Nystagmus Response Research Articles

Caloric Function After Endolymphatic Sac Surgery

To evaluate the effect of endolymphatic sac surgery on vestibular functions using caloric testing on electronystagmography (ENG). Retrospective chart review. The medical records of 21 adult patients with unilateral Ménière's disease who underwent endolymphatic sac surgery between 1998 and 2004 were reviewed. With use of ENG, the absolute value of the caloric response of the operated ear (i.e., the cool + warm irrigation response) and the degree of reduced vestibular response (RVR) rates as indicators of caloric functions were compared before and after surgery. Average follow-up was 17 (6-52) months. The mean change in RVR after surgery was found to be 2.9%. In total, there were six (28.5%) patients who had an RVR increase more than 10%. Of those, there were three (14.2%) patients who demonstrated an RVR increase more than 20%. Only one (4.8%) patient had an RVR increase more than 30%. Total loss of vestibular function was not observed in any of the patients. There were three (14.2%) patients who exhibited a decrease of more than 10% in their RVR. In two (9%) patients, the contralateral ear was shown to have less vestibular function than the operated side on ENG postoperatively (in one case, the absolute caloric nystagmus response remained the same in the operated ear, and in the other case, the response increased on the surgical side). When we evaluated the absolute caloric responses of the operated ear only, we found no statistical difference between the pre- and postoperative values (P = .219). Early results of vertigo control and hearing outcomes were comparable with those in the literature. Endolymphatic sac surgery does not appear to be a vestibular destructive procedure, and it is a therapeutic alternative for patients with Ménière's disease who have failed medical treatment. This is important given the possibility of bilateral disease in some patients.

Relationship between optokinetic nystagmus response and recognition visual acuity

The aim of this study was to investigate the relation between recognition visual acuity (RVA) and optokinetic nystagmus (OKN) response exhibited to different bar sizes with varying contrast. OKN testing was performed in 52 children aged between 3 and 11 years. The children were evaluated in two groups according to their RVA. Group I consisted of 22 eyes with RVA equal to or better than 0.1 logMAR units. Group II consisted of 30 eyes with RVA 0.2-1 logMAR units. Each subject was seated 60 cm from the screen of Ophthimus device, and was exposed to consecutive black and white stripes of seven different spatial frequencies (0.08-1.6 cycle/degree). The narrowest bar that elicited OKN was identified, and then the OKN contrast threshold at this bar size was established. Twenty-one of the 22 eyes in Group I, and 26 of the 30 eyes in Group II exhibited 1.6 cycle/degree spatial frequency (P=0.287). In Group II, 88.9% of the 18 eyes with RVA 0.2-0.5 logMAR responded at this maximum spatial frequency, whereas the corresponding figure for the 12 eyes with RVA 0.6-1 logMAR was 83.3% (P=0.531). Contrast sensitivity (CS) significantly changed with age in Group I (P=0.006). When the eyes that responded at maximum spatial frequency in the two groups were compared, the mean CS in Group II was significantly lower than that in Group I (P=0.005). The results indicate no relation between spatial frequency threshold for OKN response and RVA in children. However, the children with RVA deficits had significantly lower CS.

Anterior semicircular canal benign paroxysmal positional vertigo and positional downbeating nystagmus

Purpose The aim of this study was to describe the clinical features and video-oculographic findings in patients with anterior semicircular canal benign paroxysmal positional vertigo (BPPV). Materials and methods Study Design. This is a prospective case series. Setting. The study was set at an outpatient clinic in a general hospital. Patients. Fourteen individuals with symptoms of BPPV and positional downbeating nystagmus (pDBN) were included in the study. The diagnosis was based on a history of brief episodes of vertigo and the presence of pDBN confirmed in the video-oculographic examination during Dix-Hallpike test (DH) or head-hanging maneuver. Intervention. Patients were treated by particle repositioning maneuver and the effectiveness was evaluated at 7, 30, and 180 days posttreatment. The treatment was repeated up to 4 times if pDBN was persistent. Main Outcome Measures. The main outcome measure is the number of patients without pDBN at 30 and 180 days. Results Video-oculography showed a predominant pDBN in response to DH. Of the 14 patients, 7 had arterial hypertension, and 5 of 14 cases presented abnormalities on the caloric test. Horizontal spontaneous nystagmus was found in 3 of 14 individuals. Positional nystagmus at different positional test was observed in 5 of 14 individuals, suggesting the involvement of several canals. Of the 14 patients, 10 (71%) did not present vertigo, and the positional tests were negative at 30 days. However, 3 cases presented a positive DH with persistence of BPPV episodes and pDBN at 30 days, and another developed a contralateral posterior canal affectation. One of the patients maintained a persistent pDBN at 180 days despite the repeated maneuvers. Conclusions Video-oculography demonstrates that anterior canal BPPV is characterized by a predominant downbeating nystagmus in response to DH. These individuals may show alterations in the vestibular caloric, and they can have multicanal affectation.

Hallazgos oculográficos en 145 pacientes con vértigo posicional paroxístico benigno

ObjectiveBenign paroxysmal positional vertigo (BPPV) can be produced by specific manoeuvres and be studied by electrooculography (EOG). It allows an exhaustive study of features in the positional nystagmus. Although most of the patients with BPPV express typical nystagmus, there exists a group of them that exhibit a non typical form. In this work, a comparative study was carried out between two groups of patients. Material and methodsWe have studied retrospectively 145 patients with BPPV and Dix-Hallpicke (D-H) positive manoeuvre registered by EOG techniques. All patients were explored in a complete EOG tests. Two groups were identified: typical response/atypical response according to the features of nystagmus response. Possible alterations in other EOG tests in each group were investigated and differences between both types of response were analyzed. Results83.5% of cases showed typical nystagmus and atypical in the rest. Other EOG alterations observed in these patients, were (group typical response/atypical response): caloric hypofunction (17.3%/23%), positional nystagmus (12.4%/62.5%), alterations in visual-oculomotor function tests (10%/50%). ConclusionsFrequent alterations were observed in the classic manoeuvres and also in the rest of EOG tests. Probably atypical response in the first is due to or can explain the existence of the second.

The minimal ice water caloric test compared with established vestibular caloric test procedures

Caloric testing of the vestibular labyrinth is usually performed by classical caloric test procedures (CCTP) using water warmed to 30 degrees C and 44 degrees C. Ice water irrigation (4 degrees C) is usually not performed, although it might be useful as a bedside test. To verify the validity of the Minimal Ice Water Caloric Test (MIWCT), comparative video-oculographic investigations were performed in 22 healthy subjects using ice water (0.5 ml, 1.0 ml, 2 ml), CCTP, and cold air (27 degrees C). Frequency, amplitude, slow phase velocity (SPV), the onset, and the duration of nystagmus were documented. After addition of three ice cubes, the temperature of conventional tap water (16 degrees C) fell within 13 min to 4 degrees C. In pessimum position the subjects demonstrated no nystagmus response. Compared to CCTP, MIWCT was associated with a significantly later onset of nystagmus and a significant prolongation of the nystagmus reaction. In contrast to air stimulation (27 degrees C), a significant Spearman's correlation was noted between MIWCT (1 and 2 ml) and established CCTP in respect of essential nystagmus parameters (frequency, amplitude and SPV). Furthermore, MIWCT (0.5 and 1 ml) showed a higher sensitivity and specificity with regard to the detection of canal paresis based on Jongkees' formula compared to stimulation with air 27 degrees C. Thus, MIWCT appears to be a suitable procedure for bedside investigation of vestibular function outside the vestibular laboratory, e.g. in a hospital ward, where bedridden patients with vertigo occasionally require vestibular testing.

Electronystagmography finding in children with peripheral and central vestibular disorders

Dizziness and imbalance are uncommon in children, but often alarming for their families: a detailed interview and otoneurological examination are important for reaching a specific diagnosis and treatment. Children with vestibular disorders are thought to be difficult to examine: vestibular tests (caloric test, roto-acceleratory test, electronystagmography, opto-kinetic nystagmus, cranio-corpography and posturography) are used less frequently for children than in adult patients. The aim of the study was to determine whether the results of electronystagmography testing improve an emergency physician's diagnosis of dizziness in children. Patients were selected on the basis of the following inclusion criteria: aged between 2 and 12 years and affected with vertiginous symptoms. All patients underwent the following instrumental examinations: blood tests, audiological screening, electronystagmography of spontaneous nystagmus or provoked by vestibular and optical stimulation and electroencephalogram. The results underlines an high incidence of central vertigo (83%): 52% of the children presented a prevailing unidirectional nystagmus at labyrinth stimulation and 48% of the children an asymmetry in the response of nystagmus at optical kinetic stimulation with quality alteration of shocks. Electronystagmography can register and evaluate the qualitative and quantitative characteristics of the nystagmic response and allow to distinguish between central or peripheric vertigo: different degrees and various combinations of the abnormal responses shown in optokinetic central test were the most characteristic of electronystagmography in patients with vestibular central vertigo.

Effect of angular acceleration on the localization performance of a remembered target

Both the influence of a remembered "earth-fixed" target (RT) on the vestibulo-ocular reflex and the effect of "unilateral cold caloric vestibular stimulation" on the localization of a RT have previously been proved. As "unilateral caloric stimulation" is not a physiological stimulus, the aim of the present study was to analyze whether even physiological "bilateral vestibular stimulation" (rotation) is able to affect the RT position. The pointing error (PE) towards an RT both without and following angular acceleration was investigated in 24 healthy volunteers. Postrotatory nystagmus response was recorded by electronystagmography. Evaluation parameters were "nystagmus frequency", "total amplitude" and "velocity of the slow phase"; the horizontal and vertical PE. The fixation of an RT led to a significant reduction of about 28% in nystagmus amplitude compared to the test condition in darkness. "After rotatory stimulation" a systematic horizontal PE in the direction of the fast phase of the postrotatory nystagmus (direction of "illusory self-rotation") occurred and the magnitude of this PE increased significantly compared to the test situation "without vestibular stimulation", but showed only a non-uniform negative correlation with two of the nystagmus parameters. It has to be concluded that "after rotatory stimulation", in contrast to "unilateral cold caloric vestibular stimulation", the subjective sense of "illusory self-motion" leads to a horizontal PE in the direction of the nystagmus fast phases.

Nystagmus acceleration: A new parameter of the caloric response

Problem: To develop a new measurement of caloric-induced nystagmus that is more physiologic and less variable than the maximum slow phase speed. Methods: The study design consisted of signal analysis of intra- and intersubject normal caloric responses. The setting was the tertiary medical Otology/Neurotology referral center. Data were collected from 10 normal subjects and from 10 caloric tests on the same subject. The main outcome measure was the maximum slow phase speed and the acceleration of the nystagmus response. The latter was measured by linear fitting of the slow phase speed data of each response. The mean, standard deviation, and the coefficient of variation of maximum speed and acceleration measures were calculated. The intervention was the standard caloric test using water irrigation at 44 and 30 degrees Celsius. Data were digitally processed and analyzed both automatically and under visual control. Results: The variability of nystagmus acceleration is 50% less than that of nystagmus velocity in both inter- and intrasubject results. Conclusion: A new caloric test parameter, namely nystagmus acceleration, is introduced. This parameter measures the rate of slow phase velocity increase and therefore is more physiologic as it reflects endolymph acceleration. The degree of nystagmus acceleration variability is 50% less than that of nystagmus velocity. Most current ENG analysis algorithms can be easily modified to generate this new parameter. Significance: The study provides increased accuracy in measuring the caloric response. Support: None reported.

Vestibular destruction by slow infusion of gentamicin into semicircular canals.

Intratympanic or round window application of gentamicin is often used to alleviate disabling vertigo arising from unilateral Meniere's disease; however, treatment is often accompanied by hearing loss because the drug initially enters the cochlea before diffusing to the vestibular system. In order to enhance vestibular damage and reduce the risk of hearing loss, gentamicin was infused directly into the vestibular system. An osmotic pump containing 50, 100, 200 or 400 microg/ml of gentamicin was infused into the superior semicircular canal of the chinchilla for 7 days. Afterwards, vestibular damage was evaluated by measuring the decline in hair cell density in the utricle, saccule and superior semicircular canals. Auditory damage was assessed with distortion product otoacoustic emissions (DPOAE) and outer hair cell (OHC) and inner hair cell (IHC) loss. Infusion with the two lowest gentamicin concentrations resulted in significant hair cell loss and reduced duration of the nystagmus response, but had little or no effect on OHC or DPOAE. Higher doses of gentamicin damaged cochlear hair cells and reduced the DPOAE. In conclusion, slow infusion of a low dose of gentamicin into the semicircular canals mainly damages the vestibular hair cells and inactivates the nystagmus response without damaging cochlear hair cells or DPOAE.

Effects of visual fixation and convergence on periodic alternating nystagmus due to MS

We studied a young woman with multiple sclerosis, who developed periodic alternating nystagmus (PAN) with a period of oscillation of about three minutes. Neither visual fixation of a stationary target nor large-field optokinetic stimulation substantially influenced the cycle of PAN. During convergence, induced by fixation of a near target, PAN was suppressed by over 70%. Treatment with baclofen abolished her nystagmus, but optokinetic and pursuit responses remained impaired. Convergence during viewing a near target did not increase the response (gain) of her vestibulo-ocular reflex. We postulate that visual drives were able to suppress PAN independently of any effects on vestibular responses and were prevented from exerting effects on velocity storage and vestibular gain adjustment by demyelinating lesions affecting her pontine nuclei and cerebellar circuits.

Intérêt du traitement par exercices rotatoires chez les patients atteints de syndrome de Ménière, méthode utilisée au service d’ORL des cliniques universitaires Saint-Luc

Objective. – To date, the effectiveness of balanced rehabilitation for patients with Meniere’s syndrome has not been unanimously acknowledged by all physicians and physiotherapists. The purpose of this study is to assess the therapeutic efficacy of rotational exercises in the treatment of disequilibrium for patients with unilateral Meniere’s syndrome. Methodology. – Rotational stimuli were used to symmetrize and reduce postrotatory nystagmic response. Three reference sources were used to assess the efficacy of this management: vestibulospinal function tests: pre- and post-treatment results at the Romberg test, the Unterberger–Fukuda stepping test, the Babinski–Weil test, and gait testing with eyes closed; rotational tests: pre- and post-treatment results; and the self-perceived impact of vertigo: assessed by the Dizziness Handicap Inventory (DHI) and a scale based on the guidelines of the Japanese Society of Equilibrium Research (JSER, 1993). The JSER scale provides quantitative vertigo evaluation; the DHI reflects the patient’s perceptual evaluation of handicap. Results. – Patients required 11 sessions (mean value) to attain subjective improvement. Of the 23 patients, only seven required optokinetic stimulation (mean requirement: three sessions). Rotational tests and dynamic tests of the vestibulospinal function improved. The DHI and JSER results show that patients’ post-rehabilitation perceptual evaluation significantly improved. Conclusion. – The objective and subjective measures of disequilibrium in patients with unilateral Meniere’s syndrome were significantly improved.

Temporal analysis of the vestibular and optokinetic nystagmus

Data analysis techniques currently utilized in clinical electronystagmography may produce significantly different results. To obtain more reproducible information we adopted a new mathematical and statistical approach. Our analysis globally evaluate vestibular and optokinetic nystagmic response during its entire duration. This approach is substantially different from the traditional one adopted in the clinical setting which is often limited to the slow phase eyes velocity (SPEV) and related measurements based on local estimation. Moreover, our preliminary results show that SPEV and fast phase eyes velocity (FPEV) are related to each other and can be considered as a measurement of the efficiency of the system stimulus/response.

Linearity, symmetry and additivity of the human eye-movement response to maintained unilateral and bilateral surface galvanic (DC) vestibular stimulation.

Recent studies have shown that, although responses to long-duration, constant-current surface galvanic vestibular stimulation (GVS) show substantial interindividual variability, individual subjects show a reliable, repeatable, idiosyncratic oculomotor response pattern to GVS. It follows that GVS may be a more reliable stimulus than may have been anticipated from the literature. The aim of the present study was to examine the metrics of 3D eye-movement responses to maintained (120 s), unilateral and bilateral surface GVS. Eye movements were measured using computerised video-oculography. Two experiments were conducted: Experiment 1 examined whether the normal response is linear over increasing levels of current; and Experiment 2 examined (1) whether the normal response to surface GVS is symmetrical when comparing stimulated sides, (2) whether the normal response to surface GVS is symmetrical when the polarity of the stimulating current was reversed, and (3) whether there is additivity in the normal response to combinations of unilateral/bilateral surface GVS. Five subjects participated in Experiment 1 and eight subjects participated in Experiment 2. In both experiments, the onset of stimulation produced characteristic eye-movement responses: changes in torsional position with the upper pole of both eyes rolling towards the anode and away from the cathode; together with horizontal and torsional nystagmus with slow phases towards the anode and away from the cathode; and negligible vertical nystagmus. These responses reversed direction at stimulus offset. In the fixation condition of Experiment 1, the magnitude of ocular torsional position (OTP) and torsional nystagmus responses showed a linear relationship over conditions of increasing current strength, as did OTP, torsional and horizontal nystagmus responses in darkness. The results of Experiment 2 showed that responses to unilateral stimulation are symmetrical between stimulated sides, symmetrical between stimulating polarities, and additive (with respect to responses to bilateral stimulation). The principles derived from these findings, as well as those of recent studies, provide a foundation for future work investigating eye-movement responses to surface GVS in patients with known types of vestibular dysfunction.

Dual-directional optokinetic nystagmus elicited by the intermittent display of gratings in primary open-angle glaucoma and normal eyes

Purpose. To compare optokinetic nystagmus (OKN) responses to the intermittent display of stimuli between normal subjects and patients with primary open-angle glaucoma (POAG). Methods. Optokinetic nystagmus (OKN) was recorded in 9 glaucomatous patients and 7 normal subjects. The computergenerated stimuli displayed sinusoidal luminance gratings (16 cd/m² mean luminance, 0.5 cyc/deg) with a p/2 phase shift between successive stimulus gratings. These stimulus gratings were separated by an interstimulus interval (ISI), during which a homogeneous luminance field of 16 cd/m 2 was presented. The ISI duration and the luminance contrast were set at different values. Results. For normal subjects, dual-directional alternating OKN could be evoked in the ISI range from 33 to 100 ms. The dual-directional alternating OKN was defined as that OKN slow phase alternatively tracked in the direction of p/2 shift (forward OKN) and against the p/2 shift (reverse OKN). By contrast, for most glaucomatous eyes, nearly no reverse OKN could be evoked at any ISI value. Conclusions. The lack of reverse OKN in POAG patients in the present experiments is a meaningful finding. The occurrence of reverse OKN during a certain range of ISI duration could be related to the biphasic characteristics of the temporal impulse response in normal subjects, whereas, the lack of reverse OKN might suggest the plausible damage of magnocellular cells in POAG.

Effects of cosmonaut vestibular training on vestibular function prior to spaceflight.

The purpose of this study was to quantify the effects of repetitive Coriolis and cross-coupled stimulations, similar to the vestibular training the cosmonauts are exposed to prior to their spaceflight. on vestibular function in control subjects on Earth. Ten volunteers were passively rotated in yaw on a rotating chair while executing standardized pitch head-and-trunk movements. The chair stopped to change direction after 12 head-and-trunk movements were made. The runs were grouped in sessions of ten,which were repeated daily for 10 days. The severity of motion sickness was assessed by subjective judgment and measurements of skin pallor and salivary total protein concentration, and nystagmus was recorded. The severity of motion sickness and nystagmus decreased during cosmonaut vestibular training (CVT). One month after the end of CVT, nystagmus responses were still about 20-30% lower than control values. These results indicate that CVT induces a habituation of vestibular responses. One important implication of this experiment concerns space studies on cosmonauts who are exposed to such vestibular training prior to their spaceflight.

반고리관결석 정복술 후 최종 두위에서 기록된 안진 반응에 의한 운동치료효과 예측

Background and Objective: Benign paroxysmal positional vertigo (BPPV) is a common etiology of vertigo. Variable exercise maneuvers and treatment results of BPPV have been reported. We aim to evaluate applicability of nystagmus response in the final head position after the modified Epley maneuver as the predictor of the treatment efficacy. Materials and Methods: The nystagmus response was recorded in the final head position after the modified Epley maneuver and compared with the subsequent response obtained by Dix-Hallpike test after 1 week. The study included 12 patients with a diagnosis of BPPV based on the history and Dix-Hallpike test. Results: After the first Epley maneuver, 9 patients showed negative nystagmus response and 3 patients positive while sitting in the final head position. As compared with the results of Dix-Hallpike test after 1 week, only 2 of the 9 patients, who had no nystagmus in the final head position after the first Epley maneuver, and 2 of the 3 patients, who had nystagmus, showed positive nystagmus response. The overall results of the Epley maneuver showed that the patients who had no nystagmus in the final head position tended to be better than those who had nystagmus. Conclusion: The nystagmus response in the final head position after the modified Epley maneuver could be the predictor of the treatment efficacy. (J Clinical Otolaryngol 2001;12:41-46)

Observation of Vestibular Asymmetry in a Majority of Patients over 50 Years With Fall-Related Wrist Fractures

Previously, we have observed vestibular asymmetry in about one-third of healthy senior citizens and in about two-thirds of subjects with previous hip fractures and no other significant ailments. Wrist fractures are considered a harbinger of hip fractures. If vestibular asymmetry is correlated with falls and fractures among the elderly then it should also be reflected among subjects with wrist fractures. Sixty-six consecutive patients (mean age 67.8 years) who had sustained a fall-related wrist fracture during a 10-month period were included in the study. The frequency of head shake nystagmus among the patients was compared to that found among 49 healthy senior citizens (mean age 74.9 years). Nystagmus after head shaking, indicating asymmetric vestibular function, was found in 50 participants (76%) (p <0.001). Thirty-eight of these were graded with distinct or prominent nystagmus responses. Sixty percent of the subjects with horizontal nystagmus had a wrist fracture coinciding with the slow phase of nystagmus. Twenty-three subjects reported 30 previous fall-related fractures during the previous 10 years. Subjects with nystagmus after head shaking sustained 26 of these fractures. The frequency of signs of vestibular asymmetry was significantly higher (p <0.001) among the subjects than among healthy senior citizens. These findings suggest that an asymmetric vestibular function could be an epidemiologically important contributory factor to falls and wrist fractures among the elderly population.

Changes in horizontal oculomotor behaviour coincide with a shift in visual motion perception.

During full-field rotation of the visual field, subjects commonly experience an initial perception of object-motion which 'switches' to a perception of self-motion. We studied the characteristics of the horizontal optokinetic nystagmus responses evoked by a moving visual stimulus in these two perceptual states over a range of stimulus velocities. During self-motion perception mean eye position was found to shift more in the direction of the newly appearing stimulus elements with a slight reduction in slow phase gain in comparison to the nystagmus evoked during object-motion perception. The results may reflect a modified strategy of spatial attention with increased emphasis on anticipatory eye movements during visually induced self-motion perception.

Short-term vestibular responses to repeated rotations

To investigate the short-term vestibular habituation, we performed the 4 successive velocity step tests on 28 volunteers, the peak velocity of which was 100 deg/sec with acceleration and deceleration of 100 deg/sec2. As the repeated rotations might alter the vestibulo-ocular reflex (VOR), sinusoidal rotations at a frequency of 0.16 Hz were also given before and after the 4 successive velocity step tests to investigate the changes of gain and phase of VOR. The 28 volunteers were divided into two groups, group I and II, according to their responses to repeated rotations. In group I (25 subjects, mean age 23.7 year), the mean values of the slow cumulative eye position (SCEP), the time integral of eye velocity during nystagmus, was reduced after each trial of the 4-successive velocity step tests as follows; 403.4 +/- 29 degree (1st trial), 346.2 +/- 37 degree (2nd trial), 278.3 +/- 33 degree (3rd trial) and 256.6 +/- 36 degree (4th trial). The time constant of the nystagmus was also reduced as follows; 12.9 +/- 0.78 second (1st trial), 12 +/- 0.63 second (2nd trial), 9.7 +/- 0.78 second (3rd trial) and 9.9 +/- 0.54 second (4th trial). In group II (3 subjects, mean age 28.3 year), the mean values of SCEP gradually increased; 774.3 +/- 135 degree (1st trial), 1127 +/- 178 degree (2nd trial), 1096.3 +/- 123.4 degree (3rd trial) and 1225.7 +/- 199.7 degree (4th trial). The time constant of the nystagmus increased; 15.7 +/- 2.7 second (1st trial), 22 +/- 4.5 second (2nd trial), 22.3 +/- 3.3 second (3rd trial) and 23.3 +/- 5.7 second (4th trial). The gain of vestibulo-ocular reflex (VOR) induced by sinusoidal rotations at a frequency 0.16 Hz increased in both groups: 0.59 +/- 0.03 to 0.78 +/- 0.06 (group I) and 0.65 +/- 0.07 to 1.15 +/- 0.06 (group II). Phase changes were also observed. In group I, the phase was shifted from 0.6 +/- 0.6 degree to -0.40 +/- 0.6 degree. In group II, the phase was shifted from 3.67 +/- 1.86 degree to -0.33 +/- 0.33 degree. The repeated rotation did not induce a common nystagmic response in all subjects. Thus, person to person variations should be considered in short term vestibular habituation.

Measurement of vestibular ocular reflex (VOR) time constants with a caloric step stimulus*

A protocol is described for measuring responses to a broad-band (1-2 Hz) caloric step stimulus from which the vestibular ocular reflex (VOR) and adaptation time constants can be estimated. This novel stimulation is the caloric equivalent to a rotatory step of head acceleration. In this protocol, the ear is irrigated continuously for 5 min with water at a constant temperature. During the initial 2-min period of irrigation the subject is seated and leaning forward in a nonstimulable position (horizontal canals in a horizontal plane). This irrigation phase establishes a steady-state thermal gradient across the horizontal canal, effectively eliminating thermal dynamic properties of the caloric transmission as a confounding factor. At the end of this phase, the subject is rapidly reclined to a stimulable position (horizontal canals in vertical plane) that elicits the VOR nystagmus response to an on-step of force on the cupula. Consistent with adaptation processes, the VOR response first increases and then declines gradually over the 2-min period that the step of force is maintained. Four minutes after the onset of irrigation, the subject is rapidly returned to the nonstimulable position (off-step), which is then maintained for a final 1 min. The response after the off-step, which releases the force on the cupula, reveals reversed after-nystagmus due to adaptation. Five subjects provided caloric step responses for 26 caloric temperature conditions spanning the range from 28.4 to 43.0 degrees C. The resulting responses were fitted with an adaptation model similar to models applied to rotatory acceleration step responses. Estimates of the model parameters for robust caloric stimulation, including time constants for the VOR (18.3 sec) and for vestibular adaptation (153.2 sec), are considered in relation to corresponding values reported in the literature for rotatory and caloric vestibular stimulation. The results suggest that caloric step stimulation can be used successfully to probe VOR dynamics.