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PubMed2003

Adaptation of the vestibulo-ocular reflex, subjective tilt, and motion sickness to head movements during short-radius centrifugation.

Head movements made while the whole body is rotating at unusually high angular velocities (here with supine body position about an earth-vertical axis) result in inappropriate eye movements, sensory illusions, disorientation, and frequently motion sickness. We investigated the acquisition and retention of sensory adaptation to cross-coupled components of the vestibulo-ocular reflex (VOR) by asking eight subjects to make headturns while being rotated at 23 rpm on two consecutive days, and again a week later. The dependent measures were inappropriate vertical VOR, subjective tilt, and motion sickness in response to 90 degrees yaw out-of-plane head movements. Motion sickness was evaluated during and following exposure to rotation. Significant adaptation effects were found for the slow phase velocity of vertical nystagmus, the reported magnitude of the subjective tilt experienced during head turns, and motion-sickness scores. Retention of adaptation over a six-day rest period without rotation occurred, but was not complete for all measures. Adaptation of VOR was fully maintained while subjective tilt was only partially maintained and motion-sickness scores continued to decrease. Practical implications of these findings are discussed with particular emphasis on artificial gravity, which could be produced in weightlessness by means of a short-radius (2 m) rotator.

Adaptation, Physiological↗

PubMed2000

Context-specific adaptation of the vertical vestibuloocular reflex with regard to gravity.

We determined whether head position with regard to gravity is an important context for angular vestibuloocular reflex (aVOR) gain adaptation. Vertical aVOR gains were adapted with monkeys upright or on side by rotating the animals about an interaural axis in phase or out of phase with the visual surround for 4 h. When aVOR gains were adapted with monkeys upright, gain changes were symmetrical when tested in either on-side position (23 +/- 7%; mean +/- SD). After on-side adaptation, however, gain changes were always larger when animals were tested in the same on-side position in which they were adapted. Gain changes were 43 +/- 16% with ipsilateral side down and 9 +/- 8% with contralateral side down. The context-specific effects of head position on vertical aVOR gain were the same whether the gain was increased or decreased. The data indicate that vertical aVOR gain changes are stored in the context of the head orientation in which changes were induced. This association could be an important context for expressing the adapted state of the aVOR gain during vertical head movement.

Adaptation, Physiological↗

PubMed1999

The influence of visual input on the vestibulo-ocular reflex.

In order to stabilise a fixation target on the retina, eye movements have to compensate for head movements. During slow head movements visual feedback can control these eye movements. During fast movements of the head, mainly the vestibulo-ocular reflex (VOR) controls eye movements, as visual feedback is too slow. However, visual feedback is an important factor in controlling the VOR; e.g. the gain of the VOR depends on the distance of the target. This study investigates the influence of retinal image position during fast head movements. The experiments were carried out in five human subjects using scleral search coils. The adaptation of each eye individually to a change of retinal position of a target was examined during head shaking. The change in visual input was carried out by placing Fresnel prisms of different strengths in front of both eyes, thus inducing a change in retinal image position without changing the retinal slip. The results show, that both eyes make the appropriate corrections when the visual input changes, even during fast head-movements. These corrections did not influence the gain of the VOR. From these results we conclude, that retinal image position besides retinal slip has a major influence on the monocular eye movements even at high head rotation frequencies.

Adaptation, Ocular↗

PubMed1998

Mechanisms of airflow limitation in the nose and lungs.

The main differences in the anatomies of the upper and lower airways are the vascular characteristics and bony surroundings of the upper airways compared with the smooth muscle component and the loose-lying situation of the lower airways. Both allergic asthma and rhinitis involve a similar process of mucosal inflammation in response to allergen exposure, characterized by inflammatory cell infiltration and local release of mediators. In addition, both the upper and lower airways are under similar influences from local neuronal reflexes, exercise, posture and cyclic variations. Challenge tests have been able to demonstrate the roles of the various influences and components in the inflammatory processes of asthma and allergic rhinitis. A particular difference in the response in asthma compared with rhinitis is the degree of non-specific hyperresponsiveness, which is a characteristic of the late response or response to repeated or prolonged exposure to allergen. Responsiveness of the bronchial mucosa in asthma patients is approximately 50 times that of normal (non-allergic or non-asthmatic) subjects, whereas that of the nasal mucosa in allergic rhinitis is only 2-8 times that of control subjects. The inflammatory process involved in hyperresponsiveness is similar in both conditions, involving increased infiltration of eosinophils and subsequent increased mediator release. The greater degree of hyperresponsiveness seen in asthma appears to be a consequence of the anatomical differences between the upper vs the lower airways. Evidence is presented for the contribution of increased airway wall thickness to the hyperresponsiveness in asthma, together with other possible factors, such as decoupling of responder tissue from surrounding tissue, increased smooth muscle contractility, and smooth muscle hypertrophy.

Allergens↗

PubMed1993

The effects of head and trunk position on torsional vestibular and optokinetic eye movements in humans.

We measured torsional vestibular and optokinetic eye movements in human subjects with the head and trunk erect, with the head supine and the trunk erect, and with the head and trunk supine, in order to quantify the effects of otolithic and proprioceptive modulation. During active head movements, the torsional vestibulo-ocular reflex (VOR) had significantly higher gain with the head upright than with the head supine, indicating that dynamic otolithic inputs can supplement the semicircular canal-ocular reflex. During passive earth-vertical axis rotation, torsional VOR gain was similar with the head and trunk supine and with the head supine and the trunk erect. This finding implies that static proprioceptive information from the neck and trunk has little effect upon the torsional VOR. VOR gain with the head supine was not increased by active, self-generated head movement compared with passive, whole body rotation, indicating that the torsional VOR is not augmented by dynamic proprioceptive inputs or by an efference copy of a command for head movement. Viewing earth-fixed surroundings enhanced the torsional VOR, while fixating a chair-fixed target suppressed the VOR, especially at low frequencies. Torsional optokinetic nystagmus (OKN) evoked by a full-field stimulus had a mean slow-phase gain of 0.22 for 10 degrees/s drum rotation, but gain fell to 0.06 for 80 degrees/s stimuli. Despite this fall in gain, mean OKN slow-phase velocities increased with drum speed, reaching maxima of 2.5 degrees/s-8.0 degrees/s in our subjects. Optokinetic after nystagmus (OKAN) was typically absent.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMedDate not supplied

The value of cardiovascular autonomic function tests: 10 years experience in diabetes.

Five simple, noninvasive cardiovascular reflex tests have been used to assess autonomic function in one center over the past 10 yr. Seven hundred seventy-four diabetic subjects were tested for diagnostic and research purposes. In 543 subjects completing all five tests, abnormalities of heart rate tests occurred in 40%, while abnormal blood pressure tests occurred in less than 20%. Their results were grouped as normal (39%), early (15%), definite (18%), and severe (22%) involvement. Six percent had an atypical pattern of results. Two hundred thirty-seven diabetic subjects had the tests repeated greater than or equal to 3 mo apart: 26% worsened, 71% were unchanged, and only 3% improved. The worsening followed a sequential pattern with first heart rate and later additional blood pressure abnormalities. Comparison between a single test (heart rate response to deep breathing) and the full battery in 360 subjects showed that one test alone does not distinguish the degree or severity of autonomic damage. These tests provide a useful framework to assess autonomic neuropathy simply, quickly, and noninvasively.

Adolescent↗

PubMed1999

Preparatory balance adjustments precede withdrawal response to noxious stimulation in standing humans.

Self-initiated leg movement in standing humans is preceded by a medio-lateral preparatory balance adjustment (PBA); however, such preparatory balance control is often absent in reflex-like stepping responses evoked by whole-body instability. The presence or absence of the PBA may reflect a task-dependent modulation of the response serving to preserve lateral stability (PBA present) or avoid delay in the lifting of the foot (PBA absent). To examine whether such task-dependent modulation can occur during more stereotypical limb movements, we examined spinally-mediated withdrawal responses evoked by noxious stimulation of the foot. Results showed that rapid limb withdrawal was preceded by a large PBA when subjects were standing but not when they were supine. The PBA caused limb withdrawal to the noxious stimulation to be delayed. However, the onset of the PBA in the standing trials was equivalent in timing to the onset latency of the classic withdrawal responses recorded during the supine trials. Evidence of a preparatory balance adjustment evoked, in advance of a delayed withdrawal response, at very rapid latencies (underlying muscle activation at 70-120 ms) may raise new questions about the neural mechanisms underlying the co-ordination of balance and movement.

Adult↗

PubMedDate not supplied

Modulation by head and trunk positions of the vestibulo-spinal reflexes evoked by galvanic stimulation of the labyrinth. Observations by labyrinthine evoked EMG.

Modulation by head and trunk positions of the vestibulo-spinal reflexes was studied in the soleus muscle activities induced by galvanic stimulation of the labyrinth. The stimulation was applied using a bipolar-biaural method with the cathode on the right ear and the anode on the left ear. The intensity was 1 mA and duration 3 s. 1) When the labyrinth was stimulated with the head facing forward, soleus muscle activities increased on the right side and decreased on the left, with a latency of about 100 ms. In spite of the same stimulation, activities of the soleus muscle on both sides decreased with the head rotated to the right and increased with the head rotated to the left. 2) The responses in upper-body rotation were the same as in the case of head rotation. 3) In trunk rotation, some cases showed the same responses as with the trunk facing forward in rotation to the right and left, while others showed changes in muscular activities with the head rotation caused by rotating the trunk. 4) Changes in labyrinthine evoked EMG by head and trunk positions were considered to be due to interaction of vestibular and proprioceptive inputs on the interneurons of the spinal cord.

Ear, Inner↗

PubMed1981

Cardiovascular and autonomic reflexes in haemodialysis patients.

1. Blood pressure and heart rate responses to head-up tilt, standing, the Valsalva manoeuvre, sustained handgrip and cutaneous cold were measured in 27 haemodialysis patients (10 of whom had episodes of haemodialysis-induced hypotension) and 15 control subjects to assess autonomic nervous function. Plasma noradrenaline levels were measured at rest and during head-up tilt. 2. Mean resting supine blood pressure, heart rate and plasma noradrenaline levels were higher in haemodialysis patients than in the control subjects. There was no fall in blood pressure during head-up tilt or standing. The ratio of the R--R intervals of the thirtieth and the fifteenth heart beat after standing (30:15) was lower in the patients; this may be related to their higher resting heart rate. Head-up tilt raised plasma noradrenaline levels in both groups. Heart rate responses to the Valsalva manoeuvre were similar in the patients and control subjects. 3. Systolic blood pressure and heart rate responses to sustained handgrip were similar in both groups. Diastolic and mean blood pressure changes, however, were lower in the patients. The blood pressure and heart rate responses to cutaneous cold were similar in the patients and control subjects. 4. We conclude that generalized autonomic nervous dysfunction does not appear to cause haemodialysis-induced hypotension in patients with chronic renal failure on maintenance haemodialysis.

Adult↗

PubMed1988

Reflex constriction of human limb resistance vessels to head-down neck flexion.

The effect of head-down neck flexion on forearm and calf blood flow was determined in 10 healthy male subjects. The subject lay prone, with the neck slightly extended and the chin resting on a soft-padded support at the edge of the table. The chin support was then removed, and the subject maximally flexed and lowered the neck. This was followed by return to the initial position. Neck flexion caused a rapid decrease in blood flow in both forearm and calf; at 30 s this averaged 39 and 35%, respectively. The flow in both forearm and calf gradually recovered as the neck flexion was sustained and approached the control values at the end of 10 min. The blood flow at the ankle was unchanged, indicating that the decrease occurred in the skeletal muscles. The arterial blood pressure and heart rate were unchanged; thus the decrease in flow was due to vasoconstriction. The fact that the decrease was evident as soon as the head was lowered indicated that it was nervously mediated. Neither contraction of the flexor muscles of the neck nor venous congestion of the head, in the absence of the head-down position, altered the blood flow. Although the mechanism of the decrease in flow has not been determined, the studies demonstrate that in response to certain stimuli, the resistance vessels in the skeletal muscles of the forearm and calf undergo a similar nervously mediated vasoconstriction.

Forearm↗

PubMed2000

Effects of dimenhydrinate on computerized dynamic posturography.

OBJECTIVE: Previous studies have demonstrated that sedatives and antiemetics commonly used by patients suppress the vestibulo-ocular reflexes during electronystagmography, making it more difficult to quantify function in such patients. The effects of these medications on computerized dynamic posturography (CDP) have not been studied, and the influences, if any, on the vestibulospinal reflexes are not known. We aimed to study the influence, if any, of dimenhydrinate on CDP performance. DESIGN: A double-blinded study using a randomized protocol to compare the effects of dimenhydrinate and placebo on CDP performance in normal subjects. SETTING: A tertiary/quaternary care facility using a standardized CDP assessment protocol. METHODS: After a CDP training session (one assessment) to rule out any learning effect, 10 subjects underwent CDP assessment on 2 separate days after ingestion of either a standard single dose of dimenhydrinate or placebo. MAIN OUTCOME MEASURES: Pre- and post-medication CDP performance was measured using Sensory Organization Test (SOT) composite scores and also scores on CDP conditions particularly sensitive to measurement of vestibular impairment (SOT conditions 5 and 6). RESULTS: Analysis of data showed no significant effect on CDP performance of normal subjects after dimenhydrinate administration, although there may be a trend toward a slight effect on performance. CONCLUSIONS: There seems to be no significant effect of dimenhydrinate on CDP performance in normal subjects, although there may be a trend. Further studies using a dose causing more significant symptoms or using a higher number of subjects may clarify this.

Adult↗

PubMed1996

[Three-dimensional analysis of vestibulo-ocular reflexes--the relation between head position and eye movement].

Three-dimensional analysis of vestibulo-ocular reflexes, using a computerized eye movement analysis system, was carried out in 16 normal human subjects. We examined the relationship between head position and vestibulo-ocular reflexes. The purpose of this study was to clarify the relationship between the functional plane and the anatomical position of the semicircular canals. The subjects were stimulated by pendular rotation with their heads fixed between 40 degrees nose down and 50 degrees nose up in 10 degrees steps with respect to the orbitomeatal line and the lateral canal line. The gain in horizontal eye movement which results from the lateral canal, was the highest at 20 degrees nose down. On the other hand, the gains in vertical and torsional eye movement, which result from the vertical canals, were the lowest at 10 degrees nose up. The phase of vertical and torsional eye movement changed 180 degrees at 10 degrees nose up. These data indicate that the functional plane of the lateral canal almost coincided with the anatomical position. However, at this functional plane of the lateral canal, the influence of vertical canals was not the least. We suggest that this could be the reason why angles formed by the intersection of the lateral canal plane and the vertical canals were not 90 degrees. We could interpret this finding as showing that at the 10 degrees nose up position, the origin of eye movement alternated between the frontal and posterior canals.

Adult↗

PubMed1991

The horizontal vestibulo-ocular reflex during linear acceleration in the frontal plane of the cat.

Horizontal and vertical eye movements were recorded in alert, restrained cats that were subjected to whole-body rotations with the horizontal semicircular canals in the plane of rotation and the body centered on the axis or 45 cm eccentric from the axis of rotation. Changes in the horizontal vestibulo-ocular reflex (HVOR) due to the resultant of the linear forces (i.e., gravity and linear acceleration) acting on the otolith organs were examined during off-axis rotation when there was a centripetal acceleration along the animal's interaural axis. The HVOR time constant was slightly shortened when the resultant otolith force was not parallel to the animal's vertical axis. This effect was independent of the direction of the otolith force relative to the direction of the slow phase eye velocity. No effect on the HVOR amplitude was observed. In addition to changes in the HVOR dynamics, a significant vertical component of eye velocity was observed during stimulation of the horizontal canals when the resultant otolith force was not parallel with the animal's vertical axis. The effect was greater for larger angles between the resultant otolith force and gravity. An upward or downward component was elicited, depending on the direction of the horizontal component of eye velocity and the direction of the resultant otolith force. The vertical component was always in the direction that would tend to align the eye velocity vector with the resultant otolith force and keep the eye movement in a plane that had been rotated by the angle between the resultant otolith force and gravity.

Acceleration↗

PubMedDate not supplied

Depression of the vestibulospinal reflex by intravermal microinjection of GABAA and GABAB agonists in the decerebrate cat.

Experiments performed in decerebrate cats have shown that unilateral microinjection into the cerebellar anterior vermis of a GABAA (muscimol) or a GABAB agonist (baclofen) decreased the gain of the vestibulospinal reflex involving the ipsilateral triceps brachii (iVSR). On the contrary, the phase angle of the reflex was not significantly modified. These effects started 5 to 10 min after the injection and persisted for at least 1 to 2 h before disappearing. Just the opposite changes in gain of the VSR were obtained after local microinjection of the GABAA (bicuculline) or the GABAB antagonist (saclofen). The area on which the GABAergic agents were effective was located within the third and/or the fourth folium rostral to the fissura prima (culmen), at the laterality of 1.0 to 1.4 mm with respect to the midline. This vermal region corresponded to the zone B of the cerebellar cortex, which receives a labyrinth input and projects to the ipsilateral lateral vestibular nucleus, where it exerts a prominent inhibitory influence. It is suggested that GABA agonists inhibit the Purkinje (P)-cells' activity, thus reducing the labyrinthine-induced modulation of the firing rate of these neurons by mossy and climbing fiber afferents. Since the vermal P-cells discharge outphase with respect to the excitatory VS neurons, we may explain why a reduced output of these P-cells results in a reduced gain of the VSR. These experiments provide evidence that the cerebellar anterior vermis exerts a positive influence on the basic VSR gain.

Animals↗

PubMed1991

Observations based on 10-years' experience of non-invasive cardiovascular reflex testing of autonomic function from a rehabilitation research centre.

Observations on 1,282 measurements of non-invasive cardiovascular reflex testing of autonomic function in 1,023 control subjects and patients with different disorders are presented. It is suggested that the Valsalva manoeuvre be performed three times rather than once and the highest Valsalva ratio should be used instead of the mean or first of the three ratios. The deep breathing test gives equally reliable information when three breathing cycles are used instead of six. Symptoms were detected in up to 15% of the subjects depending on the test, although the majority of these were minor and transient. The frequency of ventricular/supraventricular extrasystoles increased especially during the Valsalva manoeuvre (+67%/+40%) and post-strain (+49%/+25%) phases and early recovery phase of the orthostatic test (+69%/+156%) and to a lesser degree during deep breathing (+18%/+50%) and handgrip (+36%/-11%) phases compared with the resting phase. However, severe cardiac rhythm disturbances were very rare. 1-2% of the younger (less than 50 years) and 10% of the older subjects were not able to perform the tests adequately. The R-R intervals on which the results are based should be selected carefully, and use of a continuous non-invasive method of blood pressure monitoring should provide further useful information.

Adolescent↗

PubMed1983

The chronic spinalized cat: a model for neuromuscular plasticity.

The ability of the hindlimbs to produce weight-supported locomotion on a motorized treadmill was assessed in cats spinalized (T12) at 2 or 12 weeks of age. The animals were assigned at random to nonexercised (NE) or exercised (E) groups, the latter being trained to walk on the treadmill for 30 minutes, five-times weekly. During the four-month recovery period, spontaneous clonus at 8 to 12 Hz and hyperactive cutaneous reflexes developed in all animals. Age of spinalization had an effect on the recovery of locomotion, as kittens in both the 2E and 2NE groups rated significantly higher than the 12E and 12NE cats. Treadmill training was not critical to the recovery process in the 2-week transected kittens, since both 2E and 2NE cats exhibited stepping with adequate weight support of the hindquarters. However, none of the 12NE cats developed weight-supported stepping, while some of the 12E cats did. Electromyographic recordings of ankle muscles from cats demonstrating the best locomotion revealed that progressive recruitment of the soleus and gastrocnemius muscles was normal despite significant changes in the fiber composition of these extensors. Kinematic analyses of these same animals revealed that gait abnormalities, such as the absence of a yield phase during stance and uncoupling of knee and ankle actions, were more common in the 2NE and 12E animals as compared to the 2E animals. It was concluded that although the spinal cord contains the basic pattern-generating circuitry essential for ambulatory locomotion, the development or recovery of this behavior was influenced by the animal's age at the time of cord transection, and to a less obvious extent by subsequent training.

Animals↗

PubMed1991

Abstinent chronic alcoholics investigated by dynamic posturography, ocular smooth pursuit and visual suppression.

Eleven male chronic alcoholic volunteers aged 44-65 years (mean 57 years) were investigated by dynamic posturography, ocular smooth pursuit and visual suppression of the vestibulo-ocular reflex (VOR). Their drinking time ranged from 8-30 years (mean 20 years) and they had been abstinent for the last 1-20 years (mean 7 years). Ocular smooth pursuit showed abnormalities in 8/11. Abnormalities were found in 5/11 in the visual suppression of the VOR. The results of dynamic posturography tests were compared to an age-matched reference material. Dynamic posturography (EquiTest) comprises a sensory organization (SO) part in which the support surface and visual surround are either stable or referenced to the patient's sway, with eyes open or closed. In the SO part the chronic alcoholics had lower equilibrium scores in all test conditions, and the differences were significant in 4 tests out of 6. In a movement coordination part the platform makes active movements, the latencies to which were significantly prolonged in the 2 larger of the 3 translational amplitudes. Adaptation to repeated tilting of the platform was estimated to be pathological in 4/10, compared to none of the controls. The abnormal pattern found in dynamic posturography correlates well with the pathology in ocular smooth pursuit and visual suppression tests, suggested to be due to alcohol induced cerebellar lesions. It is concluded that dynamic posturography is a valuable test for assessing dysequilibrium in chronic alcoholics, even abstinent.

Adult↗

PubMed1999

Effects of viewing distance on the responses of vestibular neurons to combined angular and linear vestibular stimulation.

Effects of viewing distance on the responses of vestibular neurons to combined angular and linear vestibular stimulation. The firing behavior of 59 horizontal canal-related secondary vestibular neurons was studied in alert squirrel monkeys during the combined angular and linear vestibuloocular reflex (CVOR). The CVOR was evoked by positioning the animal's head 20 cm in front of, or behind, the axis of rotation during whole body rotation (0.7, 1.9, and 4.0 Hz). The effect of viewing distance was studied by having the monkeys fixate small targets that were either near (10 cm) or far (1.3-1.7 m) from the eyes. Most units (50/59) were sensitive to eye movements and were monosynaptically activated after electrical stimulation of the vestibular nerve (51/56 tested). The responses of eye movement-related units were significantly affected by viewing distance. The viewing distance-related change in response gain of many eye-head-velocity and burst-position units was comparable with the change in eye movement gain. On the other hand, position-vestibular-pause units were approximately half as sensitive to changes in viewing distance as were eye movements. The sensitivity of units to the linear vestibuloocular reflex (LVOR) was estimated by subtraction of angular vestibuloocular reflex (AVOR)-related responses recorded with the head in the center of the axis of rotation from CVOR responses. During far target viewing, unit sensitivity to linear translation was small, but during near target viewing the firing rate of many units was strongly modulated. The LVOR responses and viewing distance-related LVOR responses of most units were nearly in phase with linear head velocity. The signals generated by secondary vestibular units during voluntary cancellation of the AVOR and CVOR were comparable. However, unit sensitivity to linear translation and angular rotation were not well correlated either during far or near target viewing. Unit LVOR responses were also not well correlated with their sensitivity to smooth pursuit eye movements or their sensitivity to viewing distance during the AVOR. On the other hand there was a significant correlation between static eye position sensitivity and sensitivity to viewing distance. We conclude that secondary horizontal canal-related vestibuloocular pathways are an important part of the premotor neural substrate that produces the LVOR. The otolith sensory signals that appear on these pathways have been spatially and temporally transformed to match the angular eye movement commands required to stabilize images at different distances. We suggest that this transformation may be performed by the circuits related to temporal integration of the LVOR.

Animals↗