Sudomotor nerve conduction velocity and central processing time of the skin conductance response

Pain‐autonomic interaction is a reliable measure of pain habituation in healthy subjects

Background

Habituation is a response decrement resulting from repeated stimuli. Reduced habituation to noxious stimuli is considered to be a proxy for central sensitization in subjects with chronic pain. Despite numerous investigations of pain habituation in relation to central sensitization, there is no consensus on the most sensitive and reliable readout, as well as analysis approach. Therefore, this study compared the usability and reliability of different readouts and habituation analysis approaches to measure pain habituation in response to repetitive heat simulation.

Methods

Three blocks of 20 contact heat stimuli were applied on the volar forearm of 20 healthy subjects on two separate visits. Habituation was assessed by three different readouts: pain ratings, contact heat evoked potentials (CHEPs) and heat‐induced sympathetic skin responses (SSRs). In addition, two different habituation analysis approaches were used: between the three stimulation blocks (between‐block) and within the first stimulation block (within‐block).

Results

Significant between‐block habituation for SSRs (p < 0.001), but not for pain ratings (p = 1.000) and CHEPs (p = 0.078) was found. There was significant within‐block habituation for pain ratings (p = 0.012) and SSRs (p < 0.001), but not for CHEPs (p = 0.246). Only the between‐block habituation of heat‐induced SSR was reliable between the two visits (first to second block: intraclass correlation coefficient [ICC] = 0.58, p = 0.030; first to third block: ICC = 0.64, p = 0.015).

Conclusion

Heat‐induced SSR as a measure of pain‐autonomic interaction revealed the strongest pain habituation and showed the highest test–retest reliability.

An Evolutionarily Threat-Relevant Odor Strengthens Human Fear Memory

Olfaction is an evolutionary ancient sense, but it remains unclear to what extent it can influence routine human behavior. We examined whether a threat-relevant predator odor (2-methyl-2-thiazoline) would contextually enhance the formation of human fear memory associations. Participants who learned to associate visual stimuli with electric shock in this predator odor context later showed stronger fear responses to the visual stimuli than participants who learned in an aversiveness-matched control odor context. This effect generalized to testing in another odor context, even after extinction training. Results of a separate experiment indicate that a possible biological mechanism for this effect may be increased cortisol levels in a predator odor context. These results suggest that innate olfactory processes can play an important role in human fear learning. Modulatory influences of odor contexts may partly explain the sometimes maladaptive persistence of human fear memory, e.g., in post-traumatic stress disorders.

Sympathetic and Parasympathetic Modulation of Pupillary Unrest

Pupillary unrest is an established indicator of drowsiness or sleepiness. How sympathetic and parasympathetic activity contribute to pupillary unrest is not entirely unclear. In this study, we investigated 83 young healthy volunteers to assess the relationship of pupillary unrest to other markers of the autonomic nervous system. Sample entropy (SE) and the established pupillary unrest index (PUI) were calculated to characterize pupil size variability. Autonomic indices were derived from heart rate, blood pressure, respiration, and skin conductance. Additionally, we assessed individual levels of calmness, vigilance, and mood. In an independent sample of 26 healthy participants, we stimulated the cardiovagal system by a deep breathing test. PUI was related to parasympathetic cardiac indices and sleepiness. A linear combination of vagal heart rate variability [root mean square of heart beat interval differences (RMSSD)] and skin conductance fluctuations (SCFs) was suited best to explain interindividual variance of PUI. Complexity of pupil diameter (PD) variations correlated to indices of sympathetic skin conductance. Furthermore, we found that spontaneous fluctuations of skin conductance are accompanied by increases of pupil size. In an independent sample, we were able to corroborate the relation of PUI with RMSSD and skin conductance. A slow breathing test enhanced RMSSD and PUI proportionally to each other, while complexity of PD dynamics decreased. Our data suggest that the slow PD oscillations (f < 0.15 Hz) quantified by PUI are related to the parasympathetic modulation. Sympathetic arousal as detected by SCFs is associated to transient pupil size increases that increase non-linear pupillary dynamics.

Physiological Correlates of Fluent and Stuttered Speech Production in Preschool Children Who Stutter

Purpose The purpose of this study was to compare physiological indices of sympathetic nervous system arousal recorded during fluent and stuttered utterances in a preschool children who stutter (CWS). Method Twenty-two 4- to 5-year-old CWS participated in the experiment. We recorded children's skin conductance response amplitude and frequency, blood pulse volume amplitude, and pulse rate as they completed a picture description task. We then compared indices of phasic sympathetic arousal recorded during stuttered versus fluent utterances. In addition, children's communication attitudes were evaluated with a self-report measure. Results We detected significantly higher sympathetic arousal during stuttered utterances compared to fluent utterances. Specifically, we found larger skin conductance responses occurring at an increased frequency and decreased blood pulse volume amplitudes during stuttered speech. The behavioral measure indicated a negative communication attitude in only one-third of the participants. Conclusion Our findings suggest that preschool CWS may exhibit higher levels of sympathetic arousal during stuttered speech compared to when they are speaking fluently. We discuss the potential impact of increased sympathetic arousal on speech regulatory mechanisms in early childhood stuttering and present questions to guide future research.

Sympathetic Nervous System Activity in Preschoolers Who Stutter

BACKGROUND

In our Dynamic Pathways, account, we hypothesized that childhood stuttering reflects an impairment in speech sensorimotor control that is conditioned by cognitive, linguistic, and emotional factors. The purpose of this study was to investigate potential differences in levels of sympathetic arousal during performance of speech and non-speech tasks between children who do and do not stutter.

METHODS

Seventy-two preschool-aged children participated in the study, 47 children who stutter (CWS; 38 boys) and 25 children who do not stutter (CWNS; 18 boys). We recorded skin conductance and blood pulse volume (BPV) signals, indices of sympathetic arousal, during higher/lower load speech tasks (structured sentence production and picture description) and non-speech tasks (jaw wagging and forceful blowing). We included a measure that reflects children's attitudes about their communication skills and a parent-report assessment of temperament.

RESULTS

We found no significant differences between preschool CWS and CWNS in phasic skin conductance response amplitude or frequency, BPV, and pulse rate for any of the experimental tasks. However, compared to CWNS, CWS had, on average, significantly higher skin conductance levels (SCL), indexing slowly changing tonic sympathetic activity, across both speech and non-speech experimental conditions. We found distinctive task-related profiles of sympathetic arousal in both groups of preschool children. Most children produced the highest levels of sympathetic arousal in the physically demanding blowing task rather than in speech, as seen in previous studies of adults. We did not find differences in temperament between the two groups of preschool children nor a relationship among behavioral indices of temperament and communication attitude and physiological measures of sympathetic arousal.

CONCLUSION

We did not find that atypically high, speech-related sympathetic arousal is a significant factor in early childhood stuttering. Rather, CWS had higher, on average, task-related tonic SCLs across speech and non-speech tasks. A relationship among behavioral measures of temperament and physiological measures of sympathetic arousal was not confirmed. Key questions for future experiments are how the typical coupling of sympathetic and speech sensorimotor systems develops over childhood and adolescence and whether task related developmental profiles follow a different course in children who continue to stutter.

Neural representation of swallowing is retained with age. A functional neuroimaging study validated by classical and Bayesian inference

We investigated the neural representation of swallowing in two age groups for a total of 51 healthy participants (seniors: average age 64 years; young adults: average age 24 years) using high spatial resolution functional magnetic resonance imaging (fMRI). Two statistical comparisons (classical and Bayesian inference) revealed no significant differences between subject groups, apart from higher cortical activation for the seniors in the frontal pole 1 of Brodmann's Area 10 using Bayesian inference. Seniors vs. young participants showed longer reaction times and higher skin conductance response (SCR) during swallowing. We found a positive association of SCR and fMRI-activation only among seniors in areas processing sensorimotor performance, arousal and emotional perception. The results indicate that the highly automated swallowing network retains its functionality with age. However, seniors with higher SCR during swallowing appear to also engage areas involved in attention control and emotional regulation, possibly suggesting increased attention and emotional demands during task performance.

Arousal electrical stimuli evoke sudomotor activity related to P300, and skin vasoconstrictor activity related to N140 in humans

OBJECTIVE

Arousal stimuli evoke bursts of skin sympathetic nerve activity (SSNA). SSNA usually contains sudomotor and vasoconstrictor neural spikes. The aim of this study was to elucidate which components of event-related potentials (ERPs) are related to sudomotor and vasoconstrictor responses comprising arousal SSNA bursts.

METHODS

We recorded SSNA from the tibial nerve by microneurography, with corresponding sympathetic skin response (SSR), sympathetic flow response (SFR), and ERPs in 10 healthy subjects. Electrical stimulation of the median nerve was used to induce arousal responses. ERPs were classified by the occurrence of SSR and SFR.

RESULTS

SSNA bursts followed by SSR were associated with larger P300 than SSNA bursts followed by no SSR. For N140, no difference in the amplitude was found between SSNA bursts with and without SSR. SSNA bursts followed by SFR were associated with larger N140 than SSNA bursts followed by no SFR. However, there were no differences in the amplitude of P300 between SSNA bursts with and without SFR.

CONCLUSIONS

Sudomotor and skin vasoconstrictor responses to arousal stimuli were differently associated with distinct ERP components.

SIGNIFICANCE

The possibility that sudomotor and skin vasoconstrictor activities comprising arousal SSNA reflect different stages of the cognitive process is suggested.

Can you give me a hand? A comparison of hands and feet as optimal anatomical sites for skin conductance recording

The fingers and feet have long been accepted as optimal anatomical recording sites for electrodermal activity. The available literature suggests that the feet are more responsive than the fingers. The present report compared skin conductance level (SCL) and responses (SCRs) from the left foot and the distal phalanges of the fingers on the nondominant hand among 19 participants. The principal results were (a) SCRs recorded from the fingers were significantly larger and more frequent with shorter latencies than SCRs from the foot, (b) SCL from the fingers was significantly higher than from the foot, (c) the fingers exhibited significantly greater discrimination conditioning than the foot, and (d) skin conductance measures recorded from the fingers and foot were significantly positively correlated. Specifically, our results demonstrate that the distal phalanges of the fingers are electrodermally more responsive than the abductor hallucis area of the foot.

An MEG compatible system for measuring skin conductance responses

We present the design of a low-cost system for recording galvanic skin conductance responses (SCRs) from humans in a magnetically shielded room (MSR) simultaneously to magnetoencephalography (MEG). Such a system was so far not available to the MEG community. Its availability is of utmost importance for neuroscience, since it will allow the concurrent assessment of the autonomic and central nervous system activity. The overall system design optimizes high signal to noise ratio (SNR) of SCRs and achieves minimal distortion of the MEG signal. Its development was based on a fiber-optic transformer, with voltage to optical transduction inside the MSR and demodulation outside the MSR. The system was calibrated and tested inside the MEG environment by using a 151-channel CTF whole head system (VSM MedTech Ltd.). MEG measurements were recorded simultaneously to SCRs from five healthy participants to test whether the developed system does not generate artifacts in the MEG data. Two measurements were performed for each participant; one without the system in the MSR, and one with the system in the MSR, connected to the participant and in operation. The data were analyzed using the time and frequency domains in separate statistical analysis. No significant differences were observed between the two sessions for any statistic index. Our results show that the system allows high quality simultaneous recordings of SCRs and MEG signals in the MSR, and can therefore be used as routine addendum to neuroscience experiments.

Emotional sweating across the body: comparing 16 different skin conductance measurement locations

Skin conductance (SC) is one of the most commonly used measures in psychophysiological studies involving emotional arousal and is traditionally measured at the fingers or the palms (i.e., the palmar locations) of the hand. Palmar skin conductance recording positions are, however, not always preferred for ambulatory recordings in real-life situations. This study quantifies the responsiveness and similarity with the finger of 16 different recording positions of skin conductance while watching emotional film fragments. Findings indicated foot, fingers and shoulders being most responsive, whereas arm, back, armpit, and thighbone were least responsive. The measurements at the foot were most similar with those of the finger. In contrast, arm, back, and armpit traces differed most from the finger trace. Taken together, foot and shoulders are the best alternatives to the finger for ambulatory measurement of skin conductance to reflect emotional arousal. These findings can help new applications using skin conductance, like automated emotion measurements, to come to fruition.

What does autonomic arousal tell us about locomotor learning?

Walking onto a stationary sled previously experienced as moving induces locomotor aftereffects (LAE, or "broken escalator phenomenon"). This particular form of aftereffect can develop after a single adaptation trial and occurs despite subjects being fully aware that the sled will not move. Here, we investigate whether such strong LAE expression may relate to arousal or fear related to instability during the gait adaptation process. Forty healthy subjects were allocated to three sled velocity groups; SLOW (0.6 m/s), MEDIUM (1.3 m/s), or FAST (2.0 m/s). Subjects walked onto the stationary sled for five trials (BEFORE), then onto the moving sled for 15 trials (adaptation or MOVING trials) and, finally, again onto the stationary sled for five trials (AFTER). Explicit warning regarding sled status was given. Trunk position, foot-sled contact timing, autonomic markers (electrodermal activity [EDA], ECG, respiratory movements) in addition to self-reported task-related confidence and state/trait anxiety were recorded. Trunk sway, EDA, and R-R interval shortening were greatest during the first MOVING trial (MOVING_1), progressively attenuating during subsequent MOVING trials. A LAE, recorded as increased gait velocity and trunk sway during AFTER_1, occurred in both MEDIUM and FAST sled velocity groups. The amplitude of forward trunk sway in AFTER_1 (an indicator of aftereffect magnitude) was related to EDA during the final adaptation trial (MOVING_15). AFTER_1 gait velocity (also an indicator of aftereffect magnitude) was related to MOVING_1 trunk sway. Hence, gait velocity and trunk sway components of the LAE are differentially related to kinematic and autonomic parameters during the early and late adaptation phase. The finding that EDA is a predictor of LAE expression indicates that autonomic arousal or fear-based mechanisms can promote locomotor learning. This could in turn explain some unusual characteristics of this LAE, namely its resistance to explicit knowledge and its generation with just a single adaptation trial.

A continuous measure of phasic electrodermal activity

Electrodermal activity is characterized by the superposition of what appear to be single distinct skin conductance responses (SCRs). Classic trough-to-peak analysis of these responses is impeded by their apparent superposition. A deconvolution approach is proposed, which separates SC data into continuous signals of tonic and phasic activity. The resulting phasic activity shows a zero baseline, and overlapping SCRs are represented by predominantly distinct, compact impulses showing an average duration of less than 2 s. A time integration of the continuous measure of phasic activity is proposed as a straightforward indicator of event-related sympathetic activity. The quality and benefit of the proposed measure is demonstrated in an experiment with short interstimulus intervals as well as by means of a simulation study. The advances compared to previous decomposition methods are discussed.

Decomposition of skin conductance data by means of nonnegative deconvolution

Skin conductance (SC) data are usually characterized by a sequence of overlapping phasic skin conductance responses (SCRs) overlying a tonic component. The variability of SCR shapes hereby complicates the proper decomposition of SC data. A method is proposed for full decomposition of SC data into tonic and phasic components. A two-compartment diffusion model was found to adequately describe a standard SCR shape based on the process of sweat diffusion. Nonnegative deconvolution is used to decompose SC data into discrete compact responses and at the same time assess deviations from the standard SCR shape, which could be ascribed to the additional process of pore opening. Based on the result of single non-overlapped SCRs, response parameters can be estimated precisely as shown in a paradigm with varying inter-stimulus intervals.

Challenging the brain: Exploring the link between effort and cortical activation

To better understand the contributions of effort on cortical activation associated with motor tasks, healthy participants with varying capacities for isolating the control of individual finger movements performed tasks consisting of a single concurrent abduction of all digits (Easy) and paired finger abduction with digits 2 and 3 abducted together concurrently with digits 4 and 5 (Hard). Brain activity was inferred from measurement using functional magnetic resonance imaging. Effort was measured physiologically using electrodermal responses (EDR) and subjectively using the Borg scale. On average, the Borg score for the Hard task was significantly higher (p=0.007) than for the Easy task (2.9+/-1.1 vs. 1.4+/-0.7, respectively). Similarly, the average normalized peak-to-peak amplitude of the EDR was significantly higher (p=0.002) for the Hard task than for the Easy task (20.4+/-6.5% vs. 12.1+/-4.9%, respectively). The Hard task produced increases in sensorimotor network activation, including supplementary motor area, premotor, sensorimotor and parietal cortices, cerebellum and thalamus. When the imaging data were subdivided based on Borg score, there was an increase in activation and involvement of additional areas, including extrastriate and prefrontal cortices. Subdividing the data based on EDR amplitude produced greater effects including activation of the premotor and parietal cortices. These results show that the effort required for task performance influences the interpretation of fMRI data. This work establishes understanding and methodology for advancing future studies of the link between effort and motor control, and may be clinically relevant to sensorimotor recovery from neurologic injury.

Sympathetic skin responses evoked by muscle contraction

INTRODUCTION

Voluntary muscle contraction is accompanied by an increase in sympathetic nerve activity. The sympathetic skin response (SSR) is a simple and non-invasive method of autonomic assessment that reflects a synchronized activity of the sweat glands. The aim of our study was to examine the possible relationship between isometric muscle contraction (IC) and changes in the SSR.

METHODS

In 11 healthy right-handed volunteers, we recorded the SSR from the palm of the hand induced by contralateral triceps IC (mSSR) of variable intensities and durations. We measured the latency, duration, amplitude, waveform and habituation index (HI) of the mSSR, in comparison to the SSR induced by supramaximal electrical stimulation (eSSR) of the brachial plexus at the axillae.

RESULTS

A single mSSR was always present at a mean latency of 1.34+/-0.5s after the onset of IC. Response amplitude, but not latency or duration, correlated positively with the intensity of IC (r=0.67; p<0.001). The latency was shorter, the duration was longer and the HI was reduced in the mSSR in comparison to the eSSRs (ANOVA; p<0.05 for all comparisons).

CONCLUSIONS

The mSSR is likely generated endogenously together with the motor commands since inputs from muscle afferents cannot account for response onset. This, together with its low level of habituation, underscores the possibilities of physiological and clinical studies using the mSSR, especially in the assessment of autonomic function in patients with nerve afferent problems.

Age differences in perception and awareness of emotion

We investigated the effects of age and gender on emotional perception and physiology using electrodermal skin conductance response (SCR) and examined whether SCR is related to subjective perceptions of emotional pictures. Older adults found pictures to be more positive and arousing than younger participants. Older women rated pictures more extremely at both ends of the valence continuum: they rated positive pictures more positively and negative pictures more negatively. Elders were less likely to show measurable SCRs. However, magnitude of SCRs when a response occurred did not differ between young and old. Subjective ratings of emotion correlated with physiological responses in younger participants, but they were unrelated in older participants. Thus, in older adults the perception of emotional events was disconnected from the physiological state induced by emotion.

Phasic electrodermal responses associated with whole-body instability: presence and influence of expectation

While much is understood about somatic contributions to postural control, there is less consideration for the potential involvement of the autonomic nervous system (ANS) as integral for maintenance of stability. The purpose of this study was to examine autonomic responses, as measured through electrodermal recordings, evoked in response to whole-body perturbations to standing balance. We hypothesized that phasic electrodermal responses (EDRs) would be consistently observed in response to evoked perturbations and that response amplitude would depend on the capacity to predict perturbation timing. Temporally unpredictable and self-initiated (predictable) backward perturbations evoked in healthy participants (n=15) elicited compensatory feet-in-place reactions with tibialis anterior activation 125.1+/-60.2 ms following perturbation onset. EDRs were consistently observed starting 1883.6+/-329.1 ms after perturbation and reaching their peak at 4016.6+/-896.9 ms. Amplitude was significantly larger in the unpredictable task (1.1+/-0.84 micromho) compared to the predictable task (0.45+/-0.55 micromho, P<0.001). Amplitude was largest in the first block of five trials (P<0.0001), then remained constant for subsequent trials in each condition. Post-hoc analysis indicated that trials with an unplanned compensatory step (3.5%) were 137.0+/-176.6% larger than feet-in-place reactions (P=0.02). Elevated EDRs during initial trials and unanticipated reactions suggest that these measures could be used to assess the perceived 'novelty' of applied perturbations, having implications for interpreting characteristics of the evoked somatic reactions. The persistence of perturbation-evoked EDRs even after thirty trials may also highlight an important role for phasic ANS responses in compensatory postural control.

Brain activity during a motor learning task: an fMRI and skin conductance study

Measuring electrodermal activity (EDA) during fMRI is an effective means of studying the influence of task-related arousal, inferred from autonomic nervous system activity, on brain activation patterns. The goals of this study were: (1) to measure reliable EDA from healthy individuals during fMRI involving an effortful unilateral motor task, (2) to explore how EDA recordings can be used to augment fMRI data analysis. In addition to conventional hemodynamic modeling, skin conductance time series data were used as model waveforms to generate activation images from fMRI data. Activations from the EDA model produced significantly different brain regions from those obtained with a standard hemodynamic model, primarily in the insula and cingulate cortices. Onsets of the EDA changes were synchronous with the hemodynamic model, but EDA data showed additional transient features, such as a decrease in amplitude with time, and helped to provide behavioral evidence suggesting task difficulty decreased with movement repetition. Univariate statistics also confirmed that several brain regions showed early versus late session effects. Partial least squares (PLS) multivariate analysis of EDA and fMRI data provided complimentary, additional insight on how the motor network varied over the course of a single fMRI session. Brain regions identified in this manner included the insula, cingulate gyrus, pre- and postcentral gyri, putamen and parietal cortices. These results suggest that recording EDA during motor fMRI experiments provides complementary information that can be used to improve the fMRI analysis, particularly when behavioral or task effects are difficult to model a priori.

State Anxiety Dependent on Perspiration during Mental Stress and Deep Inspiration

In this study, we focused on two types of perspiration, one triggered by anticipatory anxiety and the other by voluntary deep inspiration. We have previously found that the anticipation of anxiety causes increases of respiratory frequency, and that these increases are related to an activation of the temporal pole and amygdala in humans. Our interest is in the difference between the two natures of perspiration, both of which are closely related to respiratory responses. The level of sweating responses did not differ between deep and active inspiration or between three trials. This means that there was no habituation with repetitive trials for voluntary breathing. On the other hand, sweating responses during anticipation of anxiety showed habituation with repetitive trials. Habituation and nonhabituation differences with respect to these two characteristics of perspiration could be the result of differences in the related central networks. The former might be involved cortical structures associated with conscious changes of respiration. The latter was induced unconsciously, and this unconscious response is similar to the respiratory response during anticipatory anxiety, which may be related to the activation of the limbic system. Further, there was a positive correlation found between the amount of perspiration response and the state anxiety scores in deep inspiration, and also in trial 1 of the anticipatory anxiety experiment. Both types of responses were related to the individual state anxiety scores, and this may contribute to the defense mechanism with regard to adjustments to changes in the outer environment and situations.

Decrement of the skin conductance response to repeated volitional inspiration

OBJECTIVE

To examine response decrement of the recently reported inspiratory skin conductance response (SCR) [Lim CL, Seto-Poon M, Clouston PD, Morris JG. Sudomotor nerve conduction velocity and central processing time of the skin conductance response. Clin Neurophysiol 2003;114:2172-80].

METHODS

Twelve healthy adult volunteers performed 3 tasks (A) a control task of maintaining tidal breathing and then two randomized tasks, (B) a deep inspiration to a target oral pressure and (C) tapping with a finger. Each task was performed 30 times on cue every 20s in 3 runs with 5 min of rest between runs. The SCR, oral pressure, airflow, inspired volume and cue signal were recorded continuously and analysed offline. SCR amplitude was logarithmically transformed and then statistically analysed, using a linear mixed effects model, as a function of run number, trial number and absolute error between target and actual oral pressures.

RESULTS

Inspiratory efforts elicited exponentially decreasing SCR amplitude with increasing trial number during each run (P < 0.0001). After adjusting for trial number, the mean SCR amplitude of the second and the third run were, respectively, 24.2 (95% CI (0.175, 0.336), P < 0.001) and 14.4% (95% CI (0.104, 0.200), P < 0.001) of the first run amplitude.

CONCLUSIONS

Volitional deep inspiration reliably activates an SCR that exhibits response decrement with repetition, which may be habituation.

SIGNIFICANCE

The volitional inspiratory SCR may assist in the assessment of sympathetic autonomic status in patients with peripheral afferent neuropathy.