Stimulus Artifact Research Articles

A Method for Electrical Stimulus Artifact Removal Exploiting Neural Refractoriness: Validation by Contrasting Cathodic and Anodic Stimulation.

To present a novel method for removing stimulus transient that exploits the absolute refractory period of electrically excitable neural tissues. Electrical stimulation often generates significant signal artifacts that can obscure important physiological signals. Removal of the artifact and understanding latent information from these signals could provide objective measures of circuit engagement, potentially driving advancements in neuromodulation research and therapies. We conducted intracranial physiology studies on five consecutive patients with Parkinson's disease who underwent deep brain stimulation (DBS) surgery as part of their routine care. Monopolar stimuli (either cathodic or anodic) were delivered in pairs through the DBS electrode across a range of inter-stimulus intervals. Recordings from adjacent unused electrode contacts used broadband sampling and precise synchronization to generate a robust template for the stimulus transient during the absolute refractory period. These templates of stimulus transient were then subtracted from recordings at different intervals to extract and analyze the residual neural potentials. After artifact removal, the residual signals exhibited absolute and relative refractory periods with timing indicative of neural activity. Cathodic and anodic DBS pulses generated distinct patterns of local tissue activation, showing phase independence from the prior stimulus. The earliest detectable neural responses occurred at short peak latencies (ranging from 0.19 to 0.38 ms post-stimulus) and were completely or partially obscured by the stimulus artifact prior to removal. Cathodic stimuli produced stronger local tissue responses than anodic stimuli, aligning with clinical observations of lower activation thresholds for cathodic stimulation. However, cathodic and anodic pulses induced artifact patterns that were equivalent but opposite. The proposed artifact removal technique enhances prior approaches by allowing direct measurement of local tissue responses without requirements for stimulus polarity reversal, template scaling, or specialized filters. This approach could be integrated into future neuromodulation systems to visualize stimulus-evoked neural potentials that would otherwise be obscured by stimulus artifacts.

Evaluation of mechanomyogram efficacy as a tool for assessing paired-pulse inhibition of blink reflex early R1 component.

Paired-pulse stimulation provides clinically useful information regarding sensory inhibition. When supraorbital nerve stimulation is repeated within a short interval, the response to the second stimulation is reduced to varying degrees. This magnitude of change in stimulation response can be monitored by electromyogram (EMG) or by mechanomyogram (MMG) as in this report. MMG has some advantages such as being less time consuming and lacking stimulus artifact. We compared the use of MMG and EMG to validate MMG as an effective method of assessing blink reflex paired-pulse inhibition. Eight volunteers participated. Participants received electrical stimulation to the supraorbital nerve of each side. A paired-pulse paradigm was employed, varying the conditioning-test interval between 5 and 800 ms. The R1 component of the induced blink reflex was simultaneously recorded by EMG using a pair of electrodes placed on the lower eyelid and by MMG using an accelerometer placed between the electrodes. The correlation coefficient of the R1 amplitude between MMG and EMG of the grand-averaged waveforms was 0.99. The average participant r value was .91 (range .76-.99). Similar analyses were performed for the amplitude variation of the second response relative to the first response. Results correlated well, yielding r values of .97 and .86 for the grand-averaged waveform and the average for each subject. The present results demonstrate that MMG could be an alternative to EMG in assessing paired-pulse inhibition of the electrical blink reflex R1 component.

A distributed, high-channel-count, implanted bidirectional system for restoration of somatosensation and myoelectric control

Objective. We intend to chronically restore somatosensation and provide high-fidelity myoelectric control for those with limb loss via a novel, distributed, high-channel-count, implanted system. Approach. We have developed the implanted Somatosensory Electrical Neurostimulation and Sensing (iSens®) system to support peripheral nerve stimulation through up to 64, 96, or 128 electrode contacts with myoelectric recording from 16, 8, or 0 bipolar sites, respectively. The rechargeable central device has Bluetooth® wireless telemetry to communicate to external devices and wired connections for up to four implanted satellite stimulation or recording devices. We characterized the stimulation, recording, battery runtime, and wireless performance and completed safety testing to support its use in human trials. Results. The stimulator operates as expected across a range of parameters and can schedule multiple asynchronous, interleaved pulse trains subject to total charge delivery limits. Recorded signals in saline show negligible stimulus artifact when 10 cm from a 1 mA stimulating source. The wireless telemetry range exceeds 1 m (direction and orientation dependent) in a saline torso phantom. The bandwidth supports 100 Hz bidirectional update rates of stimulation commands and data features or streaming select full bandwidth myoelectric signals. Preliminary first-in-human data validates the bench testing result. Significance. We developed, tested, and clinically implemented an advanced, modular, fully implanted peripheral stimulation and sensing system for somatosensory restoration and myoelectric control. The modularity in electrode type and number, including distributed sensing and stimulation, supports a wide variety of applications; iSens® is a flexible platform to bring peripheral neuromodulation applications to clinical reality. ClinicalTrials.gov ID NCT04430218.

Removal of electrical stimulus artifact in local field potential recorded from subthalamic nucleus by using manifold denoising

Background:Deep brain stimulation (DBS) is an effective treatment for movement disorders such as Parkinson’s disease (PD). However, local field potentials (LFPs) recorded through lead externalization during high-frequency stimulation (HFS) are contaminated by stimulus artifacts, which require to be removed before further analysis. New Method:In this study, a novel stimulus artifact removal algorithm based on manifold denoising, termed Shrinkage and Manifold-based Artifact Removal using Template Adaptation (SMARTA), was proposed to remove artifacts by deriving a template for each stimulus artifact and subtracting it from the signal. Under a low-dimensional manifold assumption, a matrix denoising technique called optimal shrinkage was applied to design a similarity metric such that the template for stimulus artifacts could be accurately recovered. Result:SMARTA was evaluated using semirealistic signals, which were the combination of semirealistic stimulus artifacts recorded in an agar brain model and LFPs of PD patients with no stimulation, and realistic LFP signals recorded in patients with PD during HFS. The results indicated that SMARTA removes stimulus artifacts with a modest distortion in LFP estimates. Comparison with Existing Methods:SMARTA was compared with moving-average subtraction, sample-and-interpolate technique, and Hampel filtering. Conclusion:The proposed SMARTA algorithm helps the exploration of the neurophysiological mechanisms of DBS effects.

A PCA Based Artifact Removal Algorithm for Neural Signal Acquisition with kS/s Sampling Rate.

The contamination of stimulus artifacts during Deep Brain Stimulation (DBS) brings challenges to the signal processing, especially when the ratio of the kS/s sampling rate to the stimulation frequency is not an integer. In this work we study to deal with this problem. A transfer function is built to describe the relationship between the stimulation signal and the artifact at the acquisition site. A principal component analysis (PCA) based linear regression algorithm for eliminating the artifact is proposed. The algorithm can be used for the artifact removal with low sampling rate of the neural signal. Higher than 60% correlation coefficient of the artifact-free signal and the predetermined self-generated signal is achieved when the artifact is 60dB larger than the predetermined signal. The numerical recipe for the critical algorithm is also proposed, lowering the complexity from cubic degree to square degree.

A Clinically Viable Medial Olivocochlear Reflex Assay Using Transient-Evoked Otoacoustic Emissions.

The contralateral medial olivocochlear reflex (MOCR) strength may indicate various auditory conditions in humans, but a clinically viable assay and equipment are needed for quick, accurate, and reliable measurements. The first experiment compared an earlier version of the assay, which used a nonlinear-mode chirp stimulus, with a new assay using a linear-mode click stimulus, designed to give reliable MOCR measurements in most normal-hearing ears. The second experiment extended the improved assay on a purpose-built binaural hardware platform that used forward-pressure level (FPL) calibration for both the stimulus and the contralateral MOCR elicitor. Transient-evoked otoacoustic emission (TEOAE) tests were measured with and without a 60-dB SPL MOCR-evoking contralateral broadband noise. The normalized MOCR strength (MOCR%) was derived from the TEOAE responses for each trial pair using the complex pressure difference weighted by the TEOAE magnitude. Experiment 1 compared MOCR% within-subject and across-day using two TEOAE stimuli: nonlinear-mode chirps (50 dB SPL, bandpass 1-5 kHz, 14 ms window delayed by 2 ms) and linear-mode clicks (50 dB SPL, bandpass 0.5-2.5 kHz, 13 ms window delayed by 5 ms). TEOAE responses were analyzed in the 0.5 to 2.5 kHz band. Thirty adult participants with normal hearing (30 ears) completed the study. The TEOAE stimulus was calibrated in situ using spectral flattening, and the contralateral noise was calibrated in a coupler. Twelve TEOAE trial pairs were collected for each participant and condition. Experiment 2 used a purpose-built binaural system. The TEOAE stimuli were linear-mode clicks (50 dB SPL, bandpass 1-3 kHz, 13 ms window delayed by 5 ms), analyzed in the 1 to 3 kHz band over ~12 trial pairs. After a probe refit, an additional trial pair was collected for the two early-stopping signal-to-noise ratio criteria (15 and 20 dB). They were evaluated for single-trial reliability and test time. Nineteen adult participants with normal hearing (38 ears) completed the study. The TEOAE clicks and contralateral elicitor noise were calibrated in situ using FPL and delivered with automated timing. MOCR% for linear-mode clicks was distinguishable from measurement variability in 98% to 100% of participants' ears (both experiments), compared with only 73% for the nonlinear-mode chirp (experiment 1). MOCR detectability was assessed using the MOCR% across-subject/within-subject variance ratio. The ratio in experiment 1 for linear-mode clicks was higher (8.0) than for nonlinear-mode chirps (6.4). The ratio for linear-mode clicks (8.9) in experiment 2 was slightly higher than for the comparable linear-mode stimulus (8.0) in experiment 1. TEOAEs showed excellent reliability with high signal-to-noise ratios in both experiments, but reliability was higher for linear-mode clicks than nonlinear-mode chirps. MOCR reliability for the two stimuli was comparable. The FPL pressure response retest reliability derived from the SPL at the microphone was higher than the SPL retest reliability across 0.4 to 8 kHz. Stable results required 2 to 3 trial pairs for the linear-mode click (experiments 1 and 2) and three for the nonlinear-mode chirp (experiment 1), taking around 2 min on average. The linear-mode click assay produced measurable, reliable, and stable TEOAE and MOCR results on both hardware platforms in around 2 min per ear. The stimulus design and response window ensured that any stimulus artifact in linear mode was unlikely to confound the results. The refined assay is ready to produce high-quality data quickly for clinical and field studies to develop population norms, recognize diagnostic patterns, and determine risk profiles.

Towards optimizing single pulse electrical stimulation: High current intensity, short pulse width stimulation most effectively elicits evoked potentials

BackgroundWhile single pulse electrical stimulation (SPES) is increasingly used to study effective connectivity, the effects of varying stimulation parameters on the resulting cortico-cortical evoked potentials (CCEPs) have not been systematically explored. ObjectiveWe sought to understand the interacting effects of stimulation pulse width, current intensity, and charge on CCEPs through an extensive testing of this parameter space and analysis of several response metrics. MethodsWe conducted SPES in 11 patients undergoing intracranial EEG monitoring using five combinations of current intensity (1.5, 2.0, 3.0, 5.0, and 7.5 mA) and pulse width at each of three charges (0.750, 1.125, and 1.500 μC/phase) to study how CCEP amplitude, distribution, latency, morphology, and stimulus artifact amplitude vary with each parameter. ResultsStimulations with a greater charge or a greater current intensity and shorter pulse width at a given charge generally resulted in greater CCEP amplitudes and spatial distributions, shorter latencies, and increased waveform correlation. These effects interacted such that stimulations with the lowest charge and highest current intensities resulted in greater response amplitudes and spatial distributions than stimulations with the highest charge and lowest current intensities. Stimulus artifact amplitude increased with charge, but this could be mitigated by using shorter pulse widths. ConclusionsOur results indicate that individual combinations of current intensity and pulse width, in addition to charge, are important determinants of CCEP magnitude, morphology, and spatial extent. Together, these findings suggest that high current intensity, short pulse width stimulations are optimal SPES settings for eliciting strong and consistent responses while minimizing charge.

A Translatable System for Bi-Directional Stimulation and Evoked Response Measurement to Enable Neuronal Network Exploration

AbstractNeural stimulation therapies continue to evolve as new technologies are introduced into clinical practice. It has been over a decade since the initial descriptions of fully implantable, bidirectional neural systems, which allowed for concurrent sensing and stimulation, have been published. A major confounding issue in these types of neural recordings is the contamination of the signal of interest with electrical stimulus artifact, which can obscure short latency evoked activity and corrupt spectral analysis of longer duration signals. Approach. Here we describe the design and early preclinical evaluation of a neurostimulator with improved capabilities for sensing, with particular emphasis on managing stimulus artifact. The system was tested in three ovine deep brain stimulation (DBS) subjects, one with a DBS lead targeting the hippocampus, and two with DBS leads targeting the subthalamic nucleus (STN). All leads were externalized with percutaneous lead extensions. Main results. Results demonstrate that it was possible to record evoked potentials with a latency of 1–2 ms following stimulation in all subjects with the new system. Recordings from the hippocampal target showed clear short-latency responses exhibiting behavior consistent with evoked compound action potentials (ECAPs). In contrast, recordings from the STN target demonstrated highly resonant activity, dependent upon stimulus frequency, which could persist for 20–30 ms following individual stimuli. Both directional stimulation and directional recordings were evaluated to determine their influence on this evoked resonant neural activity (ERNA). The system was also characterized for sensing in one spinal cord stimulation (SCS) ovine subject and one sacral nerve modulation ovine subject. Significance. The bidirectional stimulation and evoked-response sensing system presented here enables sensing evoked responses elicited from stimulation, empowering continued research to expand the understanding and optimization of DBS therapy. Additionally, the example recordings from other therapy spaces demonstrate the capability of the system across neural stimulation therapies.

Electrically evoked mismatch negativity responses to loudness and pitch cues in cochlear implant users

Objective measurements could improve cochlear implant (CI) fitting, especially for CI users who have difficulty assessing their hearing impressions. In this study, we investigated the electrically evoked mismatch negativity (eMMN) brain potential as a mainly preattentive response to pitch and loudness changes. In an electrophysiological exploratory study with 21 CI users, pitch and loudness cues were presented in controlled oddball paradigms that directly electrically stimulated the CI via software. Out of them 17 valid data sets were analyzed. A pitch cue was produced by changing the stimulating CI electrodes (pairs of adjacent electrodes). A loudness cue originated from changing the stimulation amplitude on one CI electrode. MMN responses were measured unsing clinical electroencephalography recording according to a standard recording protocol. At the group level, significant eMMN responses were elicited for loudness cues and for pitch cues at basal electrode pairs but not at apical electrode pairs. The effect of deviance direction was not significant and no stimulus artifacts were observed. Recording an electrically evoked MMN in response to loudness changes in CI users is generally feasible, and is, therefore, promising to support CI fitting procedures in the future. Detection of pitch cues would require a greater electrode distance between selected electrodes for standard and deviant stimuli, especially in apical regions. A routine clinical setup can be used to measure eMMN.

Usefulness of phase gradients of otoacoustic emissions in auditory health screening: An exploration with swept tones.

Otoacoustic emissions (OAEs) are low-level sounds generated by the cochlea and widely used as a noninvasive tool to inspect cochlear impairments. However, only the amplitude information of OAE signals is used in current clinical tests, while the OAE phase containing important information about cochlear functions is commonly discarded, due to the insufficient frequency-resolution of existing OAE tests. In this study, swept tones with time-varying frequencies were used to measure stimulus frequency OAEs (SFOAEs) in human subjects, so that high-resolution phase spectra that are not available in existing OAE tests could be obtained and analyzed. The results showed that the phase of swept-tone SFOAEs demonstrated steep gradients as the frequency increased in human subjects with normal hearing. The steep phase gradients were sensitive to auditory functional abnormality caused by cochlear damage and stimulus artifacts introduced by system distortions. At low stimulus levels, the group delays derived from the phase gradients decreased from around 8.5 to 3 ms as the frequency increased from 1 to 10 kHz for subjects with normal hearing, and the pattern of group-delay versus frequency function showed significant difference for subjects with hearing loss. By using the swept-tone technology, the study suggests that the OAE phase gradients could provide highly sensitive information about the cochlear functions and therefore should be integrated into the conventional methods to improve the reliability of auditory health screening.

Criteria for differentiating left bundle branch pacing and left ventricular septal pacing: A systematic review.

BackgroundAs a novel physiological pacing technique, left bundle branch pacing (LBBP) can preserve the left ventricular (LV) electrical and mechanical synchronization by directly capturing left bundle branch (LBB). Approximately 60–90% of LBBP were confirmed to have captured LBB during implantation, implying that up to one-third of LBBP is actually left ventricular septal pacing (LVSP). LBB capture is critical for distinguishing LBBP from LVSP.Methods and resultsA total of 15 articles were included in the analysis by searching PubMed, EMBASE, Web of Science, and the Cochrane Library database till August 2022. Comparisons of paced QRS duration between LVSP and LBBP have not been uniformly concluded, but the stimulus artifact to LV activation time in lead V5 or V6 (Stim-LVAT) was shorter in LBBP than LVSP in all studies. Stim-LVAT was used to determine LBB capture with a sensitivity of 76–95.2% and specificity of 78.8–100%, which varied across patient populations.ConclusionThe output-dependent QRS transition from non-selective LBBP to selective LBBP or LVSP is direct evidence of LBB capture. LBB potential combined with short Stim-LVAT can predict LBB capture better. Personalized criteria rather than a fixed value of Stim-LVAT are necessary to confirm LBB capture in different populations, especially in patients with LBB block or heart failure.

The influence of the reference electrode location on the M−wave characteristics in the quadriceps

IntroductionIn the compound muscle action potential (M wave) recorded using the belly-tendon configuration, the contribution of the tendon electrode is assumed to be negligible compared to the belly electrode. We tested this assumption by placing the reference electrode at a distant (contralateral) site, which allowed separate recording of the belly and tendon contributions. MethodsM waves were recorded at multiple selected sites over the right quadriceps heads and lower leg using two different locations for the reference electrode: the ipsilateral (right) and contralateral (left) patellar tendon. The general parameters of the M wave (amplitude, area, duration, latency, and frequency) were measured. Results(1) The tendon potential had a small amplitude (<30%) compared to the belly potential; (2) Changing the reference electrode from the ipsilateral to the contralateral patella produced moderate changes in the M wave recorded over the innervation zone, these changes affecting significantly the amplitude of the M−wave second phase (p = 0.006); (3) Using the contralateral reference system allowed recording of short-latency components occurring immediately after the stimulus artefact, which had the same latency and amplitude (p = 0.18 and 0.25, respectively) at all recording sites over the leg. ConclusionsThe potential recorded at the “tendon” site after femoral nerve stimulation is small (compared to the belly potential), but not negligible, and makes a significant contribution to the second phase of belly-tendon M wave. Adopting a distant (contralateral) reference allowed recording of far-field components that may aid in the understanding of the electrical formation of the M wave.

A generalized framework for pacing artifact removal using a Hampel filter.

Cardiac pacing is a clinical therapy widely used for treating irregular heart rhythms. Equivalent techniques for the treatment of gastric functional motility disorders hold great potential. Accurate analysis of pacing studies is often hindered by the stimulus artifacts which are superimposed on the recorded signals. This paper presents a semi-automated artifact detection method using a Hampel filter accompanied by 2 separate artifact reconstruction methods: (i) an auto-regressive model, and (ii) weighted mean to estimate the underlying signal. The developed framework was validated on synthetic and experimental signals containing large periodic pacing artifacts alongside evoked bioelectrical events. The performance of the proposed algorithms was quantified for gastric and cardiac pacing data collected in vivo. A lower mean RMS difference was achieved by the artifact segment reconstructed using the auto-regression ([Formula: see text]), method compared to the weighted mean ([Formula: see text]) method. Therefore, a more accurate artifact reconstruction was provided by the auto-regression approach. Clinical Relevance- The ability to efficiently and accurately isolate evoked bioelectrical events by eliminating large artifacts is a critical advancement for the analysis of paced recordings. The developed framework allows more efficient analysis of preclinical pacing data and thereby contributes to the advancement of pacing as a clinical therapy.

Non-Invasive Stable Sensory Feedback for Closed-Loop Control of Hand Prosthesis.

The absence of somatotopic sensory feedback limits the function of conventional prosthetic hands. In this study, we integrated a non-invasive sensory feedback system into a commercial Bebionic hand with new customized surface stimulation electrodes. Multiple modalities of tactile and hand aperture sensory information were conveyed to the amputee via the technique of evoked tactile sensation (ETS) elicited at projected finger map (PFM) of residual limb and an additional electrotactile stimulation in the ipsilateral upper arm. A previously developed anti-stimulus artifact module was used to remove the stimulus artifact from surface electromyographic (sEMG) signals, and the filtered sEMG envelops controlled the speed of open/close of the Bebionic hand. The Ag/AgCl surface stimulation electrode in 10-mm diameter was specially designed to fit the restricted PFM areas for stable perception. We evaluated the alternating-current (AC) impedance magnitude of this electrode stimulated over 12 hours. The perceptual and upper thresholds in pulse-width over 200 days at PFM areas were recorded to assess the stability of the non-invasive sensory neural interface. The efficacy of multi-modality feedback for identification of physical properties of objects was also assessed. Results showed that the AC impedance of customized surface stimulation electrode was stable over 12 hours of stimulation. The perceptual and upper thresholds were stable over 200 days. This non-invasive sensory feedback enabled a forearm amputee to identify the compliance and length of grasped objects with an accuracy of 100 %. Results illustrated that the multi-modality sensory feedback system provided stable and sufficient sensory information for perceptual discrimination of physical features of grasped objects. Clinical Relevance- This study demonstrated a promising and novel way to restore stable sensory feedback non-invasively for commercial hand prostheses.

Stability and Effect of Parkinsonian State on Deep Brain Stimulation Cortical Evoked Potentials

ObjectivesTo characterize and compare the stability of cortical potentials evoked by deep brain stimulation (DBS) of the subthalamic nucleus (STN) across the naïve, parkinsonian, and pharmacologically treated parkinsonian states. To advance cortical potentials as possible biomarkers for DBS programming. Materials and MethodsSerial electrocorticographic (ECoG) recordings were made more than nine months from a single non-human primate instrumented with bilateral ECoG grids spanning anterior parietal to prefrontal cortex. Cortical evoked potentials (CEPs) were generated through time-lock averaging of the ECoG recordings to DBS pulses delivered unilaterally in the STN region using a chronically implanted, six-contact, scaled DBS lead. Recordings were made across the naïve followed by mild and moderate parkinsonian conditions achieved by staged injections of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxin. In addition to characterizing the spatial distribution and stability of the response within each state, changes in the amplitude and latency of CEP components as well as in the frequency content were examined in relation to parkinsonian severity and dopamine replacement. ResultsIn the naïve state, the STN DBS CEP presented as a multiphase response maximal over M1 cortex, with components attributable to physiological activity distinguishable from stimulus artifact as early as 0.45–0.75 msec poststimulation. When delivered using therapeutically effective parameters in the parkinsonian state, the CEP was highly stable across multiple recording sessions within each behavioral state. Across states, significant differences were present with respect to both the latency and amplitude of individual response components, with greater differences present for longer-latency components (all p < 0.05). Power spectral density analysis revealed a high-beta peak within the evoked response, with significant changes in power between disease states across multiple frequency bands. ConclusionsOur findings underscore the spatiotemporal specificity and relative stability of the DBS-CEP associated with different disease states and with therapeutic benefit. DBS-CEP may be a viable biomarker for therapeutic programming.

P 19 Auditory brainstem implants: Intraoperative electrophysiological measures and hearing outcome

Introduction: Auditory brainstem implants (ABI) are primarily designed for tumour related deafness in neurofibromatosis type 2 (NF2) patients. These neuro-prosthetic devices bypass the auditory nerve and produce hearing sensations by direct stimulation of the cochlear nuclei (CN). This study investigates the importance of intraoperative electrically evoked auditory brainstem responses (EABR) with regards to the auditory outcome. Patients & Methods: Out of a prospectively collected series of ABI implantations from 2005 to 2019, 22 patients (11 male, 11 female) fulfilled inclusion criteria (min. age of 15 y, NF2 diagnosis) and were analysed retrospectively for EABR and hearing outcome. EABR analysis relied on the presence and number of vertex positive peaks P1, P2 and P3 at brainstem stimulation. For post-operative hearing outcome a new Clinical ABI Outcome Classification was developed and applied at 6 to 12 months containing 4 categories: Category 1, Star Performer, with >80% speech understanding in auditory only MTP (mono- to polysyllabic) test and ability for continuous spoken conversation without any lip reading; Category 2, Good Performer, with >40 to 80% in auditory only MTP test and some speech understanding combined with lip reading; Category 3, Useful Performance, communication with some additional measures possible (hearing, lip-reading and written notes); Category 4, Non-useful Performance, no or only scarce sound reception. Results: In 22 patients, 146 EABR recordings at various sites of the implant were evaluated: A three-peak-formation was present in 7, a two-peek-formation in 115 (78,8%), and one-peak in 13 EABRs, while 11 remained without any reproducible responses. EABR waveforms showed some variation: Peak P1 developed just out of or after the stimulus artefact while peaks P2 and P3 sometimes showed melting and larger latency differences. P1 appears to correspond to wave III of the acoustically evoked ABR. Overall auditory outcome was useful (Categories 1, 2 or 3) in 95,5% of cases, with Star or Good Performance in about 77% of patients. EABR presence predicted auditory rehabilitation correctly in 95,5%. Conclusion: False positive EABR are rare and a matter of open discussion such as on lead dislocation or secondary brainstem nuclei degeneration. Overall, intra-operative reproducible EABR are highly predictive of adequate brainstem activation and useful hearing rehabilitation with ABI in deaf NF2 patients and appear indispensable for implant positioning. The most reliable peak P1 of EABR may represent immediate activity of cochlear nuclei. The importance of further peaks P2 and P3 and their anatomic correlation still need further evaluation and possibly correlation with more long-term auditory development. The presented ABI hearing classification includes the internationally accepted MTP test but goes beyond this and proves to be a universal tool to elucidate the patient's capacity for speech communication in real life.

Toward improved measurement of infant bone conduction auditory brainstem responses

Newborn hearing screening has led to improved detection and treatment of hearing loss (HL) in infants and young children, supporting improved communication outcomes in early childhood and beyond. However, current screening protocols are not equally sensitive to all forms of HL, limiting diagnostic accuracy and, ultimately, identification of appropriate treatment options. Correspondingly, treatment outcomes in children with HL remain quite variable. One fundamental and persistent challenge is the assessment of bone conduction hearing, which is essential to differentiate conductive versus sensorineural pathology. Whereas hearing via air conduction can be effectively assessed using the auditory brainstem response (ABR), a noninvasive electrophysiologic measure, bone conduction ABR measurements suffer from comparatively poor signal quality (including significant stimulus artifact), transducer acoustic output limitations, and generally lower test-retest reliability. Here we evaluated the prospective benefits of measuring infant bone conduction ABRs using a modified bone conduction transducer designed to reduce stimulus artifact and thereby improve measurement quality. Measurements obtained using a standard bone conduction transducer were compared on several dimensions to measurements obtained using the modified transducer. Improved bone conduction measurement tools are expected to support improved detection and classification of conductive and mixed HL, leading to improved treatment outcomes for this important patient population.

HP08: Deep Brain Stimulation Evoked Potentials in Children with dystonia

Pallidal Deep Brain Stimulation (DBS) is an established therapy for dystonia. Stimulation parameters are currently determined empirically. Recording the cortical response to the pallidal stimulus, known as Deep Brain Stimulation Evoked Potentials (DBSEPs) could provide a more objective measure of optimal contact selection (Tisch et al, MovDisord 2008;23:265-73; Bhanpuri et al, BrainStim 2014;7:718-26). Methods: Fifteen children with dystonia participated (age 7-19years), all with bilateral pallidal DBS implanted at least 6 months previously. Scalp EEG (10-20 system) was sampled at 5KHz. Contacts from each stimulating electrode were tested separately, using 6Hz bipolar stimulation. Pairs of electrode contacts were tested sequentially using combinations of cathodal stimulation (2 volts) with an adjacent anode, and pulse width at the patient’s therapeutic level. Each combination was recorded for 3 minutes, giving >1000 stimuli per contact pair. Therapeutic settings were restored on study completion. Offline, the DBS stimulus artefact was used to segment EEG data into epochs (10ms pre-stimulus, 150ms post-stimulus). After artefact rejection, 900-1100 epochs were averaged to obtain the DBSEP for each contact pair. Results: Clear DBSEPs were obtained in 12/15 patients on one or both sides, with peak latency of 20ms, and largest amplitude over the ipsilateral central/centro-parietal region, concordant with adult literature. In 3/15 patients no convincing DBSEP was obtained. Conclusion: To our knowledge this is the largest study of DBSEPs in dystonia and confirms the feasibility and tolerability of this technique in children. Future work will investigate potential clinical applications of DBSEPs to inform an individualised approach to DBS parameter selection.

Stimulus artifact removal to detect trigeminal sensory evoked potentials

ObjectiveLarge stimulus and myogenic artifacts usually prevent detection of sensory evoked potentials to electrical stimulation in trigeminal sensory territory (t-SEP). Stimulus Artifact (SA) removal can be obtained by means of two stimulating modes (Dual Mode Stimulation - DMS) having in common a fixed cathode alternatingly referred to opposed anodes, resulting in SAs of opposite polarity. Opposite SAs progressively cancel each other out during averaging, without interaction with the underlying bio-electrical events. MethodsUsing DMS, dermatomal t-SEP were recorded from C5/C6 scalp sites in 24 healthy volunteers after selective, electrical stimulation of five trigeminal nerve areas: supraorbital, infraorbital, superior alveolar, inferior alveolar and auriculotemporal. ResultsReproducible t-SEPs were obtained after stimulation at all sites and showed the classical W shape, without significant differences related to the stimulated area. Cortical responses were formed by a sequence of individual peaks labelled, according to polarity and mean latency, as P8, N13, P19, N27, P38. A later, less stable component followed (N55-P67), poorly defined or absent in about one third of subjects. ConclusionsThe described technique represents a novel approach, within reach of any neurophysiological unit, to record dermatomal SEPs to electrical stimulation of several, discrete areas of significant clinical interest, covering the whole trigeminal sensory territory. SignificanceDMS represents a simple and robust tool to remove SA as the main drawback that has so far prevented recording of t-SEPs in daily clinical practice.

SY5.4. Monitoring for peripheral nerve surgery

Injuries or diseases of peripheral nerves can have severe impact on function, severe disability (inability to work), social and psychological effects or chronic pain. The main reason to perform Intraoperative monitoring (IOM) during peripheral nerve surgery are to prevent nerve injuries during surgery, to identify peripheral nerves or fascicles in cases of variable anatomy, scar tissue or tumors, to determine axon continuity across a lesion, and to guide surgical decisions regarding the type of neural reconstruction to perform. Nerve action potential (NAP) recording is considered a standard method for peripheral nerve monitoring given its ability to determine the presence or absence of functional axons in a segment of nerve, it also localizes functional and non-functional nerve segments. A NAP recording requires 4.000 to 5.000 intact or regenerating axons to be obtained and its preferred to compound muscle action potential (CMAP) recordings. The presence or absence of a NAP is more important than its latency, amplitude or shape. If a NAP is recorded in a nerve or fascicle, neurolysis is performed, and nerve grafts are required in no-responding fascicles. Some technical difficulties of NAP recording are related to very short stimulation/recording distances, stimulus artifact, volume conduction from another nerve, low temperature, use of tourniquet and use of local anesthetics. CMAP recording is like that used in triggered EMG, surface or subcutaneous needle electrodes are placed over desired muscles and the exposed peripheral nerve is stimulated. Free EMG is useful to detect, in real time, trains or bursts of abnormal discharges that indicate possible nerve damage. It́s easy to use, not affected by anesthesia and has high sensitivity but low specificity. Bipolar intramuscular electrodes are preferred for EMG recordings.