Postural Stimuli Research Articles

The study of the response of postural stimuli in relation to heart rate and skin conductance in the nonlinear domain

The study of the response of postural stimuli in relation to heart rate and skin conductance in the nonlinear domain

The degree of engagement of cardiac and sympathetic arms of the baroreflex does not depend on the absolute value and sign of arterial pressure variations

Objective. The percentages of cardiac and sympathetic baroreflex patterns detected via baroreflex sequence (SEQ) technique from spontaneous variability of heart period (HP) and systolic arterial pressure (SAP) and of muscle nerve sympathetic activity (MSNA) burst rate and diastolic arterial pressure (DAP) are utilized to assess the level of the baroreflex engagement. The cardiac baroreflex patterns can be distinguished in those featuring both HP and SAP increases (cSEQ++) and decreases (cSEQ−−), while the sympathetic baroreflex patterns in those featuring a MSNA burst rate decrease and a DAP increase (sSEQ+−) and vice versa (sSEQ−+). The present study aims to assess the modifications of the involvement of the cardiac and sympathetic arms of the baroreflex with age and postural stimulus intensity. Approach. We monitored the percentages of cSEQ++ (%cSEQ++) and cSEQ−− (%cSEQ−−) in 100 healthy subjects (age: 21–70 years, 54 males, 46 females), divided into five sex-balanced groups consisting of 20 subjects in each decade at rest in supine position and during active standing (STAND). We evaluated %cSEQ++, %cSEQ−−, and the percentages of sSEQ+− (%sSEQ+−) and sSEQ−+ (%sSEQ−+) in 12 young healthy subjects (age 23 ± 2 years, 3 females, 9 males) undergoing incremental head-up tilt. Main results. We found that: (i) %cSEQ++ and %cSEQ−− decreased with age and increased with STAND and postural stimulus intensity; (ii) %sSEQ+− and %sSEQ−+ augmented with postural challenge magnitude; (iii) the level of cardiac and sympathetic baroreflex engagement did not depend on either the absolute value of arterial pressure or the direction of its changes. Significance. This study stresses the limited ability of the cardiac and sympathetic arms of the baroreflex in controlling absolute arterial pressure values and the equivalent ability of both positive and negative arterial pressure changes in soliciting them.

The study of the response of postural stimuli in relation to Heart rate and Skin conductance in the nonlinear domain

The study of the response of postural stimuli in relation to Heart rate and Skin conductance in the nonlinear domain

Categorizing the Role of Respiration in Cardiovascular and Cerebrovascular Variability Interactions.

Respiration disturbs cardiovascular and cerebrovascular controls but its role is not fully elucidated. Respiration can be classified as a confounder if its observation reduces the strength of the causal relationship from source to target. Respiration is a suppressor if the opposite situation holds. We prove that a confounding/suppression (C/S) test can be accomplished by evaluating the sign of net redundancy/synergy balance in the predictability framework based on multivariate autoregressive modelling. In addition, we suggest that, under the hypothesis of Gaussian processes, the C/S test can be given in the transfer entropy decomposition framework as well. Experimental protocols: We applied the C/S test to variability series of respiratory movements, heart period, systolic arterial pressure, mean arterial pressure, and mean cerebral blood flow recorded in 17 pathological individuals (age: 64±8 yrs; 17 males) before and after induction of propofol-based general anesthesia prior to coronary artery bypass grafting, and in 13 healthy subjects (age: 27±8 yrs; 5 males) at rest in supine position and during head-up tilt with a table inclination of 60°. Respiration behaved systematically as a confounder for cardiovascular and cerebrovascular controls. In addition, its role was affected by propofol-based general anesthesia but not by a postural stimulus of limited intensity. The C/S test can be fruitfully exploited to categorize the role of respiration over causal variability interactions. The application of the C/S test could favor the comprehension of the role of respiration in cardiovascular and cerebrovascular regulations.

Changes in Activity of Spinal Postural Networks at Different Time Points After Spinalization.

Postural limb reflexes (PLRs) are an essential component of postural corrections. Spinalization leads to disappearance of postural functions (including PLRs). After spinalization, spastic, incorrectly phased motor responses to postural perturbations containing oscillatory EMG bursting gradually develop, suggesting plastic changes in the spinal postural networks. Here, to reveal these plastic changes, rabbits at 3, 7, and 30 days after spinalization at T12 were decerebrated, and responses of spinal interneurons from L5 along with hindlimb muscles EMG responses to postural sensory stimuli, causing PLRs in subjects with intact spinal cord (control), were characterized. Like in control and after acute spinalization, at each of three studied time points after spinalization, neurons responding to postural sensory stimuli were found. Proportion of such neurons during 1st month after spinalization did not reach the control level, and was similar to that observed after acute spinalization. In contrast, their activity (which was significantly decreased after acute spinalization) reached the control value at 3 days after spinalization and remained close to this level during the following month. However, the processing of postural sensory signals, which was severely distorted after acute spinalization, did not recover by 30 days after injury. In addition, we found a significant enhancement of the oscillatory activity in a proportion of the examined neurons, which could contribute to generation of oscillatory EMG bursting. Motor responses to postural stimuli (which were almost absent after acute spinalization) re-appeared at 3 days after spinalization, although they were very weak, irregular, and a half of them was incorrectly phased in relation to postural stimuli. Proportion of correct and incorrect motor responses remained almost the same during the following month, but their amplitude gradually increased. Thus, spinalization triggers two processes of plastic changes in the spinal postural networks: rapid (taking days) restoration of normal activity level in spinal interneurons, and slow (taking months) recovery of motoneuronal excitability. Most likely, recovery of interneuronal activity underlies re-appearance of motor responses to postural stimuli. However, absence of recovery of normal processing of postural sensory signals and enhancement of oscillatory activity of neurons result in abnormal PLRs and loss of postural functions.

Assessment of the Coupling Strength of Cardiovascular Control via Joint Symbolic Analysis during Postural Challenge in Recreational Athletes.

Short-term cardiovascular control, comprising cardiac baroreflex and mechanisms governing cardiac contractility and vascular properties, links heart period (HP) and systolic arterial pressure (SAP) fluctuations. It is activated during postural challenge and this activation is traditionally quantified via linear tools such as HP-SAP squared coherence function. In this study the ability of a nonlinear bivariate tool based on joint symbolic analysis (JSA) approach was tested against HP-SAP coherence function during orthostatic challenge in recreational athletes. We studied 9 men healthy cycling practitioners (age: 20-40 yrs) at rest in supine condition (REST) and during active standing (STAND). The JSA method is based on the definition of symbolic HP and SAP patterns and on the evaluation of the rate of their simultaneous occurrence in both HP and SAP series. HP-SAP squared coherence was computed in the low frequency band (LF, from 0.04 to 0.15 Hz). We found the expected response to the postural stimulus, namely the increase of sympathetic modulation and the contemporaneous vagal withdrawal. However, only JSA was able to detect the expected increase of association between HP and SAP variability series over slow time scales. This result suggests that recreational athletes have more relevant nonlinear interactions between HP and SAP that might be missed by traditional linear tools and might require nonlinear tools to be efficiently described.

Are changes in the stomatognatic system able to modify the eye balance in dyslexia?

ObjectivesTo clarify the link between eye muscle function and oral information by comparing 21 dyslexic readers (DR) and 14 normal readers (NR). MethodsChanges in vertical heterophoria (VH) were measured using the Maddox Rod Test performed according to oral modifications and postural conditions. The Spearman correlation was used to assess whether reading delay was correlated with the lability index. ResultsOverall, 50% of NR children and 81% of DR experienced at least one variation in visual perception (p = 0.053). Among DR, the less reading delay they had, the higher their index of lability (p = 0.026), whereas there was no significant correlation among NR. Changes in the Maddox Test were more frequent in DR than in NR after the addition of sensory and postural stimuli, except for one specific posture. For sensory stimuli, the mean lability index was 1.35 in NR and 4.19 in DR, (p = 0.001). For postural stimuli, it was 0.71 and 2.61, (p = 0.003). ConclusionsIt is possible to modify visual perception by changing sensory or mechanical stimuli. Changes are more frequent in DR than in NR. Postural control can be improved with guided oral stimulations. SignificanceThese results reinforce the importance of professional cooperation in the care of dyslexic readers.

High-Intensity Exercise Mitigates Cardiovascular Deconditioning During Long-Duration Bed Rest.

Head-down-tilt bed rest (HDT) mimics the changes in hemodynamics and autonomic cardiovascular control induced by weightlessness. However, the time course and reciprocal interplay of these adaptations, and the effective exercise protocol as a countermeasure need further clarification. The overarching aim of this work (as part of a European Space Agency sponsored long-term bed rest study) was therefore to evaluate the time course of cardiovascular hemodynamics and autonomic control during prolonged HDT and to assess whether high-intensity, short-duration exercise could mitigate these effects. A total of n = 23 healthy, young, male participants were randomly allocated to two groups: training (TRAIN, n = 12) and non-training (CTRL, n = 11) before undergoing a 60-day HDT. The TRAIN group underwent a resistance training protocol using reactive jumps (5–6 times per week), whereas the CTRL group did not perform countermeasures. Finger blood pressure (BP), heart rate (HR), and stroke volume were collected beat-by-beat for 10 min in both sitting and supine positions 7 days before HDT (BDC−7) and 10 days after HDT (R+10), as well as on the 2nd (HDT2), 28th (HDT28), and 56th (HDT56) day of HDT. We investigated (1) the isolated effects of long-term HDT by comparing all the supine positions (including BDC−7 and R+10 at 0 degrees), and (2) the reactivity of the autonomic response before and after long-term HDT using a specific postural stimulus (i.e., supine vs. sitting). Two-factorial linear mixed models were used to assess the time course of HDT and the effect of the countermeasure. Starting from HDT28 onwards, HR increased (p < 0.02) and parasympathetic tone decreased exclusively in the CTRL group (p < 0.0001). Moreover, after 60-day HDT, CTRL participants showed significant impairments in increasing cardiac sympathovagal balance and controlling BP levels during postural shift (supine to sitting), whereas TRAIN participants did not. Results show that a 10-day recovery did not compensate for the cardiovascular and autonomic deconditioning following 60-day HDT. This has to be considered when designing rehabilitation programs—not only for astronauts but also in general public healthcare. High-intensity, short-duration exercise training effectively minimized these impairments and should therefore deserve consideration as a cardiovascular deconditioning countermeasure for spaceflight.

Behavioural responses to facial and postural expressions of emotion: An interpersonal circumplex approach.

While the recognition of emotional expressions has been extensively studied, the behavioural response to these expressions has not. In the interpersonal circumplex, behaviour is defined in terms of communion and agency. In this study, we examined behavioural responses to both facial and postural expressions of emotion. We presented 101 Romanian students with facial and postural stimuli involving individuals ('targets') expressing happiness, sadness, anger, or fear. Using an interpersonal grid, participants simultaneously indicated how communal (i.e., quarrelsome or agreeable) and agentic (i.e., dominant or submissive) they would be towards people displaying these expressions. Participants were agreeable-dominant towards targets showing happy facial expressions and primarily quarrelsome towards targets with angry or fearful facial expressions. Responses to targets showing sad facial expressions were neutral on both dimensions of interpersonal behaviour. Postural versus facial expressions of happiness and anger elicited similar behavioural responses. Participants responded in a quarrelsome-submissive way to fearful postural expressions and in an agreeable way to sad postural expressions. Behavioural responses to the various facial expressions were largely comparable to those previously observed in Dutch students. Observed differences may be explained from participants' cultural background. Responses to the postural expressions largely matched responses to the facial expressions.

A Markov Chain Analysis of Emotional Exchange in Voice-to-Voice Communication: Testing for the Mimicry Hypothesis of Emotional Contagion

Mimicry is a central plank of the emotional contagion theory; however, it was only tested with facial and postural emotional stimuli. This study explores the existence of mimicry in voice-to-voice communication by analyzing 8,747 sequences of emotional displays between customers and employees in a call-center context. We listened live to 967 telephone inter-actions, registered the sequences of emotional displays, and analyzed them with a Markov chain. We also explored other propositions of emotional contagion theory that were yet to be tested in vocal contexts. Results supported that mimicry is significantly present at all levels. Our findings fill an important gap in the emotional contagion theory; have practical implications regarding voice-to-voice interactions; and open doors for future vocal mimicry research.

Postural stability in young adults with Down syndrome in challenging conditions.

To evaluate postural control and performance in subjects with Down syndrome (SwDS), we measured postural sway (COP) in quiet stance in four 20-second tests: with eyes open or closed and on hard or foam surface. Ten SwDS and eleven healthy subjects participated, aged 29.8 (4.8) and 28.4 (3.9), respectively. The time-series recorded with the sampling rate of 100 Hz were used to evaluate postural performance (COP amplitude and mean velocity) and strategies (COP frequency, fractal dimension and entropy). There were no intergroup differences in the amplitude except the stance on foam pad with eyes open when SwDS had larger sway. The COP velocity and frequency were larger in SwDS than controls in all trials on foam pad. During stances on the foam pad SwDS increased fractal dimension showing higher complexity of their equilibrium system, while controls decreased sample entropy exhibiting more conscious control of posture in comparison to the stances on hard support surface. This indicated that each group used entirely different adjustments of postural strategies to the somatosensory challenge. It is proposed that the inferior postural control of SwDS results mainly from insufficient experience in dealing with unpredictable postural stimuli and deficit in motor learning.

Postural prioritization defines the interaction between a reaction time task and postural perturbations

Concurrent demands for postural and cognitive control processes are now known to induce interference, e.g., information processing speed may decrease during postural adjustment. It is less clear whether postural control may, at least in many situations, take precedence over cognitive control ("postural prioritization"). The purpose of this study was to determine if postural dual-task effects are the result of a postural prioritization effect. Twelve young subjects (6 female; 24.1 +/- 4.1) performed a discrete choice reaction time (RT) task in combination with a platform perturbation. To assess the effect of postural prioritization on RT and center of pressure (COP) parameters, destabilizing perturbations were randomly interspersed with non-destabilizing perturbations. Furthermore, stimulus order and the time interval of the RT stimulus relative to the platform perturbation were manipulated. COP and RT data obtained in these manipulations were compared to single-task baseline data. The results suggested that, irrespective of the degree of threat to postural stability, postural task processes are prioritized. Furthermore, anticipation of a postural stimulus negatively affects RT. However, once a perturbation commences subsequent RTs are speeded. Postural reactions were unaffected by a concurrent RT task, however. The RT stimulus acted as a cue to initiate biomechanical adaptations for an upcoming perturbation.

Neuroendocrine Responses to a Cold Pressor Stimulus in Polydipsic Hyponatremic and in Matched Schizophrenic Patients

Schizophrenia, many believe, reflects an enhanced vulnerability to psychological stress. Controlled exposure to stressors, however, has produced inconclusive results, particularly with regards to neurohormones. Some of the variability may be attributable to the nature and psychological significance of the stimulus and failure to control physiologic confounds. In addition, it is possible that the heterogeneity of schizophrenia is an important factor. In a carefully designed study and in a controlled setting, we measured the neuroendocrine response of eight polydipsic hyponatremic (PHS), seven polydipsic normonatremic (PNS), and nine nonpolydipsic normonatremic (NNS) (ie normal water balance) schizophrenic in-patients as well as 12 healthy controls (HC) to two different stressors: one of which appears to influence neuroendocrine secretion through its psychological (cold pressor) and the other (upright posture) through its systemic actions. Subjects in the three psychiatric groups were stabilized and acclimated to the research setting, and all received saline to normalize plasma osmolality. Following the cold pressor, plasma adrenocorticotropin and cortisol levels showed a more prolonged rise in PHS patients relative to PNS patients. NNS patients, in contrast, exhibited blunted responses relative to both of the polydipsic groups and the HC. Peak vasopressin responses were also greater in PHS and blunted in NNS patients. Responses to the postural stimulus were similar across patient groups. These findings provide a mechanism for life threatening water intoxication in schizophrenia; help to reconcile conflicting findings of stress responsiveness in schizophrenia; and potentially identify a discrete patient subset with enhanced vulnerability to psychological stress.

Reflex control of the spine and posture: a review of the literature from a chiropractic perspective.

ObjectiveThis review details the anatomy and interactions of the postural and somatosensory reflexes. We attempt to identify the important role the nervous system plays in maintaining reflex control of the spine and posture. We also review, illustrate, and discuss how the human vertebral column develops, functions, and adapts to Earth's gravity in an upright position. We identify functional characteristics of the postural reflexes by reporting previous observations of subjects during periods of microgravity or weightlessness.BackgroundHistorically, chiropractic has centered around the concept that the nervous system controls and regulates all other bodily systems; and that disruption to normal nervous system function can contribute to a wide variety of common ailments. Surprisingly, the chiropractic literature has paid relatively little attention to the importance of neurological regulation of static upright human posture. With so much information available on how posture may affect health and function, we felt it important to review the neuroanatomical structures and pathways responsible for maintaining the spine and posture. Maintenance of static upright posture is regulated by the nervous system through the various postural reflexes. Hence, from a chiropractic standpoint, it is clinically beneficial to understand how the individual postural reflexes work, as it may explain some of the clinical presentations seen in chiropractic practice.MethodWe performed a manual search for available relevant textbooks, and a computer search of the MEDLINE, MANTIS, and Index to Chiropractic Literature databases from 1970 to present, using the following key words and phrases: "posture," "ocular," "vestibular," "cervical facet joint," "afferent," "vestibulocollic," "cervicocollic," "postural reflexes," "spaceflight," "microgravity," "weightlessness," "gravity," "posture," and "postural." Studies were selected if they specifically tested any or all of the postural reflexes either in Earth's gravity or in microgravitational environments. Studies testing the function of each postural component, as well as those discussing postural reflex interactions, were also included in this review.DiscussionIt is quite apparent from the indexed literature we searched that posture is largely maintained by reflexive, involuntary control. While reflexive components for postural control are found in skin and joint receptors, somatic graviceptors, and baroreceptors throughout the body, much of the reflexive postural control mechanisms are housed, or occur, within the head and neck region primarily. We suggest that the postural reflexes may function in a hierarchical fashion. This hierarchy may well be based on the gravity-dependent or gravity-independent nature of each postural reflex. Some or all of these postural reflexes may contribute to the development of a postural body scheme, a conceptual internal representation of the external environment under normal gravity. This model may be the framework through which the postural reflexes anticipate and adapt to new gravitational environments.ConclusionVisual and vestibular input, as well as joint and soft tissue mechanoreceptors, are major players in the regulation of static upright posture. Each of these input sources detects and responds to specific types of postural stimulus and perturbations, and each region has specific pathways by which it communicates with other postural reflexes, as well as higher central nervous system structures. This review of the postural reflex structures and mechanisms adds to the growing body of posture rehabilitation literature relating specifically to chiropractic treatment. Chiropractic interest in these reflexes may enhance the ability of chiropractic physicians to treat and correct global spine and posture disorders. With the knowledge and understanding of these postural reflexes, chiropractors can evaluate spinal configurations not only from a segmental perspective, but can also determine how spinal dysfunction may be the ultimate consequence of maintaining an upright posture in the presence of other postural deficits. These perspectives need to be explored in more detail.

Postural control in the rabbit maintaining balance on the tilting platform.

A deviation from the dorsal-side-up body posture in quadrupeds activates the mechanisms for postural corrections. Operation of these mechanisms was studied in the rabbit maintaining balance on a platform periodically tilted in the frontal plane. First, we characterized the kinematics and electromyographic (EMG) patterns of postural responses to tilts. It was found that a reaction to tilt includes an extension of the limbs on the side moving down and flexion on the opposite side. These limb movements are primarily due to a modulation of the activity of extensor muscles. Second, it was found that rabbits can effectively maintain the dorsal-side-up body posture when complex postural stimuli are applied, i.e., asynchronous tilts of the platforms supporting the anterior and posterior parts of the body. These data suggest that the nervous mechanisms controlling positions of these parts of the body can operate independently of each other. Third, we found that normally the somatosensory input plays a predominant role for the generation of postural responses. However, when the postural response appears insufficient to maintain balance, the vestibular input contributes considerably to activation of postural mechanisms. We also found that an asymmetry in the tonic vestibular input, caused by galvanic stimulation of the labyrinths, can affect the stabilized body orientation while the magnitude of postural responses to tilts remains unchanged. Fourth, we found that the mechanisms for postural corrections respond only to tilts that exceed a certain (threshold) value.

Changes of plasma calcitonin gene–related peptide levels in postmenopausal women

OBJECTIVE: Our purpose was to investigate whether the secretion of a cardiovascular hormone, calcitonin gene - related peptide, is modified in climateric women according to cardiovascular adaptive responses. STUDY DESIGN: Plasma calcitonin gene–related peptide levels were measured in climateric women in a basal condition (n = 15), in response to an upright position (n = 8), and during hot flushes (n = 12). The effect of hormonal replacement therapy on plasma calcitonin gene–related peptide was also studied (n = 9). Plasma calcitonin gene–related peptide levels were measured by a specific radioimmunoassay after an acidic extraction. RESULTS: Plasma calcitonin gene–related peptide levels in postmenopausal women were significantly lower than in the control group (p < 0.01). After the women assumed an upright posture, a lack of plasma calcitonin gene–related peptide increase was observed in control fertile women, who showed the typical significant hormonal increase (p < 0.01). In all patients the occurrence of hot flushes was associated with a significant and rapid increase of plasma calcitonin gene - related peptide levels. After 3 months of hormonal replacement therapy basal plasma calcitonin gene–related peptide levels returned to the range of healthy fertile women. CONCLUSIONS: The current data show that the secretion of plasma calcitonin gene–related peptide is lower in postmenopausal women and its response to postural stimulus is impaired. Vasomotor changes are associated with an increase of plasma levels of this cardiovascular hormone. An effect of hormonal replacement therapy on calcitonin gene–related peptide secretion has been shown. (Am J Obstet Gynecol 1996;175:638-42.)

Norepinephrine and epinephrine responses to postural stimulus in orthostatic hypotension due to brainstem ischemic lesions

Norepinephrine (NE) and epinephrine (E) responses to upright posture were investigated in 25 patients with orthostatic hypotension due to brainstem ischemic lesions and in 25 control subjects. In controls the postural stimulus induced constantly a rise in NE urinary excretion and a reduction in E excretion, while in patients with orthostatic hypotension it caused a depression in NE urinary excretion and a rise in E urinary excretion; the last alteration was noticed in all but one patient. The E discharge induced by posture in patients with orthostatic hypotension may be involved in the reduction of vascular peripheral resistance and then in postural fall of blood pressure displayed by such patients.

Altered cardiovascular and neurohumoral responses to head-up tilt after heart-lung transplantation.

Heart-lung transplantation results in afferent and efferent denervation of the transplanted organs including interruption of the central connections from the low-pressure receptors in the atria and pulmonary veins. We investigated whether the cardiovascular and neurohumoral responses to the postural stimulus of head-up tilt were affected after transplantation. Responses in eight heart-lung transplant recipients were studied and compared with those in eight normal subjects matched for age and sex during passive head-up tilt at 45 degrees for 1 hour. The transplant group had a higher initial heart rate (99 +/- 2 versus 68 +/- 2 beats/min, p less than 0.001) and diastolic blood pressure (88 +/- 5 versus 76 +/- 2 mm Hg, p less than 0.05) than did the control group. The increases in heart rate and diastolic blood pressure during head-up tilt were similar in the two groups. Systolic blood pressure remained unchanged. The decrease in cardiac output (30% versus 18%, p less than 0.05) and the increase in systemic vascular resistance (52% versus 28%, p less than 0.05) were greater in the transplant group. Baseline levels of norepinephrine, epinephrine, vasopressin, and plasma renin activity were similar in the two groups. Atrial natriuretic peptide concentrations were higher in the transplant group (26 +/- 3.8 versus 9.7 +/- 1.6 pmol/l, p less than 0.001). During head-up tilt, plasma norepinephrine levels increased to a greater extent in the transplant group than in the control group (83% versus 53%, p less than 0.01), indicating an increased sympathetic response. In contrast, plasma renin activity increased significantly in the control group but not in the transplant group.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperreacivity of the Central Sympathetic Nervous System in Vibration-Induced White Finger

Vibration exposed workers with and without vibration-induced white finger (VWF), subjects with primary Raynaud's phenomenon and non-exposed controls were investigated in the present studies. The applied stimuli comprised cold stimuli, vibration exposure, and postural stimuli. The digital vasomotor responses to these stimuli were objectively quantified by measurements of finger systolic blood pressure and relative capillary blood flow rate in the skin of the finger. The results indicate that hyperreactivity of the central sympathetic nervous system may function as an/the abnormal element of the reflex mechanisms triggering the predominant portion of the vasoconstriction in episodic arterial closure of VWF. The hyperreactivity of the central sympathetic nervous system may be induced by prolonged exposure to hand-arm vibration, possibly by activation of the vibration elicited central sympathetic vasoconstrictor reflex mechanisms.

Influence of central set on human postural responses.

1. The effect of central set on automatic postural responses was studied in humans exposed to horizontal support-surface perturbations causing forward sway. Central set was varied by providing subjects with prior experience of postural stimulus velocities or amplitudes under 1) serial and random conditions, 2) expected and unexpected conditions, and 3) practiced and unpracticed conditions. In particular, the influence of central-set conditions was examined on the pattern and magnitude of six leg and trunk electromyograph (EMG) activations and associated ankle torque responses to postural perturbations with identical stimulus parameters. 2. The scaling of initial agonist integrated EMG (IEMG) and torque responses to postural perturbation amplitude disappeared when perturbation amplitudes were randomized. This finding suggests that the initial magnitude of postural responses were centrally set to anticipated postural perturbation amplitudes based on sequential experience with the stimulus. 3. Expectation of postural stimulus amplitude had a significant effect on initial torque responses; subjects overresponded when a larger perturbation was expected and underresponded when a smaller perturbation was expected. Expectation of postural stimulus velocity had a smaller effect on initial torque responses, and subjects consistently overresponded when the velocity of the perturbation was unexpected. This difference in amplitude and velocity expectation may be because of the capacity to encode stimulus velocity, but not amplitude information, into the earliest postural responses of the current trial. The relative strength of amplitude and velocity central-set effects varied widely with individual subjects. 4. Central-set conditions did not affect initial EMG response latencies (100 +/- 20 ms, mean +/- SD) or the relative onset of proximal and distal agonists and antagonists. Unexpected or unpracticed stimulus amplitudes, however, were associated with significant late activation of ankle antagonist, tibialis. Thus errors in initial response magnitude because of central-set effects appear to be partially corrected by reciprocal antagonist activity. Agonist IEMG, however, did not always reflect significant changes in torque responses with central-set conditions. 5. Expectation of postural stimulus amplitude and velocity had two different types of effects on the magnitudes of postural responses: 1) a directionally specific, central-set effect consisting of either increased or decreased responses, depending on expectation of stimulus amplitude; and 2) a nonspecific enhanced response to novel stimulus velocities with a gradual reduction when a velocity was presented repeatedly. Two different neural mechanisms are proposed for these two adaptive effects. 6. Reduction of postural response magnitude and antagonist activity during practice may be partially explained by adaptive mechanisms based on expectation because of prior experience with stimulus velocity and amp