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PubMed2006

Impaired viscerosomatic reflexes and abdominal-wall dystony associated with bloating.

BACKGROUND & AIMS: Abdominal bloating is a frequent complaint in irritable bowel syndrome (IBS), but its underlying mechanism remains uncertain. Our aim was to determine whether the abdominal wall, specifically its adaptation to intra-abdominal volumes, plays a role. METHODS: In 12 patients complaining of abdominal bloating (8 IBS and 4 functional bloating) and in 12 healthy controls, the effect of colonic gas load (24 mL/min rectal gas infusion for 1 hour) on perception (measured by a 0-6 scale), abdominal girth, and muscular activity was tested. With the participants sitting on an ergonomic chair and the trunk erect, multichannel electromyography was measured via bipolar surface electrodes located over the upper and lower rectus abdominis, and the external and internal oblique bilaterally. RESULTS: In healthy controls, colonic gas loads produced subjective symptoms (score, 3.0 +/- 0.3), objective abdominal distention (girth increment, 6 +/- 1 mm), and increased the activity of the abdominal muscles (external oblique activity, 11% +/- 3% in; P < .05 vs basal). At the same infused gas volumes, the patients developed significantly more symptoms (score, 4.5 +/- 0.4) and abdominal distention (11 +/- 1 mm; P < .05 vs healthy for both). These abnormal responses were associated with failed tonic contraction of the abdominal wall (external oblique activity change, -1% +/- 4%; P value not significant vs basal) and paradoxic relaxation of the internal oblique (activity reduction, 26% +/- 7%; P < .01 vs basal). CONCLUSIONS: In patients with bloating, abdominal perception and distention in response to intra-abdominal volume increments are exaggerated markedly and associated with muscular dystony of the abdominal wall.

Abdominal Muscles↗
PubMed1987

Testing the vestibular-ocular reflexes: abnormalities of the otolith contribution in patients with neuro-otological disease.

Conventional vestibular rotation testing with the head centered on the axis stimulates the semicircular canals evoking compensatory eye movements. If the head is placed forwards of the axis in an eccentric position the otoliths are also stimulated by a tangential linear acceleration acting laterally to the skull. In normal subjects the additional otolithic stimulus evokes compensatory eye movements with a higher gain than with head centred, particularly for high frequency (greater than 0.1 Hz) stimuli. The responses with head centred and eccentric in various patients with known/suspected neuro-otological abnormalities have been compared. Patients with vestibular neurinectomies who have asymmetrical head centred responses showed greater asymmetry with head eccentric at higher stimulus frequencies. Some patients with cerebellar lesions showed abnormally enhanced or depressed and asymmetrical responses with head eccentric in comparison with head centred responses, which could be normal. The enhancing effects could be specific to low frequency stimuli. All patients who showed abnormal responses with head eccentric also had positional nystagmus provoked by the gravity acceleration vector when the head was tilted laterally. The direction of the positional nystagmus with respect to the gravity vector was not necessarily the same as the direction of the effect on eye movements of lateral acceleration during eccentric oscillation. Patients with benign paroxysmal vertigo or chronic linear vertigo in whom otolithic abnormalities are suspected were not found to have abnormal responses with head eccentric. We conclude that this method of testing may be useful in elucidating pathophysiology but is not a decisive clinical test for the presence of disordered otolith function.

Acceleration↗
PubMed2001

Translational vestibulo-ocular reflex evoked by a "head heave" stimulus.

The gain and symmetry of vestibulo-ocular reflexes for high-frequency, high-acceleration movements of the head are altered following unilateral vestibular lesions. These changes have been well characterized for rotational head movements (thrusts), and provide reliable markers of dysfunction in individual semicircular canals. Alterations in the vestibulo-ocular reflex (VOR) evoked by lateral, whole-body translations have also been observed. In an approach directed at the development of a bedside test of otolith function, we have recorded (scleral search coil) the VOR evoked by brief, high-acceleration lateral translations (heaves). We delivered these stimuli manually and also developed a "head sled" device that minimizes any rotational contaminating component of the stimulus. Our geometrical analysis of the stimuli enables us to take into account the translational and rotational components of the movement, and to calculate an ideal response required for stabilization of images on the fovea at different fixation distances. We observed a tracking response (visually assisted VOR) that was close to ideal for image stabilization when these methods were used to analyze responses to slow, low-amplitude lateral translations of the head. When applied to rapid, high-acceleration (0.5 g) translations, the VOR was found to be less than compensatory in subjects with normal vestibular function. In a patient with unilateral vestibular hypofunction following intratympanic gentamicin injections, both the rotational and the translational VOR were asymmetric. Responses for translations toward the treated side had lower gain than those for translations toward the normal side. These findings provide a basis for further development of this technique as a clinical test and as a method for quantitative evaluation of otolith function.

Adult↗
PubMedDate not supplied

Head slippage during broad-frequency rotational chair testing.

Broad-frequency rotational chair testing is employed in clinical and research settings to evaluate the response of the vestibulo-ocular reflex (VOR) over a range of frequencies. Accurate computation of the gain and phase of the VOR is dependent upon the assumption that the subject's head is rigidly coupled to the rotating chair over the range of frequencies employed. We tested this assumption by examining head slippage in 20 normal subjects using a standard rotational chair over the frequency range 0.025 to 2 Hz. Measurements were made with the subjects' head optimally restrained according to our standard clinical protocol and with the head minimally restrained. Head slippage was expressed as gain and phase of the head with respect to the chair. Computation of these parameters was made by comparing the signal received from a biteblock-mounted rate sensor rigidly coupled to the skull with that of the chair's tachometer. We found highly significant slippage of the head with respect to the chair occurred at 0.5, 1 and 2 Hz, even with the head optimally restrained, leading to increased gain and a phase lag. Gain and phase were highly variable for both conditions at 2 Hz indicating inadequate head fixation using our methods. Below 0.5 Hz, minimal head slippage occurred whether the head was restrained or simply rested against a contoured headrest. Consideration of these results may lead to changing some practices currently employed in broad-frequency rotational chair testing at frequencies of 0.5 Hz and above.

Adult↗
PubMed1996

Hip impact velocities and body configurations for voluntary falls from standing height.

Fall dynamics have largely been ignored in the study of hip fracture etiology and in the development of hip fracture prevention strategies. In this study, we asked the following questions: (1) What are the ranges of hip impact velocities associated with a sideways fall from standing height? (2) What are the ranges of body configurations at impact? and (3) How do protective reflexes such as muscle activation or using an outstretched hand influence fall kinematics? To answer these questions, we recruited six young healthy athletes who performed voluntary sideways falls on a thick foam mattress. Several categories of falls were investigated: (a) muscle-active vs muscle-relaxed falls; (b) falls from a standing position or from walking; and (c) falls in which an outstretched arm was used to break the fall. Each fall was videotaped at 60 frames s(-1). Fall kinematics parameters were obtained by digitizing markers placed on anatomical points of interest. The mean value for vertical hip impact velocity was 2.75 ms(-1) (+ or - 0.42 ms(-1) [S.D.]). The mean value for trunk angle (the angle between the trunk and the vertical) was 17.3 degrees (+ or - 11.5 degrees [S.D.]). We found a 38 percent reduction in the trunk angle at impact, and a 7 percent reduction in hip impact velocity for relaxed vs muscle-active falls. Finally, regarding the. falls in which an outstretched arm was used, only two out of the six subjects were able to break the fall with their arm or hand. For the remaining subjects hip impact occurred first, followed by contact of the arm or hand.

Accidental Falls↗
PubMed1996

The contribution of the vertical semicircular canals to high-velocity horizontal vestibulo-ocular reflex (VOR) in normal subjects and patients with unilateral vestibular nerve section.

We have examined to what extent the vertical semicircular canals contribute to the nonlinearity of the horizontal VOR imposed by the driving of primary vestibular afferents into inhibitory cutoff at high velocities of head rotation (Ewald's second law). The gain (eye velocity/head velocity) of the horizontal component of the VOR with the head pitched down 30 degrees and pitched up 30 degrees was examined during constant-velocity rotations in normal subjects and patients following unilateral vestibular nerve section. In normal subjects, VOR gain decreases as chair velocity increases from 60-300 degrees/s when the head is pitched up, but VOR gain remains constant when the head is pitched down. This finding implies that the mechanism by which the gain of the horizontal VOR gain remains constant at all velocities of rotation depends upon the pattern of labyrinthine stimulation. Following unilateral nerve section, we found that the directional preponderance (DP) in horizontal VOR depends upon whether the head is pitched up 30 (mean asymmetry = 5%) or pitched down 30 degrees (mean asymmetry = 20%). This is what is expected based on the degree to which the lateral and vertical semicircular canals sense horizontal head acceleration with the head in different degrees of pitch. Hence, following unilateral vestibular lesions, the DP of horizontal VOR gain is most easily elicited at high velocities of head rotation and with the head pitched down 30 degrees. Evidence for DP at the bedside using the "head-shaking nystagmus" technique may be optimally elicited with the head pitched down 30 degrees.

Acceleration↗
PubMed1999

A linear canal-otolith interaction model to describe the human vestibulo-ocular reflex.

A control systems model of the vestibulo-ocular reflex (VOR) originally derived for yaw rotation about an eccentric axis (Crane et al. 1997) was applied to data collected during ambulation and dynamic posturography. The model incorporates a linear summation of an otolith response due to head translation scaled by target distance, adding to a semi-circular canal response that depends only on angular head rotation. The results of the model were compared with human experimental data by supplying head angular velocity as determined by magnetic search coil recording as the input for the canal branch of the model and supplying linear acceleration as determined by flux gate magnetometer measurements of otolith position. The model was fit to data by determining otolith weighting that enabled the model to best fit the data. We fit to the model experimental data from normal subjects who were: standing quietly, walking, running, or making active sinusoidal head movements. We also fit data obtained during dynamic posturography tasks of: standing on a platform sliding in a horizontal plane at 0.2 Hz, standing directly on a platform tilting at 0.1 Hz, and standing on the tilting platform buffered by a 5-cm thick foam rubber cushion. Each task was done with the subject attending a target approximately 500, 100, or 50 cm distant, both in light and darkness. The model accurately predicted the observed VOR response during each test. Greater otolith weighting was required for near targets for nearly all activities, consistent with weights for the otolith component found in previous studies employing imposed rotations. The only exceptions were for vertical axis motion during standing, sliding, and tilting when the platform was buffered with foam rubber. In the horizontal axis, the model always fit near target data better with a higher otolith component. Otolith weights were similar with the target visible and in darkness. The model predicts eye movement during both passive whole-body rotation and free head movement in space implying that the VOR is controlled by a similar mechanism during both situations. Factors such as vision, proprioception, and efference copy that are available during head free motion but not during whole-body rotation are probably not important to gaze stabilization during ambulation and postural stabilizing movement. The linearity of the canal-otolith interaction was tested by re-analysis of the whole body rotation data on which the model is based (Crane et al. 1997). Normalized otolith-mediated gain enhancement was determined for each axis of rotation. This analysis uncovered minor non-linearities in the canal-otolith interaction at frequencies above 1.6 Hz and when the axis of rotation was posterior to the head.

Adult↗
PubMed2005

Spatio-temporal separation of roll and pitch balance-correcting commands in humans.

This study was designed to provide evidence for the hypothesis that human balance corrections in response to pitch perturbations are controlled by muscle action mainly about the ankle and knee joints, whereas balance corrections for roll perturbations are controlled predominantly by motion about the hip and lumbro-sacral joints. A dual-axis rotating support surface delivered unexpected random perturbations to the stance of 19 healthy young adults through eight different directions in the pitch and the roll planes and three delays between pitch and roll directions. Roll delays with respect to pitch were no delay, a short 50-ms delay of roll with respect to pitch movements, (chosen to correspond to the onset time of leg muscle stretch reflexes), and a long 150-ms delay between roll and pitch movements (chosen to shift the time when trunk roll velocity peaks to the time when trunk peak pitch velocity normally occurs). Delays of stimulus roll with respect to pitch resulted in delayed roll responses of the legs, trunk, arms, and head consistent with stimulus delay without any changes in roll velocity amplitude. Delayed roll perturbations induced only small changes in the pitch motion of the legs and trunk; however, major changes were seen in the time when roll motion of the trunk was arrested. Amplitudes and directional sensitivity of short-latency (SL) stretch reflexes in ankle muscles were not altered with increasing roll delay. Small changes to balance correcting responses in ankle muscles were observed. SL stretch reflexes in hip and trunk muscles were delayed, and balance-correcting responses in trunk muscles became split into two distinct responses with delayed roll. The first of these responses was small and had a directional responsiveness aligned more along the pitch plane. The main, larger, response occurred with an onset and time-to-peak consistent with the delay in trunk roll displacement and its directional responsiveness was roll oriented. The sum of the amplitudes of these two types of balance-correcting responses remained constant with roll delay. These results support the hypothesis that corrections of the body's pitch and roll motion are programmed separately by neural command signals and provide insights into possible triggering mechanisms. The evidence that lower leg muscle balance-correcting activity is hardly changed by delayed trunk roll also indicates that lower leg muscle activity is not predominant in correcting roll motion of the body. Lower leg and trunk muscle activity appears to have a dual action in balance corrections. In trunk muscles the main action is to correct for roll perturbations and the lesser action may be an anticipatory stabilizing reaction for pitch perturbations. Likewise, the small changes in lower leg muscle activity may result from a generalized stabilizing reaction to roll perturbations, but the main action is to correct for pitch perturbations.

Adult↗
PubMed1995

Vertical, horizontal, and torsional eye movement responses to head roll in the squirrel monkey.

The vestibulo-ocular reflex (VOR) serves to stabilize images on the retina by rotating the eyes in the direction which opposes angular (aVOR) or linear (IVOR) head movement. The aVOR responds to rotations in any plane. Head rotations about the naso-occipital axis (roll) are accompanied by compensatory torsional eye movements, with gains typically less than 0.7. However, geometric considerations suggest that the response should not be restricted to torsion, and that horizontal, vertical, and torsional response components should depend upon eye position relative to the axis of rotation. Since eye position can differ for the two eyes (e.g., during convergence), the response to head roll should be accordingly disconjugate. Further, because the eyes are typically displaced from the axis of rotation, head roll entails a calculable translation of the eyes in space, and compensation for this component of motion is expected to add to the response to angular motion. The translational response component should be modulated by fixation distance. To test these geometric considerations in the aVOR, we investigated the three-dimensional ocular responses of squirrel monkeys to head roll. Torsional aVOR responses were accompanied by vertical components which were modulated by horizontal gaze position, and by horizontal components which were modified by vertical gaze position. The vertical response components were often appropriately disconjugate, and even opposing, yielding responses that appeared "see-saw" in character.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2005

Kinematics of vertical saccades during the yaw vestibulo-ocular reflex in humans.

PURPOSE: Listing's law (LL) constrains the rotational axes of saccades and pursuit eye movements to Listing's plane (LP). In the velocity domain, LL is ordinarily equivalent to a tilt in the ocular velocity axis equal to half the change in eye position, giving a tilt angle ratio (TAR) of 0.5. This study was undertaken to investigate vertical saccade behavior after the yaw vestibulo-ocular reflex (VOR) had driven eye torsion out of LP, an initial condition causing the position and velocity domain formulations of LL to differ. METHODS: Binocular eye and head motions were recorded with magnetic search coils in eight humans. With the head immobile, LP was determined for each eye, and mean TAR was 0.50 +/- 0.07 (mean +/- SD) for horizontal and 0.45 +/- 0.11 for vertical saccades. The VOR was evoked by transient, whole-body yaw at 2800 deg/s2 peak acceleration, capable of evoking large, uninterrupted VOR slow phases. Before rotation, subjects viewed a target at eye level, 20 degrees up, or 20 degrees down. In two thirds of the trials, the target moved upward or downward at systematically varying times, triggering a vertical saccade during the horizontal VOR slow phase. RESULTS: Because the head rotation axis was generally misaligned with LP, the eye averaged 3.6 degrees out of LP at vertical saccade onset. During the saccade, eye position continued to depart LP by an average 0.8 degrees. The horizontal TAR at saccade onset was 0.29 +/- 0.07. At peak saccade velocity 35 +/- 3 ms later, the vertical TAR was 0.45 +/- 0.07, statistically similar to that of head fixed saccades. Saccades did not return to LP. CONCLUSIONS: Although they did not observe the position domain formulation of LL, vertical saccades, during the VOR, observed the half-angle velocity domain formulation of LL.

Adult↗
PubMed1995

Influence of aging on leg muscle reflex responses to stance perturbation.

The effect of age on latency and amplitude of leg muscle responses to stance perturbations was studied in 75 control subjects. They stood upright on a platform and were displaced by toe-up (upward tilt) and toe-down (downward tilt) platform rotations. Perturbations were induced during free and supported stance (holding on to a stable structure). Surface electromyograms (EMG) of the soleus (Sol) and tibialis anterior (TA) were recorded and latency and area of responses were measured. Body sway variables during stance with open or closed eyes were also recorded. Upward tilt evoked a short-latency response (SLR) in Sol and a long-latency response (LLR) in TA. Downward tilt evoked a medium-latency response (MLR) in TA and a LLR in Sol. This pattern of EMG responses was similar in both young and elderly subjects, although there were some differences in latency and amplitude. There was a significant relationship between latency of all responses and age. Slope of the regression lines of TA LLR, TA MLR, and Sol LLR was steeper than that of Sol SLR. Area of Sol SLR was unrelated to age, but a positive trend was identified in the other responses, significant for TA LLR. Under supported-stance condition, amplitude of TA MLR, TA LLR, and Sol LLR was decreased to a similar extent in both young and elderly subjects. There was a weak relationship between age and most body sway variables. A significant relationship was found between most sway variables and latency of Sol SLR and LLR, chiefly with eyes closed. Neither TA MLR nor LLR were significantly correlated with sway variables, but a trend was present for TA MLR with eyes closed.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed2003

Dependence of adaptation of the human vertical angular vestibulo-ocular reflex on gravity.

We determined the spatial dependence of adaptive gain changes of the vertical angular vestibulo-ocular reflex (aVOR) on gravity in five human subjects. The gain was decreased for 1 h by sinusoidal oscillation in pitch about a spatial vertical axis in a subject-stationary surround with the head oriented left-side down. Gains were tested by sinusoidal oscillation about a spatial vertical axis while subjects were tilted in 15 degrees increments from left- to right-side down positions through the upright. Changes in gain of the vertical component of the induced eye movements were expressed as a percentage of the preadapted values for the final analysis. Vertical aVOR gain changes were maximal in the position in which the gain had been adapted and declined progressively as subjects were moved from this position. Gain changes were plotted as a function of head orientation and fit with a sine function. The bias level of the fitted sines, i.e., the gravity-independent gain change, was -29+/-10% (SD). The gains varied around this bias as a function of head position by +/-18+/-6%, which were the gravity-dependent gain changes. The gravity-dependent gain changes induced by only 1 h of adaptation persisted, gradually declining over several days. We conclude that there is a component of the vertical aVOR gain change in humans that is dependent on the head orientation in which the gain was adapted, and that this dependence can persist for substantial periods.

Adaptation, Physiological↗
PubMed1991

Early onset cerebellar ataxia with retained tendon reflexes. Clinical, electrophysiological and MRI observations in comparison with Friedreich's ataxia.

Fourteen patients with the clinical diagnosis of early onset cerebellar ataxia with retained tendon reflexes (EOCA) were examined and compared with 11 patients with Friedreich's ataxia (FA). The mean age of onset in EOCA was 15.9 +/- 6.0 yrs (FA: 14.0 +/- 5.7 yrs). Annual progression rate and the percentage of patients who were wheelchair-bound was lower in EOCA as compared with FA, although the difference did not reach statistical significance. The latency until becoming wheelchair-bound, however, was significantly longer in EOCA than in FA. The segregation ratio in EOCA was significantly lower than 0.25. Clinically, EOCA and FA patients presented with a progressive cerebellar syndrome. Associated symptoms, such as muscle wasting, sensory disturbances, foot deformity, scoliosis and electrocardiographic abnormalities were encountered less frequently in EOCA than in FA patients. The electrophysiological findings in EOCA were variable and pointed to axonal degeneration in peripheral nerves and central pathways. Posturographic measurements revealed a higher incidence of anteroposterior sway direction in EOCA as compared with FA, suggesting a cerebellar type of ataxia in EOCA. Eleven out of the 14 EOCA patients had cerebellar atrophy in MRI. The characteristic MRI finding in FA was upper cervical cord shrinkage and only minor atrophy of the cerebellum. The demonstration of cerebellar atrophy in the majority of EOCA patients supports the view that EOCA is distinct from FA. It is uncertain, however, whether EOCA is a homogenous disease entity or a group of phenotypically similar syndromes.

Action Potentials↗
PubMed1989

[Study of the veno-arteriolar reflex in young and elderly subjects by recording of TcPO2].

The variations' measurement of transcutaneous oxygen tension (TcPO2) looks as an interesting local vasomotor reflex (L.V.R.) exploration, particularly on lower limbs arterial occlusive disease. However the possible impacts of physiological ageing upon this reflex have not clearly been evaluated. But they can be better analysed with a dynamic measure of TcPO2 on the first intermetatarsal space of the patient, successively recumbent, sitting standing and walking on a moving walkway. TcPO2 is measured with a Radiometer TCM3 oxygen monitor. Two groups of patients, without any arterial occlusive lesion, have been tested like that: 20 young (mean age: 23.3 +/- 1.8 years) and 16 older subjects (mean age: 68.3 +/- 6 years). In recumbent position, TcPO2 is about 74.05 +/- 5.6 mmHg in young and 62.2 +/- 4.7 mmHg in older subjects (P less than 0.01). From recumbent to sitting position, after 15 seconds TcPO2 is elevating to a maximal value at 1 min 30 sec. (85 +/- 6 mmHg in young--71.5 +/- 5 mmHg in older subjects--P less than 0.01). Whilst in young TcPO2 decreases slowly to a stable value, at 5 minutes, higher than in recumbent (+ 6.3 +/- 0.5 mmHg), in the older TcPO2 increases ever to a stable value higher than in recumbent (+ 12 +/- 0.7 mmHg). For the last ones there is no TcPO2 adaptation from recumbent to sitting position because of the vascular ageing, while the LVR increased from recumbent to standing position. During the walk, the LVR disappears for both of them. These measurements confirm the decrease of vascular reactivity with ageing and oblige to compare the results on same age people, during the arterial occlusive disease exploration, by evaluating the graph kinetics.

Adult↗
PubMed1997

Medium-latency stretch reflexes of foot and leg muscles analysed by cooling the lower limb in standing humans.

1. In standing subjects, an ankle-dorsiflexing perturbation of the supporting surface evokes a short-latency response (SLR) and a medium-latency response (MLR) to stretch in both soleus (Sol) and flexor digitorum brevis (FDB) muscles. The SLR is the counterpart of the monosynaptic reflex, whilst the MLR might be either mediated by Ia fibres, the delay being due to a long-loop central circuit, or by fibres of slower conduction velocity. Since small afferents are slowed more than large ones by low temperature, a greater latency increment for the MLR than the SLR induced by cooling of the limb would point to a peripheral origin of the MLR. 2. In nine subjects, one limb was cooled by circulating water in a tube wrapped around it for about 120 min. Perturbations were delivered to the same limb prior to and during cooling, and after rewarming. EMG was recorded by surface electrodes from the Sol and FDB muscles. 3. The mean increase in latency of MLRs was significantly greater than that of SLRs in both muscles. On average, the Sol SLR increased from 42.4 to 47.0 ms and the Sol MLR from 72.0 to 82.3 ms. The FDB SLR increased from 58.1 to 66.5 ms and the FDB MLR from 94.9 to 110.5 ms. The mean difference (MLR minus SLR) increased from 29.6 to 35.2 ms for Sol, and from 36.8 to 43.9 ms for FDB at the end of cooling. After 30 min of rewarming, the responses of both muscles recovered towards control values. 4. The greater latency increment of the MLRs than of the SLRs favours the hypothesis of a slower conduction velocity of the responsible afferent fibres. The most likely candidate fibres are the spindle group II afferents.

Adult↗
PubMed1999

Sensory organization for balance: specific deficits in Alzheimer's but not in Parkinson's disease.

BACKGROUND: The cause of frequent falling in patients with dementia of the Alzheimer type (AD) is not well understood. Distraction from incongruent visual stimuli may be an important factor as suggested by their poor performance in tests of shifting visual attention in other studies. The purpose of this study was to determine whether AD patients have difficulty maintaining upright balance under absent and/or incongruent visual and other sensory conditions compared to nondemented healthy elderly persons and individuals with Parkinson's disease (PD). METHODS: Seventeen healthy older adults, 15 medicated PD subjects, and 11 AD subjects underwent the Sensory Organization Test protocol. The incidence of loss of balance ("falls"), and the peak-to-peak amplitude of body center of mass sway during stance in the six sensory conditions were used to infer the ability to use visual, somatosensory, and vestibular signals when they provided useful information for balance, and to suppress them when they were incongruent as an orientation reference. Vestibular reflex tests were conducted to ensure normal vestibular function in the subjects. RESULTS: AD subjects had normal vestibular function but had trouble using it in condition 6, where they had to concurrently suppress both incongruent visual and somatosensory inputs. All 11 AD subjects fell in the first trial of this condition. With repeated trials, only three AD subjects were able to stay balanced. AD subjects were able to keep their balance when only somatosensory input was incongruent. In this condition, all AD subjects were able to maintain balance whereas some falls occurred in the other groups. In all conditions, when AD subjects did not fall, they were able to control as large a sway as the healthy controls, except when standing with eyes closed in condition 2: AD subjects did not increase their sway whereas the other groups did. In the PD group, the total fall incidence was similar to the AD group, but the distribution was generalized across more sensory conditions. PD subjects were also able to improve with repeated trials in condition 6. CONCLUSION: Patients with dementia of the Alzheimer type have decreased ability to suppress incongruent visual stimuli when trying to maintain balance. However, they did not seem to be dependent on vision for balance because they did not increase their sway when vision was absent. Parkinsonian patients have a more general balance control problem in the sensory organization test, possibly related to difficulty changing set.

Accidental Falls↗
PubMed2003

Adaptive changes in the angular VOR: duration of gain changes and lack of effect of nodulo-uvulectomy.

Alterations in the gain of the vertical angular vestibulo-ocular reflex (VOR) are dependent on the head position in which the gain changes were produced. We determined how long gravity-dependent gain changes last in monkeys after four hours of adaptation, and whether the adaptation is mediated through the nodulus and uvula of the vestibulocerebellum. Vertical VOR gains were adaptively modified by rotation about an interaural axis, in phase or out of phase with the visual surround. Vertical VOR gains were modified with the animals in one of three orientations: upright, left-side down, or right-side down. Monkeys were tested in darkness for up to four days after adaptation using sinusoidal rotation about an interaural axis that was incrementally tilted in 10 degrees steps from vertical to side down positions. Animals were unrestrained in their cages in normal light conditions between tests. Gravity-dependent gain changes lasted for a day or less after adaptation while upright, but persisted for two days or more after on-side adaptation. These data show that gravity-dependent gain changes can last for prolonged periods after only four hours of adaptation in monkeys, as in humans. They also demonstrate that natural head movements made while upright do not provide an adequate stimulus for rapid recovery of vertical VOR gains that were induced on side. In two animals, the nodulus and uvula were surgically ablated. Vertical gravity-dependent gain changes were not significantly different before and after surgery, indicating that the nodulus and uvula do not have a critical role in producing them.

Adaptation, Physiological↗
PubMed1986

Long-lasting excitability changes of soleus alpha-motoneuron induced by midpontine stimulation in decerebrate, standing cat.

Stimulation of the dorsal portion of the caudal tegmental field (DTF) in the pons resulted in hyperpolarization of extensor alpha-motoneurons (alpha-MNs) that persisted for several minutes after cessation of the stimulation. The resulting inhibition of alpha-MN discharge led to a progressive reduction in the number of active motor units. Renshaw cells, persistently active at high levels of extensor muscle tone, were abruptly silenced by DTF stimulation. Active discharge was renewed at the time of cessation of the stimulation but at a frequency reduced in proportion to the persistently lowered level of extensor muscle tone. Ia primary afferents were tonically active during the high extensor tonus of reflex standing. DTF stimulation was accompanied by a brief, slight increase in Ia discharge frequency followed by a reduction in frequency variably correlated to the magnitude of extensor force reduction. Orthodromically elicited Ia EPSPs in the soleus alpha-MNs were reduced in peak voltage, time to peak, and half width during the hyperpolarization accompanying DTF stimulation. All of these parameters recovered beyond their prestimulus values with the cessation of DTF stimulation in spite of the persisting hyperpolarization. Antidromically initiated invasion of the somatodendritic (SD) segment of the motoneuron membrane was delayed and sometimes blocked during DTF stimulation. At the same time, the peak voltage of the SD action potential was reduced. There was an immediate recovery of these changes on termination of DTF stimulation, although the poststimulus hyperpolarization persisted. Intracellular injection of depolarizing current steps during DTF stimulation revealed a depression of membrane excitability that persisted during the hyperpolarization that followed the termination of the DTF stimulation. Depolarizing and hyperpolarizing steps of intracellular current were used to demonstrate a reduction of cellular input resistance during DTF stimulation. The resistance values rapidly returned to prestimulus levels following the cessation of DTF stimulation. It was demonstrated that the degree of resistance change is greater and that the magnitude of DTF-induced hyperpolarization is smaller for low-resistance cells than for high-resistance cells. Iontophoretically induced increase in intracellular Cl- resulted in a reversal of both Ia IPSPs and the hyperpolarization induced by DTF stimulation. The hyperpolarization enduring after DTF stimulation was not affected by the Cl- injection.

Action Potentials↗