Conrad Wall

Normal subject postural sway during the Romberg test.

Analysis of fixed force platform recordings of Romberg tests performed by 132 normal subjects demonstrated no statistically significant sex or age effect for adults aged 20 through 49 years. There was a strong stabilizing influence of vision upon postural control in most, but not all, normal subjects. Comparison trials showed no statistically significant differences between trials for the standard Romberg eyes-open maneuver. However, a significant improvement was demonstrated for the second trial for all other Romberg maneuvers. Tests repeated over a period of five consecutive days yielded results with large variances within normal limits and no systematic individual or group trends. Normal percentiles and confidence intervals for the 95th percentile were calculated for use as a normal data base. These percentile distributions derived from a relatively large population of normal subjects provide a normal statistical base for comparison with postural sway from abnormal subjects.

Balance prostheses for postural control

There is a clear need for a prosthesis that improves postural stability in the balance impaired. Such a device would be used as a temporary aid during recovery from ablative inner-ear surgery and as a permanent prosthesis for those elderly prone to falls. Research using a one-axis device that estimates body tilt and displays it to vestibulopathic subjects via an array of tactile vibrators has demonstrated feasibility. The noninvasive, vibrotactile display of body tilt helped the balance-impaired subjects to reduce their body sway during standardized tests. The motion sensor array is comprised of three MEMS linear accelerometers and three MEMS rate gyros whose sensitive axes are aligned along three orthogonal directions to provide six-degree-of-freedom (dof) motion information.

Temporal Bone Studies of the Human Peripheral Vestibular System: 1. Normative Vestibular Hair Cell Data

Quantitative studies of the vestibular system with serially sectioned human temporal bones have been limited because of difficulty in distinguishing hair cells from supporting cells and type I from type II hair cells. In addition, there is only a limited amount of normative data available regarding vestibular hair cell counts in humans. In this study, archival temporal bone sections were examined by Nomarski (differential interference contrast) microscopy, which permitted visualization of the cuticular plate and stereociliary bundle so as to allow unambiguous identification of hair cells. The density of type I, type II, and total numbers of vestibular hair cells in each of the 5 sense organs was determined in a set of 67 normal temporal bones that ranged from birth through 100 years of age. The mean total densities at birth were 76 to 79 cells per 0.01 mm2 in the cristae, 68 cells per 0.01 mm2 in the utricle, and 61 cells per 0.01 mm2 in the saccule. The ratio of type I to type II hair cells at birth was 2.4:1 in the cristae and 1.3:1 in the maculae. There was a highly significant age-related decline in all sense organs for total, type I, and type II hair cell densities that was best fit by a linear regression model. The cristae lost type I cells with advancing age at a significantly greater rate than the maculae, whereas age-related losses for type II cells occurred at the same rate for all 5 sense organs. Hair cell densities in the cristae were significantly higher at the periphery than at the center. There were no significant sex or interaural differences for any of the counts. Mathematical models were developed to calculate the mean and 95% prediction intervals for the total, type I, and type II hair cell densities in each sense organ on the basis of age. There was overall good agreement between the hair cell densities determined in this study and those reported by others using surface preparation techniques. Our data and related models will serve as a normative database that will be useful for comparison to counts made from subjects with known vestibular disorders.

Vibrotactile Tilt Feedback Improves Dynamic Gait Index: a Fall Risk Indicator in Older Adults

The purpose of this study was to determine the effectiveness of vibrotactile feedback of body tilt in improving dynamic gait index (DGI) a fall risk indicator in community dwelling older adults. Twelve healthy elderly subjects (three males and nine females, age 79.7+/-5.4 yrs) were tested in an institutional balance rehabilitation laboratory to investigate changes between the feedback off and on conditions. Subjects were acutely exposed to a vibrotactile display that indicated the magnitude and direction of their body tilt from the vertical. DGI and mediolateral (ML) sway were determined during locomotion with, and without, vibrotactile tilt feedback (VTTF). All subjects were at risk for falls based on their initial DGI Score (range: 15-19, mean 17.4+/-1.56), which was taken with the vibratory stimulus turned off. Subjects learned to use the trunk tilt information from the vibrotactile feedback vest through 20-30 min of gait and balance training consisting of activities that challenged their balance. Subjects were then retested on the DGI. Statistically significant changes were demonstrated for the DGI total score while using the vibrotactile tilt feedback. DGI total scores improved from 17.1+/-0.4 to 20.8+/-0.3 (p<0.05). We conclude that vibrotactile tilt feedback improves both control of mediolateral sway during gait and dynamic gait index. Both are fall risk indicators for this population.

Effects of Age, Sex and Stimulus Parameters upon Vestibulo-ocular Responses to Sinusoidal Rotation

We evaluated the vestibulo-ocular reflex (VOR) in 50 normal human subjects, uniformly distributed in age 20-59 years, and evenly distributed in sex by decade, using sinusoidal rotation over the frequency range of 0.005 to 1.0 Hz and three peak amplitudes of 25, 50, and 100 deg/sec. Age variations. The gain magnitude of the normal human VOR appears to decrease with increasing age. This trend becomes statistically significant at p less than 0.05 for the 0.005 and 0.01 Hz test points. Sex-related variations. There were no statistically significant differences in gain magnitude between the sexes but females had higher average gains than males for the lowest test frequencies. The phase data showed a trend with the average phase points for females being less than those for males. This difference was statistically significant at p less than 0.05 for the 0.005 Hz test frequency. Dynamic range. Increasing the stimulus amplitude by a factor of four yielded a small but statistically significant decrease in gain magnitude, thus suggesting a mild saturation-type of nonlinearity.

Recovery from perturbations during paced walking

The aim of the current study was to develop a safe, standardized, stability test and to explore a set of metrics to characterize the recovery of gait stability in healthy individuals following a single mechanical perturbation during steady locomotion. Balance perturbations were mechanically applied to the right foot of 12 healthy subjects during paced walking by translating a platform embedded in a 12 m walkway diagonally (+45/-135 degrees ) relative to the direction of travel approximately 200 ms after heel strike. We examined the medio-lateral (ML) displacement of the sternum before, during and after the platform translation. Measurements of ML position of the right and left shanks in relation to the position of the sternum were used as step-by-step estimates of the moment arm controlling ML motion of the body. We hypothesized that when gait is perturbed in the single stance phase of the step cycle via a translation of the support surface, a series of steps after the perturbation input will be altered reflecting an effort by the CNS to maintain the center of mass (COM) within the base of support and to stabilize the upper body for continued gait. Specifically, if the foot is perturbed laterally during mid-stance a widening of the upcoming step will occur and if the foot is perturbed medially a narrowing of the upcoming step will occur. This behavior was frequent for most subjects. Recovery of non-perturbation behavior was achieved on the third step after the platform translation. An additional strategy was seen for some subjects during lateral perturbation inputs. Instead of widening the upcoming step, these subjects acquired the support to stabilize the body by putting their left foot down very quickly with minimal change in stance width. The recovery profiles of the sternum, though directionally asymmetric, were similar in shape among subjects and roughly proportional to the magnitude of the platform translation. Five to six steps were required for complete recovery in the subjects tested in this study.

Vestibular evoked myogenic potentials versus vestibular test battery in patients with Meniere's disease.

Objective: The present study was undertaken to assess the sensitivity of vestibular evoked myogenic potentials testing to side-of-disease in Ménière’s disease patients and to test the hypothesis that information supplied by vestibular evoked myogenic potentials is complementary to that provided by a conventional vestibular test battery. Study Design: Prospective cohort study. Setting: Large specialty hospital, department of otolaryngology. Subjects: Twenty consenting adults (9 men and 11 women) with unilateral Ménière’s disease by American Academy of Otolaryngology–Head and Neck Surgery diagnostic criteria. Interventions: All subjects underwent bilateral vestibular evoked myogenic potentials testing using ipsilateral broadband click and short-toneburst stimuli at 250, 500, and 1,000 Hz. All subjects also underwent electronystagmography and sinusoidal vertical axis rotation testing. Main Outcome Measures: Accuracy of side-of-disease assignment by vestibular evoked myogenic potentials, caloric asymmetry, and multivariate analysis. Results: Side-of-disease assignment was most accurate using caloric asymmetry with a 5% interaural difference criterion, achieving 85% correct assignment. The next best method was vestibular evoked myogenic potentials using 250-Hz toneburst stimuli, achieving 80% correct assignment. The least accurate method was caloric asymmetry using a traditional 30% interaural difference limen, achieving 55% correct assignment. Comparison of 5% interaural difference criterion and vestibular evoked myogenic potentials using 250-Hz toneburst stimuli showed discordant results, but in no case did both 5% interaural difference criterion and vestibular evoked myogenic potentials using 250-Hz toneburst stimuli make an incorrect assignment. Conclusion: Vestibular evoked myogenic potentials threshold was shown to be highly sensitive to side-of-disease in unilateral Ménière’s disease. We observed instances of discordance in side-of-disease assignment by caloric asymmetry and vestibular evoked myogenic potential methods but no case in which both methods were incorrect. This supports the hypothesis that vestibular evoked myogenic potentials supplies information complementary to that provided by other components of the vestibular test battery.

Eye movements in response to electric stimulation of the human posterior ampullary nerve.

Objectives: The concept of a vestibular implant to restore balance, similar to that of a cochlear implant to restore hearing in deaf patients, has been investigated in animal models. It remains to be shown, however, that electric stimulation of the human end organ or its vestibular nerve branches is capable of eliciting a nystagmic eye movement response. Methods: Three subjects were given electric stimulation of their posterior ampullary nerve, which was surgically exposed under local anesthesia, by a procedure developed by Gacek. The stimulus was a multiphasic, charge-balanced train of electric pulses. Results: In all subjects, a pulse repetition rate of 200 pulses per second produced a robust vertical nystagmus without any apparent change in the slow component velocity of the preexisting horizontal nystagmus. Conclusions: We have been able to replicate in humans a finding somewhat similar to that of Suzuki and Cohen in monkeys for electric stimulation of the posterior semicircular canal. The similarity is an eye movement with a large, predominant vertical component. The difference is that we saw no horizontal response component, and were not able to measure a torsional response, because we used 2-dimensional video methods. In addition, we found a robust nystagmus with slow component velocities that are large enough to compensate for vertical head movements. This is an essential step in demonstrating the feasibility of a vestibular prosthesis using electric stimulation.

Vibrotactile display coding for a balance prosthesis

Preliminary experiments have demonstrated the potential usefulness of a precursor balance prosthesis that displays the tilt of the subject using tactile vibrators (tactors) which arc in contact with the subjects skin. The device consists of a motion sensing system mounted on the head or body whose signals are converted into estimates of head or body tilt. Tilt is displayed to the subject by coding the tilt estimate into signals that are sent to the tactors using one of several schemes. Because full blown, end-to-end balance experiments are relatively time consuming and expensive, and because there are many possible display schemes, we have developed a quantitative means to evaluate the display step separately. We used a modified version of the manual control critical tracking task (CTT) to help us make an initial selection of the more promising vibrotactile display schemes for further evaluation. The classic CTT is a compensatory form of tracking in which the operator attempts to control an increasingly unstable system using a joystick to regulate a tracking error signal (system minus joystick) that is visually displayed as a dot on a cathode ray tube. Our modification added vibrotactile display of the error signal. For a given subject and vibrotactile display scheme, the level of difficulty at which the subjects lost control, called the critical lambda (/spl lambda//sub c/), was highly repeatable. Four different coding schemes were evaluated using an array of 16 vibrators that were attached to the lower backs of 11 healthy subjects. The first scheme, called interval-based coding, modulated the interval between pulses that were sent to single tactors mounted on the subjects right and left side. A greater tracking error magnitude was displayed as a faster pulse rate. A positive error was displayed on the right side while a negative one was displayed on the left. The remaining three schemes, called position-based coding, used a horizontal row of 14 tactors. Tracking error magnitude was mapped to position of the activated vibrator so that an error near zero corresponded to a vibrator near the center of the back. The three position-based schemes tested used three, four, or seven tactors per side. Averaged over all subjects, the value of /spl lambda/c for the interval-based scheme was significantly less than it was for each of the three position-based schemes. There was no significant change in /spl lambda//sub c/ as the number of position-based tactors was increased from three to seven per side. The prediction of better actual balance performance using position-based relative to interval-based vibrotactile display was validated by a preliminary study of six normal subjects that compared the body sway produced during quiet standing while providing head tilt estimates using both display modes. Our study provides basic characterization using /spl lambda//sub c/ for several vibrotactile display schemes in human subjects. The quantitative CTT measure of performance can logically be extended to other applications of vibrotactile displays and to other kinds of display schemes used for rehabilitation.

Influence of gravity on cat vertical vestibulo-ocular reflex

SummaryThe vertical vestibulo-ocular reflex (VOR) was recorded in cats using electro-oculography during sinusoidal angular pitch. Peak stimulus velocity was 50°/s over a frequency range from 0.01 to 4.0 Hz. To test the effect of gravity on the vertical VOR, the animal was pitched while sitting upright or lying on its side. Upright pitch changed the cats orientation relative to gravity, while on-side pitch did not. The cumulative slow component position of the eye during on-side pitch was less symmetric than during upright pitch. Over the mid-frequency range (0.1 to 1.0 Hz), the average gain of the vertical VOR was 14.5% higher during upright pitch than during on-side pitch. At low frequencies (<0.05 Hz) changing head position relative to gravity raised the vertical VOR gain and kept the reflex in phase with stimulus velocity. These results indicate that gravity-sensitive mechanisms make the vertical VOR more compensatory.