Stimulus Artifact Research Articles

Measuring Sharp Waves and Oscillatory Population Activity With the Genetically Encoded Calcium Indicator GCaMP6f.

GCaMP6f is among the most widely used genetically encoded calcium indicators for monitoring neuronal activity. Applications are at both the cellular and population levels. Here, we explore two important and under-explored issues. First, we have tested if GCaMP6f is sensitive enough for the detection of population activity with sparse firing, similar to the sensitivity of the local field potential (LFP). Second, we have tested if GCaMP6f is fast enough for the detection of fast network oscillations critical for the encoding and consolidation of memory. We have focused this study on the activity of the hippocampal network including sharp waves (SWs), carbachol-induced theta oscillations, and interictal-like spikes. We compare simultaneous LFP and optical GCaMP6f fluorescent recordings in Thy1-GCaMP6f mouse hippocampal slices. We observe that SWs produce a clear population GCaMP6f signal above noise with an average magnitude of 0.3% ΔF/F. This population signal is highly correlated with the LFP, albeit with a delay of 40.3 ms (SD 10.8 ms). The population GCaMP6f signal follows the LFP evoked by 20 Hz stimulation with high fidelity, while electrically evoked oscillations up to 40 Hz were detectable with reduced amplitude. GCaMP6f and LFP signals showed a large amplitude discrepancy. The amplitude of GCaMP6f fluorescence increased by a factor of 28.9 (SD 13.5) between spontaneous SWs and carbachol-induced theta bursts, while the LFP amplitude increased by a factor of 2.4 (SD 1.0). Our results suggest that GCaMP6f is a useful tool for applications commonly considered beyond the scope of genetically encoded calcium indicators. In particular, population GCaMP6f signals are sensitive enough for detecting synchronous network events with sparse firing and sub-threshold activity, as well as asynchronous events with only a nominal LFP. In addition, population GCaMP6f signals are fast enough for monitoring theta and beta oscillations (<25 Hz). Faster calcium indicators (e.g., GCaMP7) will further improve the frequency response for the detection of gamma band oscillations. The advantage of population optical over LFP recordings are that they are non-contact and free from stimulation artifacts. These features may be particularly useful for high-throughput recordings and applications sensitive to stimulus artifact, such as monitoring responses during continuous stimulation.

Wavelet-Based Muscle Artefact Noise Reduction for Short Latency rTMS Evoked Potentials.

This paper presents a new method of reducing the noise in the EEG response signal recorded during repetitive transcranial magnetic stimulation (rTMS). This noise is principally composed of the residual stimulus artefact and millivolt amplitude compound muscle action potentials (CMAP) recorded from the scalp muscles and precludes analysis of the cortical evoked potentials, especially during the first 20-ms post stimulus. The proposed method uses the wavelet transform with a fourth-order Daubechies mother wavelet and a novel coefficient reduction algorithm based on cortical amplitude thresholds. Four other mother wavelets as well as digital filtering have been tested, and the Coiflets 2 and 3 also found to be effective with Coiflet 3 results marginally better than Daubechies 4. The approach has been tested using data recorded from 16 normal subjects during a study of cortical sensitivity to rTMS at different cortical locations using stimulation amplitudes, frequencies, and sites typically used in clinical practice to treat major depressive disorder.

Vibrotactile piezoelectric stimulation system with precise and versatile timing control for somatosensory research

BackgroundTactile stimulations systems are useful for studying the somatosensory system in children because they are innocuous and safe. Stimulators based on piezoelectric actuator are useful, but there is still a need for such systems capable of providing accurate and versatile control of timing and pattern of activation. New methodWe have implemented a vibrotactile stimulating system useful for behavioral and electroencephalography (EEG) and magnetoencephalography (MEG) research. Our design goal was to create a system capable of providing up to five independently controlled mechanical stimulations with precise timing. We developed a Graphic User Interface (GUI) in LabVIEW, which controls a commercially available piezoelectric braille stimulator using an Arduino based controller. We made a customized braille stimulator based on the Metec Braille device. ResultsOur system can control up to five tactile stimulators with independent timing control and negligible errors. Although it can be directly used for behavioral and EEG research, the piezoelectric stimulators in our system generate stimulus artifacts that interfere with MEG recordings. A moving averaging subtraction algorithm we developed can remove the artifact. The stimulator can be used to measure somatic evoked magnetic fields from the somatosensory cortex of a child without the artifact. Comparison to existing methodsOur system provides an accurate independent control of one or more piezoelectric actuators using a GUI-based easy-to-control programming approach based on recent advances in embedded systems and software. Versatility and precise stimulation timing distinguish our system compared to existing somatic stimulators. ConclusionsOur system may be useful for somatic research.

Intelligent closed-loop deep brain stimulation modulation platform with multi-sensor information feedback

This paper describes an intelligent closed-loop deep brain stimulation modulation platform with multi-sensor information input feedback, adaptive, interactive, and high integration. This platform has 16 channels of data acquisition channels and 4 channels of stimulation channels. Four channels of the acquisition channel are used for intracranial LFPs acquisition, and 12 channels are used for acquisition of extracranial signals (EEG, ECG and EMG). The output waveform of stimulation channels is a configurable and arbitrary waveform. The stimulation voltage is in constant voltage mode, the stimulation voltage range is -5 V ∼ +5 V, and the impedance range of the stimulation load is 500 Ω ∼ 5 KΩ. This study achieved simultaneously the stimulation at the recording, which effectively reduces the influence of the stimulation artifacts on the recorded signal. The platform constructs multi-information closed-loop deep brain stimulation for Parkinson's disease from three aspects: motor behavior signal feedback, extracranial electrical signal feedback, and deep brain field potential signal feedback. A modular portable and universal closed-loop neuromodulation research platform has been formed. This study completed the acquisition and recording and adaptive closed-loop algorithm and stimulation function, effectively eliminating the stimulus artifacts, adapting to wide dynamic range brain impedance, and enhancing the stability of stimulus output. It has the characteristics of compact structure, small size, low power consumption and stable operation.

Micro-channel sieve electrode for concurrent bidirectional peripheral nerve interface. Part B: stimulation

Objective. Successful use of a prosthetic limb by an amputee is facilitated by haptic feedback—both a sense of touch and proprioception. Stimulating afferent fibers within peripheral nerves has been shown to provide somatosensation enabling amputees to modulate the control of prosthetic limbs. Peripheral nerve interfaces (PNIs) have also been used to decode patients’ motor intentions. It seems ideal to use PNIs to record efferent fibers for motor control while stimulating afferent fibers to create concurrent sensory feedback. However, while many PNIs claim to be bi-directional, few can both stimulate and record at the same time due to stimulation artifacts which are orders of magnitude larger than the recorded motor signals. This study uses computational modelling to compare the stimulation artifact at threshold levels of stimulation for thin-film transverse intrafascicular multichannel electrodes (tfTIMEs) with micro-channel sieve electrodes. Approach. Finite element models of micro-channel sieves and tfTIMESs were used to solve for electric fields generated during peripheral nerve stimulation. Electrophysiological responses were simulated using axon models. Stimulation artifacts were calculated for stimuli eliciting axonal action potentials. Simulations were carried out for multiple micro-channel geometries and electrode configurations. Main results. Stimulation artifacts generated for threshold stimulation currents are lower for micro-channel devices compared to tfTIMEs. Consequently, stimulus artifacts at threshold currents were substantially higher for the tfTIME. Micro-channel width has a moderate impact on recruitment thresholds and stimulus artifacts. Using the micro-channel sieve in bipolar and tripolar stimulation configurations greatly decreases stimulation artifacts particularly for optimized contact placements (CPs). Electroneurogram (ENG) signals from the companion paper were incorporated showing a great improvement in signal-to-artifact ratio for the micro-channel electrode compared to tfTIMEs. Significance. Stimulating regenerated nerve tissue using micro-channel sieve electrodes can decrease stimulation artifacts and elicit neuronal responses at low stimulation amplitudes. Further analysis provides clues to optimal implementations of micro-channel devices. Finally, stimulation artifacts for simulated tfTIME devices were 2–3 orders of magnitude larger than ENG levels. In contrast, for some micro-channel configurations stimulation artifacts were 3–4 orders of magnitude smaller than ENG levels.

Neural tracking of the speech envelope in cochlear implant users

Objective. When listening to speech, the brain tracks the speech envelope. It is possible to reconstruct this envelope from EEG recordings. However, in people who hear using a cochlear implant (CI), the artifacts caused by electrical stimulation of the auditory nerve contaminate the EEG. The objective of this study is to develop and validate a method for assessing the neural tracking of speech envelope in CI users. Approach. To obtain EEG recordings free of stimulus artifacts, the electrical stimulation is periodically interrupted. During these stimulation gaps, artifact-free EEG can be sampled and used to train a linear envelope decoder. EEG recordings obtained during audible and inaudible (i.e. sub-threshold) stimulation were used to characterize the artifacts and their influence on the envelope reconstruction. Main results. The present study demonstrates for the first time that neural tracking of the speech envelope can be measured in response to ongoing electrical stimulation. The responses were validated to be truly neural and not affected by stimulus artifact. Significance. Besides applications in audiology and neuroscience, the characterization and elimination of stimulus artifacts will enable future EEG studies involving continuous speech in CI users. Measures of neural tracking of the speech envelope reflect interesting properties of the listener’s perception of speech, such as speech intelligibility or attentional state. Successful decoding of neural envelope tracking will open new possibilities to investigate the neural mechanisms of speech perception with a CI.

A 2.7-$\mu$ W Neuromodulation AFE With 200 mVpp Differential-Mode Stimulus Artifact Canceler Including On-Chip LMS Adaptation

We present a 65-nm CMOS neuromodulation SoC with a stimulus artifact canceler allowing simultaneous recording and stimulation. A first-order ${\Delta \Sigma }$ -modulated switched capacitor DAC injects artifact canceling charges at the LNA input, handling up to 200 mV $_{\text{pp}}$ differential-mode artifacts with only a 10% noise penalty. An on-chip LMS engine adapts tap coefficients of an integrated FIR filter that reconstructs the artifact signals. Residual artifacts are further canceled with another FIR/LMS filter in the digital backend. This canceler achieves a state-of-the-art effective dynamic range of 77 dB while retaining low power consumption. The AFE including the canceler consumes $2.7~ {\mu }\text{W}$ of power and occupies 0.18 mm2 of area per channel.

Measurements of bone conduction auditory brainstem response with the new audiometric bone conduction transducer Radioear B81

Objective: To compare recordings of bone conduction (BC) stimulated auditory brainstem response (ABR) obtained using the newer BC transducer Radioear B81 and the conventional BC transducer Radioear B71. Balanced electromagnetic separation transducer (BEST) design found in the B81 may influence the ABR magnitudes and latencies, as well as electrical artefacts.Design: ABRs to tone burst stimuli of 500 Hz, 2000 Hz, 4000 Hz, click stimulation, and broad-band chirp stimulation at 20 and 50 dB nHL were recorded. For each device, stimulus and intensity level, the ABR Jewett wave V amplitude and latency were obtained. The device-related electrical stimulus artefacts on the ABR recordings were also analysed by calculating the Hilbert envelope of the peri-stimulus recording segments.Study sample: Twenty-three healthy adults with normal hearing were included in the study.Results: The ABRs obtained by the B81 were similar to that of the B71 in terms of ABR wave V amplitude and latency. However, the B81 produced smaller electrical artefacts than B71 and this difference was statistically significant. Conclusions: The BC transducer Radioear B81 provides ABRs comparable to Radioear B71 while causing smaller artefacts.

T97. A revisit of the clinically used methods for intraoperative nerve action potential recordings in nonhuman primate

Compound nerve action potential recordings are often used intraoperatively to assist the surgeon in decision making on surgical approaches during nerve repair surgeries. The methods used clinically are based on surgical recordings (Robert et al., 2009). Although these methods are of great simplicity as handheld electrodes are applied to a nerve dissected and isolated from surrounding tissues, implementation of the methods in the clinical setting remains challenging. Here, we report findings from an experiment where similar recordings were made in nonhuman primate in order to improve our understanding and the performance of the methods. Recordings were made from the median nerve at the upper arm of an adult female pigtail monkey under TIVA (pentobarbital) and full NMB (pancuronium). Core body temperature was maintained near 38 °C. Nerve action potentials were induced by constant current stimulation and recorded using laboratory equipment. IONM electrodes were used: triple and double hook electrodes (Cadwell) for stimulation and recording, respectively. Using the same recording configuration as in the clinical situation, we failed to record recognizable compound action potentials across a short conduction length of a nerve (3–4 cm) that was isolated and suspended from surrounding tissues as large stimulus artifacts obscured the neuronal signal. The stimulus artifacts were suppressed and the compound nerve action potential became unmasked after a saline-soaked gauze was placed underneath the nerve between the stimulating and recording electrodes to bridge the nerve and the surrounding tissues (i.e., ‘bridge grounding’). Furthermore, compound action potentials were of small amplitude when all prongs of the stimulation electrode were placed underneath the nerve. However, the potentials increased markedly in size with another modification, i.e. when the nerve was placed between the middle prong and the outer 2 prongs of the stimulation electrode (i.e., ‘sandwich stimulation’) after reshaping of the prongs. The recordings were made at room temperature (about 23 °C) without use of warm saline irrigation, at which the compound action potentials preserved good amplitudes but slowed in conduction, leading to a better separation of compound action potentials from stimulus artifacts. We reversely translated the clinical methods of nerve compound action potential recording to a laboratory setting in order to better understand the factors affecting performance. We observed that compound nerve action potentials could be reliably recorded with IONM electrodes when 2 modifications, the ‘bridge grounding’ and ‘sandwich stimulation’, were made. Confirmation and additional experiments are underway to revisit the clinically used recording techniques in the laboratory setting.

S182. Visual inputs control of sustained muscle contraction

Very few is known about visual influences on volitional motor control. It is speculated that visual attention mediated by the dorsal stream might play an important role in motor planning in grasping and reaching tasks while little is known about the influence of visual inputs on motor control. The cortical silent period (cSP) to transcranial magnetic stimulation (TMS) is a measure of cortical motor inhibition that is strongly related to the degree of muscular activation. We aimed at investigating the effect of alteration of visual inputs on the descending control of muscle activation during a sustained contraction in healthy subjects by measuring the relationship between cSP duration and the level of electromyographic (EMG) activity on a target muscle. We obtained motor evoked potential (MEP) and cSP to TMS at a stimulus intensity of 130% of resting motor threshold while subjects were maintaining a unilateral first dorsal interosseous (FDI) muscle contraction. In the control condition, subjects had their two hands visible on a convenient table. In the test condition, the hand under examination was hidden from the subject’s view inside a custom-made box that had a mirror reflecting the contralateral (inactive) hand. Subjects were instructed to maintain the same amount of muscle contraction in the two conditions, of about 30% of their maximum. We recorded 5 trials for each condition and calculated the average value for MEP onset latency, MEP peak-to-peak amplitude, MEP termination and CSP duration (from stimulus artefact to the moment in which any EMG activity resumed after inhibition). Subjects were not aware of the aims of the study. They were only instructed on generating and maintaining the same voluntary strength of the FDI contraction in the two conditions. We performed a Wilcoxon non-parametric test for related variables, comparing control and test conditions. Statistical significance was set to two-tailed p < 0.05. Fifteen healthy subjects were included (mean age 29.6 ± 4.6 years, F 74%, right handedness 98.1%). Mean MEP onset latency (24.1 ± 2.2 ms vs 23.9 ± 2.1 ms) and peak-to-peak amplitude (2.3 ± 0.9 mV vs 2.5 ± 0.6 mV) were not significantly different in the two study conditions (p > 0.05 for the two comparisons). However, CSP duration shortened significantly when subjects had their hand out of their view (114.4 ± 9.7 ms vs 83.6 ± 9.3 ms p = 0.004). A trend of an increased background EMG activity was found in the test condition with respect to baseline, although not statistically significant (706 ± 488 uV vs 1074 ± 804 uV p = 0.1). Visual inputs contribute to modulate descending control of force during a sustained muscle contraction. In our case, it is possible that vision of an inactive hand when trying to keep a sustained contraction might have modified corticospinal output in a way that inhibition was diminished.

F128. Excitability-tests using multi-channel contraction force recordings: Development of a novel setup to study distal motor excitability properties

In animal models of motor neuron disease downstream pathophysiological events were shown to occur at distal nerve terminal branches. Recently, distal excitability-tests were developed which have the potential to assess changes in ion-channel activity in these branches. In these tests, electrical conditioning and test stimuli are delivered to distal motor axon branches near the muscle. The muscle responses are then recorded by assessing threshold-changes for eliciting a given muscle force as CMAP-recording would be too much affected by electrical stimulus artifacts. With current methods insight into direction of force remains unknown when using a single force transducer. Therefore, we present a novel method to study distal excitability by monitoring force in multiple directions. Excitability properties were obtained by stimulating the median nerve at wrist level and the motor point over the thenar muscles. Thumb contraction force was recorded using four transducers, which were configured in such a way to measure force in left–right and up–down direction. With this configuration the transducers enclosed a square in the middle which was used to position the interphalangeal joint of the thumb. Using multiple force transducers the optimal recording direction was determined by rotating the transducers to maximize the response in a single-channel and minimize the response in the others. For excitability testing, the single-channel with the maximum response was selected. Five excitability recordings were successfully performed in the median nerve at wrist level (n = 3) and motor point (n = 2) in a single test subject. The maximum baseline to peak force during supramaximal nerve and motor point stimulation was approximately 2–3 N. The mean threshold for 50% maximum force was 2.4 mA at wrist level, and right-shifted for the motor point with a mean threshold of 10.2 mA. The mean strength-duration time constant was 0.476 ms at the wrist level and 0.211 ms at the motor point. At the motor point, the current-threshold relationship could not be completed for hyperpolarizing conditioning stimuli over 50%, most likely due to high currents. In the recovery cycle, the thresholds from 2.5 ms to 100 ms were lower for motor point stimulation than stimulation at wrist level. Although minimal, small force responses were also observed perpendicular to main recording direction. Measuring excitability with multiple force transducers is potentially useful to optimize distal excitability recording.

Recovery of early neural spikes from stimulation electrodes using a DC-coupled low gain high resolution data acquisition system

BackgroundNeural responses to electrical stimulation provide valuable information to probe and study the network function. Especially, recording neural responses from the stimulated site provides improved neural interfacing method. However, it is difficult to measure short-delayed responses at the stimulated electrode due to the saturation of the amplifier after stimulation which is called “stimulus artifact”. Despite the advances in handling stimulation artifacts, it is still very challenging to deal with the artifacts if one tries to stimulate and record from the same electrode. New methodIn this paper, we developed a system consisting of 24 bit ADC and low gain DC-amplifier which allows us to record the entire responses including saturation-free stimulus artifact and neural responses with excellent resolution. ResultsOur approach showed saturation-free recording after stimulation, which makes it possible to recover neural spike as early as in 2 ms at the stimulating electrode with digital elimination methods. Comparison with Existing methodsWith our system we could record neural signals after stimulation that was difficult with high gain and high pass filtered recording system due to amplifier saturation. ConclusionsOur new system can enhance interface performance with its higher robustness and with simple system configuration.

Tracking small sensory nerve action potentials in human axonal excitability studies

BackgroundExcitability studies on normal and diseased human axons in vivo have been greatly enhanced by fast non-invasive threshold-tracking techniques, using surface stimulation and recording. Although sensory axons are often more affected in disease, most studies to date have focussed on motor axons, because of technical difficulties in resolving pathologically small nerve volleys in the presence of noise and stimulus artefact. New methodsThis paper describes techniques for tracking low-amplitude compound action potentials, using a battery-powered, isolated preamplifier of simple construction with high common mode rejection (>125 dB [balanced inputs]) and low noise (<0.4 μV referred to inputs [shorted]). ResultsWe demonstrate the preamplifier’s capability by tracking targets as small as 2 μV for a full range of excitability measurements without the usual distortion due to residual stimulus artefact and without the need for clamping, additional filtering or ensemble averaging. Comparison with existing methodsIn practice, threshold-tracking studies have been unable to study sensory axons when the maximal compound sensory action potential was less than about 15 μV. The techniques and amplifier in the present study allow measurements to be made from nerve with maximal responses less than half that size, and we present three recordings in patients with pathologically small nerve action potentials ≤7 μV. ConclusionsBased on measurements of stimulus artefact distortion, noise and the performance in experiments, we conclude that the techniques described here will facilitate the study of diseased axons for which the sensory potentials have high thresholds and may be only a few microvolts in amplitude.

Mapping direct and indirect activation of cortical neurons by weak electrical pulses with massively parallel MEA-based electrical stimulation and recording

Mapping direct and indirect activation of cortical neurons by weak electrical pulses with massively parallel MEA-based electrical stimulation and recording

Temporal filters of electrically stimulated retinal ganglion cells

Event Abstract Back to Event Temporal filters of electrically stimulated retinal ganglion cells Larissa Hoefling1*, Philipp Berens2, 3, 4 and Günther Zeck1 1 Natural and Medical Sciences Institute, Germany 2 Institut für Ophthalmologische Forschung, Universität Tübingen, Germany 3 Werner Reichardt Centrum für Integrative Neurowissenschaften (CIN), Universität Tübingen, Germany 4 Bernstein Center for Computational Neuroscience, Germany Motivation Degenerative diseases like Retinitis Pigmentosa lead to blindness due to the degeneration of photoreceptors, while leaving the network of bipolar, horizontal, amacrine and ganglion cells largely intact. This remaining network can be stimulated electrically to evoke visual percepts, an approach that is already used in retinal implants. We stimulate rd10 mouse retinal ganglion cells (RGCs) using a smooth electrical stimulus waveform to mimic physiological retinal activity, and simultaneously record their responses to map temporal electrical filters (TEF). The TEFs could guide the development of more efficient stimulation protocols for retinal prostheses. Methods We use a high-density CMOS-based microelectrode array to simultaneously deliver smoothed white Gaussian noise stimuli via 1024 capacitive stimulation electrodes and record on 4225 recording electrodes from adult rd10 mouse retina prepared ex vivo and interfaced with the chip in flat-mount epiretinal configuration. Our approach allows recording of single neuron activity during stimulation without interference from stimulus artefacts. Custom-written data analysis software is used to recover the temporal electrical receptive fields of RGCs. Results Using a physiologically plausible smoothed white Gaussian noise stimulus, we are able to activate retinal ganglion cells at low current densities (≤1.5 mA/cm²). Ganglion cell activity is evoked and modulated in a reliable manner for repetitive stimulus presentations. We found two distinct types of TEFs for ON and OFF cells, respectively, suggesting that ON and OFF cells may be activated selectively. Conclusion We present and discuss smooth electrical stimulation as an alternative to pulsatile stimuli for electrically activating retinal ganglion cells in degenerating mouse retina. Acknowledgements Acknowldegement. This research is supported by a grant of the Baden Württemberg Stiftung (RetNetControl, FKZ: NEU 013) Keywords: Electrical Stimulation, receptive field properties, Temporal filters, retinal processing, linear nonlinear models, Retinal Ganglion Cells, CMOS-MEAs Conference: MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays, Reutlingen, Germany, 4 Jul - 6 Jul, 2018. Presentation Type: Poster Presentation Topic: Neural Networks Citation: Hoefling L, Berens P and Zeck G (2019). Temporal filters of electrically stimulated retinal ganglion cells. Conference Abstract: MEA Meeting 2018 | 11th International Meeting on Substrate Integrated Microelectrode Arrays. doi: 10.3389/conf.fncel.2018.38.00082 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 18 Mar 2018; Published Online: 17 Jan 2019. * Correspondence: Ms. Larissa Hoefling, Natural and Medical Sciences Institute, Reutlingen, Germany, larissa.hoefling@student.uni-tuebingen.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Larissa Hoefling Philipp Berens Günther Zeck Google Larissa Hoefling Philipp Berens Günther Zeck Google Scholar Larissa Hoefling Philipp Berens Günther Zeck PubMed Larissa Hoefling Philipp Berens Günther Zeck Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

A new approach for the determination of ECAP thresholds

Background: Electrically evoked compound action potentials (ECAPs) of the auditory nerve are routinely recorded for testing the cochlear implant integrity and its functional connection to the auditory system. The response thresholds derived from ECAP recordings are widely used as a helpful guide in the fitting of the dynamic range of electric stimulation, although they may not always predict the behavioral thresholds of individuals well. Conventionally, this threshold is based on the identification of a minimum N peak and maximum P peak and linear extrapolation of the resulting amplitude growth function (AGF). As an alternative, a new procedure involving numeric signal processing and requiring less user intervention is presented here.Data acquisition: In 12 adults implanted with MED-EL FLEX28 electrodes, two series of ECAPs were recorded immediately after implantation: (i) a full profile involving all 12 channels across the whole stimulus range in steps of 200 current units and (ii) a high resolution section (20 records in the immediate neighborhood of the threshold) of the AGF in one selected channel.Data treatment: It was observed that N and P wave latencies do not depend on stimulus intensity. Fixed time windows were hence defined for stimulus plus noise and noise alone regions. In these windows, the variance of the compound signal representing response and noise is extracted, whereas the noise variance is extracted from the tail of the curve following this time window. The base line is corrected by fitting an exponential function to reduce stimulus or amplifier artifacts. The response threshold is then derived from the response to noise ratio which should exceed the limit of 6 dB.Results: The ECAP thresholds obtained from the new procedure coincide well with those determined by the conventional linear extrapolation of the AGF and they correlate to a greater degree with psychometric thresholds than the existing approach.Conclusions: The new ECAP algorithm looks promising and may reduce the need for user intervention in determining thresholds.

Electrical stimulus artifact cancellation and neural spike detection on large multi-electrode arrays.

Simultaneous electrical stimulation and recording using multi-electrode arrays can provide a valuable technique for studying circuit connectivity and engineering neural interfaces. However, interpreting these measurements is challenging because the spike sorting process (identifying and segregating action potentials arising from different neurons) is greatly complicated by electrical stimulation artifacts across the array, which can exhibit complex and nonlinear waveforms, and overlap temporarily with evoked spikes. Here we develop a scalable algorithm based on a structured Gaussian Process model to estimate the artifact and identify evoked spikes. The effectiveness of our methods is demonstrated in both real and simulated 512-electrode recordings in the peripheral primate retina with single-electrode and several types of multi-electrode stimulation. We establish small error rates in the identification of evoked spikes, with a computational complexity that is compatible with real-time data analysis. This technology may be helpful in the design of future high-resolution sensory prostheses based on tailored stimulation (e.g., retinal prostheses), and for closed-loop neural stimulation at a much larger scale than currently possible.

Reliability of surface electromyography measurements from the suprahyoid muscle complex.

Assessment of swallowing musculature using motor evoked potentials (MEPs) can be used to evaluate neural pathways. However, recording of the swallowing musculature is often invasive, uncomfortable and unrealistic in normal clinical practice. To investigate the possibility of using the suprahyoid muscle complex (SMC) using surface electromyography (sEMG) to assess changes to neural pathways by determining the reliability of measurements in healthy participants over days. Seventeen healthy participants were recruited. Measurements were performed twice with one week between sessions. Single-pulse (at 120% and 140% of the resting motor threshold (rMT)) and paired-pulse (2ms and 15ms paired pulse) transcranial magnetic stimulation (TMS) were used to elicit MEPs in the SMC which were recorded using sEMG. ≈50% of participants (range: 42-58%; depending on stimulus type/intensity) had significantly different MEP values between day 1 and day 2 for single-pulse and paired-pulse TMS. A large stimulus artefact resulted in MEP responses that could not be assessed in four participants. The assessment of the SMC using sEMG following TMS was poorly reliable for ≈50% of participants. Although using sEMG to assess swallowing musculature function is easier to perform clinically and more comfortable to patients than invasive measures, as the measurement of muscle activity using TMS is unreliable, the use of sEMG for this muscle group is not recommended and requires further research and development.

Design and application of a novel gastric pacemaker.

Omnipresent bioelectrical events known as slow waves are responsible for coordinating motility in the gastrointestinal tract. Functional motility diseases, such as gastroparesis, are associated with slow wave dysrhythmias. Electrical stimulation is a potential therapy to correct abnormal slow wave patterns. We present the design and application of a new gastric pacemaker. Real-time changes to the stimulation parameters such as period, amplitude and pulse width were applied using a graphical user interface, which communicated with the microcontroller to deliver the stimulus. The new pacemaker allows the voltage, delivered current and resistance between pacing electrodes to be continuously monitored. The pacing device was applied experimentally and was able to modulate and entrain gastric slow wave activity. After the onset of pacing, the direction of slow wave propagation was altered. Furthermore, the mean velocity and amplitude of slow wave activity increased from 4.7±1.5 to 5.4±1.3 mm/s, and from 1.1±1.1 to 1.7±0.9 mV, respectively. A simplified bidomain electrical model was used to simulate the recorded stimulus artifact. The model illustrated a new approach to evaluate if the stimulus has been delivered to the gastric tissue. The new pacing device and model will be used to investigate the mechanisms that allow pacing to entrain slow wave activity.

Effects of Stimulus Polarity and Artifact Reduction Method on the Electrically Evoked Compound Action Potential.

Previous research from our laboratory comparing electrically evoked compound action potential (ECAP) artifact reduction methods has shown larger amplitudes and lower thresholds with cathodic-leading forward masking (CathFM) than with alternating polarity (AltPol). One interpretation of this result is that the anodic-leading phase used with AltPol elicits a less excitatory response (in contrast to results from recent studies with humans), which when averaged with responses to cathodic-leading stimuli, results in smaller amplitudes. Another interpretation is that the latencies of the responses to anodic- and cathodic-leading pulses differ, which when averaged together, result in smaller amplitudes than for either polarity alone due to temporal smearing. The purpose of this study was to separate the effects of stimulus polarity and artifact reduction method to determine the relative effects of each. This study used a within-subjects design. ECAP growth functions were obtained using CathFM, anodic-leading forward masking (AnodFM), and AltPol for 23 CI recipients (N = 13 Cochlear and N = 10 Advanced Bionics). N1 latency, amplitude, slope of the amplitude growth function, and threshold were compared across methods. Data were analyzed separately for each manufacturer due to inherent differences between devices. N1 latencies were significantly shorter for AnodFM than for CathFM and AltPol for both Cochlear and Advanced Bionics participants. Amplitudes were larger for AnodFM than for either CathFM or AltPol for Cochlear recipients; amplitude was not significantly different across methods for Advanced Bionics recipients. Slopes were shallowest for CathFM for Cochlear subjects, but were not significantly different among methods for Advanced Bionics subjects. Thresholds with AltPol were significantly higher than both FM methods for Cochlear recipients; there was no difference in threshold across methods for the Advanced Bionics recipients. For Cochlear devices, the smaller amplitudes and higher thresholds observed for AltPol seem to be the result of latency differences between polarities. These results suggest that AltPol is not ideal for managing stimulus artifact for ECAP recordings. For the Advanced Bionics group, there were no significant differences among methods for amplitude, slope, or threshold, which suggests that polarity and artifact reduction method have little influence in these devices. We postulate that polarity effects are minimized for symmetrical biphasic pulses that lack an interphase gap, such as those used with Advanced Bionics devices; however, this requires further investigation.