Subjective visual horizontal in the upright posture and asymmetry in roll-tilt perception: independent measures of vestibular function

The subjective visual horizontal (SVH) was measured in the upright position and at 10, 20, and 30 degrees of head and body tilt to the right and left. Normal subjects (n=25) were tested on two separate occasions with an interval of 1-14 days. Test variables considered were the SVH in the upright position, the perception of tilt to the right and left, calculated on the basis of the SVH in the upright and tilted positions, and the asymmetry in tilt perception. There was no correlation between the perception of tilt to the right and to the left r=0.10). Neither was there any correlation between the SVH in the upright position, representing a resting asymmetry, and the asymmetry in tilt perception, i.e. the response asymmetry (r=0.17). However, for each variable, there was a high correspondence between data obtained at test and retest (r ranged from 0.68 to 0.89, p<0.001), suggesting that the independence between variables is not due to noise. Findings are discussed taking into consideration the possible roles of otoliths and semicircular canals in the formation of the SVH. In an attempt to explain the independence between the two measures of asymmetry it is hypothesized that while the otoliths must be essential for the perception of static lateral tilt, the SVH in the upright position to a considerable degree reflects semicircular canal function.     

Tilted perception of the subjective 'upright' in unilateral loss of vestibular function

OBJECTIVE: To demonstrate that unilateral vestibular loss (UVL) may cause an erroneous perception of the subjective (bodily) vertical. STUDY DESIGN: Comparison of patients with UVL with age-matched controls. SETTING: The study was performed in a clinical neurophysiology laboratory. PATIENTS: Seven patients with unilateral, mixed acute, and chronic losses of vestibular function (six surgical), two patients with bilaterally absent vestibular function, one patient with a plugged posterior canal, and twenty-two healthy individuals. INTERVENTIONS: Subjects were seated, whole body restrained, in a flight simulator. The simulator executed intermittent stepwise tilts, in roll, up to 28 degrees that subjects had to correct to maintain an "earth upright" attitude using the joystick. Subjects performed in both "calm" conditions and also when the simulator was oscillating in roll at 1 Hz, 4 degrees peak displacement to simulate "turbulence." The purpose of the turbulence was to destabilize (or mask) somatosensory cues to uprightness. MAIN OUTCOME MEASURES: Accuracy of corrections of subjective attitude to earth upright after tilts. RESULTS: All subjects accepted the initial attitude of the simulator as "upright." In response to imposed tilts, normal subjects (n = 22) corrected their attitude to a mean 0.4 degrees SD 1.5 degrees (tilted slightly rightward) in calm and 0.7 degrees SD 1.3 degrees in turbulence. On average, corrections were normometric. All patients with UVL responded to imposed tilts by corrections that left them 'flying' tilted slightly to the side of their lesion, mean 3.2 degrees SD 2.5 degrees when calm, 6.4 degrees SD 2.7 degrees in turbulence (p<0.01). Their corrections were hypometric in response to imposed tilts to the lesioned side (i.e., undershooting true upright) and hypermetric in response to tilts to the intact side. CONCLUSIONS: Unilateral vestibular loss causes a "vestibular perception" of an erroneous tilt of the body that is probably caused by an imbalance of otolith signals and apparently never fully compensates. The tilt is enhanced when rapid perturbations of posture make somatosensory cues difficult to interpret. An erroneous perception of upright may contribute to vestibular ataxia, which is provoked when motion context involves rapid change.     

Otolith function assessed with the subjective postural horizontal and standardised stance and gait tasks

If otolith function is essential to maintain upright standing while moving along slanted or uneven surfaces, subjects with an otolith deficit should have difficulty judging whether the inclination of the surface on which they are standing is tilted or not. We tested this judgement and compared it with the ability to control trunk sway during standardised stance and gait tests. Thirteen patients with unilateral vestibular nerve neurectomy at least 6 months prior to testing and 39 age-matched controls were asked to move a dynamic posturography platform on which they were standing back to their subjective 'horizontal' position after the platform had been slowly tilted at 0.4 degrees/s to 5 degrees in 8 different directions. Normal subjects left the platform deviated in pitch (forwards-backwards) at about 0.7 degrees on describing the platform as levelled off for all directions of tilt. Patients showed larger deviations of about 1.3 degrees in pitch with significant differences for forward right tilt (1.58+/-0.73 degrees compared to 0.73+/-0.11 degrees for normals; mean and SEM) and for forward left. Roll (lateral) deviations were about 0.4 degrees for normals and 0.5 degrees larger for the patients (for example, for backward left, 1.13+/-0.24 degrees compared to 0.4+/-0.07 degrees in normals). Except for a tendency towards greater deviations to the lesion side of patients with eyes closed, no differences were noted between tests under eyes open and closed conditions. However, for backward and roll tilts patients needed to steady themselves first by grasping a handrail when tested with eyes closed. Stance tests on foam showed increases in roll and pitch trunk sway with respect to controls. Patients had significantly larger trunk roll sway deviations during 1-legged stance tests and during gait trials. For stance trials, the patients lost their balance control prior to the end of the standard 20-second recording time. We conclude that a unilateral loss of otolith inputs due to nerve resection permanently impairs the ability to judge whether the support surface is horizontal, and leads to excessive trunk sway when standing on a compliant surface as well as excessive trunk roll sway during gait.     

Outcomes of uvulopalatopharyngoplasty with harmonic scalpel after failure of continuous positive airway pressure in sleep apnea syndrome

The subjective visual horizontal (SVH) was measured by means of a small rotatable luminous line in darkness in the upright body position and at 10, 20 and 30° of body tilt to the right and left prior to, and during a follow-up period after, stapedotomy in 12 patients with otosclerosis. In the acute stage after surgery, SVH in the upright body position was significantly tilted away from the operated side. In addition, the perception of roll tilt towards the operated side (K_op) was significantly increased after stapedotomy, while the perception of roll tilt towards the healthy side (K_he) showed a slight but not significant reduction. After exclusion of two outliers, a statistically significant correlation was found between changes in K_op and in K_he. The slope of the regression line was 1.8 : 1, probably corresponding to a preference of the utricle for ipsilateral as opposed to contralateral head tilt. In four patients there was a weak (&;lt;1°/s) spontaneous nystagmus, not systematically related to the side of surgery, while in most cases there were no nystagmus or subjective vertigo symptoms. These specific changes in the subjective horizontal show that the otolithic effects on perception can be dissociated from canal effects. Further, the results are opposite to those for patients with unilateral loss of vestibular function. The tilt of SVH after stapedotomy indicates an increase in resting activity of utricular afferents. In addition, based on recent theories on otolith function, we suggest that an increased activity in saccular afferents is of major importance for the changes in roll-tilt perception because of its interaction with the utricle on the central nervous level.     

Function of semicircular canals, utricles and saccules in deaf children

OBJECTIVE: To study vestibular function in deaf children. MATERIAL AND METHODS: In 36 deaf children the function of the semicircular canals, saccules and utricles was measured by means of caloric testing, recordings of vestibular-evoked myogenic potentials (VEMPs) and measurements of the subjective visual horizontal (SVH) at different body tilts, respectively. RESULTS: In total, 30% of subjects had caloric hypo- or areflexia and 24% had a caloric asymmetry. VEMPs were weak or absent bilaterally in 22% of cases, and asymmetric in 19%. Regarding the utricle, 17% of subjects had a pathologically reduced perception of roll tilt to both sides and 25% had an asymmetry. In total, 30% of subjects were pathologic in all 3 tests and 30% were completely normal. Semicircular canal function correlated best with the function of the saccule. If hearing was better than 90 dB (pure-tone average of 0.5, 1.0 and 2.0 kHz) vestibular function was often normal. For hearing levels of 100-120 dB, otolith function declined significantly. CONCLUSIONS: Vestibular function tends to be preserved up to a point where hearing is nearly extinct. Hearing level correlates more closely with otolith function, especially that of the utricle, than with semicircular canal function.     

Chronic unilateral loss of otolith function revealed by the subjective visual vertical during off center yaw rotation

Assessing the subjective visual vertical, SVV, in a static upright position is an easy clinical test in which a deviation of some 10 degrees from true vertical indicates an acute loss of unilateral (otolithic) vestibular function on the side to which the SVV is tilted. Because this deviation of the SVV is compensated during the following months, patients with chronic unilateral vestibular loss do no longer differ from normal subjects. This study presents an experimental set-up that allows for clear detection of compensated chronic loss of unilateral otolithic function by testing the SVV. 21 normals and 17 unilaterally vestibular deafferentiated (UVD) patients (vestibular neurectomies) were first rotated on a human centrifuge about an earth vertical yaw axis through the midsagittal plane of the head (240 degrees/s). This induced tilts of the gravito-inertial force (GIF) vectors, which differed at the two inner ears by 8 degrees. During constant velocity rotation, the subjects were moved in pseudo-randomized steps laterally up to 16 cm apart from the rotation axis, inducing roll tilts of the GIF vectors up to 16 degrees. Normal subjects set their SVV to pre-centrifugation values at positions with the midsagittal plane of their head close to the rotation axis, while chronic UVD patients indicated pre-centrifugation values during positions with the rotation axis 5.9 +/- 2.5 cm paramedian on the side of the intact ear. Tilts of the GIF vectors shifted the SVV with a gain of 0.70 in normals and only 0.32 in UVD patients. Roll gains for laterally directed GIF vectors relative to the intact inner ear did not differ from medially directed roll gains in the UVD patients. The roll gains observed in this experimental set-up were lower than those observed with static body tilts or during eccentric rotation with a larger radius, which might be at least partially due to conflicting stimulation between otolithic and extra-vestibular cues.     

Subjective visual vertical and subjective visual horizontal measures in patients with chronic dizziness

OBJECTIVES: We aimed to find the frequency of otolith organ pathologies in the clinical picture of common dizziness etiologies in the chronic stage. METHOD: Subjective visual vertical and subjective visual horizontal measures were assessed in patients who had persistent or recurrent dizziness at least 2 months after the acute period. Every patient was tested in three head positions: neutral, right, or left deviation in the roll plane. Test results were compared with those of the control group. RESULTS: Seventy-three patients and 18 controls were examined. Fifty-eight of the patients had peripheral vestibular disease; 15 of them had central vestibular disease. Left subjective visual horizontal (SVH) and right SVH measures of the peripheral group were significantly different from those of the control group (p < .01). There was no difference in any test between the peripheral and central groups. When we put a cut off point for abnormality (0, 1) according to mean +/- 2 SD of the control group, the peripheral and central groups had very high significant differences from the control group. Approximately 25 to 50% of our patients had pathologic subjective visual vertical or SVH measures according to test type. CONCLUSION: These results showed that the otolith system must be evaluated in the chronic period of dizziness, especially in patients who frequently visit their physician, and modifications in treatment programs must be conducted.     

Function of semicircular canals,utricles and saccules in deaf children

Objective To study vestibular function in deaf children.

Material and Methods In 36 deaf children the function of the semicircular canals, saccules and utricles was measured by means of caloric testing, recordings of vestibular-evoked myogenic potentials (VEMPs) and measurements of the subjective visual horizontal (SVH) at different body tilts, respectively.

Results In total, 30% of subjects had caloric hypo- or areflexia and 24% had a caloric asymmetry. VEMPs were weak or absent bilaterally in 22% of cases, and asymmetric in 19%. Regarding the utricle, 17% of subjects had a pathologically reduced perception of roll tilt to both sides and 25% had an asymmetry. In total, 30% of subjects were pathologic in all 3 tests and 30% were completely normal. Semicircular canal function correlated best with the function of the saccule. If hearing was better than 90 dB (pure-tone average of 0.5, 1.0 and 2.0 kHz) vestibular function was often normal. For hearing levels of 100–120 dB, otolith function declined significantly.

Conclusions Vestibular function tends to be preserved up to a point where hearing is nearly extinct. Hearing level correlates more closely with otolith function, especially that of the utricle, than with semicircular canal function.     

Subjective Visual Horizontal During Follow-up After Unilateral Vestibular Deafferentation with Gentamicin

The subjective visual horizontal (SVH) was measured by means of a small, rotatable, luminous line in darkness in the upright head and body position and at 10, 20 and 30° of tilt to the right and left before, and repeatedly during a follow-up period of 1 year after intratympanic gentamicin instillations in 12 patients with recurrent vertigo attacks. This treatment caused a loss of the bithermal caloric responses on the diseased side. Shortly after treatment there was a significant tilt of SVH towards the treated side (group mean=10.6°). Repeated testing made it possible to characterize mathematically the changes with time for SVH. For the group of patients as a whole this otolithic component of vestibular compensation was best described by a power function, SVH=8.65t^-0.16 degrees, where t is time in days after maximum tilt of SVH. After 1 year, SVH was still significantly tilted towards the treated side (group mean=3.16°). Gentamicin treatment also caused a significant reduction in the perception of head and body tilt towards the deafferented side, while the perception of tilt towards the healthy side did not show any significant changes. During follow-up there was a gradual improvement in the perception of tilt towards the treated side. However, a significant asymmetry in roll-tilt perception was still present 1 year after deafferentation. There was no correlation between SVH in the upright position and roll-tilt perception, suggesting that these parameters are to some extent dependent on different afferent input from the vestibular organ. They were also found to be complementary for the detection of vestibular disturbance.     

Vibration-induced shift of the subjective visual horizontal: a sign of unilateral vestibular deficit.

BACKGROUND: Vibration to the head or neck excites vestibular and neck muscle spindle afferents. Can such vibrations improve the sensitivity of the subjective visual horizontal (SVH) test to chronic unilateral deficit of the vestibular system? DESIGN: Controlled experimental study. SETTING: Tertiary referral center. PATIENTS AND CONTROLS: Thirteen healthy subjects and 23 patients with chronic unilateral vestibular deficits after vestibular neurectomy or neurolabyrinthitis. Results of head-impulse test showed unilateral loss of function of all 3 semicircular canals in 14 patients and loss of anterior and lateral semicircular canals in 9 patients. INTERVENTION: Unilateral vibration (92 Hz; 0.6-mm amplitude) applied to sternocleidomastoid muscle (SCM) or mastoid bone. MAIN OUTCOME MEASURE: Results of SVH test (in degrees). RESULTS: Without vibration, 13 of 23 patients and all healthy subjects had SVH of less than 3 degrees (sensitivity, 43%; specificity, 100%). During vibration to the ipsilesional SCM, SVH increased to greater than 3 degrees in 21 of 23 patients but in only 1 of 13 healthy subjects (sensitivity, 91%; specificity, 92%). The patient group had significantly greater SVH shifts to the ipsilesional side than did healthy subjects in response to SCM and mastoid bone vibration on either side. The SVH shift during vibration to the ipsilesional SCM was significantly greater than that during vibration to the contralesional muscle (P<.001) or to the mastoid bone on either side (P<.05). The vibration-induced SVH shift was significantly greater in those patients with loss of 3 semicircular canals than in those with loss of 2 (P<.01). CONCLUSIONS: The sensitivity of the SVH test to chronic unilateral vestibular deficits can be improved by applying vibration to the SCM. The magnitude of vibratory SVH shift is related to the extent of unilateral deficit of the otolithic organs, vertical canals, or both.     

Human ocular counter-rolling and roll tilt perception during off-vertical axis rotation after spaceflight

Ocular counter-rolling (OCR) induced by whole body tilt in roll has been explored after spaceflight as an indicator of the adaptation of the otolith function to microgravity. It has been claimed that the overall pattern of OCR responses during static body tilt after spaceflight is indicative of a decreased role of the otolith function, but the results of these studies have not been consistent, mostly due to large variations in the OCR within and across individuals. By contrast with static head tilt, off-vertical axis rotation (OVAR) presents the advantage of generating a sinusoidal modulation of OCR, allowing averaged measurements over several cycles, thus improving measurement accuracy. Accordingly, OCR and the sense of roll tilt were evaluated in seven astronauts before and after spaceflight during OVAR at 45 degrees/s in darkness at two angles of tilt (10 degrees and 20 degrees). There was no significant difference in OCR during OVAR immediately after landing compared to preflight. However, the amplitude of the perceived roll tilt during OVAR was significantly larger immediately postflight, and then returned to control values in the following days. Since the OCR response is predominantly attributed to the shearing force exerted on the utricular macula, the absence of change in OCR postflight suggests that the peripheral otolith organs function normally after short-term spaceflight. However, the increased sense of roll tilt indicates an adaptation in the central processing of gravitational input, presumably related to a re-weighting of the internal representation of gravitational vertical as a result of adaptation to microgravity.     

Ocular Tilt Reaction

Visual disturbances after acoustic neuroma surgery are very rare complications. We present a patient with diplopia after a middle fossa approach for resection of an acoustic neuroma. The patient presented ocular torsion and tilts of subjective visual vertical which is typical for the ocular tilt reaction. It is generally agreed that this reaction is secondary to a dysfunction of the tonic bilateral vestibular inputs that stabilize the eyes and head in a normal upright position in the roll plane. It has also been described after denervation of the utricle and injury of the brain stem.     

Ocular counter-rolling after prolonged alteration in the direction of gravity

To investigate the dynamics of otolith and oculomotor function, we subjected volunteers to a lateral body tilt for a period of two hours and analyzed ocular counter-rolling. Six healthy adult volunteers were roll-tilted along the naso-occipital axis at a speed of 0 degree/sec. from the 0 degree earth vertical position to the 90 degrees lateral tilt position. After two hours, the volunteers were returned to the 0 degree earth vertical position. Ocular counter-rolling was recorded using an infrared CCD camera. The video recordings were made in the dark while the volunteers had their eyes open. Recordings were obtained (1) as the volunteers was moved from the 0 degree earth vertical position to the 90 degrees lateral position, (2) 5 minutes after reaching the lateral position, (3) during the roll-back from the 90 degrees lateral position to the 0 degree upright position, two hours after the lateral tilt loading, and (4) 5 minutes after reaching the final upright position. The occurrence of ocular counter-rolling between the 0 degree upright position and the 30 degrees tilt position was confirmed during both roll movements from 0 degree to 90 degrees and from 90 degrees to 0 degree. The counter-rolling was most noticeable between the 0 degree and the 30 degrees positions. No differences in ocular counter-rolling during the roll-tilt and the roll-back situations were observed. These results indicate that the two-hour lateral tilt position did not produce any functional changes in the hair cells and/or the otolith-oculomotor system. All of the subjects exhibited ocular counter-counter-rolling during the initial stage of their roll-back to a normal upright position. This phenomenon might result from the additional bending of the hairs by inertia during the initiation of the backward roll motion.     

主观视觉垂直线和水平线检查软件的研发与应用

目的 自主研发主观视觉垂直线(SVV)、主观视觉水平线(SVH)检查软件。主要用于判断双侧耳石器功能的对称性和优势偏向侧,评估耳石器功能的静态代偿状态,指导前庭康复。 方法 研发SVV/SVH检查软件,结合消除视觉参考的桶状设备及暗环境检查室,收集80例健康志愿者检查数据,确定正常参考值范围。研究方法为横断面研究。 结果 80例健康志愿者,SVV检查平均值偏离度范围为:-3.00°~3.00°,其平均值的均值(±标准差)为:-0.29(±1.42)°;SVH检查平均值偏离度范围为:-4.00°~3.50°,其平均值的均值(±标准差)为:-0.38(±1.84)°。 结论 自主研发的SVV/SVH软件95%正常值参考范围分别为:-3.13°~2.55°,-4.06°~3.30°;此检查软件和设备可以用于前庭耳石器机能状态的评估,与其他前庭检查设备如前庭诱发肌源性检查、双温试验、甩头试验等相结合,全面评估患者的前庭功能和前庭代偿状态,指导前庭康复。     

The human vertical optokinetic afternystagmus and the effect of gravity during pitch tilt

BACKGROUND: Findings concerning human vertical optokinetic afternystagmus (VOKAN) are often not consistent and in some cases even contradictory. METHOD: VOKAN was studied in nine normal subjects using corneo-retinal potential technique and an infrared video camera detection apparatus (ISCAN). VOKAN responses were measured at three angles of pitch head (and body) tilt: 0 degrees (upright), 90 degrees (supine) and 135 degrees (45 degrees below horizontal). The optokinetic stimulus was stripe movement with a velocity of 60 degrees/s. RESULTS: Upward slow phase velocity (SP-Up) VOKAN was evoked in 6/9 subjects (67%) at 0 degrees tilt and at 90 degrees tilt, and in 4/9 (44%) at 135 degrees tilt. Downward slow phase velocity (SP-Down) VOKAN was elicited in 3/9 subjects (33%) at 0 degrees tilt; in 2/9 subjects (22%) at 90 degrees tilt and in 0/9 subjects (0%) at 135 degrees tilt. CONCLUSION: The pitch tilt, as a gravity effect, influences significantly on the appearance and the duration of VOKAN.     

Ocular counterrolling in response to static and dynamic tilting: implications for human otolith function

Nineteen subjects underwent rotation about the naso-occipital axis to examine ocular counterrolling (OCR) responses in both dynamic and static conditions. Dynamic rotation consisted of tilt to 90 degrees right and left at constant velocity of 3 degrees /s, with acceleration at 0.2 degrees /s(2). Static rotation (stepwise tilting) consisted of one minute at steps 30 degrees, 60 degrees, 90 degrees, 60 degrees, 30 degrees to each side. OCR amplitudes at each of the above head tilts in dynamic vs. static tilt showed highly significant differences, with dynamic tilt exceeding that of static. Although OCR disconjugacy was greater in static than dynamic in most subjects, that difference was not statistically significant. Possible explanations for the disparity in the responses to dynamic and static tilt in humans lie in vestibular experiments leading to the conclusion that the otolith membrane in cats moves not as a unit, but rather in patches (28). This is supported by work showing the otolith membrane in bullfrogs does not move en bloc (1). In addition, hair cell responses are found to be amplified during motion as compared to the responses during static positioning (20). Functionally, it is suggested that the otolith system may perform better in moving than in static conditions.     

Abnormal Tilt Perception During Centrifugation in Patients with Vestibular Migraine

Vestibular migraine (VM), defined as vestibular symptoms caused by migraine mechanisms, is very common but poorly understood. Because dizziness is often provoked in VM patients when the semicircular canals and otolith organs are stimulated concurrently (e.g., tilting the head relative to gravity), we measured tilt perception and eye movements in patients with VM and in migraine and normal control subjects during fixed-radius centrifugation, a paradigm that simultaneously modulates afferent signals from the semicircular canals and otoliths organs. Twenty-four patients (8 in each category) were tested with a motion paradigm that generated an inter-aural centrifugal force of 0.36 G, resulting in a 20° tilt of the gravito-inertial force in the roll plane. We found that percepts of roll tilt developed slower in VM patients than in the two control groups, but that eye movement responses, including the shift in the eye’s rotational axis, were equivalent in all three groups. These results demonstrate a change in vestibular perception in VM that is unaccompanied by changes in vestibular-mediated eye movements and suggest that either the brain’s integration of canal and otolith signals or the dynamics of otolith responses are aberrant in patients with VM.     

3D eye movement analysis during sinusoidal off vertical axis rotation in human subjects

OBJECTIVE: It has been reported that the eye movements induced by off vertical axis rotation (OVAR) are composed of two components: a bias component (BIC) and a modulation component (MOC). It was suggested that the MOC compensates for changes in head orientation with respect to gravity and that the BIC is related to the velocity storage mechanism. However, the characteristics of the otolith organs remain obscure. In order to investigate these characteristics, we performed a 3D analysis of eye movements induced by human sinusoidal s-OVAR, and compared the result with that of eye movements induced by constant c-OVAR. MATERIAL AND METHODS: Fifteen healthy human subjects participated in this study. OVAR stimulation was produced by rotation following tilt. After a 30 degrees tilt, the subject was rotated sinusoidally at 0.05 and 0.02 Hz at 50 degrees/s peak velocity. Eye movements were recorded using an infrared charge-coupled device camera, and the updated version of our standard computerized image recognition method was used to analyze them three-dimensionally. RESULTS: The eye movements induced by s-OVAR consisted of two kinds of sine waves: low- and high-frequency components. The low-frequency component was related to the change in angular acceleration, and the eye movements were found to derive from the semicircular canals. In contrast, the high-frequency component was related to the change in head position, and the eye movements were found to derive from the otolith organs. CONCLUSION: s-OVAR may prove to be a useful tool for simultaneously assessing the functions of both the otolith organs and the semicircular canals.     

前庭神经炎患者虚拟现实辅助主观垂直水平视觉研究

目的 通过观察前庭神经炎患者主观垂直、水平视觉测试结果,为临床诊断提供依据.方法 45名无眩晕的健康自愿者作为健康对照组,46名单侧前庭神经炎患者作为VN组,均佩戴VR眼镜,分别完成头正中位的主观垂直视觉和主观水平视觉试验.结果 健康对照组主观视觉水平线、主观视觉垂直线的偏斜角度为-2°～2°.以此为判定阳性与阴性的标...     

Direction and angle of active head tilts influencing the Purkinje effect and the inhibition of postrotatory nystagmus I and II

Postrotatory nystagmus I and II (P I, P II) were evoked in four normal humans by velocity steps (prior velocity of rotating chair 90 degrees/s). 4 s after the stop the head was actively tilted by the subject 90 degrees forwards, backwards, to the shoulder of the previous direction of rotation -'ipsilaterally', or to the other shoulder-'contralaterally'. In control trials, the head was kept in the previous erect position. Compared with the control experiments, P I was significantly reduced by all head tilts. Inhibition of P I was strongest with forward and weakest with backward tilts. This difference is explained by the inclination of the utricular base by 30 degrees backward with respect to the horizontal of the skull and by the elastic properties of the sensory matrix. A smaller amplitude (45 degrees) of head tilt about the roll axis leads to a weaker inhibition (28.5%) than a 90 degree tilt, which corresponds to the difference of the sine of the tilt angle and thereby reflects the mechanical force acting on the receptor layer.