Combining physical training with transcranial direct current stimulation to improve gait in Parkinson’s disease: a pilot randomized controlled study

Objective:

To improve gait and balance in patients with Parkinson’s disease by combining anodal transcranial direct current stimulation with physical training.

Design:

In a double-blind design, one group (physical training; n = 8) underwent gait and balance training during transcranial direct current stimulation (tDCS; real/sham). Real stimulation consisted of 15 minutes of 2 mA transcranial direct current stimulation over primary motor and premotor cortex. For sham, the current was switched off after 30 seconds. Patients received the opposite stimulation (sham/real) with physical training one week later; the second group (No physical training; n = 8) received stimulation (real/sham) but no training, and also repeated a sequential transcranial direct current stimulation session one week later (sham/real).

Setting:

Hospital Srio Libanes, Buenos Aires, Argentina.

Subjects:

Sixteen community-dwelling patients with Parkinson’s disease.

Interventions:

Transcranial direct current stimulation with and without concomitant physical training.

Main measures:

Gait velocity (primary gait outcome), stride length, timed 6-minute walk test, Timed Up and Go Test (secondary outcomes), and performance on the pull test (primary balance outcome).

Results:

Transcranial direct current stimulation with physical training increased gait velocity (mean = 29.5%, SD = 13; p < 0.01) and improved balance (pull test: mean = 50.9%, SD = 37; p = 0.01) compared with transcranial direct current stimulation alone. There was no isolated benefit of transcranial direct current stimulation alone. Although physical training improved gait velocity (mean = 15.5%, SD = 12.3; p = 0.03), these effects were comparatively less than with combined tDCS + physical therapy ( p < 0.025). Greater stimulation-related improvements were seen in patients with more advanced disease.

Conclusions:

Anodal transcranial direct current stimulation during physical training improves gait and balance in patients with Parkinson’s disease. Power calculations revealed that 14 patients per treatment arm (α = 0.05; power = 0.8) are required for a definitive trial.

The effects of vibrotactile biofeedback training on trunk sway in Parkinson's disease patients

BACKGROUND

Postural instability in Parkinson's disease (PD) can lead to falls, injuries and reduced quality of life. We investigated whether balance in PD can improve by offering patients feedback about their own trunk sway as a supplement to natural sensory inputs. Specifically, we investigated the effect of artificial vibrotactile biofeedback on trunk sway in PD.

METHODS

Twenty PD patients were assigned to a control group (n = 10) or biofeedback group (n = 10). First, all patients performed two sets of six gait tasks and six stance tasks (pre-training assessment). Subsequently, all subjects trained six selected tasks five times (balance training). During this training, the feedback group received vibrotactile feedback of trunk sway, via vibrations delivered at the head. After training, both groups repeated all twelve tasks (post-training assessment). During all tasks, trunk pitch and roll movements were measured with angular velocity sensors attached to the lower trunk. Outcomes included sway angle and sway angular velocity in the roll and pitch plane, and task duration.

RESULTS

Overall, patients in the feedback group had a significantly greater reduction in roll (P = 0.005) and pitch (P < 0.001) sway angular velocity. Moreover, roll sway angle increased more in controls after training, suggesting better training effects in the feedback group (P < 0.001).

CONCLUSIONS

One session of balance training in PD using a biofeedback system showed beneficial effects on trunk stability. Additional research should examine if these effects increase further after more intensive training, how long these persist after training has stopped, and if the observed effects carry over to non-trained tasks.

Trunk sway measures of postural stability during clinical balance tests: effects of a unilateral vestibular deficit

This research evaluated whether quantified measures of trunk sway during clinical balance tasks are sensitive enough to identify a balance disorder and possibly specific enough to distinguish between different types of balance disorder. We used a light-weight, easy to attach, body-worn apparatus to measure trunk angular velocities in the roll and pitch planes during a number of stance and gait tasks similar to those of the Tinetti and CTSIB protocols. The tasks included standing on one or two legs both eyes-open and closed on a foam or firm support-surface, walking eight tandem steps, walking five steps while horizontally rotating or pitching the head, walking over low barriers, and up and down stairs. Tasks were sought, which when quantified might provide optimal screening for a balance pathology by comparing the test results of 15 patients with a well defined acute balance deficit (sudden unilateral vestibular loss (UVL)) with those of 26 patients with less severe chronic balance problems caused by a cerebellar-pontine-angle-tumour (CPAT) prior to surgery, and with those of 88 age- and sex-matched healthy subjects. The UVL patients demonstrated significantly greater than normal trunk sway for all two-legged stance tasks especially those performed with eyes closed on a foam support surface. Sway was also greater for walking while rotating or pitching the head, and for walking eight tandem steps on a foam support surface. Interestingly, the patients could perform gait tasks such as walking over barriers almost normally, however took longer. CPAT patients had trunk sway values intermediate between those of UVL patients and normals. A combination of trunk sway amplitude measurements (roll angle and pitch velocity) from the stance tasks of standing on two legs eyes closed on a foam support, standing eyes open on a normal support surface, as well as from the gait tasks of walking five steps while rotating, or pitching the head, and walking eight tandem steps on foam permitted a 97% correct recognition of a normal subject and a 93% correct recognition of an acute vestibular loss patient. Just over 50% of CPAT patients could be classified into a group with intermediate balance deficits, the rest were classified as normal. Our results indicate that measuring trunk sway in the form of roll angle and pitch angular velocity during five simple clinical tests of equilibrium, four of which probe both stance and gait control under more difficult sensory conditions, can reliably and quantitatively distinguish patients with a well defined balance deficit from healthy controls. Further, refinement of these trunk sway measuring techniques may be required if functions such as preliminary diagnosis rather than screening are to be attempted.

Differential diagnosis of proprioceptive and vestibular deficits using dynamic support-surface posturography

The objective of this study was to evaluate how effective dynamic support-surface posturography could be as a diagnostic tool in patients with balance disorders (proprioceptive or vestibular deficits). Specifically, we studied whether measures of trunk control and simple toe-up rotational perturbations, selected using statistical techniques, could provide a better diagnostic yield than either the analysis of lower-body movements or use of a "nulled" ankle input paradigm. The test subjects were 15 control subjects, five patients with bilateral peripheral vestibular loss (VL) and five patients with selective bilateral, lower-leg proprioceptive loss (PL). Amplitudes and onset latencies of bursts of EMG activity in upper and lower-leg muscles, paraspinals and trapezius muscles, concurrent changes in ankle torque, and peak amplitudes of upper-leg, lower-leg, and trunk angular-velocities were measured. Stimuli included three different types of sudden movements of the support surface, a "nulled" ankle input paradigm, a simple toe-up rotation paradigm, and a combined toe-up rotation and backwards translation of the support surface. All stimuli were tested under eyes-open and eyes-closed conditions. For each type of movement and condition the diagnostic classification accuracy (i.e. the overall sensitivity and specificity) was calculated based on those posturography measures providing the highest diagnostic separation between the three populations. Both patient groups showed increased trunk sway, changed support-surface reaction forces and muscle amplitudes compared with controls for toe-up and "nulled" test conditions. Measures providing the greatest diagnostic utility were the amplitude of trunk-angular velocity (increased in VL subjects, less so in PL), the amplitude of balance-correcting paraspinal responses (increased in VL subjects, decreased in PL subjects), the amplitude of trapezius stabilising responses (increased in both patient groups) for simple toe-up rotations under eyes-closed conditions. We conclude, that diagnosis of balance disorders using dynamic posturography is best achieved using measures of trunk control following pure toe-up rotational perturbations tested under eyes-closed conditions.

Balance control analysis as a method for screening and identifying balance deficits

We propose a two-step clinical evaluation procedure to identify the possible etiology and laterality of a balance deficit. Step 1 employs a minimum clinical test battery, developed in our labs, to screen for the balance deficit by examining changes to trunk sway for standard clinical stance and gait tests. Step 2 characterizes pathophysiological components in balance corrections, as well as deficits brought about by aging, using biomechanical and electromyographic (EMG) responses to multidirectional stance perturbations. This is best accomplished by delivering stance perturbations to patients standing on a support surface that is tipped in four directions: forwards to the left and right, and backwards to the left and right. This review provides an overview of the two procedures and proposes for the screening procedure a minimum clinical test battery with a score, termed the balance control index (BCI).

Vestibular influences on human postural control in combinations of pitch and roll planes reveal differences in spatiotemporal processing

The present study examined the influence of bilateral peripheral vestibular loss (BVL) in humans on postural responses to multidirectional surface rotations in the pitch and roll planes. Specifically, we examined the effects of vestibular loss on the directional sensitivity, timing, and amplitude of early stretch, balance correcting, and stabilizing reactions in postural leg and trunk muscles as well as changes in ankle torque and trunk angular velocity following multidirectional rotational perturbations of the support surface. Fourteen normal healthy adults and five BVL patients stood on a dual axis rotating platform which rotated 7.5 degrees at 50 degrees/s through eight different directions of pitch and roll combinations separated by 45 degrees. Directions were randomized within a series of 44 perturbation trials which were presented first with eyes open, followed by a second series of trials with eyes closed. Vestibular loss did not influence the range of activation or direction of maximum sensitivity for balance correcting responses (120-220 ms). Response onsets at approximately 120 ms were normal in tibialis anterior (TA), soleus (SOL), paraspinals (PARAS), or quadriceps muscles. Only SOL muscle activity demonstrated a 38- to 45-ms delay for combinations of forward (toe-down) and roll perturbations in BVL patients. The amplitude of balance correcting responses in leg muscles between 120 and 220 ms was, with one exception, severely reduced in BVL patients for eyes open and eyes closed conditions. SOL responses were decreased bilaterally for toe-up and toe-down perturbations, but more significantly reduced in the downhill (load-bearing) leg for combined roll and pitch perturbations. TA was significantly reduced bilaterally for toe-up perturbations, and in the downhill leg for backward roll perturbations. Forward perturbations, however, elicited significantly larger TA activity in BVL between 120 and 220 ms compared to normals, which would act to further destabilize the body. As a result of these changes in response amplitudes, BVL patients had reduced balance correcting ankle torque between 160 and 260 ms and increased torque between 280 and 380 ms compared to normals. There were no differences in the orientation of the resultant ankle torque vectors between BVL and normals, both of which were oriented primarily along the pitch plane. For combinations of backward (toe-up) and roll perturbations BVL patients had larger balance correcting and stabilizing reactions (between 350 and 700 ms) in PARAS than normals and these corresponded to excessive trunk pitch and roll velocities. During roll perturbations, trunk velocities in BVL subjects after 200 ms were directed along directions different from those of normals. Furthermore, roll instabilities appeared later than those of pitch particularly for backward roll perturbations. The results of the study show that combinations of roll and pitch surface rotations yield important spatiotemporal information, especially with respect to trunk response strategies changed by BVL which are not revealed by pitch plane perturbations alone. Our results indicate that vestibular influences are earlier for the pitch plane and are directed to leg muscles, whereas roll control is later and focused on trunk muscles.

Trunk sway measures of postural stability during clinical balance tests: effects of age

BACKGROUND

The major disadvantage of current clinical tests that screen for balance disorders is a reliance on an examiner's subjective assessment of equilibrium control. To overcome this disadvantage we investigated, using quantified measures of trunk sway, age-related differences of normal subjects for commonly used clinical balance tests.

METHODS

Three age groups were tested: young (15-25 years; n = 48), middle-aged (45-55 years; n = 50) and elderly (65-75 years; n = 49). Each subject performed a series of fourteen tasks similar to those included in the Tinetti and Clinical Test of Sensory Interaction in Balance protocols. The test battery comprised stance and gait tasks performed under normal, altered visual (eyes closed), and altered proprioceptive (foam support surface) conditions. Quantification of trunk sway was performed using a system that measured trunk angular velocity and position in the roll (lateral) and pitch (fore-aft) planes at the level of the lower back. Ranges of sway amplitude and velocity were examined for age-differences with ANOVA techniques.

RESULTS

A comparison between age groups showed several differences. Elderly subjects were distinguished from both middle-aged and young subjects by the range of trunk angular sway and angular velocity because both were greater in roll and pitch planes for stance and stance-related tasks (tandem walking). The most significant age group differences (F = 30, p <.0001) were found for standing on one leg on a normal floor or on a foam support surface with eyes open. Next in significance was walking eight tandem steps on a normal floor (F = 13, p <.0001). For gait tasks, such as walking five steps while rotating or pitching the head or with eyes closed, pitch and roll velocity ranges were influenced by age with middle-aged subjects showing the smallest ranges followed by elderly subjects and then young subjects (F = 12, p <.0001). Walking over a set of low barriers also yielded significant differences between age groups for duration and angular sway. In contrast, task duration was the only variable significantly influenced when walking up and down a set of stairs. An interesting finding for all tasks was the different spread of values for each population. Population distributions were skewed for all ages and broadened with age.

CONCLUSIONS

Accurate measurement of trunk angular sway during stance and gait tasks provides a simple way of reliably measuring changes in balance stability with age and could prove useful when screening for balance disorders of those prone to fall.

Auditory perception and speech identification in children with cochlear implants tested with the EARS protocol

The performance of children who receive a cochlear implant may be dependent on both age of the child at implantation and the amount of experience with the implant. In the present study, changes in auditory perception and speech identification were investigated with experience of 71 children who had received a cochlear implant. The children were divided into three groups, those above and those below the age of 7 years at the time of implantation, and those aged 3 years or younger deafened by meningitis. The children received either the Nucleus 22, the Nucleus 24 or the Med El C40+ implant. The test material was a reduced form of the EARS evaluation protocol developed by Med El into a multi-language format. Tests were performed pre-operatively, within 2-5 days of first fitting of the speech processor, then at 1, 3 and 6 months and every 6 months thereafter, for a total period of 24 months. The results indicated that all children showed improvement after 6-12 months. The rate of improvement differed between age groups. Children over 7 years of age had pre-operatively higher test scores than younger children, presumably because of their previous experience with hearing aids. These children showed an immediate post-operative drop in performance that recovered 1-3 months later. The children aged under 7 years started at lower performance levels but approached those of the older children after 12 months' cochlear implant use because their post-operative drop was less significant and their performance improved faster. Children who had been deafened by meningitis and implanted at the age of 3 or less made little progress over the first 6 months but approached test levels of the under-7-year-olds by 18 months or later. All three components of the evaluation protocol employed the Listening Progress Profile (LiP), the Monosyllabic-Trochee-Polysyllabic Test (MTP) and the Meaningful Auditory Integration Scale (MAIS) and proved to be valuable in demonstrating improvement in performance of cochlear implant children in all age groups once the immediate post-operative drop had been overcome.

Is lower leg proprioception essential for triggering human automatic postural responses?

It is unknown to what extent automatic postural responses are triggered by lower leg proprioception. This issue was addressed by studying postural control in five carefully selected patients with subtle diabetic polyneuropathy (restricted to the lower legs) and 15 healthy subjects. All patients had bilaterally absent Achilles tendon reflexes and weak or absent patella tendon reflexes, but muscle strength was fully preserved. Subjects were tested while standing on a supporting, movable force-plate. The contribution of lower leg proprioception to automatic postural responses was investigated by randomly exposing the subjects to either a 4 degrees 'toe-up' rotational perturbation ('normal ankle input'), a simultaneous 4-cm rearward translation and 4 degrees toe-up rotation ('enhanced ankle input'), or a simultaneous 4-cm rearward translation and 4 degrees 'toe-down' rotation ('nulled ankle input'). We recorded surface EMG (stretch reflexes and balance-correcting responses) from leg and trunk muscles, ankle torque and angular velocities of the upper and lower legs and trunk. We argued that automatic postural responses that have abnormally small amplitudes in patients and are modulated in controls with the velocity of different types of ankle rotations must receive a major input from lower leg proprioception. Conversely, automatic postural responses that are weakly modified in amplitude or onset by different ankle perturbations and are present despite nulled ankle inputs and, finally, are unaffected in patients with distal polyneuropathy must be triggered or modulated by inputs other than from lower leg proprioception. Normal postural synergies and strategies were maintained in patients, although within a given synergy the timing and amplitude of some automatic postural responses were abnormal. A few automatic postural responses appeared to be triggered or modulated by lower leg proprioception. Thus, early stretch reflexes in soleus and medial gastrocnemius were severely diminished in patients, while in controls these stretch reflexes were modulated by different ankle perturbations. Furthermore, balance-correcting responses in tibialis anterior were diminished and delayed in patients, while in controls these balance-correcting responses were modulated by different ankle perturbations. Other automatic postural responses were apparently not triggered or modulated by lower leg proprioception, but likely received a major input from more proximal sensory systems. Thus, in both groups prominent balance-correcting responses were present in several muscles (soleus, gastrocnemius, quadriceps, paraspinals and trapezius) during the 'nulled ankle input' condition, where ankle position was stabilised over the first 250 ms. During the 'enhanced ankle input' condition, where prominent ankle dorsiflexion occurred during the first 200 ms, amplitudes of balance-correcting responses were only marginally weaker in patients than in controls. We analysed body segment displacements to unveil the potential nature of proximal triggers for automatic postural responses. As opposed to the 'inverted pendulum' concept of postural control, early movement occurred in the knees, hips and trunk well before the onset of automatic postural responses. For example, during the 'nulled ankle input' condition, the lower leg moved forward with early knee flexion, followed by knee extension. The trunk extended backwards at 80 ms, which was followed by forward flexion. The absent stretch reflex and weaker balance-correcting responses in patients produced changed trunk velocity profiles (mainly a reduced initial backward motion of the trunk), but lower-body segment movements showed no consistent differences between the two groups. Considering these body segment displacements, any automatic postural response with an onset within the first 200 ms could well be triggered by receptors located at the knee, hip or trunk. (ABSTRACT TRUNCATED)

Directional sensitivity of stretch reflexes and balance corrections for normal subjects in the roll and pitch planes

A large body of evidence has been collected which describes the response parameters associated with automatic balance corrections in man to perturbations in the pitch plane. However, perturbations to human stance can be expected from multiple directions. The purpose of the present study was to describe the directional sensitivities of muscle responses re-establishing disturbed stance equilibrium in normal subjects. The contributions of stretch reflex and automatic balance-correcting responses to balance control, and concomitant biomechanical reactions, were examined for combinations of pitch and roll perturbations of the support surface. More specifically, muscle responses, initial head accelerations and trunk velocities were analyzed with the intention of identifying possible origins of directionally specific triggering signals and to examine how sensory information is used to modulate triggered balance corrections with respect to direction. Fourteen healthy adults were required to stand on a dual-axis rotating platform capable of delivering rotational perturbations with constant amplitude (7.5 degrees ) and velocity (50 degrees /s) through multiple directions in the pitch and roll planes. Each subject was randomly presented with 44 support surface rotations through 16 different directions separated by 22.5 degrees first under eyes-open, and then, for a second identical set of rotations, under eyes-closed conditions. Bilateral muscle activities from tibialis anterior, soleus, lateral quadriceps and paraspinals were recorded, averaged across direction, and areas calculated over intervals with significant bursts of activity. Trunk angular velocity and ankle torque data were averaged over intervals corresponding to significant biomechanical events. Stretch reflex (intervals of 40-100, 80-120 ms) and automatic balance-correcting responses (120-220, 240-340 ms) in the same muscle were sensitive to distinctly different directions. The directions of the maximum amplitude of balance-correcting activity in leg muscles were oriented along the pitch plane, approximately 180 degrees from the maximum amplitude of stretch responses. Ankle torques for almost all perturbation directions were also aligned along the pitch plane. Stretch reflexes in paraspinal muscles were tuned along the 45 degrees plane but at 90 degrees to automatic balance corrections and 180 degrees to unloading responses in the same muscle. Stretch reflex onsets in paraspinal muscles were observed at 60 ms, as early as those of soleus muscles. In contrast, unloading reflexes in released paraspinal muscles were observed at 40 ms for perturbations which caused roll of the trunk towards the recorded muscle. Onsets of trunk roll velocities were earlier and more rapid than those observed for pitch velocities. Trunk pitch occurred for pure roll directions but not vice versa. When considered together, early stretch and unloading of paraspinals, and concomitant roll and pitch velocities of the trunk requiring a roll-and-pitch-based hip torque strategy, bring into question previous hypotheses of an ankle-based trigger signal or ankle-based movement strategies for postural balance reactions. These findings are compatible with the hypothesis that stretch-, force- and joint-related proprioceptive receptors at the level of the trunk provide a directionally sensitive triggering mechanism underlying a, minimally, two-stage (pitch-based leg and pitch-and-roll-based trunk) balance-correcting strategy. Accelerometer recordings from the head identified large vertical linear accelerations only for pitch movements and angular roll accelerations only during roll perturbations with latencies as early as 15 ms. Thus, it appears that balance corrections in leg and trunk muscles may receive strong, receptor-dependent (otolith or vertical canal) and directionally sensitive amplitude-modulating input from vestibulospinal signals.

[The usefulness of intraoperatively registered, electrically evoked stapedius reflex for the programming of cochlear implants in children]

The programming of a cochlear implant speech processor used by young children is often difficult, especially when the stimulus level associated with maximum auditory loudness (MAL) needs to be determined. Excessively high stimulation should be avoided as this can have a traumatic effect. The aim of this study was to determine if a relationship exists between the intraoperatively determined electrical stapedius reflex threshold (ESRT) and the postoperatively determined MAL and hearing threshold for 27 patients, each having one of three implant types. The question of whether the ESRT provides a practical technique to simplify, improve and accelerate speech processor programming was investigated. For the monopolar stimulation mode used for the Med-El and Clarion implant systems, the average MAL and threshold was expressed as a percentage of the average ESRT across all electrodes. For the "common ground" stimulation mode used for the Nucleus implant system, a parabolic transformation was used to relate MAL and ESRT to one another. These transformations between MAL values calculated from the ESRT and the actual MAL values, determined psychoacoustically, diverged considerably from one another. Therefore, it was not possible to determine the MAL from the ESRT with certainty. The ESRT does, however, provide a means to estimate an approximate upper boundary for the MAL, apart from its use to control implant function. The determination of the exact MAL will still need to be determined using behavioural techniques.

An overview of the clinical use of dynamic posturography in the differential diagnosis of balance disorders

Dynamic posturography comprises a series of balance control tests which help physicians overcome numerous diagnostic and treatment challenges arising when examining patients complaining of a debilitating balance disorder. These challenges include the specific differential diagnosis, documentation of symptoms and assessment of functional disability. It must be determined whether the cause of the disability is an organic sensory, deficit, a central nervous system (CNS) lesion or a non-organic (that is, possibly psychogenic or just overtly simulated) disorder. This review is targeted towards providing the reader (a) an overview of the effects sensorimotor deficits have on balance control, specifically vestibulospinal and proprioceptive reflex deficits; and, (b) how these effects may be assessed objectively in a clinical setting to differentiate between various organic and non-organic balance-disorders. The techniques used to study these effects are based on the analysis of both rapid balance-correcting and slow balance-stabilizing responses to fast and slow movements in the pitch plane of the support surface on which the test subject stands.

Recovery of vestibulo-ocular reflex-function in subjects with an acute unilateral peripheral vestibular deficit

The centrally controlled compensation for a reduced horizontal vestibulo-ocular reflex (VOR) gain caused by a unilateral afferent deficit is usually studied following a selective surgical procedure which completely lesions the vestibular nerve or blocks the horizontal semicircular canal. The more common, unilateral, vestibular deficit encountered clinically, is a partial loss of peripheral vestibular function, following which peripheral recovery and/or central compensation may occur. We investigated changes of the VOR gain in response to a sudden, idiopathic, unilateral vestibular deficit in 64 subjects by examining the responses to low-frequency, whole-body, rotations about an earth vertical axis with different accelerations (5, 20 and 40 deg/sec2) during in- and out-patient visits separated by 4 months in an attempt to identify changes brought about by peripheral recovery and by central compensation processes. Peripheral function was assumed to be measured by the response to caloric irrigation. It improved some 30% on average between the two visits. VOR responses for rotations towards the deficit side also improved between the two visits. Most improvement occurred for 20 deg/sec2 accelerations. However, the correlation coefficient between rotation and caloric responses was always less than 0.6. Unlike caloric responses which improved over time, responses for rotations to the intact side did not change between the visits. For this reason, the majority of observed VOR rotation responses were nearly symmetrical at the time of the second visit, despite being below normal levels. These findings suggest that both peripheral recovery and central compensation processes help restore symmetrical VOR function for head rotations after a partial unilateral vestibular deficit. However the improvement of VOR response symmetry, particularly to slow (< 40 deg/sec2) accelerations, is largely independent of the recovery of peripheral sensitivity.

[Infrared and video oculography--alternatives to electrooculography?]

With the introduction of each new technique for registering eye movements, the question arises concerning whether these provide a reliable and accurate alternative for the diagnosis of labyrinthine dysfunction when compared to the most commonly used electro-oculographic technique. To answer this question we compared mean slow-phase velocity (SPV) using three different recording techniques:electro-oculography (EOG), video-oculography (VOG) and infrared oculography (IROG) during four different types of examinations. The examinations were the eye target tracking test (ETT), horizontal and vertical optokinetic nystagmus (OKN) tests and the rotating chair test of the horizontal vestibulo-ocular reflex (HVOR). For the ETT tests the VOG provided consistently higher mean SPV values with low variance, presumably because of the accuracy and stability of the calibration. For the horizontal OKN and VOR rotating chair tests no significant differences were found between the mean SPV obtained with the different recording techniques, although the IROG recordings were associated with a larger variance. Vertical OKN mean SPV was consistently lower with IROG and VOG techniques presumably because of non-linearities and resolution limitations inherent in these techniques. These results indicate that the EOG technique despite its disadvantages of noise, time to apply and variability of calibration still provides an inexpensive, reliable and accurate means of measuring slow-phase eye movements.

Principles underlying real-time nystagmus analysis of horizontal and vertical eye movements recorded with electro-, infra-red-, or video-oculographic techniques

New methods for separating fast and slow phases of human nystagmus in real time are presented that are adaptive to the time-dependent noise properties of the input eye movement signal and therefore applicable to different recording techniques and directions. The methods employ a statistical filter technique to track slow-phase eye movements, uninfluenced by fast phases and blink artifacts, and fuzzy-logic techniques to identify fast-phase eye movements. Because these two techniques are decoupled from one another, highly accurate phase separation and slow-phase velocity profiles are achieved. In addition, the tracking of the variance of slow-phase and fast-phase eye movement recording permits a quality control of the analysis for different recording techniques and a variety of ocular nystagmus responses. Because blinks impose different eye velocity profiles on the recordings, depending on the type of recording technique and direction (horizontal, vertical), blink detection and its effect on fast-phase amplitude must be individually adjusted to each recording technique. Results are illustrated in the context of simultaneously recorded video-oculographic, infra-red, and electro-oculographic recordings of vestibulo-ocular reflex and optokinetic reflex responses causing horizontal or vertical eye movements.

Interactions between vestibular and proprioceptive inputs triggering and modulating human balance-correcting responses differ across muscles

Interactions between proprioceptive and vestibular inputs contributing to the generation of balance corrections may vary across muscles depending on the availability of sensory information at centres initiating and modulating muscle synergies, and the efficacy with which the muscle action can prevent a fall. Information which is not available from one sensory system may be obtained by switching to another. Alternatively, interactions between sensory systems and the muscle to which this interaction is targeted may be fixed during neural development and not switchable. To investigate these different concepts, balance corrections with three different sets of proprioceptive trigger signals were examined under eyes-open and eyes-closed conditions in the muscles of normal subjects and compared with those of subjects with bilateral peripheral vestibular loss. The different sets of early proprioceptive inputs were obtained by employing three combinations of support surface rotation and translation, for which ankle inputs were nulled, normal or enhanced, the knees were either locked or in flexion, and the trunk was either in flexion or extension. Three types of proprioceptive and vestibulospinal interactions were identified in muscles responses. These interactions were typified by the responses of triceps surae, quadriceps, and paraspinal muscles. The amplitudes of stretch responses at 50 ms after the onset of ankle flexion in triceps surae muscles were related to the velocity of ankle stretch. The amplitude of balance-correcting responses at 100 ms corresponded more with stretch of the biarticular gastrocnemius when the knee was re-extended at 60 ms. Absent stretch reflexes at 50 ms in triceps surae with nulled ankle inputs caused a minor, 12-ms delay in the onset of balance-correcting responses in triceps surae muscles. Vestibular loss caused no change in the amplitude of balance-correcting responses, but a negligible decrease in onset latency in triceps surae even with nulled ankle inputs. Stretch responses in quadriceps at 80 ms increased with the velocity of knee flexion but were overall lower in amplitude in vestibular loss subjects. Balance-correcting responses in quadriceps had amplitudes which were related to the directions of initial trunk movements, were still present when knee inputs were negligible and were also altered after vestibular loss. Stretch and unloading responses in paraspinals at 80 ms were consistent with the direction of initial trunk flexion and extension. Subsequent balance-correcting responses in paraspinals were delayed 20 ms in onset and altered in amplitude by vestibular loss. The changes in the amplitudes of ankle (tibialis anterior), knee (quadriceps) and trunk (paraspinal) muscle responses with vestibular loss affected the amplitudes and timing of trunk angular velocities, requiring increased stabilizing tibialis anterior, paraspinal and trapezius responses post 240 ms as these subjects attempted to remain upright. The results suggest that trunk inputs provide an ideal candidate for triggering balance corrections as these would still be present when vestibular, ankle and knee inputs are absent. The disparity between the amplitudes of stretch reflex and automatic balance-correcting responses in triceps surae and the insignificant alteration in the timing of balance-correcting responses in these muscles with nulled ankle inputs indicates that ankle inputs do not trigger balance corrections. Furthermore, modulation of balance corrections normally performed by vestibular inputs in some but not all muscles is not achieved by switching to another sensory system on vestibular loss. We postulate that a confluence of trunk and upper-leg proprioceptive input establishes the basic timing of automatic, triggered balance corrections which is then preferentially weighted by vestibular modulation in muscles that prevent falling. (ABSTRACT TRUNCATED)

Evaluation of electrically elicited stapedius reflex threshold measured through three different cochlear implant systems

OBJECTIVE

To evaluate intraoperative electrically elicited stapedius reflex thresholds (ESRTs) measured through three different cochlear implant systems: the Nucleus Mini 22, the Clarion Enhanced Bipolar, and the Med-El Combi-40.

SUBJECTS AND METHODS

Relations between intraoperative ESRT and postoperative maximum comfort level (MCL) were examined in seven children (4 Nucleus, 2 Clarion, and 1 Med-El) and one adult (Clarion).

RESULTS

Preliminary results indicated most ESRTs were either higher or both higher and lower (across the electrode array within a subject) than MCLs. All systems provided satisfactory means for measuring ERSTs.

CONCLUSION

It is recommended that hand-held systems have a direct readout to the programming station and that audio and visual feedback be improved for all units.

The control of head movements during human balance corrections

The assumption that the CNS regulates head stability during human balance corrections is explored in this review (an outgrowth of discussions initiated during the Head/Neck meeting held in Vail, Colorado, USA, July 1995). Two major questions were considered. First, how do the vestibulocollic (VCR) and cervicocollic (CCR) reflexes interact with intrinsic mechanical properties of the head neck system to control head position during balance corrections? Second, how is this interaction affected by factors such as vestibular loss, aging, and changes in behavioral goals or central set? The authors conclude that head velocities observed during balance corrections depend to a large extent on the movements of the head-neck mass-viscoelastic system whose properties could be altered by cocontracting the neck muscles. For experiments involving stance perturbations, much of the corrective response in neck muscles appeared to be triggered by trunk and leg proprioceptive signals, and a major role for the VCR was not established. Evidence consistent with a role for the vestibular system was found in other experimental paradigms in which the head was perturbed directly. In these paradigms the VCR modulates the amplitude of functionally stabilizing responses and damps mechanically induced instability of the head and neck.

[Follow-up of caloric test response after acute peripheral vestibular dysfunction]

This study examined retrospectively the spontaneous recovery of patients with an acute peripheral vestibular deficit in order to determine whether the caloric test response and with it vestibular function improves over time. The caloric bithermal was tested three times on 79 patients who were hospitalised with an acute deficit. The first test was recorded on emergency admission by observing nystagmus beats under the Frenzel glasses. Two to five days later a complete electronystagmus (ENG) examination was performed. A second ENG was performed, on average, 4 months later. 46% of the patients recovered a normal caloric canal paresis value (less than 32%). By comparing the canal paresis values in the first and second ENG an improvement exceeding 30% was demonstrated in 50% of the patients and there was no correlation between the extent of the canal paresis deficit and the amount of recovery. A simultaneous cochlear deficit had no influence on the recovery of vestibular function.

Estimating net joint torques from kinesiological data using optimal linear system theory

Net joint torques (NJT) are frequently computed to provide insights into the motor control of dynamic biomechanical systems. An inverse dynamics approach is almost always used, whereby the NJT are computed from 1) kinematic measurements (e.g., position of the segments), 2) kinetic measurements (e.g., ground reaction forces) that are, in effect, constraints defining unmeasured kinematic quantities based on a dynamic segmental model, and 3) numerical differentiation of the measured kinematics to estimate velocities and accelerations that are, in effect, additional constraints. Due to errors in the measurements, the segmental model, and the differentiation process, estimated NJT rarely produce the observed movement in a forward simulation when the dynamics of the segmental system are inherently unstable (e.g., human walking). Forward dynamic simulations are, however, essential to studies of muscle coordination. We have developed an alternative approach, using the linear quadratic follower (LQF) algorithm, which computes the NJT such that a stable simulation of the observed movement is produced and the measurements are replicated as well as possible. The LQF algorithm does not employ constraints depending on explicit differentiation of the kinematic data, but rather employs those depending on specification of a cost function, based on quantitative assumptions about data confidence. We illustrate the usefulness of the LQF approach by using it to estimate NJT exerted by standing humans perturbed by support-surface movements. We show that unless the number of kinematic and force variables recorded is sufficiently high, the confidence that can be placed in the estimates of the NJT, obtained by any method (e.g., LQF, or the inverse dynamics approach), may be unsatisfactorily low.

Recent advances in clinical neurotology

In recent years, owing to significant technological developments and an increased number of investigators entering the field, there have been spectacular advances in our understanding of the basic anatomy and physiology of the vestibular system. Unfortunately, these advances in basic science are slow to impact the clinical management of patients. We have selected a few important advances in clinical neurotology that have impacted the diagnosis and treatment of patients with vestibular disorders. This material was originally presented at the "Mechanisms of Vestibular Function and Dysfunction" symposium of the 1994 Neural Control of Movement meeting in Waikoloa, Hawaii.

Differential control of leg and trunk muscle activity by vestibulo-spinal and proprioceptive signals during human balance corrections

Knowledge about how proprioceptive signals trigger and modulate human balance corrections has important implications for the rehabilitation of postural and gait disorders, and increases our understanding of normal interactions between these sensory systems. We used combinations of support-surface rotation and rearward translation to examine the triggering effects of ankle and knee movements on balance corrections. By comparing the responses in normal subjects to those in persons with a bilateral peripheral vestibular deficit, we determined the modulating influence of vestibular inputs on balance responses. Differences in normal and vestibular-loss responses under the different proprioceptive conditions revealed four general findings. First, ventral leg muscle responses are strongly modulated by vestibulo-spinal inputs and by proprioceptive inputs from the ankle and knee. Second, triceps surae muscle responses are initially dependent on ankle inputs, and after 100 ms are modulated by knee inputs; they are not altered by vestibular loss. Third, paraspinal responses in vestibular-loss subjects are enhanced because of unstable trunk sway induced by the lack of ventral leg-muscle activity. Fourth, the earliest possible triggering signal for establishing the timing of interlink muscle activity appears to be knee flexion and/or trunk rotation on the pelvis. These results indicate that a confluence of knee and trunk proprioceptive and vestibulo-spinal inputs, rather than either input alone, is involved in establishing the muscle synergy underlying normal balance corrections.

Synergies and strategies underlying normal and vestibulary deficient control of balance: implication for neuroprosthetic control

Future developments of neuroprosthetic control will probably permit locomotion and posture to be maintained without the aid of crutches and will therefore require some form of balance control. Three fundamental questions will arise. First, the question of the location of imbalance-sensing transducers must be assessed. Secondly, the synergy, which is the relative amplitude and timing of muscle activity, and/or the strategy of joint torques required to re-establish a stable posture for different types of balance disturbances must be addressed. Thirdly, the control laws that map either trunk muscle activity or imbalance-sensing transducer outputs into multi-joint postural control of standing by paraplegic individuals must be generated. The most appropriate means of gathering the relevant information applicable to neuroprosthetic control systems is through the detailed analysis of normal and non-normal human models. In order to gain such detailed insights into normal balance control and its dependence on head angular and linear accelerations, the synergy and strategy of balance corrections in normal subjects or patients with vestibular deficits were investigated for two types of support surface perturbation, a dorsiflexion rotation (ROT) and a rearward translation (TRANS). These experimentally induced perturbations to upright stance were adjusted to cause equal amplitudes of ankle dorsiflexion, thus providing additional information about the role of lower leg proprioception on balance control. Synergies defined on the basis of peak cross-correlations of each recorded muscle's EMG to that of the largest muscle response were significantly different for TRANS and ROT. Translation synergies consisted of a sequential coactivation at several levels (soleus and abdominals some 30 msec before hamstrings, and trapezius some 15 msec before paraspinals), whereas the sequential activation of paraspinals and tibialis anterior dominated the balance synergy to ROT. Likewise, response strategies, defined using cross-correlations of joint torques, differed. That for TRANS was organised as a multi-link strategy with neck torques leading those of all other joints by 40 msec or more; hip joint lead ankle torques by 30 msec. That for ROT was organised around hip and ankle torques without a major correlation to neck torques. Vestibulary deficient subjects developed weaker synergies with respect to subjects with normal balance systems under eyes-open conditions and there was no clear synergy with eyes closed. Consequently, hip torques were delayed some 180 msec with respect to ankle torques, and correlations to neck torques were completely out of phase under eyes-closed conditions. Fundamental changes in TRANS synergies and strategies also occurred in vestibulary deficient subjects for eyes-open and eyes-closed conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

[New indications for the rotating chair test for side localization and determination of central compensation in vestibular function disorder]

The horizontal vestibulo-ocular reflex was studied in normals and 35 patients with an acute or compensated unilateral peripheral vestibular deficit (PVD) in order to determine the efficacy of different response measures obtained from a rotating chair test in localizing the deficit side and defining its central compensation. The profiles of chair velocity and the visual fixation period were chosen in such a way that the slow phase velocity profiles were comparable to those obtained during caloric tests. Mean slow phase eye velocity measured during the culmination period elicited by constant chair acceleration of 5 degrees/s2 over 40 s was significantly reduced for rotations towards the side of an acute or compensated PVD. Per-rotatory gain asymmetry (after correcting for spontaneous nystagmus) was not always specific for the side of the deficit and often not significantly different from normal values in the compensated stage of the deficit. In combination both parameters (SPV and gain asymmetry) were reliable indicators (sensitivity 88%) of the underlying pathology and correctly localized the deficit side during the compensated state. The decay of post rotatory SPV followed a more rapid time course than normal during acute and compensated stages of the deficit. It may be concluded from those results that the rotating chair test is a valuable indicator of the presence and side of a peripheral vestibular deficit even when it has been centrally compensated.

Human auditory-evoked potentials before and after magnetic resonance imaging

The effect of magnetic fields on auditory brainstem responses (ABRs) was examined under two conditions. The first involved recording ABRs before and immediately after magnetic resonance imaging (MRI). ABRs were then recorded as the static magnetic field was increased stepwise to 2T. No significant changes in ABR latencies were noted under either condition. These results indicate that MRI has no measurable effect on the transmission latencies of auditory brainstem neural pathways.

Diagnosis and surgery of cerebellopontine-angle tumors

The present report is based on the analysis of clinical data collected from 80 patients with cerebellopontine-angle tumors. Special reference is made to the history of the patient, the neurootologic test battery, particularly to the sensitivity and specificity of audiological and vestibular diagnostic procedures. The postoperative findings are evaluated especially with respect to postoperative morbidity and the preservation of facial nerve and auditory functions.

The effect of age on the visuo- and vestibulo-ocular reflexes of elderly patients with vertigo

To evaluate the effect of age and vestibular deficit on the vestibulo-ocular and associated visual reflexes, rotating chair (VOR), eye tracking test (ETT) and optokinetic (OKN) responses were investigated using comparisons between three populations: young normals, vertiginous elderly patients, and age-matched normal elderly subjects. The gain of ETT and OKN responses were reduced for both elderly populations as compared to young normals, but less so for the vertiginous elderly. VOR gain was more significantly reduced in the vertiginous elderly, though VOR time constant was as reduced as for the normal elderly. These results indicate that visual inputs, even in the elderly, are probably employed to compensate for a vestibular deficit.

Documentation of the recovery course and deficit side localization of an acute unilateral vestibular deficit using four-quadrant diagrams of slow phase velocity

The horizontal vestibulo-ocular reflex was studied in normals and in patients with an acute or compensated unilateral peripheral vestibular deficit (PVD) in order to determine the efficacy of various response measures of deficit side and central compensation for high rotation velocities. The profiles of whole body rotation and the visual fixation period was chosen to yield slow phase velocity (SPV) profiles comparable with those obtained with caloric irrigation. The chair rotation direction producing the smaller amplitude of slow phase velocity measured over culmination period obtained with 40 s of 5 degrees/s2 constant acceleration to 200 degrees/s represented a lateralizing sign of all acute, and 85% of all compensated PVD cases.

Classification of peripheral and central (pontine infarction) vestibular deficits. Selection of a neuro-otological test battery using discriminant analysis

The results obtained from a complete neuro-otological test battery were examined statistically in order to select measurement variables which would optimally indicate significant differences between four groups: normal patients, patients with partially compensated unilateral peripheral vestibular deficit, patients with an acoustic neurinoma and patients with central (brainstem) vestibular deficit. A stepwise-discriminant analysis was performed on measurements of slow-phase velocity obtained from each test. The primary measurements selected to assign a subject optimally to one population were the canal paresis (CP) of the caloric test, the eye-tracking gain contralateral to the deficit for a 15 deg/s stimulus, the gain asymmetry for optokinetic nystagmus with a 30 deg/s stimulus, and the level of spontaneous nystagmus. The resulting classifications were 100% correct for normal and central deficit patients. However, the division between peripheral deficit and acoustic neurinoma patients overlapped causing about 30% false classifications of neurinoma patients: some 20% of the peripheral deficit patients were classified as normal. If the CP was not available the discriminant analysis substituted the rotating chair response for 5 deg/s2, in place of CP. This substitution caused a 10 to 20% decrease in classification accuracy.

[Medical informatics systems exemplified by the diagnosis of equilibrium disorders]

An interdisciplinary field, namely the differential diagnosis of balance disorders and vertigo, is used to describe how a medical expert system can be developed using modern computer analysis, medical expertise, and human pattern recognition techniques. The advantages, results, and unresolved issues of close cooperation between biomedical engineers and physicians are described. The aim of this cooperation was to ensure that complicated data were presented in simple graphic form and that large amounts of diagnostic data were optimally linked together for the generation of a recommended diagnosis. Similar techniques may usefully be employed in other areas of medicine.

Shifts in auditory brainstem response latencies following plasma-level-controlled aminoglycoside therapy

The ototoxic effect of plasma-level-controlled netilmicin and tobramycin was monitored in 20 intensive care patients, using auditory brainstem responses (ABRs). ABRs were recorded at the onset and after 6 days of treatment. Significant shifts in the ABR wave V latency were observed independent of the type of medication received or whether the patients were intubated or not. Shifts were also observed in wave I latencies. These latter changes were only significant for the complete population, probably because this potential is more difficult to elicit and measure than is wave V. The I-V interlatency time was greater for netilmicin than for tobramycin. These results indicate that the cumulative effects of aminoglycoside ototoxicity may be different for the central and peripheral nervous systems, and therefore ABR measurements may be the most accurate way of controlling ototoxicity in intensive care patients.

Characteristics of electrically evoked 'auditory' brainstem responses elicited with the nucleus 22-electrode intracochlear implant

Electrically evoked auditory brainstem responses (EABRs) were measured in cochlear implant patients fitted with the Nucleus 22 electrode system. The typical response waveform consisted of a series of two to three peaks. The largest peak was similar in form to the wave V of acoustically evoked ABRs and was most prominent for stimulus intensities nearly equal to the patients' maximum comfortable (MC) behavioural stimulus level for the test electrode. The first identifiable wave V amplitude was observed at stimulus levels greater than the patients' psychophysical threshold. With increasing stimulus intensity, wave V amplitude increased rapidly to plateau at a level highly correlated with the patients' MC level at the EABR stimulus rate of 17/s. Wave V peak latency was generally shorter than normal ABRs (4.0 cf. 5.5 ms) and varied with electrode position: apical electrodes had shorter latencies than basal electrodes by approximately 0.4 ms. These results suggest that EABRs can be used as an objective estimate of a patient's electrode-specific MC level, once the correlation of EABR growth functions at 17/s to those at clinically employed rates of 250/s has been determined. EABRs may indicate differences in nerve action potential generation for apical and basal electrodes.

Automatic electronystagmus analysis and documentation: recent advances in the study of vestibular, optokinetic and pursuit tracking function

A normal or pathologically altered peripheral vestibular system and associated brainstem structures can be diagnosed from the pattern of eye movement responses elicited by appropriate stimuli. Recent advances in two stages crucial to an accurate assessment of pathological or normal responses are described in this article. The first stage involves the automatic analysis of electronystagmus signals to yield the main parameters of clinical and scientific interest, slow phase eye velocity and fast phase frequency. Since four algorithms based on the first derivative of eye position perform this task remarkably well on-line, it is not necessary to employ features of the stimulus to separate the slow and fast phases of nystagmus. Examples are used liberally to illustrate the accuracy, advantages and limitations of the algorithms. The second stage involves a numerical and graphical comparison of measurements from a patient's analyzed responses with normal responses. This documentation phase permits immediate recognition of normal, borderline, or pathological optokinetic, eye tracking, caloric and rotating chair test results. Selected examples of pathological responses illustrate the documentation technique.

The role of stretch and vestibulo-spinal reflexes in the generation of human equilibrating reactions

Equilibrating reactions in standing humans were examined for evidence that either vestibulo-spinal or proprioceptive long loop stretch reflexes from ankle muscles, or both, are responsible for the control and organization of rapid postural responses. Specifically, the hypothesis was tested that the same postural response could be evoked by rotation of the support surface that mimics the ankle rotation occurring during support surface translations. Rotation perturbations evoked postural responses in leg and trunk muscles that were different in strategy, synergy and coactivation from translation responses, even though the short-latency response in the stretched triceps surae muscles was equal in latency and size. Movement patterns consisted of a stiffening strategy and hardly any compensating ankle rotation for rotation stimuli, and a multi-link strategy with motion focused about the neck, hip and ankle joints for translation stimuli. Dorsiflexion rotations caused earlier and stronger responses in tibialis anterior and quadriceps muscles just post to the onset of paraspinal muscles, whereas rearward translation activated soleus and abdominals strongest, both just prior to hamstring muscles. Correlated activation strengths of agonist and antagonist activity was a common feature for both types of perturbation, albeit, only in the ankle muscles for rotations and only in the trunk muscles for translations. These data suggest that sensory inputs, other than those generated in the lower leg predominate, in the triggering and modulation of equilibrating reactions. Possible candidates are those of the vestibular system or proprioceptive inputs from the trunk.

Indicators of the influence a peripheral vestibular deficit has on vestibulo-spinal reflex responses controlling postural stability

For a controlled sway stabilization task, the areas underlying EMG responses in ankle and neck muscles, as well as amplitudes of ankle torque responses, were shown to be significantly correlated with the clinically defined extent of a patient's peripheral vestibular deficit. The responses, elicited by ankle dorsiflexion of the support surface on which the subject stood, were statistically examined in order to select those measurements which would best indicate differences between a normal, a patient with a unilateral deficit, or one with a bilateral deficit. For this purpose, a stepwise discriminant analysis was performed on measurements of head and trunk angular accelerations in addition to muscle EMG and ankle torque signals. The primary measurements selected to optimally assign a subject to a population were the periods of ankle torque and neck extensor activity associated with correcting for the imposed body displacement backwards and maintaining upright head position respectively. The resulting division into populations was 100% correct. However, within the population of unilateral deficit patients, the technique failed to correctly identify those with acute from those with compensated deficit. This technique of investigating vestibulo-spinal reflex responses is more specific and sensitive than Romberg tests, because it will quantify and specify the underlying cause of the patient's balance and ambulatory disorder.

Long-term modifications of vertical and horizontal vestibulo-ocular reflex dynamics in man. I. After acute unilateral peripheral vestibular paralysis

Horizontal (HOR) and vertical (VERT) vestibulo-ocular reflex (VOR) responses to whole-body triangular velocity profiles with constant accelerations of 10, 15 and 20 deg/s2 were studied in two populations: normals, and patients with acute unilateral peripheral vestibular paralysis. The effect of this type of unilateral deficit on VOR gain and long time constant were determined as well as the time course of the compensation processes for HOR and VERT VOR dynamics. In the patient population, HOR VOR gain was asymmetric post deficit, being, on average, 50% and 75% of normal for rotations toward and away from the deficit, respectively. For the VERT VOR, on average, a symmetric 66% reduction occurred. The VERT VOR time constant was marginally affected by the deficit. HOR time constants were reduced for both directions of rotation. HOR and VERT VOR gain was within normal limits 1-3 months following an acute paralysis; time constants required a longer recovery period. Our results indicate that a unilateral deficit causes a markedly different alteration for HOR VOR dynamics compared to the effect on VERT VOR.

Postural coactivation and adaptation in the sway stabilizing responses of normals and patients with bilateral vestibular deficit

The experiments were designed to test two hypotheses and their corollaries: 1. That adaptation of EMG responses to support surface rotations is due to a decrease in the gain of proprioceptively triggered long-loop stretch reflexes (Nashner 1976), and that the adaptation is dependent on a normally functioning vestibular system (Nashner et al. 1982); 2. That EMG responses to rotations are generated primarily by vestibulo-spinal reflexes triggered by head accelerations (Allum and Pfaltz 1985) and comprise a coactivation of opposing leg muscles (Allum and Büdingen 1979). Adaptation with successive dorsi-flexive rotations of the support surface was investigated in the EMG responses of the ankle muscles, soleus (SOL) and tibialis anterior (TA), as well as the neck muscles, trapezius (TRAP) and splenius capitis (SPLEN CAP), both for normal subjects and for patients with bilateral peripheral vestibular deficit. Both normals and patients who first received the stimulus with their eyes open demonstrated decreasing activation at medium latency (ML), that is, with an onset at about 125 ms, and long latency (LL) responses with an onset ca 200 ms. This was the case for both ankle and neck muscles when the EMG response areas for the first 3 and second 7 of 10 trials were compared. Ankle muscle responses in the patients were diminished in area with respect to normals both with the eyes open and with the eyes closed. Ankle torque recordings from the patients were also smaller in amplitude, and these attenuated differently from normal torque responses. Functional coupling of the opposing ML and LL SOL and TA muscle responses was confirmed by the nearly coincident onset times and significantly correlated EMG response areas. At ML, ankle torque was highly correlated with TA activity when the influence of SOL was controlled. At LL, SOL activity was highly correlated with torque when the influence of TA was controlled. The delay of torque adaptation beyond the period of ML activity in normals, but not in the patients was attributed to the proportionally balanced coactivated muscle patterns producing a consistent force output and level of stability in normals. The results indicate that the adaptation in EMG response amplitudes during a sway stabilisation task is not dependent on a normally functioning vestibular system nor on visual inputs but rather appears to be due to a generalized habituation in the postural control system.(ABSTRACT TRUNCATED AT 400 WORDS)

[Vestibulospinal reflexes in differentiating between organic and functional (psychogenic) causes of vertigo]

Recent investigations comparing the postural responses of normals and patients with bilateral peripheral vestibular deficit to a sudden backwards tilt of the support surface about the ankle joints have permitted an objective definition of the timing and amplitude of vestibulo-spinal reflex (VSR) EMG responses in ankle and neck muscles. The muscle contractions result in a sway stabilizing action correcting for induced body tilt, whose effect may be observed in ankle torque, and body and head angular acceleration records. The same techniques were used to monitor VSR responses of 10 patients with suspected simulated, aggravated, or psychogenic vertigo--vertigo of functional origin. Two cases, presented in detail, illustrate the identification of weak reflex responses for patients aggravating an organic VSR pathology during the Romberg test and the identification voluntarily controlled destabilizing responses for patients with a simulated or psychogenic vertigo. Recording the vertical vestibulo-ocular reflex in the same rotation plane as induced tilt is shown to provide valuable additional information on a possible peripheral vestibular pathology. It is concluded that VSR responses time-locked to a vestibular stimulus provide an improved means of identifying a functionally based vertigo.

Neural correlates of isometric force in the "motor" thalamus

The relationship between single cell activity in the "motor" thalamus and the generation of isometric force between the fingers has been investigated in 2 monkeys. Neurons related to the task were found in the thalamic motor regions VLo, VPLo, and VA where microstimulation occasionally elicited motor reactions in hand and fingers. 58% of these 55 neurons, designated "typical", showed modulation of their discharge patterns with force similar to neurons in precentral cortex and could be assigned to one of 5 discharge patterns described for the motor cortex. Only a small percentage of the thalamic neurons were found to have phasic activity. The other "atypical" neurons (42%) had discharge patterns with complex sequences of phasic and tonic activation with respect to force. For 18 typical and atypical neurons with tonic and phasic-tonic modulation of their firing rate with force significant regression coefficients between firing rate and static force were observed. The mean index of force sensitivity (rate-force slope) was 54.5 Hz/N for the neurons increasing their discharge rate with force, i.e. approximately that of precentral cells. Neurons tested for their sensory properties had receptive fields located on hand and/or fingers and were activated mainly by stimulation of muscle and joint receptors. The characteristics of these thalamic neurons are compared to those of precentral cells recorded under identical experimental conditions and are discussed in relation to the known input-output relationships of the motor thalamic nuclei. The data strongly support the hypothesis that parameters of movement, in particular force are represented by the activity of neurons in the "motor" thalamus.

Visual and vestibular contributions to pitch sway stabilization in the ankle muscles of normals and patients with bilateral peripheral vestibular deficits

Vestibular, visual, and proprioceptive influences on muscle activity correcting for backwards body tilt were investigated in normals and patients with bilateral peripheral vestibular deficits. Body tilt was induced by a dorsi-flexion rotation of the feet about the ankle joints while the subject stood on a force measuring platform. Ankle muscle activity and torque were monitored as upright stance was reestablished, and correlated with head angular accelerations and neck muscle activity. In normals with eyes closed, soleus stretch reflex activity at 50-80 ms was followed by two bursts of tibialis anterior (TA) EMG activity at ca 80 and 125 ms from the onset of 36 deg/s, 3 deg amplitude platform rotations. Neck muscle activity rotated the head backwards at the same time as TA activity rotated the body forwards about the ankle joints. Under the influence of vision, i.e. eyes open, slight increases in the second burst of TA activity, and ankle torque were observed. When the subjects sat, and were instructed to activate TA rapidly on onset of the platform movement, TA EMG activity increased gradually at ca. 150 ms and not as a burst. In patients with long-lasting bilateral vestibular deficits, both bursts of TA activity were significantly less than normal with eyes closed. Consequently sway correcting torques were abnormally low and all but one of the patients fell over backwards. With eyes open, TA activity was slightly less than, and ankle torques were approximately equal to normal values. In contrast to normals, TA responses obtained in standing and sitting positions were not significantly different. Neck EMG activity varied from normal, consisting of a long burst 100 ms in duration. The present data indicate that a coordinated pattern of ankle, and neck muscle activity occurs during the first 150 ms following induced backward tilt. Ankle muscle activity corrects for the body sway, and neck muscle activity attempts to stabilise the head with respect to earth fixed coordinates. It is proposed that the vestibulo-spinal reflex system predominantly underlies the genesis and coordination of this muscle activity.

Compensation for intrinsic muscle stiffness by short-latency reflexes in human triceps surae muscles

The incremental torque resisting rotation of the foot about the ankle joint was studied in normal seated subjects. Prior to each rotation, subjects were required to activate triceps surae (TS) muscles and maintain a constant plantar flexion torque (range 6-14 N X m) on a platform whose position was controlled by a torque motor. Subjects were instructed to increase torque as rapidly as possible once rotation commenced. Rotations ranged from 0.5 to 14 degrees amplitude and from 20 to 300 degrees/s maximum velocity. The torque in response to rotations stretching TS muscles and releasing tibialis anterior (TA) muscles increased steeply and then rapidly decreased with stretch velocity. At approximately 60 ms from stretch onset, the torque reduction terminated, torque then increased again until it began to level off at approximately 120 ms. A further large increase in torque occurred at 180 ms. A burst of short-latency (SL) electromyographic (EMG) activity in soleus (SOL) commenced at 40 ms, and was followed by a second burst at approximately 68 ms, provided that stretch deceleration started later than 20 ms after stretch onset. A period of sustained EMG activity in SOL commenced at approximately 130 ms (long-latency (LL) activity). Incremental torque in response to stretch of TA and release of TS muscles initially showed a step decrease followed by a reversal of the torque trajectory back toward base line. This change was arrested at 60 ms and torque then remained approximately constant until a large increase in torque at 180 ms. Ischemia was used to reduce SL EMG reflexes without significantly modifying the background EMG activity. A comparison between torque curves under control and ischemic conditions indicated that SL EMG activity in TS muscles recruited the force responsible for terminating the torque reduction coincident with decreasing stretch velocity. The torque response prior to the onset of force recruited by SL activity was attributed to the intrinsic properties of active muscle fibers. Thereafter, until the onset of LL activity, the torque response was attributed to intrinsic and reflex-recruited force. Torque in these two time periods was compared under a variety of stretch conditions in order to test the hypothesis that force recruited by segmental reflexes compensates for the non-linear stretch properties of active TS muscles. The relationships of SL EMG amplitudes and areas to stretch velocity and acceleration were also examined.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of stabilizing reflex responses in tibialis anterior muscles following ankle flexion perturbations of standing man

Reflex activity in human ankle muscles in response to 36 deg./s dorsi-flexion rotations of the feet was investigated in subjects standing upright and when leaning back so as to preactivate ankle flexor muscles. Short latency stretch reflex activity in soleus and inhibition in tibialis anterior muscles occurred at 50 ms from ankle rotation onset. Two prominent bursts of tibialis activity followed at 83 and 131 ms. and preceded large stabilizing ankle torques. Head movements commenced 20 ms after foot rotations and acquired accelerations exceeding 100 deg./s2 within 60 ms. It is suggested that the tibialis anterior activity is either a vestibulospinal reflex resulting from the head movement, or a stretch reflex only present during standing, since this activity was not observed when seated subjects received identical foot rotations.

The mechanical effectiveness of short latency reflexes in human triceps surae muscles revealed by ischaemia and vibration

The resistance to stretch provided by short latency (SL) reflexes in human triceps surae muscles was investigated under three experimental conditions: control, ischaemia, and with 100 Hz vibration applied to the Achilles tendon. Incremental changes in plantar flexion force always showed a strong initial resistance followed by yielding in response to rapid dorsiflexion of the foot about the ankle joint. These changes were attributed to inherent stiffness of the triceps surae muscles. The force curves for each experimental condition diverged during the yield phase some 20 ms after the onset of SL EMG reflexes. During ischaemia, SL EMG reflexes were reduced to 8% of control values and yielding continued until the onset of medium latency EMG activity whereas the yielding was interrupted by SL action in the control situation. The difference between the ischaemia and control force curves was attributed to force recruited by SL reflexes under normal stretch conditions. Vibration reduced the SL EMG reflex amplitude to 20% of control values and produced with it a reduced force response.

Presumed reflex responses of human first dorsal interosseus muscle to naturally occurring twitch contractions of physiological tremor

Surface EMG activity from the first dorsal interosseus, time-locked to twitch contractions in the same muscle, was investigated using the technique of spike-triggered averaging. A decrease in surface EMG activity was observed during the rising phase of twitch force, consistently followed by two bursts of activity with roughly constant onset latencies. One burst occurred at 45 msec from the onset of the single motor unit potential used to trigger the averager and was near the peak of the twitch contraction. The other burst occurred at 77 msec during the falling phase of the twitch contraction. If it is assumed that the chance synchronization of motor units contributes to physiological tremor [1, 5, 20], then the observed relative strengths of the two bursts of EMG activity indicate that their net action could enhance tremor.

Stiffness regulation provided by short-latency reflexes in human triceps surae muscles

The triceps surae muscles of normal human subjects were rapidly stretched and released by rotating the foot about the ankle joint with a torque motor. Following the initial intrinsic resistance, the yielding observed in incremental force records was more rapid for stretch than for release responses. Short-latency EMG responses elicited by stretch recruit force, to compensate for the yielding and to maintain the total (intrinsic plus reflex) resistance constant as the prior force level changes.