Stimulus Frequency Research Articles

Self-powered multi-parameter sensing system without decoupling algorithm needed based on flexible triboelectric nanogenerator

The rapid development of wireless sensor networks (WSN) and Internet of things (IoT) bring an urgent need for wireless sensor nodes featured with flexibility, self-powering and multi-sensing functionalities. Recently, self-powered sensors based on micro-energy harvesting methods are intensively studied as a potential solution. However, their single sensing function and poor anti-interference capability have become one of the major bottlenecks obstructing their widespread applications and commercialization. This work proposes a flexible textile-based, switch-integrated triboelectric nanogenerator (S-I-TENG), which is able to scavenge mechanical energy and generate an oscillating signal containing sensing information. With a facile circuit configuration based on triboelectric effect, frequency modulation and voltage divider to improve anti-interference capability, external stimulus frequency, capacitive and resistive sensing information with high accuracy can be easily obtained and decoded without any complicated calculation or decoupling algorithm required. Results showed that the self-powered multiparameter sensing system has a high sensitivity of 759.47 Hz/%RH and 0.01096/℃ for humidity and temperature sensing, and accuracy of over 94% and 96%, respectively. The S-I-TENG is adhered to an insole to scavenge human walking energy, which is then used to sense the ambient relative humidity, temperature, and human walking frequency concurrently and display them in real-time.

Multichannel Electrochemical Impedance Spectroscopy and Equivalentcircuit Synthesis of a Kw-Scale Vanadium Redox Flow Battery

An original multichannel Electrochemical Impedance Spectroscopy (EIS) system operating at high bias current and suitable for kW-class Vanadium Redox Flow Batteries (VRFBs) is presented. Power and signal connections, whose stray parameters affected measurements, required a careful optimization and calibration in the implementation of the measurement chain. In any cases, electromagnetic interferences set an upper limit to the applied stimulus frequency that limited the set of measurement data at each bias point. The work provides early results obtained with this EIS system, which were used to identify an equivalent circuit of the whole stack in which each cell is represented with a dynamic Thévenin equivalent, i.e. the series of a voltage source, a variable resistor and a variable RC loop. The dependence of the equivalent parameters on the operating conditions has been experimentally analyzed, confirming that only the RC-loop resistance is strongly affected by mass transport. In addition, more complex equivalent circuits have been proposed, i.e. a Randles cell modified with CPE, a Randles cell modified with CPE and Warburg element and a two-loop model of ZARC elements. The inclusion of the constant phase element constitutes a substantial improvement in the fit, Fig. 1. Conversely, the addition of the Warburg element, which aims to model the mass transfer in the electrochemical process, does not produce significant effects for the frequencies at which we have experimental data. To our knowledge, this is the first dynamic equivalent circuit of a whole stack that has been validated against EIS measurements taken on a real industrial-scale VRFB. Building on the measurement methodology here presented, advanced online state of health of industrial flow batteries can be developed and implemented.

Investigation of Spatiotemporal Profiles of Single-Pulse TMS-Evoked Potentials with Active Stimulation Compared with a Novel Sham Condition.

Identifying genuine cortical stimulation-elicited electroencephalography (EEG) is crucial for improving the validity and reliability of neurophysiology using transcranial magnetic stimulation (TMS) combined with EEG. In this study, we evaluated the spatiotemporal profiles of single-pulse TMS-elicited EEG response administered to the left dorsal prefrontal cortex (DLPFC) in 28 healthy participants, employing active and sham stimulation conditions. We hypothesized that the early component of TEP would be activated in active stimulation compared with sham stimulation. We specifically analyzed the (1) stimulus response, (2) frequency modulation, and (3) phase synchronization of TMS–EEG data at the sensor level and the source level. Compared with the sham condition, the active condition induced a significant increase in TMS-elicited EEG power in the 30–60 ms time interval in the stimulation area at the sensor level. Furthermore, in the source-based analysis, the active condition induced significant increases in TMS-elicited response in the 30–60 ms compared with the sham condition. Collectively, we found that the active condition could specifically activate the early component of TEP compared with the sham condition. Thus, the TMS–EEG method that was applied to the DLPFC could detect the genuine neurophysiological cortical responses by properly handling potential confounding factors such as indirect response noises.

Improved Resistive Switching with Low-Power Synaptic Behaviors of ZnO/Al2O3 Bilayer Structure.

In this work, the resistive switching behavior of bilayer ZnO/Al2O3-based resistive-switching random access memory (RRAM) devices is demonstrated. The polycrystalline nature of the ZnO layer confirms the grain boundary, which helps easy oxygen ion diffusion. Multilevel resistance states were modulated under DC bias by varying the current compliance from 0.1 mA to 0.8 mA, the SET operations where the low resistance state of the memristor device was reduced from 25 kΩ to 2.4 kΩ. The presence of Al2O3 acts as a redox layer and facilitates oxygen vacancy exchange that demonstrates stable gradual conductance change. Stepwise disruption of conductive filaments was monitored depending on the slow DC voltage sweep rate. This is attributed to the atomic scale modulation of oxygen vacancies with four distinct reproducible quantized conductance states, which shows multilevel data storage capability. Moreover, several crucial synaptic properties such as potentiation/depression under identical presynaptic pulses and the spike-rate-dependent plasticity were implemented on ITO/ZnO/Al2O3/TaN memristor. The postsynaptic current change was monitored defining the long-term potentiation by increasing the presynaptic stimulus frequency from 5 Hz to 100 Hz. Moreover, the repetitive pulse voltage stimulation transformed the short-term plasticity to long-term plasticity during spike-number-dependent plasticity.

Mining logical circuits in fungi

Living substrates are capable for nontrivial mappings of electrical signals due to the substrate nonlinear electrical characteristics. This property can be used to realise Boolean functions. Input logical values are represented by amplitude or frequency of electrical stimuli. Output logical values are decoded from electrical responses of living substrates. We demonstrate how logical circuits can be implemented in mycelium bound composites. The mycelium bound composites (fungal materials) are getting growing recognition as building, packaging, decoration and clothing materials. Presently the fungal materials are passive. To make the fungal materials adaptive, i.e. sensing and computing, we should embed logical circuits into them. We demonstrate experimental laboratory prototypes of many-input Boolean functions implemented in fungal materials from oyster fungi P. ostreatus. We characterise complexity of the functions discovered via complexity of the space-time configurations of one-dimensional cellular automata governed by the functions. We show that the mycelium bound composites can implement representative functions from all classes of cellular automata complexity including the computationally universal. The results presented will make an impact in the field of unconventional computing, experimental demonstration of purposeful computing with fungi, and in the field of intelligent materials, as the prototypes of computing mycelium bound composites.

Integrating Visual Information into the Auditory Cortex Promotes Sound Discrimination through Choice-Related Multisensory Integration.

An increasing number of studies have shown that cross-modal interaction can occur in early sensory cortices. Yet, how neurons in sensory cortices integrate multisensory cues in perceptual tasks and to what extent this influences behavior is largely unclear. To investigate, we examined visual modulation of auditory responses in the primary auditory cortex (A1) in a two-alternative forced-choice task. During the task, male rats were required to make a behavioral choice based on the pure tone frequency (low vs high) of the self-triggered stimulus to get a water reward. The result showed that the presence of a noninformative visual cue did not uniformly influence auditory response, with frequently enhancing just one of them. Closely correlated with behavioral choice, the visual cue mainly enhanced responsiveness to the auditory cue indicating a movement direction contralateral to A1 being recorded. Operating in this fashion provided A1 neurons a superior capability to discriminate sound during multisensory trials. Concomitantly, behavioral data and decoding analysis revealed that visual cue presence could speed the process of sound discrimination. We also observed this differential multisensory integration effect in well-trained rats when tested with passive stimulation and under anesthesia, albeit to a much lesser extent. We did not see this differentially integrative effect while recording in A1 in another similar group of rats performing a free-choice task. These data suggest that auditory cortex can engage in meaningful audiovisual processing, and perceptual learning can modify its multisensory integration mechanism to meet task requirements.SIGNIFICANCE STATEMENT In the natural environment, visual stimuli are frequently accompanied by auditory cues. Although multisensory integration has traditionally been seen as a feature of associational cortices, recent studies have shown that cross-modal inputs can also influence neuronal activity in primary sensory cortices. However, exactly how neurons in sensory cortices integrate multisensory cues to guide behavioral choice is still unclear. Here, we describe a novel model of multisensory integration used by A1 neurons to shape auditory representations when rats performed a cue-guided task. We found that a task-irrelevant visual cue could specifically enhance the response of neurons in sound guiding to the contralateral choice. This differentially integrative model facilitated sound discrimination and behavioral choice. This result indicates that task engagement can modulate multisensory integration.

Speech-Evoked Envelope Following Responses in Children and Adults.

Envelope following responses (EFRs) could be useful for objectively evaluating audibility of speech in children who are unable to participate in routine clinical tests. However, relative to adults, the characteristics of EFRs elicited by frequency-specific speech and their utility in predicting audibility in children are unknown. EFRs were elicited by the first (F1) and second and higher formants (F2+) of male-spoken vowels /u/ and /i/ and by fricatives /ʃ/ and /s/ in the token /suʃi/ presented at 15, 35, 55, 65, and 75 dB SPL. The F1, F2+, and fricatives were low-, mid-, and high-frequency dominant, respectively. EFRs were recorded between the vertex and the nape from twenty-three 6- to 17-year-old children and 21 young adults with normal hearing. Sensation levels of stimuli were estimated based on behavioral thresholds. In children, amplitude decreased with age for /ʃ/-elicited EFRs but remained stable for low- and mid-frequency stimuli. As a group, EFR amplitude and phase coherence did not differ from that of adults. EFR sensitivity (proportion of audible stimuli detected) and specificity (proportion of inaudible stimuli not detected) did not vary between children and adults. Consistent with previous work, EFR sensitivity increased with stimulus frequency and level. The type of statistical indicator used for EFR detection did not influence accuracy in children. Adultlike EFRs in 6- to 17-year-old typically developing children suggest mature envelope encoding for low- and mid-frequency stimuli. EFR sensitivity and specificity in children, when considering a wide range of stimulus levels and audibility, are ~77% and ~92%, respectively. https://doi.org/10.23641/asha.21136171.

Skin Pain Sensation Under Mechanical Stimulus: Wind-Up and Ramp-Off

Abstract Researchers have been studying the pain sensation extensively in the past few decades. Quantitative simulation and theoretical modeling of pain sensation based on experimental results are necessary for pain research. Many theories have been proposed to explain the mechanism of pain from molecular, cellular, and neuron network perspectives. But some phenomena in pain sensation are not fully understood, including wind-up and ramp-off. This paper focused on the theoretical model of wind-up and ramp-off phenomena in the pain sensation. With the addition of the transduction model, the generation mechanism of wind-up and ramp-off is better explained. The simulations were carried out to analyze the skin pain sensation under the mechanical stimulus, consisting of four different parts: the mechanical model of skin, transduction, transmission, modulation, and perception. The stress distribution on the skin was obtained based on the elastic theory. And the modified Hodgkin and Huxley model and the mathematical model of gate control theory were utilized to analyze the process of transduction, modulation, and perception, respectively. The numerical experiments demonstrated the wind-up occurs with a frequent stimulus of 1 Hz and 2 Hz, and ramp-off appears with the withdrawal of constant mechanical stimulus, which could contribute to the understanding of the pain sensation mechanism.

A subject-independent SSVEP-based BCI target detection system based on fuzzy ordering of EEG task-related components

BCI systems are recognized as a means of interaction between the brain and the computer. In this research, a subject-independent BCI target detection system based on SSVEP was developed using the information in the task-related components and their fuzzy memberships to the regarding task. To assess the proposed system, an online available SSVEP data with four stimulus frequencies of 5, 6, 7 and 8 Hz from five participants were used. First, the task-related components of the SSVEP data were extracted using the TRCA algorithm. Then the membership of each task-related component was determined to each stimulus frequency, and the components were ordered fuzzily based on these membership values. Finally, a new concept named “Isolated Group” was defined and the final data label was decided by the identification of the isolated groups. Based on the results, observation of the detected isolated groups, showed that the isolated groups’ components were mostly related to the stimulus frequency, based on their FFT plots. The classification results obtained using Leave-One-Subject-Out method were 100 %, 79.17 %, 70.83 %, 70.83 % and 95.83 % (mean: 83.33 %) for five participants, respectively. In summary, the fuzzy ordering of task-related components was used to pick the components with the most important information for target detection. Although the detection of stimulus frequency in BCI applications has been widely studied, most current target detection systems require an intensive training process for every individual. In contrast, in this research‘s method, only a concise training process for determining criteria weights in terms of an optimization step, is needed.

Manganese-enhanced magnetic resonance imaging detects activation of limbic structures in response to auditory stimuli of different frequencies

PurposeAs we are exposed to stress on a daily basis, it is important to detect and treat stress during the subclinical period. However, methods to quantify and confirm stress are currently unavailable, and the detection of subclinical stressors is difficult. This study aimed to determine whether manganese-enhanced magnetic resonance imaging (MEMRI) could be used to assess stress in rat brains. MethodsWe exposed male Wistar/ST rats bred in a specific pathogen-free environment to ultrasound stimuli (22 kHz and 55 kHz) for 10 days and then assessed brain activities using MEMRI, the light/dark box test, and ΔFosB immunohistochemical staining. ResultsIn the MEMRI assessments, exposure at 22 kHz activated the periaqueductal gray, while exposure at 55 kHz specifically enhanced activity in the nucleus accumbens core and the orbitofrontal cortex. The exploratory behavior of the 55-kHz group increased sharply, while that of the 22-kHz group showed a lower exploratory value. ΔFosB expression increased in the orbitofrontal cortex, nucleus accumbens, periaqueductal gray, and amygdaloid nucleus in the 22-kHz group. ConclusionUltrasound stimuli at 22 kHz suppressed weight gain in rats and excessive ΔFosB induction in the nucleus accumbens caused excessive sensitization of the neural circuit, thereby contributing to pathological behavior. We thus demonstrated that MEMRI can be useful to objectively assess the pathophysiology of stress-related disorders.

Physiological noise facilitates multiplexed coding of vibrotactile-like signals in somatosensory cortex

Neurons can use different aspects of their spiking to simultaneously represent (multiplex) different features of a stimulus. For example, some pyramidal neurons in primary somatosensory cortex (S1) use the rate and timing of their spikes to, respectively, encode the intensity and frequency of vibrotactile stimuli. Doing so has several requirements. Because they fire at low rates, pyramidal neurons cannot entrain 1:1 with high-frequency (100 to 600 Hz) inputs and, instead, must skip (i.e., not respond to) some stimulus cycles. The proportion of skipped cycles must vary inversely with stimulus intensity for firing rate to encode stimulus intensity. Spikes must phase-lock to the stimulus for spike times (intervals) to encode stimulus frequency, but, in addition, skipping must occur irregularly to avoid aliasing. Using simulations and invitro experiments in which mouse S1 pyramidal neurons were stimulated with inputs emulating those induced by vibrotactile stimuli, we show that fewer cycles are skipped as stimulus intensity increases, as required for rate coding, and that intrinsic or synaptic noise can induce irregular skipping without disrupting phase locking, as required for temporal coding. This occurs because noise can modulate the reliability without disrupting the precision of spikes evoked by small-amplitude, fast-onset signals. Specifically, in the fluctuation-driven regime associated with sparse spiking, rate and temporal coding are both paradoxically improved by the strong synaptic noise characteristic of the intact cortex. Our results demonstrate that multiplexed coding by S1 pyramidal neurons is not only feasible under invivo conditions, but that background synaptic noise is actually beneficial.

From target to actor: Contagion of honesty and deception across buyer–supplier negotiations

AbstractUnderstanding negotiators' decision‐making processes in buyer–supplier relationships has been of key interest to behavioral operations and supply chain management researchers. We hypothesize that through the exposure to various counterparts in the supply chain network, negotiators' behaviors are influenced by others' behaviors and prone to behavioral contagion—where the target adopts the behavior he/she is subjected to and exhibits it toward a nonpartisan counterpart. This paper examines such contagion where targets turn into actors of honesty and deception across buyer–supplier negotiations. We analyzed text responses from two studies employing scenario‐based negotiation experiments with 350 and 424 individuals with B2B sales experience and tested for the effects of previously received behavior (honesty/deception) and different frequency levels of the received behavior. Our results show contagion effects for honesty and deception in both studies. These effects are largely independent from the frequency with which the behaviors have been received, suggesting that a low stimulus frequency suffices to induce contagion of honest and deceptive behaviors across buyer–supplier negotiations. Interestingly, in contrast to our hypothesized key mechanism based on the extant literature, neither injunctive nor descriptive social norms mediated the relationship between received and acted behaviors. We offer alternative explanations and discuss implications for theory and practice.

Age and Sex-Related Changes in Retinal Function in the Vervet Monkey.

Among the deficits in visual processing that accompany healthy aging, the earliest originate in the retina. Moreover, sex-related differences in retinal function have been increasingly recognized. To better understand the dynamics of the retinal aging trajectory, we used the light-adapted flicker electroretinogram (ERG) to functionally assess the state of the neuroretina in a large cohort of age- and sex-matched vervet monkeys (N = 35), aged 9 to 28 years old, with no signs of obvious ocular pathology. We primarily isolated the cone–bipolar axis by stimulating the retina with a standard intensity light flash (2.57 cd/s/m2) at eight different frequencies, ranging from 5 to 40 Hz. Sex-specific changes in the voltage and temporal characteristics of the flicker waveform were found in older individuals (21–28 years-old, N = 16), when compared to younger monkeys (9–20 years-old, N = 19), across all stimulus frequencies tested. Specifically, significantly prolonged implicit times were observed in older monkeys (p < 0.05), but a significant reduction of the amplitude of the response was only found in old male monkeys (p < 0.05). These changes might reflect ongoing degenerative processes targeting the retinal circuitry and the cone subsystem in particular. Altogether, our findings corroborate the existing literature in humans and other species, where aging detrimentally affects photopic retinal responses, and draw attention to the potential contribution of different hormonal environments.

Improving the Classification of Real-World SSVEP Data in Brain-Computer Interface Speller Systems Using Deep Convolutional Neural Networks

Purpose: Brain-Computer Interface (BCI) Speller systems help people with mobility impairments improve their cognitive and physical abilities. Steady-State Visual Evoked Potential (SSVEP) signals have been used to build high-speed BCI speller systems. SSVEP signals are a subtype of Visual Evoked Potential (VEP), a form of co-frequency, and the harmonics response elicited by a specific frequency stimulus. Noise and artifacts are critical issues for target detection in SSVEP-based BCI systems.&#x0D; Materials and Methods: Thus, it is essential to provide target detection techniques that operate well in the presence of noises. One solution for overcoming the noise impact is to employ approaches that automatically extract the appropriate features for target detection from the training data. Deep Convolutional Neural Network (DCNN) was utilized in this study to automatically extract features from SSVEP data in noisy conditions. Moreover, the BETA database, which contains SSVEP data from 70 individuals collected outside of the electromagnetic shielding room, was used. In this regard, a suitable DCNN structure for target stimulus frequency identification was first designed. The network was pre-trained with part of the data from the BETA database. Finally, at the single-subject level, this pre-trained network was retrained and evaluated.&#x0D; Results: The results showed that after retraining, the accuracy and Information Transfer Rate (ITR) increased (p-value &lt; 0.01) for all participants.&#x0D; Conclusion:.The enhancement in accuracy and ITR are 25.72% and 43.10 bpm, respectively.

Variability in the Estimated Amplitude of Vowel-Evoked Envelope Following Responses Caused by Assumed Neurophysiologic Processing Delays.

Vowel-evoked envelope following responses (EFRs) reflect neural encoding of the fundamental frequency of voice (f0). Accurate analysis of EFRs elicited by natural vowels requires the use of methods like the Fourier analyzer (FA) to consider the production-related f0 changes. The FA's accuracy in estimating EFRs is, however, dependent on the assumed neurophysiological processing delay needed to time-align the f0 time course and the recorded electroencephalogram (EEG). For male-spoken vowels (f0 ~ 100Hz), a constant 10-ms delay correction is often assumed. Since processing delays vary with stimulus and physiological factors, we quantified (i) the delay-related variability that would occur in EFR estimation, and (ii) the influence of stimulus frequency, non-f0 related neural activity, and the listener's age on such variability. EFRs were elicited by the low-frequency first formant, and mid-frequency second and higher formants of /u/, /a/, and /i/ in young adults and 6- to 17-year-old children. To time-align with the f0 time course, EEG was shifted by delays between 5 and 25ms to encompass plausible response latencies. The delay-dependent range in EFR amplitude did not vary by stimulus frequency or age and was significantly smaller when interference from low-frequency activity was reduced. On average, the delay-dependent range was < 22% of the maximum variability in EFR amplitude that could be expected by noise. Results suggest that using a constant EEG delay correction in FA analysis does not substantially alter EFR amplitude estimation. In the present study, the lack of substantial variability was likely facilitated by using vowels with small f0 ranges.

A novel form of transcutaneous electrical nerve stimulation for the reduction of dysesthesias caused by spinal nerve dysfunction: A case series.

BackgroundCurrent therapeutic interventions for dysesthesias caused by spinal cord dysfunctions are ineffective. We propose a novel intervention using transcutaneous electrical nerve stimulation (TENS) for dysesthesias, and we present an in-depth case series.Patients and methodsConventional high-frequency TENS and the novel dysesthesia-matched TENS (DM-TENS) were applied to 16 hands of nine patients with spinal cord dysfunction. The dysesthesia-matched TENS’ stimulus intensity and frequency matched the intensity and somatosensory profile of the patients’ dysesthesias. The Short-Form McGill Pain Questionnaire version-2 (SF-MPQ2) and quantitative sensory testing (QST) were applied during electrical stimulation/no stimulation. We determined intraclass correlation coefficients (ICCs) to evaluate the reliability of the setting and the effects on the dysesthesias and the change in subjective dysesthesia between each patient’s baseline without TENS and DM-TENS.ResultsWe were able to apply electrical stimulation matching the patients’ subjective dysesthesia for 14 hands (eight patients). TENS could not be applied for the remaining patient due to severe sensory deficits. Compared to the patients’ baseline and high-frequency TENS, the DM-TENS provided significant decreases in tingling/pins-and-needles and numbness on the SF-MPQ2, and it significantly improved the dynamic and static mechanical detection on QST. Regarding the reliability of the dysesthesia-matched TENS settings, the ICCs (1,5) were intensity, 0.95; frequency, 1.00; and effect on dysesthesia, 0.98.ConclusionDM-TENS improved the dysesthesias and mechanical hypoesthesia caused by spinal cord dysfunction. The effectiveness of DM-TENS particularly for tingling and numbness was clearly higher and was reliable within the patients. These results may suggest an effective treatment of dysesthesias in patients with spinal cord dysfunction.Clinical trial registration[https://rctportal.niph.go.jp/s/detail/um?trial_id=UMIN000045332], identifier [UMIN000045332].

Retinal ganglion cell desensitization is mitigated by varying parameter constant excitation pulse trains.

Retinal prostheses partially restore vision in patients blinded by retinitis pigmentosa (RP) and age-related macular degeneration (AMD). One issue that limits the effectiveness of retinal stimulation is the desensitization of the retina response to repeated pulses. Rapid fading of percepts is reported in clinical studies. We studied the retinal output evoked by fixed pulse trains vs. pulse trains that have variable parameters pulse-to-pulse. We used the current clamp to record RGC spiking in the isolated mouse retina. Trains of biphasic current pulses at different frequencies and amplitudes were applied. The main results we report are: (1) RGC desensitization was induced by increasing stimulus frequency, but was unrelated to stimulus amplitude. Desensitization persisted when the 20 Hz stimulation pulses were applied to the retinal ganglion cells at 65 μA, 85 μA, and 105 μA. Subsequent pulses in the train evoked fewer spikes. There was no obvious desensitization when 2 Hz stimulation pulse trains were applied. (2) Blocking inhibitory GABAA receptor increased spontaneous activity but did not reduce desensitization. (3) Pulse trains with constant charge or excitation (based on strength-duration curves) but varying pulse width, amplitude, and shape increased the number of evoked spikes/pulse throughout the pulse train. This suggests that retinal desensitization can be partially overcome by introducing variability into each pulse.

The influence of imagery vividness and internally-directed attention on the neural mechanisms underlying the encoding of visual mental images into episodic memory.

Attention can be directed externally toward sensory information or internally toward self-generated information. Using electroencephalography (EEG), we investigated the attentional processes underlying the formation and encoding of self-generated mental images into episodic memory. Participants viewed flickering words referring to common objects and were tasked with forming visual mental images of the objects and rating their vividness. Subsequent memory for the presented object words was assessed using an old-new recognition task. Internally-directed attention during image generation was indexed as a reduction in steady-state visual evoked potentials (SSVEPs), oscillatory EEG responses at the frequency of a flickering stimulus. The results yielded 3 main findings. First, SSVEP power driven by the flickering word stimuli decreased as subjects directed attention internally to form the corresponding mental image. Second, SSVEP power returned to pre-imagery baseline more slowly for low- than high-vividness later remembered items, suggesting that longer internally-directed attention is required to generate subsequently remembered low-vividness images. Finally, the event-related-potential difference due to memory was more sustained for subsequently remembered low- versus high-vividness items, suggesting that additional conceptual processing may have been needed to remember the low-vividness visual images. Taken together, the results clarify the neural mechanisms supporting the encoding of self-generated information.

Effects of Stimulus Frequency, Intensity, and Sex on the Autonomic Response to Transcutaneous Vagus Nerve Stimulation

This study aimed to determine how transcutaneous vagus nerve stimulation (tVNS) alters autonomic nervous activity by comparing the effects of different tVNS frequencies and current intensities. We also investigated the sex-dependent autonomic response to tVNS. Thirty-five healthy adult participants were stimulated using a tVNS stimulator at the left cymba conchae while sitting on a reclining chair; tVNS-induced waveform changes were then recorded for different stimulus frequencies (Experiment 1: 3.0 mA at 100 Hz, 25 Hz, 10 Hz, 1 Hz, and 0 Hz (no stimulation)) and current intensities (Experiment 2: 100 Hz at 3.0 mA, 1.0 mA, 0.2 mA (below sensory threshold), and 0 mA (no stimulation)) using an electrocardiogram. Pulse widths were set at 250 µs in both experiment 1 and 2. Changes in heart rate (HR), root-mean-square of the difference between two successive R waves (RMSSD), and the ratio between low-frequency (LF) (0.04–0.15 Hz) and high-frequency (HF) (0.15–0.40 Hz) bands (LF/HF) in spectral analysis, which indicates sympathetic and parasympathetic activity, respectively, in heart rate variability (HRV), were recorded for analysis. Although stimulation at all frequencies significantly reduced HR (p = 0.001), stimulation at 100 Hz had the most pronounced effect (p = 0.001) in Experiment 1 and was revealed to be required to deliver at 3.0 mA in Experiment 2 (p = 0.003). Additionally, participants with higher baseline sympathetic activity experienced higher parasympathetic response during stimulation, and sex differences may exist in the autonomic responses by the application of tVNS. Therefore, our findings suggest that optimal autonomic changes induced by tVNS to the left cymba conchae vary depending on stimulating parameters and sex.

Frequency-dependent depression of the NTS synapse affects the temporal response of the antihypertensive effect of auricular vagus nerve stimulation (aVNS)

Objective. Auricular vagus nerve stimulation (aVNS) has recently emerged as a promising neuromodulation modality for blood pressure (BP) reduction due to its ease of use although its efficacy is still limited compared to direct baroreflex stimulation. Previous studies have also indicated that synaptic depression of nucleus tractus solitarius (NTS) in the baroreflex pathway depends on stimulus frequency. However, the nature of this frequency dependence phenomenon on antihypertensive effect has been unknown for aVNS. We aimed to investigate the antihypertensive effect of aVNS considering frequency-dependent depression characteristic in the NTS synapse. We explored NTS activation and BP reduction induced by aVNS and by direct secondary neuron stimulation (DS). Approach. Both protocols were performed with recording of NTS activation and BP response with stimulation for each frequency parameter (2, 4, 20, 50, and 80 Hz). Main results. The BP recovery time constant was significantly dependent on the frequency of DS and aVNS (DS—2 Hz: 8.17 ± 4.98; 4 Hz: 9.73 ± 6.3; 20 Hz: 6.61 ± 3.28; 50 Hz: 4.93 ± 1.65; 80 Hz: 4.00 ± 1.43, p < 0.001, Kruskal–Wallis (KW) H-test/aVNS—2 Hz: 4.02 ± 2.55; 4 Hz: 8.13 ± 4.05; 20 Hz: 6.40 ± 3.16; 50 Hz: 5.18 ± 2.37; 80 Hz: 3.13 ± 1.29, p < 0.05, KW H-test) despite no significant BP reduction at 2 Hz compared to sham groups (p > 0.05, Mann–Whitney U-test). Significance. Our observations suggest that the antihypertensive effect of aVNS is influenced by the characteristics of frequency-dependent synaptic depression in the NTS neuron in terms of the BP recovery time. These findings suggest that the antihypertensive effect of aVNS can be improved with further understanding of the neurological properties of the baroreflex associated with aVNS, which is critical to push this new modality for clinical interpretation.