Respiratory-related evoked potential elicited by expiratory occlusion

Neural mechanisms of respiratory interoception

Respiratory interoception is one of the internal bodily systems that is comprised of different types of somatic and visceral sensations elicited by different patterns of afferent input and respiratory motor drive mediating multiple respiratory modalities. Respiratory interoception is a complex system, having multiple afferents grouped into afferent clusters and projecting into both discriminative and affective centers that are directly related to the behavioral assessment of breathing. The multi-afferent system provides a spectrum of input that result in the ability to interpret the different types of respiratory interceptive sensations. This can result in a response, commonly reported as breathlessness or dyspnea. Dyspnea can be differentiated into specific modalities. These respiratory sensory modalities lead to a general sensation of an Urge-to-Breathe, driven by a need to compensate for the modulation of ventilation that has occurred due to factors that have affected breathing. The multiafferent system for respiratory interoception can also lead to interpretation of the sensory signals resulting in respiratory related sensory experiences, including the Urge-to-Cough and Urge-to-Swallow. These behaviors are modalities that can be driven through the differentiation and integration of multiple afferent input into the respiratory neural comparator. Respiratory sensations require neural somatic and visceral interoceptive elements that include gated attention and detection leading to respiratory modality discrimination with subsequent cognitive decision and behavioral compensation. Studies of brain areas mediating cortical and subcortical respiratory sensory pathways are summarized and used to develop a model of an integrated respiratory neural network mediating respiratory interoception.

Cortical Sources of Respiratory Mechanosensation, Laterality, and Emotion: An MEG Study

Airway obstruction activates mechanoreceptors that project to the cerebral cortices in humans, as evidenced by scalp encephalography recordings of cortical neuronal activation, i.e., respiratory-related evoked potential (RREP). However, neural evidence of both high spatial and temporal resolution of occlusion-elicited cortical activation in healthy individuals is lacking. In the present study, we tested our hypothesis that inspiratory mechanical stimuli elicit neural activation in cortical structures that can be recorded using magnetoencephalography (MEG). We further examined the relationship between depression and respiratory symptoms and hemispheric dominance in terms of emotional states. A total of 14 healthy nonsmoking participants completed a respiratory symptom questionnaire and a depression symptom questionnaire, followed by MEG and RREP recordings of inspiratory occlusion. Transient inspiratory occlusion of 300 ms was provided randomly every 2 to 4 breaths, and approximately 80 occlusions were collected in every study participant. Participants were required to press a button for detection when they sensed occlusion. Respiratory-related evoked fields (RREFs) and RREP peaks were identified in terms of latencies and amplitudes in the right and left hemispheres. The Wilcoxon signed-rank test was further used to examine differences in peak amplitudes between the right and left hemispheres. Our results showed that inspiratory occlusion elicited RREF M1 peaks between 80 and 100 ms after triggering. Corresponding neuromagnetic responses peaked in the sensorimotor cortex, insular cortex, lateral frontal cortex, and middle frontal cortex. Overall, the RREF M1 peak amplitude in the right insula was significantly higher than that in the left insula (p = 0.038). The RREP data also showed a trend of higher N1 peak amplitudes in the right hemisphere compared to the left (p = 0.064, one-tailed). Subgroup analysis revealed that the laterality index of sensorimotor cortex activation was significantly different between higher- and lower-depressed individuals (−0.33 vs. −0.02, respectively; p = 0.028). For subjective ratings, a significant relationship was found between an individual’s depression level and their respiratory symptoms (Spearman’s rho = 0.54, p = 0.028, one-tailed). In summary, our results demonstrated that the inspiratory occlusion paradigm is feasible to elicit an RREF M1 peak with MEG. Our imaging results showed that cortical neurons were activated in the sensorimotor, frontal, middle temporal, and insular cortices for the M1 peak. Respiratory occlusion elicited higher cortical neuronal activation in the right insula compared to the left, with a higher tendency for right laterality in the sensorimotor cortex for higher-depressed rather than lower-depressed individuals. Higher levels of depression were associated with higher levels of respiratory symptoms. Future research with a larger sample size is recommended to investigate the role of emotion and laterality in cerebral neural processing of respiratory sensation.

The test-retest reliability of the respiratory-related evoked potential

The respiratory-related evoked potential (RREP) is an established technique to study the neural processing of respiratory sensations. We examined the test-retest reliability of the RREP during an unloaded baseline condition (no dyspnea) and an inspiratory resistive loaded breathing condition (dyspnea) over a one-week period. RREPs were evoked by short inspiratory occlusions (150 ms) while EEG was continuously measured. The mean amplitudes of the RREP components Nf, P1, N1, P2, and P3 were studied. For the no dyspnea condition, moderate test-retest reliability for Nf (intraclass correlation coefficient ICC: 0.73) and P1 (ICC: 0.74), good test-retest reliability for N1 (ICC: 0.89) and P3 (ICC: 0.76), and excellent test-retest reliability for P2 (ICC: 0.92) was demonstrated. For the dyspnea condition, moderate test-retest reliability was found for Nf (ICC: 0.69) and P1 (ICC: 0.57) and good test-retest reliability for N1 (ICC: 0.77), P2 (ICC: 0.84), and P3 (ICC: 0.77). This indicates that the RREP components Nf, P1, N1, P2, and P3, elicited by inspiratory occlusions, show adequate reliability in a test-retest study design with or without parallel sustained resistive load-induced dyspnea.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

Respiratory cortical processing to inspiratory resistances during wakefulness in children with the obstructive sleep apnea syndrome

Children with the obstructive sleep apnea syndrome (OSAS) have impaired respiratory afferent cortical processing during sleep that persists after treatment of OSAS. However, it is unknown whether this impairment is present during wakefulness and, if so, whether it improves after OSAS treatment. We hypothesized that children with OSAS, during wakefulness, have abnormal cortical processing of respiratory stimuli manifested by blunted respiratory-related evoked potentials (RREP) and that this resolves after OSAS treatment. We measured RREP during wakefulness in 26 controls and 21 children with OSAS before and after treatment. Thirteen participants with OSAS repeated testing 3-6 mo after adenotonsillectomy. RREP were elicited by interruption of inspiration by total occlusion and 30 and 20 cmH2O/l per s resistances. Nf at Fz latency elicited by occlusion was longer in children with OSAS at baseline compared with controls (78.8 ± 24.8 vs. 63.9 ± 19.7 ms, P = 0.05). All other peak amplitudes and latencies were similar between the two groups. After OSAS treatment, Nf at Fz latency elicited by 30 cmH2O/l per s decreased significantly (before, 88 ± 26 vs. after, 71 ± 25 ms, P = 0.02), as did that elicited by 20 cmH2O/l per s (85 ± 27 vs. 72 ± 24 ms, P = 0.004). The amplitude of N1 at Cz elicited by occlusion increased from -3.4 ± 5.6 to -7.4 ± 3 μV (P = 0.049) after treatment. We concluded that children with OSAS have partial delay of respiratory afferent cortical processing during wakefulness that improves after treatment.

Emotions and neural processing of respiratory sensations investigated with respiratory-related evoked potentials

OBJECTIVE

Patients with respiratory diseases such as asthma and chronic obstructive pulmonary disease frequently experience respiratory sensations, which are often perceived as unpleasant or threatening. However, the accurate perception of respiratory sensations is important for the management and treatment of these diseases. Emotions can substantially influence the perception of respiratory sensations and might affect the course of respiratory diseases, but the underlying neural mechanisms are poorly understood. The respiratory-related evoked potential (RREP) recorded from the electroencephalogram is a noninvasive technique that allowed first studies to examine the impact of emotions on the neural processing of respiratory sensations.

METHODS

In this review, we will briefly introduce the importance of the perception of respiratory sensations and the influence of emotions on respiratory perception. We then provide an overview on the technique of RREP and present a systematic review on recent findings using this technique in the context of emotions.

RESULTS AND CONCLUSIONS

The evidence currently available from studies in healthy individuals suggests that short-lasting emotional states and anxiety affect the later RREP components (N1, P2, P3) related to higher-order neural processing of respiratory sensations, but not the earlier RREP components (Nf, P1) related to first-order sensory processing. We conclude with a discussion of the implications of this work for future research that needs to focus on respiratory patient groups and the associated clinical outcomes.

Transneuronal tracing of airways-related sensory circuitry using herpes simplex virus 1, strain H129

Sensory input from the airways to suprapontine brain regions contributes to respiratory sensations and the regulation of respiratory function. However, relatively little is known about the central organization of this higher brain circuitry. We exploited the properties of the H129 strain of herpes simplex virus 1 (HSV-1) to perform anterograde transneuronal tracing of the central projections of airway afferent nerve pathways. The extrathoracic trachea in Sprague-Dawley rats was inoculated with HSV-1 H129, and tissues along the neuraxis were processed for HSV-1 immunoreactivity. H129 infection appeared in the vagal sensory ganglia within 24 h and the number of infected cells peaked at 72 h. Brainstem nuclei, including the nucleus of the solitary tract and trigeminal sensory nuclei were infected within 48 h, and within 96 h infected cells were evident within the pons (lateral and medial parabrachial nuclei), thalamus (ventral posteromedial, ventral posterolateral, submedius, and reticular nuclei), hypothalamus (paraventricular and lateral nuclei), subthalamus (zona incerta), and amygdala (central and anterior amygdala area). At later times H129 was detected in cortical forebrain regions including the insular, orbital, cingulate, and somatosensory cortices. Vagotomy significantly reduced the number of infected cells within vagal sensory nuclei in the brainstem, confirming the main pathway of viral transport is through the vagus nerves. Sympathetic postganglionic neurons in the stellate and superior cervical ganglia were infected by 72 h, however, there was no evidence for retrograde transynaptic movement of the virus in sympathetic pathways in the central nervous system (CNS). These data demonstrate the organization of key structures within the CNS that receive afferent projections from the extrathoracic airways that likely play a role in the perception of airway sensations.

Blunted respiratory-related evoked potential in awake obstructive sleep apnoea subjects: a NEP technique study

OBJECTIVE

Respiratory-related evoked potentials (RREP) elicited by transmural pressure in obstructive sleep apnoea (OSA) subjects have reported conflicting data. Different features of pressure stimuli and/or in the timing of stimuli application seem to account for these contradictory results. The negative expiratory pressure (NEP) technique, highly reproducible in terms of rise time and pressure values, allows to minimize the methodological confounding factors. We determined whether the afferent activity from the upper airway (UA) is altered in OSA subjects.

METHODS

RREP potentials were examined in 10 OSA and in 12 non-apnoeic awake subjects by means of the NEP technique.

RESULTS

All controls showed a cortical response to all pressure stimuli. All OSA subjects showed responses to -5 and -10 cmH(2)O whereas six of them showed no responses to -1 cmH(2)O. The amplitude of the P22, N45 and P85 components of the RREP was significantly reduced in OSA with respect to the controls in response to both the -5 and -10 cmH(2)O stimuli. We found no significant differences in latencies.

CONCLUSIONS

Awake OSA subjects had a raised threshold to pressure stimuli and blunted respiratory-related evoked potentials.

SIGNIFICANCE

These data indicate a deficit in afferent activity in the UA.

Respiratory-related evoked potentials in children with asthma

AIMS OF THE STUDY

Respiratory-related evoked potentials (RREPs) are a method of recording brain activities in response to respiratory stimuli. Although data in childhood are scarce, the absence of the early P1 component of RREPs has been reported in children with a history of life-threatening asthma. This study was focused on the presence, latencies, and amplitudes of the P1, N1, P2, and N2 components of the RREPs in a paediatric series of asthmatic patients.

PATIENTS AND METHODS

RREPs were recorded in 21 patients with stable asthma, age range 8-17 years, 11 healthy children, age range 6-16 years, and 24 healthy adults, age range 20-28 years. The signals from left (C3-Cz) and right (C4-Cz) central (rolandic) location were recorded separately, using surface electrodes. Evoked responses to two series of 80 consecutive mid-inspiratory occlusions were averaged. Recordings were analysed manually.

RESULTS

All 4 RREPs components were significantly more often absent in asthmatic children than in healthy children and adults (P1, p=0.01; N1, p=0.008; P2, p=0.008, N2, p=0.01). The latencies and amplitudes of the four components were similar in patients and healthy subjects.

CONCLUSION

RREPs components were less frequently present in children with asthma than in healthy subjects. This finding should promote the recording of RREPs in other acute and chronic respiratory diseases in children in order to search for possible electroclinical correlations.

Detection threshold for inspiratory resistive loads and respiratory-related evoked potentials

The relationship between detection threshold of inspiratory resistive loads and the peaks of the respiratory-related evoked potential (RREP) is unknown. It was hypothesized that the short-latency and long-latency peaks of the RREP would only be elicited by inspiratory loads that exceeded the detection threshold. The detection threshold for inspiratory resistive loads was measured in healthy subjects with inspiratory-interruption or onset load presentations. In a separate protocol, the RREPs were recorded with resistive loads that spanned the detection threshold. The loads were presented in stimulus attend and ignore sessions. Onset and interruption load presentations had the same resistive load detection threshold. The P1, Nf, and N1 peaks of the RREP were observed with loads that exceeded the detection threshold in both attend and ignore conditions. The P300 was present with loads that exceeded the detection threshold only in the attend condition. No RREP components were elicited with subthreshold loads. The P1, Nf, and P300 amplitudes varied with resistive load magnitude. The results support the hypothesis that there is a resistive load threshold for eliciting the RREPs. The amplitude of the RREP peaks vary as a function of load magnitude. The cognitive P300 RREP peak is present only for detectable loads and when the subject attends to the stimulus. The absence of the RREP with loads below the detection threshold and the presence of the RREP elicited by suprathreshold loads are consistent with the gating of these neural measures of respiratory mechanosensory information processing.

Respiratory evoked potentials and occlusion elicited sympathetic skin response

INTRODUCTION

Neurophysiological study of respiratory structures usually relies upon diaphragm electromyography and phrenic nerve conduction study, which do not assess the afferent sensory pathways.

OBJECTIVE

To assess the feasibility of respiratory evoked potentials (REPs) and sympathetic skin responses (SSRs) elicited by inspiratory occlusion.

METHODS

REPs and SSRs were studied in 12 healthy adults. REPs were elicited by inspiratory occlusions triggered by the physician within 1 s after the onset of a respiratory effort. They were recorded from C3, C4 and Cz needle electrodes (referenced to Fz). Each individual trial consisted of two superimposed 30-sweep averaged responses to inspiratory occlusions. SSRs were recorded from surface electrodes placed on the subject's hand and elicited by similar inspiratory occlusions.

RESULTS

Reproducible REPs and SSRs were obtained in all subjects. Mean latencies of initial P1 and N1 cortical responses were 41 and 72 ms, respectively. SSRs were similar to those usually elicited by peripheral nerve electrical stimulation.

CONCLUSION

Brief occlusion of inspiration induces cortical and sympathetic activation, both are easily recordable. Since REPs are considered to be the neurophysiological substrate of certain types of respiratory sensations and are altered in different chronic respiratory diseases, they, in addition to SSR, represent attractive new techniques that may provide better understanding of respiratory dysfunction.

Upper airway afferents are sufficient to evoke the early components of respiratory-related cortical potentials in humans

Repeated inspiratory occlusions in humans elicit respiratory-related cortical potentials, the respiratory counterpart of somatosensory-evoked potentials. These potentials comprise early components (stimulus detection) and late components (cognitive processing). They are considered as the summation of several afferent activities from various part of the respiratory system. This study assesses the role of the upper airway as a determinant of the early and late components of the potentials, taking advantage of the presence of a tracheotomy in patients totally or partially deafferented. Eight patients who could breathe either through the mouth or through a tracheotomy orifice (whole upper airway bypassed) were studied (4 quadriplegic patients with phrenic pacing, 4 patients with various sources of inspiratory pump dysfunction). Respiratory-related evoked potentials were recorded in CZ-C3 and CZ-C4. They were consistently present after mouth occlusions, with a first positive P1 and a first negative N1 components of normal latencies (P1: 40.4 ± 6.1 ms in CZ-C3 and 47.6 ± 7.6 ms in CZ-C4; N1: 84.4 ± 27.1 ms in CZ-C3 and 90.2 ± 17.4 ms in CZ-C4) and amplitudes. Tracheal occlusions did not evoke any cortical activity. Therefore, in patients with inspiratory pump dysfunction, the activation of upper airway afferents is sufficient to produce the early components of the respiratory-related evoked cortical potentials. Per contra, in this setting, pulmonary afferents do not suffice to evoke these components.

A review of the evidence for P2 being an independent component process: age, sleep and modality

This article reviews the event-related potential (ERP) literature in relation to the P2 waveform of the human auditory evoked potential. Within the auditory evoked potential, a positive deflection at approximately 150-250 ms is a ubiquitous feature. Unlike other cognitive components such as N1 or the P300, remarkably little has been done to investigate the underlying neurological correlates or significance of this waveform. Indeed until recently, many researchers considered it to be an intrinsic part of the 'vertex potential' complex, involving it and the earlier N1. This review seeks to describe the evidence supportive of P2 being the result of independent processes and highlights several features, such as its persistence from wakefulness into sleep, the general consensus that unlike most other EEG phenomena it increases with age, and the fact that it can be generated using respiratory stimuli.

Sleep and respiratory stimulus specific dampening of cortical responsiveness in OSAS

The application of inspiratory occlusion stimuli produces cortical responses called respiratory-related evoked potentials (RREPs). During wakefulness the RREP waveform consists of early P1 and Nf components, an N1 and a P300. During non-REM sleep the predominant component is an N550, best seen in the averages of elicited K-complexes. Obstructive sleep apnea syndrome (OSAS) patients have been previously shown to have a normal wake RREP but to have a reduced amplitude N550 and a smaller proportion of elicited K-complexes than controls. The present study tested the hypothesis that this reflects a sleep-specific dampening peculiar to inspiratory effort-related stimuli, by assessing both respiratory and auditory evoked potentials (AEPs) during wakefulness and non-REM sleep in OSAS patients and controls. Auditory tones were presented in an oddball sequence during wakefulness and as a monotonous series during stage 2 sleep. Inspiratory occlusions, delivered for 500 msec via an nCPAP mask were also presented during wakefulness and stage 2 sleep, every three to five breaths. Data were collected from ten OSAS patients and ten controls. There were no significant differences in the amplitudes of the auditory N1 and P3 or the respiratory P1, Nf, N1 or P3 components during wakefulness. The amplitude of the auditory N550 and the proportion of elicited K-complexes did not differ between groups for auditory stimuli presented during stage 2 sleep. The respiratory N550 and K-complex elicitation rate were both significantly reduced in the OSAS group, despite there being no differences in the mask occlusion pressure response to the occlusion. The results confirm a blunted cortical response to inspiratory occlusions that is specific to sleep. The absence of significant group differences in the responses to auditory stimuli highlight that the sleep-related differences seen in OSAS patients are specific to the processing of inspiratory effort related stimuli.

Respiratory-related evoked potential and upper airway transmural pressure change by using the negative expiratory pressure (NEP) device

OBJECTIVE

Several studies have previously shown respiratory related evoked potentials (RREP) in humans elicited by mechanical stimuli applied on upper airways (UA). According to us, heterogeneous findings, concerning latencies and amplitudes, have been reported because of the different timing of stimuli application during the respiratory cycle and/or features of pressure stimuli. Therefore we evaluated the cortical response evoked by transmural pressure changes at the mouth induced by a negative expiratory pressure (NEP) device.

METHODS

RREP were recorded in 22 healthy non-obese, non-snoring volunteers. The subjects were studied awake in seated position during quiet breathing. Three different pressure levels were applied, in a random order, 200 ms after the beginning of expiration. Cortical electrical responses were recorded from scalp electrodes at Fz, Cz, and Pz scalp location (international 10-20 system) referenced to the linked earlobes.

RESULTS

RREP responses consisted of two negative (N45, N120) and two positive (P22, P85) waves. There was no significant effect of pressure or electrode on component latencies. The P22 wave (PRESSURE: F(df 2,42)=6.66, P<0.01), the N45 wave (PRESSURE: F(df 2,42)=16.51, P<0.001), and the P85 wave (PRESSURE: F(df2,42)=15.15, P<0.001) increased significantly theyr amplitude with increasing from pressure stimuli 1 to 10 cmH2O.

CONCLUSIONS

The present results suggest that the UA NEP application in humans is a reliable way of evoking cortical responses. The experimental features that we described allow us to minimize the confounding factors in evaluating RREPs. The NEP device appears to be a useful tool for investigation of the neurobiology of UA sensation in humans.

Evidence of a sleep-specific blunted cortical response to inspiratory occlusions in mild obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) patients have elevated non-rapid eye movement (REM) sleep arousal thresholds to inspiratory loading. To test the hypothesis that this is due to sleep-specific dampening of cortical responses to inspiratory effort, respiratory-related evoked potentials (RREPs) were evaluated in six mild OSAS patients and six age- and body mass index-matched controls during wakefulness and Stage 2 non-REM sleep. Electroencephalogram was recorded from six scalp sites (Fz, FCz, Cz, CPz, Pz, and O(2)). Electrooculogram, electromyogram, and mask pressure signals were also recorded. During sleep, pharyngeal pressure was recorded using a Millar pressure catheter placed 2 cm below the glottis. The RREP waveform was broadly similar in the two groups during wakefulness, but was markedly different during Stage 2 non-REM sleep. During wakefulness, only the N1 component showed reduced amplitude in the OSAS group. During sleep, the occlusion stimulus elicited fewer K-complexes in the OSAS patients. In addition, the N550 component in the average of K-complex responses was smaller in amplitude in the OSAS group. The data suggest that patients with mild OSAS have a "blunted" response to the respiratory occlusion stimulus. This appears not to be related to compromised mechanoreceptor function, as the RREP was normal in the patients when they were awake.

Expiratory resistive loaded breathing in humans increases fluctuations of force production in submaximal isometric quadriceps contractions

This study demonstrated that expiratory resistive loading (ERL) induced an increase in force fluctuation during a unilateral, submaximal isometric contraction of the non-dominant left vastus lateralis (VL), but did not effect force fluctuation during complex bilateral contractions. The increase in force fluctuation in the unilateral left VL contraction during ERL was not accompanied by alterations of average force production, motor unit activation (median power frequency) or airflow rate when compared to the bilateral contraction. Inspiratory RL (IRL) did not significantly affect force fluctuation in unilateral or bilateral contractions. The results concur with previous reports of ERL, but not IRL, effecting VL function and suggest that patients with obstructive diseases may also be vulnerable to reduced fine motor control.

Changes in electromyogram during upper limb muscle contraction induced by resistive loaded breathing in humans

Expiratory only resistive loaded breathing (RL) reduces high energy electromyogram (EMG) power (EH) during an isometric contraction of a leg extensor muscle, but not an arm flexor. An interaction between afferent activity during expiratory RL and inspiratory non-loaded phases of breathing, which the contraction spanned, could have accounted for the reduced EH in these long contractions. Therefore this study tested the hypothesis that brief arm extensor muscle contractions (70% of maximal force), performed during a single phase of expiratory RL, would also exhibit reduced EH. Surprisingly, EH in triceps, but not biceps brachii was reduced significantly when the contraction was performed during inspiratory RL rather than expiratory RL. The results suggest that either (a) short and prolonged contractions or (b) motor drive to arm and leg extensors are affected differently by RL.

Myogenic and scalp signals evoked by midinspiratory airway occlusion

A somatosensory potential that is evoked by transient added inspiratory load has previously been described (Davenport PW, Friedman WA, Thompson FJ, and Franzen O. J Appl Physiol 60: 1843-1848, 1986). This evoked potential is novel because it arises in response to a stimulus that also evokes a muscle response, and so this potential could contain myogenic components. The present study was undertaken to define the relationship between the scalp response and other physiological responses that are evoked by airway occlusion. Evoked signals were recorded from the scalp, scalenus anterior, masseter, and electrooculogram. Responses to a 200-ms midinspiratory occlusion were recorded in 12 healthy volunteers. Evoked responses were reliably recorded at C(3)-C(Z) and C(4)-C(Z) and from the skin overlying the scalenus anterior in 11 of these subjects. The onset latencies were 15.7 +/- 3.1 at C(3)-C(Z), 15.9 +/- 2.1 at C(4)-C(Z), and 17.6 +/- 5.5 ms at scalenus anterior. In nine subjects, the masseter response appeared to coincide with the mouth pressure trace, and this was interpreted as movement artifact. No consistent electrooculogram or frontal electroencephalogram response was recorded. Because of the similarity in onset latency at C(3)-C(Z), C(4)-C(Z), and scalenus anterior, it was concluded that the myogenic signal may contribute to the scalp response and should be viewed as a potential source of artifact in experiments of this nature.