold of detection and the just-noticeable difference of electrocutaneous stimulation current in five stroke survivors with unilateral sensory deficits. We show significantly greater (40%) stimulation currents are needed for initial perception on the paretic hand compared to the non-paretic hand. We also show significantly greater percent changes in stimulation current (140%) are needed for reliable incremental perception on the paretic hand compared to the non-paretic hand. Lastly, we show little correlation between electrocutaneous discrimination performance and clinical sensory assessments of light-touch and spatial mechanoperception. These findings can help guide the implementation of artificial sensory feedback as an assistive or rehabilitative intervention for individuals experiencing sensory loss after a stroke.Clinical Relevance- Our results can help guide the implementation of electrical stimulation as an assistive or rehabilitative intervention for individuals with sensory loss after stroke.

Delayed Muscle Activity in Stroke Survivors with Upper-Limb Hemiparesis.

Stroke is the leading cause of disability worldwide, and nearly 80% of stroke survivors suffer from upper-limb hemiparesis. Myoelectric exoskeletons can restore dexterity and independence to stroke survivors with upper-limb hemiparesis. However, the ability of patients to dexterously control myoelectric exoskeletons is limited by an incomplete understanding of the electromyographic (EMG) hallmarks of hemiparesis, such as muscle weakness and spasticity. Here we show that stroke survivors with upper-limb hemiparesis suffer from delayed voluntary muscle contraction and delayed muscle relaxation. We quantified the time constants of EMG activity associated with initiating and terminating voluntary hand grasps and extensions for both the paretic and non-paretic hands of stroke survivors. We found that the initiation and termination time constants were greater on the paretic side for both hand grasps and hand extensions. Notably, the initiation time constant during hand extension was approximately three times longer for the paretic hand than for the contralateral non-paretic hand (0.618 vs 0.189 s). We also show a positive correlation between the initiation and termination time constants and clinical scores on the Modified Ashworth Scale. The difficulty stroke survivors have in efficiently modulating their EMG presents a challenge for appropriate control of assistive myoelectric devices, such as exoskeletons. This work constitutes an important step towards understanding EMG differences after stroke and how to accommodate these EMG differences in assistive myoelectric devices. Real-time quantitative biofeedback of EMG time constants may also have broad implications for guiding rehabilitation and monitoring patient recovery.Clinical Relevance- After a stroke, muscle activity changes, and these changes make it difficult to use muscle activity to drive assistive and rehabilitative technologies. We identified slower muscle contraction and muscle relaxation as a key difference in muscle activity after a stroke. This quantifiable difference in muscle activity can be used to develop better assistive technologies, guide rehabilitation, and monitor patient recovery.

Automated Quantifiable Assessments of Sensorimotor Function Using an Instrumented Fragile Object.

Accurate assessment of hand dexterity plays a critical role in informing rehabilitation and care of upper-limb hemiparetic stroke patients. Common upper-limb assessments, such as the Box and Blocks Test and Nine Hole Peg Test, primarily evaluate gross motor function in terms of speed. These assessments neglect an individual's ability to finely regulate grip force, which is critical in activities of daily living, such as manipulating fragile objects. Here we present the Electronic Grip Gauge (EGG), an instrumented fragile object that assesses both gross and fine motor function. Embedded with a load cell, accelerometer, and Hall-effect sensor, the EGG measures grip force, acceleration, and relative position (via magnetic fields) in real time. The EGG can emit an audible "break" sound when the applied grip force exceeds a threshold. The number of breaks, transfer duration, and applied forces are automatically logged in real-time. Using the EGG, we evaluated sensorimotor function in implicit grasping and gentle grasping for the non-paretic and paretic hands of 3 hemiparetic stroke patients. For all participants, the paretic hand took longer to transfer the EGG during implicit grasping. For 2 of 3 participants, grip forces were significantly greater for the paretic hand during gentle grasping. Differences in implicit grasping forces were unique to each participant. This work constitutes an important step towards more widespread and quantitative measures of sensorimotor function, which may ultimately lead to improved personalized rehabilitation and better patient outcomes.

Non-invasive central nervous system assessment of a porcine model of neuropathic pain demonstrates increased latency of somatosensory-evoked potentials.

BACKGROUND: The study of chronic pain and its treatments requires a robust animal model with objective and quantifiable metrics. Porcine neuropathic pain models have been assessed with peripheral pain recordings and behavioral responses, but thus far central nervous system electrophysiology has not been investigated. This work aimed to record non-invasive, somatosensory-evoked potentials (SEPs) via electroencephalography in order to quantitatively assess chronic neuropathic pain induced in a porcine model. NEW METHOD: Peripheral neuritis trauma (PNT) was induced unilaterally in the common peroneal nerve of domestic farm pigs, with the contralateral leg serving as the control for each animal. SEPs were generated by stimulation of the peripheral nerves distal to the PNT and were recorded non-invasively using transcranial electroencephalography (EEG). The P30 wave of the SEP was analyzed for latency changes. RESULTS: P30 SEPs were successfully recorded with non-invasive EEG. PNT resulted in significantly longer P30 SEP latencies (p < 0.01 [n = 8]) with a median latency increase of 14.3 [IQR 5.0 - 17.5] ms. Histological results confirmed perineural inflammatory response and nerve damage around the PNT nerves. COMPARISON WITH EXISTING METHOD(S): Control P30 SEPs were similar in latency and amplitude to those previously recorded invasively in healthy pigs. Non-invasive recordings have numerous advantages over invasive measures. CONCLUSIONS: P30 SEP latency can serve as a quantifiable neurological measure that reflects central nervous system processing in a porcine model of chronic pain. Advancing the development of a porcine chronic pain model will facilitate the translation of experimental therapies into human clinical trials.

Differential Activation of Canal and Otolith Afferents by Acoustic Tone Bursts in Rats.

Vestibular evoked myogenic potentials (VEMPs) are routinely used to test otolith function, but which specific vestibular afferent neurons and central circuits are activated by auditory frequency VEMP stimuli remains unclear. To examine this question, we analyzed the sensitivity of individual vestibular afferents in adult Sprague-Dawley rats to tone bursts delivered at 9 frequencies (125-4000 Hz) and 3 intensity levels (60, 70, 80 dB SL re: acoustic brainstem response (ABR) threshold). Afferent neuron tone sensitivity was quantified by the cumulative probability of evoking a spike (CPE). Based on a threshold CPE of 0.1, acoustic stimuli in the present study evoked responses in 78.2 % (390/499) of otolith afferent neurons vs. 48.4 % (431/891) of canal afferent neurons. Organ-specific vestibular inputs to the central nervous system in response to tone bursts differ based on intensity and frequency content of the stimulus. At frequencies below 500 Hz, tone bursts primarily activated both otolith afferents, even at the highest intensity tested (80 dB SL re ABR threshold). At 1500 Hz, however, tone bursts activated the canal and otolith afferents at the moderate and high intensities tested (70, 80 dB SL), but activated only otolith afferents at the low intensity tested (60 dB SL). Within an end organ, diversity of sensitivity between individual afferent neurons correlated with spontaneous discharge rate and regularity. Examination of inner ear fluid mechanics in silico suggests that the frequency response and preferential activation of the otolith organs likely arise from inner ear fluid motion trapped near the oval and round windows. These results provide insight into understanding the mechanisms of sound activation of the vestibular system and developing novel discriminative VEMP testing protocols and interpretative guidelines in humans.

Biomechanics of Third Window Syndrome.

Third window syndrome describes a set of vestibular and auditory symptoms that arise when a pathological third mobile window is present in the bony labyrinth of the inner ear. The pathological mobile window (or windows) adds to the oval and round windows, disrupting normal auditory and vestibular function by altering biomechanics of the inner ear. The most commonly occurring third window syndrome arises from superior semicircular canal dehiscence (SSCD), where a section of bone overlying the superior semicircular canal is absent or thinned (near-dehiscence). The presentation of SSCD syndrome is well characterized by clinical audiological and vestibular tests. In this review, we describe how the third compliant window introduced by a SSCD alters the biomechanics of the inner ear and thereby leads to vestibular and auditory symptoms. Understanding the biomechanical origins of SSCD further provides insight into other third window syndromes and the potential of restoring function or reducing symptoms through surgical repair.

Wave Mechanics of the Vestibular Semicircular Canals.

The semicircular canals are biomechanical sensors responsible for detecting and encoding angular motion of the head in 3D space. Canal afferent neurons provide essential inputs to neural circuits responsible for representation of self-position/orientation in space, and to compensatory circuits including the vestibulo-ocular and vestibulo-collic reflex arcs. In this work we derive, to our knowledge, a new 1D mathematical model quantifying canal biomechanics based on the morphology, dynamics of the inner ear fluids, and membranous labyrinth deformability. The model takes the form of a dispersive wave equation and predicts canal responses to angular motion, sound, and mechanical stimulation. Numerical simulations were carried out for the morphology of the human lateral canal using known physical properties of the endolymph and perilymph in three diverse conditions: surgical plugging, rotation, and mechanical indentation. The model reproduces frequency-dependent attenuation and phase shift in cases of canal plugging. During rotation, duct deformability extends the frequency bandwidth and enhances the high frequency gain. Mechanical indentation of the membranous duct at high frequencies evokes traveling waves that move away from the location of indentation and at low frequencies compels endolymph displacement along the canal. These results demonstrate the importance of the conformal perilymph-filled bony labyrinth to pressure changes and to high frequency sound and vibration.