Helen S. Cohen

Habituation and adaptation of the vestibuloocular reflex: a model of differential control by the vestibulocerebellum

SummaryWe habituated the dominant time constant of the horizontal vestibuloocular reflex (VOR) of rhesus and cynomolgus monkeys by repeated testing with steps of velocity about a vertical axis and adapted the gain of the VOR by altering visual input with magnifying and reducing lenses. After baseline values were established, the nodulus and ventral uvula of the vestibulocerebellum were ablated in two monkeys, and the effects of nodulouvulectomy and flocculectomy on VOR gain adaptation and habituation were compared. The VOR time constant decreased with repeated testing, rapidly at first and more slowly thereafter. The gain of the VOR was unaffected. Massed trials were more effective than distributed trials in producing habituation. Regardless of the schedule of testing, the VOR time constant never fell below the time constant of the semicircular canals (≈5 s). This finding indicates that only the slow component of the vestibular response, the component produced by velocity storage, was habituated. In agreement with this, the time constant of optokinetic after-nystagmus (OKAN) was habituated concurrently with the VOR. Average values for VOR habituation were obtained on a per session basis for six animals. The VOR gain was adapted by natural head movements in partially habituated monkeys while they wore ×2.2 magnifying or ×0.5 reducing lenses. Adaptation occurred rapidly and reached about ±30%, similar to values obtained using forced rotation. VOR gain adaptation did not cause additional habituation of the time constant. When the VOR gain was reduced in animals with a long VOR time constant, there were overshoots in eye velocity that peaked at about 6–8 s after the onset or end of constant-velocity rotation. These overshoots occurred at times when the velocity storage integrator would have been maximally activated by semicircular canal input. Since the activity generated in the canals is not altered by visual adaptation, this finding indicates that the gain element that controls rapid changes in eye velocity in the VOR is separate from that which couples afferent input to velocity storage. Nodulouvulectomy caused a prompt and permanent loss of habituation, returning VOR time constants to initial values. VOR gain adaptation, which is lost after flocculectomy, was unaffected by nodulouvulectomy. Flocculectomy did not alter habituation of the VOR or of OKAN. Using a simplified model of the VOR, the decrease in the duration of vestibular nystagmus due to habituation was related to a decrement in the dominant time constant of the velocity storage integrator (1/h0). Nodulouvulectomy, which reversed habituation, would be effected by decreasing h0, thereby increasing the VOR time constant. Small values of h0 would cause velocity storage to approach an ideal integrative process, leading the system to become unstable. By controlling the VOR time constant through habituation, the nodulus and uvula can stabilize the slow component of the VOR. VOR gain adaptation was related to a modification of the direct vestibular path gain g1, without altering the coupling to velocity storage g0 or its time constant (1/h0). The mismatched direct- and indirect-pathway gains simulated the overshoots in the dynamic response to a step in velocity, that were observed experimentally. We conclude that independent distributed elements in the VOR modify its dynamic response, under control of separate parts of the vestibulocerebellum.

Increased independence and decreased vertigo after vestibular rehabilitation

OBJECTIVE: We sought to determine the effectiveness in decreasing some symptoms, such as vertigo, and increasing performance of daily life skills after vestibular rehabilitation. STUDY DESIGN AND SETTING: Patients who had chronic vertigo due to peripheral vestibular impairments were seen at a tertiary care center. They were referred for vestibular rehabilitation and were assessed on vertigo intensity and frequency with the use of the Vertigo Symptom Scale, the Vertigo Handicap Questionnaire, the Vestibular Disorders Activities of Daily Living Scale, and the Dizziness Handicap Inventory. They were then randomly assigned to 1 of 3 home program treatment groups. RESULTS: Vertigo decreased and independence in activities of daily living improved significantly. Improvement was not affected by age, gender, or history of vertigo. CONCLUSION: For many patients a simple home program of vestibular habituation head movement exercises is related to reduction in symptoms and increasing independence in activities of daily living.

Vestibular Rehabilitation Reduces Functional Disability

Vertigo caused by vestibular disorder may be successfully treated with a physical therapy program of graded exercises to habituate the patient to the vertiginous stimulus and to increase the range of motion through which the patient can tolerate moving. Performance on daily self-care tasks is an important indicator of the patients tolerance for head movement and the success of treatment. In this study, self-care skill in subjects with labyrinthine and brainstem lesions before and after receiving vestibular rehabilitation was examined. Subjects improved significantly after physical therapy, demonstrating greater independence in their abilities to care for themselves. These data provide further support for the value of vestibular rehabilitation procedures.

Efficacy of Treatments for Posterior Canal Benign Paroxysmal Positional Vertigo

Objective: This study was performed to determine the relative effectiveness of several passive head maneuvers for treating benign paroxysmal positional vertigo.

Effectiveness of treatments for benign paroxysmal positional vertigo of the posterior canal.

Objective: To determine which common, nonpharmacological, nonsurgical treatments are most effective for treatment of benign paroxysmal positional vertigo (BPPV). Study Design: Prospective, randomized, sham-controlled. Patients: Patients (n = 124) with BPPV of the posterior semicircular canal. Setting: Tertiary care center. Interventions: Random assignment to one of five groups: modified canalith repositioning maneuver (CRP), modified liberatory maneuver (LM), sham maneuver, Brandt and Daroffs exercise, and vertigo habituation exercises. Subjects received a standard educational lecture about BPPV and the purpose of the intervention. No vestibular-suppressant medication or special instructions for head positioning were used. Post-tests were given at 1 week after treatment and at approximately 3 months and 6 months later. Main Outcome Measures: Vertigo intensity and frequency. Results: Multilevel analyses showed that vertigo decreased significantly after LM, CRP, and Brandt-Daroff exercise; those three groups did not differ significantly. The habituation group did not differ from sham, Brandt-Daroff, LM, or CRP groups. Changes in scores were maintained throughout the 6-month follow-up period. Conclusion: LM, CRP, and exercises are all effective interventions; patient education plus the sham maneuver, however, had some beneficial effect. These results support two possible mechanisms of BPPV: displaced otoconia and a neural mechanism affecting interpretation of semicircular canal signals.

Application of the Vestibular Disorders Activities of Daily Living Scale

Objective Existing scales of functional performance are either insufficiently sensitive or omit some important daily life tasks. This paper demonstrates that a new scale of self‐perceived disablement in the vestibularly impaired population—the Vestibular Disorders Activities of Daily Living Scale (VADL)—differentiates between disabled and healthy persons and evaluates the associations of this assessment with other measures of vestibular disorders.

Benign Paroxysmal Positional Vertigo and Comorbid Conditions

Purpose: To determine the prevalence of comorbid disease in patients with benign paroxysmal positional vertigo (BPPV) and the relationship of comorbid disease to symptoms of vertigo, disequilibrium, and anxiety. Procedures: Patients who had posterior semicircular canal BPPV and who had been referred for vestibular rehabilitation at a tertiary care center completed a health status questionnaire and the Vertigo Symptom Scale, answered questions about level of vertigo, and were tested on computerized dynamic posturography. Results: Subjects had high rates of diabetes, mild head trauma, and probable sinus disease. Balance was generally impaired, worse in diabetics and subjects with significant vestibular weakness. Subjects who smoked or had had mild head trauma had higher levels of anxiety. Conclusions: Comorbid conditions, particularly diabetes, mild head trauma, and sinus disease, are unusually prevalent in BPPV patients. Message: Patients with comorbid disease are at risk for having increased vertigo, anxiety, and disequilibrium compared to other patients.

Decreased Ataxia and Improved Balance after Vestibular Rehabilitation

OBJECTIVES: To determine the effects of vestibular rehabilitation on gait ataxia and balance. SETTING: Tertiary care center. STUDY DESIGN: Subjects were patients with chronic vertigo due to peripheral vestibular impairments, referred for vestibular rehabilitation. They were assessed on the Timed Up and Go test, ataxia during a path integration test, computerized dynamic posturography, level of vertigo, independence in activities of daily living, and psychological locus of control. They were randomly assigned to three home program treatment groups. RESULTS: Ataxia decreased significantly, and posturography scores and time to perform Timed Up and Go improved significantly, for all subjects. Improvements were significantly related to scores on the ambulation subtest of the Vestibular Disorders Activities of Daily Living Scale, decreases in vertigo, and increases in locus of control. CONCLUSIONS: For many patients, a simple home program of vestibular habituation head movement exercises is related to reduced symptoms of imbalance during stance and gait. (Otolaryngol Head Neck Surg 2004;130:418–25.)

Side-lying as an alternative to the Dix-Hallpike test of the posterior canal.

Objective: Limitations in passive or active range of motion preclude testing some patients suspected of benign paroxysmal positional vertigo of the posterior semicircular canal (BPPV) with the Dix-Hallpike maneuver. The goal of this study was to determine if an alternative test, brisk side-lying with the nose turned 45° away from the tested side, yields the same results as the Dix-Hallpike maneuver. Study Design: Prospective, within-groups. Setting: Diagnostic laboratory at a tertiary care center. Patients: The 61 patients, seen before their physicians had determined their diagnoses, were all referred by their physicians for objective diagnostic tests. All subjects complained of vertigo elicited by up or down rotations of the head or turning over in bed, often provocative positions for BPPV. Methods: Results from the Dix-Hallpike maneuver and sidelying maneuver were compared. Group 1 was tested with the Dix-Hallpike maneuver followed up by side-lying; Group 2 was tested in reverse order. Main Outcome Measures: Slow-phase eye velocity of nystagmus. Results: With the groups collapsed to eliminate possible order effects, no significant differences were found between the tests. Significantly more subjects had no response to testing than minimal or stronger responses. Conclusions: Side-lying is a valid alternative test to the Dix-Hallpike maneuver, which could be useful when range-of-motion limitations or other problems preclude use of the Dix-Hallpike maneuver.

Variable practice with lenses improves visuo-motor plasticity ☆

Novel sensorimotor situations present a unique challenge to an individuals adaptive ability. Using the simple and easily measured paradigm of visual-motor rearrangement created by the use of visual displacement lenses, we sought to determine whether an individuals ability to adapt to visuo-motor discordance could be improved through training. Subjects threw small balls at a stationary target during a 3-week practice regimen involving repeated exposure to one set of lenses in block practice (x 2.0 magnifying lenses), multiple sets of lenses in variable practice (x 2.0 magnifying, x 0.5 minifying and up-down reversing lenses) or sham lenses. At the end of training, adaptation to a novel visuo-motor situation (20-degree right shift lenses) was tested. We found that (1) training with variable practice can increase adaptability to a novel visuo-motor situation, (2) increased adaptability is retained for at least 1 month and is transferable to further novel visuo-motor permutations and (3) variable practice improves performance of a simple motor task even in the undisturbed state. These results have implications for the design of clinical rehabilitation programs and countermeasures to enhance astronaut adaptability, facilitating adaptive transitions between gravitational environments.