Awake Humans Research Articles

Investigation of vocal fold impact stress in human subjects.

The primary purpose of the study was to explore a methodology for measuring vocal fold impact stress (SI) in awake humans, and to provide information about the general magnitude of SIs that may occur at the midpoint of the membranous vocal folds during phonation. A secondary purpose was to examine the potential use of the electroglottographic closed quotient (EGG CQ) to indirectly reflect SI. Seven male and 13 female adults were enrolled as subjects, of whom 18 had normal larynges and normal voices, 1 had nodules, and 1 had vocal fold paresis and bowing. Subjects attempted to produce 3 different voice types (pressed, normal, breathy), at 3 different pitches (low, medium, high) and 3 different loudness levels (quiet, medium, loud). For a first set of trials, only EGG data were collected. For a second set, a sensor was also introduced to the midmembranous glottis for the collection of SI data. The primary findings were that (1) endolaryngeal sensor placement was achieved during phonation trials for 17 of 20 subjects; however, grossly consistent anteroposterior positioning was accomplished, and analyzable data were obtained, for only 7 subjects; (2) SIs ranged from less than 1 kPa to about 3 kPa for those 7 subjects; and (3) no relation was detected between simultaneous CQs and SIs for individual data, although a relation was reported in a prior canine study. One possible reason for the failure to show such a relation in the present study was subtle variations in vertical as well as anteroposterior positioning of the sensor during the trials. Future studies should focus on developing a methodology for ensuring invariant 3-dimensional sensor positioning between the membranous folds, so that the stability of both SI and simultaneous CQ data can be improved.

Midazolam effects on prepulse inhibition of the acoustic blink reflex.

The eye-blink response following sudden acoustic noise bursts is part of the startle reflex. The magnitude of the startle response can be attenuated by presentation of a weak stimulus before the 'startle-eliciting' stimulus (prepulse inhibition, PPI). PPI is a stable finding in awake humans but may be altered by anaesthetic drugs. We investigated whether the application of benzodiazepines altered the magnitude of PPI in healthy male volunteers. In an open-label noncontrolled investigation, the effect of the benzodiazepine agonist midazolam on PPI was assessed in the absence and presence of the antagonist flumazenil. After an initial control period of 60 min three consecutive periods, each of 60 min, with progressively increasing concentrations of midazolam were studied (0. 02, 0.06, 0.14 mg kg-1 h-1 ). A final 60 min period during the administration of flumazenil (0.004 mg kg-1 h-1 ) and while the agonist was still present was also studied. Drug was administered intravenuously as a combination of bolus, 50% of total dose and continuous infusion over the 60 min period. Electromyographic (EMG) response of the right orbicularis oculi muscle was used to assess the startle response to noise bursts of 50 ms duration (95 dB(A)). Noise bursts were randomly preceded by nonstartling prepulses (800 Hz sinus, 50 ms duration, 65 dB(A), prepulse to noise interval 120 ms). The magnitude of PPI was calculated by dividing the EMG response to nonprepulsed stimuli by the response to prepulsed stimuli for each individual and period. Eleven subjects participated in the study, two of them were excluded from the statistical analysis because startle responses could not be reliably elicited (final sample size n=9). The magnitude of PPI was inversely related to the concentration of midazolam. This relationship was described by a sigmoidal Emax model, giving an Emax of 0.65+/-0.13, an ED50 of 33.9+/-10.9 ng ml-1 and gamma of 3.5+/-1.0. During infusion of flumazenil and in the presence of midazolam, the magnitude of PPI increased by 0.11 (95% CI, 0-0.22, P</=0.04), which is consistent with its mode of action as a benzodiazepine antagonist. In healthy male volunteers the magnitude of PPI varies according to agonism and antagonism of benzodiazepine receptors, suggesting that the assessment of PPI may be potentially useful to monitor the sedative effect of benzodiazepines in the clinical setting.

Stimulated changes in localized cerebral energy consumption under anesthesia.

Focal changes in the cerebral metabolic rate of glucose utilization (CMRglc) are small (10-40%) during sensory activation in awake humans, as well as in awake rodents and primates (20-50%). They are significantly larger (50-250%) in sensory activation studies of anesthetized rats and cats. Our data, in agreement with literature values, show that in the resting anesthetized state values of CMRglc are lower than in the resting nonanesthetized state whereas the final state values, reached upon activation, are similar for the anesthetized and nonanesthetized animals. The lower resting anesthetized state values of CMRglc explain why the increments upon activation from anesthesia are larger than when starting from the nonanesthetized conditions. Recent 13C NMR measurements in our laboratory have established a quantitative relationship between the energetics of glucose oxidation, CMRglc (oxidative), and the flux of the glutamate/gamma-aminobutyric acid/glutamine neurotransmitter cycle, Vcycle. In both the resting awake value of CMRglc(oxidative), and its increment upon stimulation, a large majority (approximately 80%) of the brain energy consumption is devoted to Vcycle. In the differencing methods of functional imaging, it is assumed that the incremental change in the measured signal represents the modular activity that supports the functional response. However, the same amount of activity must be present during the response to stimulation, irrespective of the initial basal state of the cortex. Thus, whereas the incremental signals of DeltaCMRglc can localize neurotransmitter activity, the magnitude of such activity during the response is represented by the total localized CMRglc, not the increment.

Spinal Cord-Evoked Potentials and Muscle Responses Evoked by Transcranial Magnetic Stimulation in 10 Awake Human Subjects

Transcranial magnetic stimulation (TCMS) causes leg muscle contractions, but the neural structures in the brain that are activated by TCMS and their relationship to these leg muscle responses are not clearly understood. To elucidate this, we concomitantly recorded leg muscle responses and thoracic spinal cord-evoked potentials (SCEPs) after TCMS for the first time in 10 awake, neurologically intact human subjects. In this report we provide evidence of direct and indirect activation of corticospinal neurons after TCMS. In three subjects, SCEP threshold (T) stimulus intensities recruited both the D wave (direct activation of corticospinal neurons) and the first I wave (I1, indirect activation of corticospinal neurons). In one subject, the D, I1, and I2 waves were recruited simultaneously, and in another subject, the I1 and I2 waves were recruited simultaneously. In the remaining five subjects, only the I1 wave was recruited first. More waves were recruited as the stimulus intensity increased. The presence of D and I waves in all subjects at low stimulus intensities verified that TCMS directly and indirectly activated corticospinal neurons supplying the lower extremities. Leg muscle responses were usually contingent on the SCEP containing at least four waves (D, I1, I2, and I3).

Incremental responses in the auditory cortex and the auditory periphery

When probe tones are added to background sounds, the perception or detectability of the probe is influenced by the time delay from the onset of the background to the onset of the probe. In general, the shorter the time delay, the harder it is to detect or perceive the probe as a separate signal. Possible physiological counterparts of these phenomena were investigated by adding 40-ms-long probe tones to 200-ms-long background tones. The corresponding increment in response was measured in the auditory nerve of anesthetized gerbils, using single-unit and modulation following responses, and in the auditory cortex of awake humans, using a 37-channel Magnetoencephalogram (MEG) and a single equivalent current-dipole model. The auditory nerve responses appear to obey superposition such that the increment in response is independent of the time delay. In contrast, the increment in the MEG response was significantly smaller for small delays than for long delays. The results suggest that the cortical responses behave differently than the peripheral responses, and more closely resemble those obtained psychophysically, although differences in experimental paradigms may also affect comparisons. Hence the difference between peripheral and central effects may reflect a functionally significant change in temporal processing occurring along the auditory pathway.

Regulation of cerebral oxygen delivery.

Under normal conditions, it is generally accepted that for the adult mammalian cortex, almost all the energy required for cerebral ATP generation is supplied by oxidation of glucose through the tri-carboxylic acid cycle (Siesjo, 1978), and cerebral metabolic rates of oxygen and glucose use (i.e., CMRO2 and CMRglc, respectively) are regionally modified to fulfil metabolic needs through regulation of cerebral blood flow (CBF) and volume (CBV) (Roy and Sherrington, 1890). However, some positron emission tomography (PET) results have reported that during brain activation of awake humans CBF increases by a greater fraction than CMRO2 (Fox and Raichle, 1986; Fox et al., 1988). In the last few decades, ever since in vivo cortical measurements could be made of CBF and/or CMRO2, the mechanisms for the proportionality of changes between CBF and CMRO2 due to physiological perturbations have remained intangible, and the measured stoichiometries have varied reflecting a variety of experimental methods and/or conditions. However, regionally tight relationship between CMRO2 and CBF has been observed at rest with a ratio of about ~1:1 (Kety et al., 1947–8; Raichle et al., 1976).

Multichannel EEG analysis of respiratory evoked-potential components during wakefulness and NREM sleep

Airway occlusion in awake humans produces a somatosensory evoked response called the respiratory-related evoked potential (RREP). In the present study, 29 channel evoked-potential recordings were obtained from seven men who were exposed to 250-ms inspiratory airway occlusions during wakefulness, stage 1, stage 2, and slow-wave sleep. The RREP recorded during wakefulness was similar to previous reports, with the unique observation of an additional short-latency positive peak with a mean latency of 25 ms. Short-latency RREP components were maintained in non-rapid-eye-movement (NREM) sleep. The clearly seen N1 vertex and late positive complex components during wakefulness were markedly attenuated during NREM sleep, and two large negative components (N300 and N550) dominated the sleep RREP. These findings indicate the maintenance of central nervous system monitoring of respiratory afferent information during NREM sleep, presumably to facilitate protective arousal responses to pathophysiological respiratory phenomena.

Imaging the mind

At the forefront of cognitive neuroscience research in normal humans are the new techniques of functional brain imaging: positron emission tomography (PET) and, more recently, magnetic resonance imaging (MRI). The signal used by PET is based on the fact that changes in the cellular activity of the brain of normal, awake humans and laboratory animals are accompanied almost invariably by changes in local blood flow. This robust, empirical relationship has fascinated scientists for well over a hundred years. PET provided a level of precision in the measurement of blood flow that opened up the modern era of functional human brain mapping. Further, the discovery with PET that these changes in blood flow are unaccompanied by quantitatively similar changes in oxygen consumption has paved the way for the explosive rise in the use of MRI in functional brain imaging. The remarkable success of this enterprise is a fitting tribute to men like Michel Ter-Pogossian. He pioneered the use of positron emitting radionuclides in biology and medicine when most had abandoned them in favor of more conventional nuclear medicine radionuclides. Importantly, also, he welcomed into his laboratory young scientists with a broad range of talents, many of whom subsequently became leaders in imaging the mind in research centers throughout the world.

No Evidence of Sevoflurane-Induced Renal Injury in Volunteers

No Evidence of Sevoflurane-Induced Renal Injury in Volunteers

Carbon dioxide and the cerebral circulation.

Carbon dioxide and the cerebral circulation.

Behind the scenes of functional brain imaging: a historical and physiological perspective.

At the forefront of cognitive neuroscience research in normal humans are the new techniques of functional brain imaging: positron emission tomography and magnetic resonance imaging. The signal used by positron emission tomography is based on the fact that changes in the cellular activity of the brain of normal, awake humans and laboratory animals are accompanied almost invariably by changes in local blood flow. This robust, empirical relationship has fascinated scientists for well over a hundred years. Because the changes in blood flow are accompanied by lesser changes in oxygen consumption, local changes in brain oxygen content occur at the sites of activation and provide the basis for the signal used by magnetic resonance imaging. The biological basis for these signals is now an area of intense research stimulated by the interest in these tools for cognitive neuroscience research.

Influence of low dose dopamine on the heart rate and ventilatory responses to sustained isocapnic hypoxia.

In awake humans, acute isocapnic hypoxia causes a rapid increase in both ventilation and heart rate. Although the acute ventilatory response is mediated primarily through a stimulatory drive from the carotid body, the heart rate response is more complex. There is apparently a heart rate depressing drive arising from the carotid bodies (CB), and a stimulation arising from a central effect of hypoxia, stimulation from vagal reflexes from the increased pulmonary ventilation and perhaps an increased drive from the aortic bodies (see ref. 1 for a review).

Influence of cerebral blood flow on the ventilatory response to hypoxia in humans.

The purpose of this study was to quantify the possible reduction in ventilation that could be attributed to changes in cerebral blood flow (CBF) with hypoxia to determine whether it could be of sufficient magnitude to underlie hypoxic ventilatory decline (HVD). Six subjects underwent 20 min of isocapnic hypoxia (end-tidal PO2, 50 mmHg). An index of CBF was obtained using transcranial Doppler ultrasound of the middle cerebral artery. The CBF sensitivities to hypoxia and hypercapnia were obtained from the percentage changes in CBF between the last 3 min of the hypoxic or hypercapnic exposure and the 3 min period prior to the exposure. The magnitude of HVD during hypoxia was estimated by fitting a simple model of the ventilatory response to the hypoxic stimulus. The predicted fall in expiratory ventilation (VE) due to a reduction in brain PCO2 generated by the increase in CBF with hypoxia in all subjects was less than the measured magnitude of HVD (33%). Thus, the results from this study suggest that, in awake humans, changes in CBF during acute isocapnic hypoxia are quantitatively insufficient to underlie HVD in humans.

A new aspect of the carotid body function controlling hypoxic ventilatory decline in humans.

Ventilatory response to eucapnic sustained mild hypoxia was measured in one patient with unilateral and three patients with bilateral carotid body (CB) resection (defined UR and BR, respectively). The profile of ventilatory response in UR patient was initially augmented then gradually declined (biphasic pattern) as generally seen in normal subjects although the absolute magnitude was substantially low. On the other hand, biphasic pattern was disappeared in all three BRs. Lack of hypoxic ventilatory decline (HVD) in the late period of sustained hypoxia was in marked contrast to that reported in the anaesthetized and CB-denervated animals whose ventilation was severely depressed lower than the pre-hypoxic control level. In view of recent knowledge that the analogous mild hypoxia in normal animals and humans elicits an useful adaptation to economize energy expenditure with maintaining reversible excitability in control of respiration, BR patients were considered to have lost this ability. We conclude that in awake humans the CB not only stimulates ventilation but also controls the degree of subsequent HVD during sustained hypoxia.

Effects of stimulus intensity on laryngeal long latency responses in awake humans

Percutaneous electrical stimulation applied to the internal branch of the superior laryngeal nerve (ISLN) results in two long latency laryngeal adductor responses in awake humans: an ipsilateral thyroarytenoid (TA) R1 muscle response at 16 ms, and later bilateral TA R2 muscle responses at 60 ms. The purpose of this study was to determine whether a functional relationship existed between the R1 and R2 responses by gradually increasing the level of electrical stimulation from threshold to supramaximal levels. R1 amplitude increased linearly with stimulation intensity in 9 of the 11 subjects, whereas R2 only had a positive linear relationship in 3 subjects and a negative relationship with stimulation intensity in 1 subject. Significant negative relationships were found between response latency and stimulation intensity in 3 subjects for the R1 responses and 3 other subjects for the R2 responses. Overall, R1 amplitudes increased systematically, whereas R2 responses varied in latency and amplitude with increasing stimulus intensity. Neither the latencies nor the amplitudes of the two responses were related after adjusting for stimulation intensity within subjects by using partial correlation coefficients. The R1 and R2 responses were functionally unrelated and most likely have different neural components. (Otolaryngol Head Neck Surg 1997;117:521-9.)

Effects of stimulus intensity on laryngeal long latency responses in awake humans.

Percutaneous electrical stimulation applied to the internal branch of the superior laryngeal nerve (ISLN) results in two long latency laryngeal adductor responses in awake humans: an ipsilateral thyroarytenoid (TA) R1 muscle response at 16 ms, and later bilateral TA R2 muscle responses at 60 ms. The purpose of this study was to determine whether a functional relationship existed between the R1 and R2 responses by gradually increasing the level of electrical stimulation from threshold to supramaximal levels. R1 amplitude increased linearly with stimulation intensity in 9 of the 11 subjects, whereas R2 only had a positive linear relationship in 3 subjects and a negative relationship with stimulation intensity in 1 subject. Significant negative relationships were found between response latency and stimulation intensity in 3 subjects for the R1 responses and 3 other subjects for the R2 responses. Overall, R1 amplitudes increased systematically, whereas R2 responses varied in latency and amplitude with increasing stimulus intensity. Neither the latencies nor the amplitudes of the two responses were related after adjusting for stimulation intensity within subjects by using partial correlation coefficients. The R1 and R2 responses were functionally unrelated and most likely have different neural components.

Ventilatory stability to transient CO2 disturbances in hyperoxia and normoxia in awake humans.

Modarreszadeh and Bruce (J. Appl. Physiol. 76: 2765-2775, 1994) proposed that continuous random disturbances in arterial PCO2 are more likely to elicit ventilatory oscillation patterns that mimic periodic breathing in normoxia than in hyperoxia. To test this hypothesis experimentally, in nine awake humans we applied pseudorandom binary inspired CO2 fraction stimulation in normoxia and hyperoxia to derive the closed-loop and open-loop ventilatory responses to a brief CO2 disturbance in terms of impulse responses and transfer functions. The closed-loop impulse response has a significantly higher peak value [0.143 +/- 0.071 vs. 0.079 +/- 0.034 (SD) l . min-1 . 0.01 l CO2-1, P = 0.014] and a significantly shorter 50% response duration (42.7 +/- 13.3 vs. 72.3 +/- 27.6 s, P = 0.020) in normoxia than in hyperoxia. Therefore, the ventilatory responses to transient CO2 disturbances are less damped (but generally not oscillatory) in normoxia than in hyperoxia. For the closed-loop transfer function, the gain in normoxia increased significantly (P < 0.0005), while phase delay decreased significantly (P < 0.0005). The gain increased by 108.5, 186.0, and 240.6%, while phase delay decreased by 26.0, 18.1, and 17.3%, at 0. 01, 0.03, and 0.05 Hz, respectively. Changes in the same direction were found for the open-loop system. Generally, an oscillatory ventilatory response to a small transient CO2 disturbance is unlikely during wakefulness. However, changes in parameters that lead to additional increases in chemoreflex loop gain are more likely to initiate oscillations in normoxia than in hyperoxia.

Respiration response curve analysis of heart rate variability.

A noninvasive study was conducted on intact awake humans to characterize the dynamic response of the heart to the vagus during slow-, comfortable-, and fast-paced respiration (8, 12, and 18 breaths/min), under both sitting and standing conditions. The respiration response curve (RRC) of respiration-associated vagal effects on the heart was estimated, and characteristics of entrainment and frequency dependence on respiration were demonstrated. It was shown that the degree of entrainment and magnitude of phase resetting decrease with increase of pacing rate from 8 to 18 breaths/min. Further, the RRC was examined by overlapping equivalent phase shifts in different respiration cycles. This examination of the RRC can help us not only to find the common pattern underlying the RRC during different respiration cycles but also to perceive its variation related to degree of entrainment.

Cough and Other Reflexes on Irritation of Airway Mucosa in Man

Both human and animal studies show that irritation of airway mucosa elicits a variety of reflex responses such as coughing, apnoea, and laryngeal closure. Most of the information concerning these reflex responses were obtained in anesthetized conditions with little applicability to awake conditions. Various aspects of cough and other reflexes on irritation of the airway mucosa are discussed. Studies on awake humans showed that stimulation of the laryngeal mucosa with a small amount of distilled water during wakefulness causes elicitation of the expiration reflex, cough reflex, and swallowing reflex while other types of responses are scarcely observed. In addition, the duration of these responses is remarkably short. In contrast, the same stimulation causes more variant, prolonged, and exaggerated responses during a light depth of anesthesia. An increase in depth of anesthesia abolishes expiratory efforts such as coughing and the expiration reflex whereas the apnoeic reflex and laryngeal closure reflex are resistant to the depressant effect of anesthesia. Also, the respiratory reflex responses to airway irritation varied, depending on the site of stimulation: both laryngeal and tracheal stimulation cause vigorous respiratory responses whereas bronchial stimulation causes little or no respiratory responses. These results indicate not only that the types and magnitude of reflex responses is greatly modified by the central nervous state but also that the site of stimulation is crucial for determining the pattern of respiratory responses elicited by airway stimulation in humans.

Somatic evoked high-frequency magnetic oscillations reflect activity of inhibitory interneurons in the human somatosensory cortex

High-frequency potential oscillations in the range of 300–900 Hz have recently been shown to concur with the primary response (N20) of the somatosensory cortex in awake humans. However, the physiological mechanisms of the high-frequency oscillations remained undetermined. We addressed the issue by analyzing magnetic fields during wakefulness and sleep over the left hemisphere to right median nerve stimulation with a wide bandpass (0.1–2000 Hz) recording with subsequent high-pass (> 300 Hz) and low-pass (< 300 Hz) filtering. With wide bandpass recordings, high-frequency magnetic oscillations with the main signal energy at 580–780 Hz were superimposed on the N20m during wakefulness. Isofield mapping at each peak of the high-pass filtered and isolated high-frequency oscillations showed a dipolar pattern and the estimated source for these peaks was the primary somatosensory cortex (area 3b) very close to that for the N20m peak. During sleep, the high-frequency oscillations showed dramatic diminution in amplitude while the N20m amplitude exhibited a moderate increment. This reciprocal relation between the high-frequency oscillations and the N20m during a wake-sleep cycle suggests that they represent different generator substrates. We speculate that the high-frequency oscillations represent a localized activity of the GABAergic inhibitory interneurons of layer 4, which have been shown in animal experiments to respond monosynaptically to thalamo-cortical input with a high-frequency (600–900 Hz) burst of short duration spikes. On the other hand, the underlying N20m represents activity of pyramidal neurons which receive monosynaptic excitatory input from the thalamus as well as a feed-forward inhibition from the interneurons.