Clinical Neurophysiology 125 (2014) 627–634
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Ocular vestibular evoked myogenic potentials: The effect of head and body tilt in the roll plane Rachael L. Taylor a,1, Minzhi Xing a,1, Deborah A. Black b, G. Michael Halmagyi a, Miriam S. Welgampola a,⇑ a b
Institute of Clinical Neurosciences, Royal Prince Alfred Hospital, Central Clinical School, University of Sydney, Sydney, NSW, Australia Faculty of Health Sciences, University of Sydney, NSW, Australia
See Editorial, pages 439–441
a r t i c l e
i n f o
Article history: Available online 19 November 2013 Keywords: oVEMP Otolith organs Static head tilt Gravitational acceleration
h i g h l i g h t s Head and body tilt below the horizontal plane results in a significant decrease in the amplitudes of
oVEMPs evoked by both air-conducted (AC) and bone-conducted (BC) stimulation. The asymmetry ratio of the BC oVEMP was unaffected by head tilt in the roll plane, implying that BC
stimuli and static tilt each activate different populations of afferents. oVEMPs are optimally recorded in the upright position.
a b s t r a c t Objective: To explore effects of whole-head/body tilt in the roll plane on ocular-vestibular evoked myogenic potentials (oVEMP). Methods: Twenty healthy subjects were randomly tilted in an Eply Omniax rotator across a series of eight angles from 0° to 360° (at 45° separations) in the roll plane. At each position, oVEMPs to air-conducted (AC) and bone-conducted (BC) stimulation were recorded from unrectified infra-orbital surface electromyography during upward gaze. oVEMP amplitudes, latencies and amplitude asymmetry were compared across each angle of orientation. Results: Head orientation had a significant effect on oVEMP reflex amplitudes for both AC and BC stimulation (p < 0.001). For both stimuli there was a trend for lower amplitudes with increasing angular departure from the upright position. Mean amplitudes decreased by 42.6–56.8% (AC) and 23.2–25.5% (BC) when tilted 180°. Roll-plane tilt had a significant effect on amplitude asymmetry ratios recorded in response to AC stimuli (p < 0.001), indicating a trend for lower amplitudes from the dependent (down) ear. Amplitude asymmetry ratios for BC stimuli were unaffected by head and body orientation. Conclusions: The results confirm an effect of head and body orientation on oVEMP reflexes recorded in response to air- and bone-conducted stimuli. Significance: The upright position yields an optimal oVEMP response. Ó 2013 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction The utricle and saccule contain the sensory hair cells for detecting linear head acceleration and head orientation with respect to gravity. Deflection of hair cell stereocilia occurs during head movement due to the inertia of the otoconial membrane, and during
⇑ Corresponding author. Tel.: +61 295158820; fax: +61 295158347. 1
E-mail address: [email protected] (M.S. Welgampola). Equal contribution.
head tilt as a result of gravitational shearing forces. For the saccule, the macula is aligned in the sagittal plane. Therefore hair cells are maximally sensitive to vertical head acceleration and gravity when the head is upright or at 180° from upright. Conversely, utricular hair cells are oriented approximately orthogonal to the saccule and are responsive to horizontal head acceleration and head tilt in roll- and pitch-planes. However, while each utricle is responsive to roll plane tilt in both directions, the degree of afferent excitation is asymmetrical. Single unit studies in cats and squirrel monkeys indicate a greater proportion of afferents are excited by ipsilateral head tilt (when the ear is tilted toward the ground) (Curthoys and
1388-2457/$36.00 Ó 2013 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clinph.2013.08.031
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Markham, 1971; Fernandez et al., 1972; Fernandez and Goldberg, 1976). Some otolith afferents are also sensitive to non-physiological stimuli (Young et al., 1977; McCue and Guinan, 1994; Murofushi and Curthoys, 1997; Curthoys et al., 2006; Xu et al., 2009; Zhu et al., 2011; Curthoys and Vulovic, 2011). Loud air-conducted (AC) sound and bone-conducted (BC) stimulation give rise to a phase-locked increase in the firing rate of irregular afferents innervating type I hair cells of the utricle and saccule (Curthoys et al., 2006; Curthoys and Vulovic, 2011). A recently developed test of the otolith-ocular reflex pathways exploits the use of AC and BC stimulation to elicit a recordable response from the extraocular eye muscles, coined the ocular vestibular evoked myogenic potential, or oVEMP (Todd et al., 2007; Iwasaki et al., 2007). When recorded infraorbitally in response to AC sound and positive polarity BC stimulation at Fz, the oVEMP represents an excitatory, contralaterally dominant surface potential (Todd et al., 2007; Iwasaki et al., 2007). Whether the oVEMP to AC or BC stimulation represents predominantly saccular or utricular activation is controversal. Given the marked asymmetry in responses recorded from patients with superior vestibular neuritis, the oVEMP is thought to be mediated by the superior vestibular nerve (Iwasaki et al., 2009; Manzari et al., 2010; Shin et al., 2012). The superior nerve contains all afferents originating from the utricle as well as a small proportion of afferents from the anterior region of the saccule (Lorente de No, 1933). Based on this pattern of innervation, it is proposed that oVEMPs are predominantly utricular in origin (Manzari et al., 2010; Curthoys and Vulovic, 2011; Shin et al., 2012). However, other studies provide evidence that the stimulus frequency (Todd et al., 2009; Zhang et al., 2011, 2012), modality (AC or BC) and site of BC application (Rosengren et al., 2005; Lin et al. 2010, Jombik et al., 2011) may all be important factors in determining which otolith hair cells are preferentially stimulated. A contribution from the canal afferents is also possible, given that some studies have demonstrated their acoustic sensitivity (Young et al., 1977; Xu et al., 2009; Zhu et al., 2011). The proportion of afferents from each end organ that are activated by different types of AC and BC stimuli is yet to be determined. Given the dual sensitivity of the otolith organs to both physiological (gravitational acceleration) and non-physiological (AC and BC) stimuli, we hypothesized the oVEMP may be modulated by head and body orientation. Previous researchers have used AC stimuli to investigate the effects of head position on the cervical Vestibular Evoked Myogenic Potential (cVEMP; Colebatch et al., 1994). Ito et al. (2007) studied subjects in upright, supine, prone, left and right lateral positions, but reported no significant change in corrected cVEMP amplitudes. However, Shojaku et al. (2008) found that under the condition of microgravity imposed during parabolic flight, cVEMP amplitudes were significantly greater than in normal gravity or hypergravity. They suggested that dislocation of the saccular otoconia from the sensory epithelium could increase the mobility of the otolith hair cells, giving rise to an augmented response. During the course of our manuscript preparation a study was published by Iwasaki et al. (2012), which specifically examined oVEMP symmetry across five head orientations (upright and tilted at 45° and 90° to the left or right). Smaller amplitudes were recorded from the lower ear in response to bone-conducted stimulation, yet amplitudes to air-conducted sound were symmetrical across all angles of orientation. The authors suggested that activation of different populations of otolith afferents might account for the different results obtained for the two stimuli. In the present study we investigate the effects of gravity on oVEMP amplitude, latency and symmetry over an extended (360°) range of head and body rotation in the roll plane. If the
oVEMP indeed represents significant utricular activation, we expected the greatest effect on oVEMP amplitudes to occur when the head is oriented orthogonal to the gravitational acceleration vector (90° roll plane tilts to the left and right). 2. Methods 2.1. Subjects Participants were selected following a pilot recording of oVEMPs in the upright position to ensure peak–peak amplitudes of at least 2 lV in response to AC sound. A total of 20 healthy volunteers aged 21–52 (32.1 ± 9.1) with no history of hearing loss or vertigo were recruited and studied in accordance with the guidelines of the Helsinki declaration. 2.2. Stimuli Stimuli were generated at a rate of 5/s by the evoked potential stimulator of a Medelec Synergy EMG/EP system (Viasys Health Care Systems, Old Woking, UK). Air-conducted (AC) 0.1 ms clicks of alternating polarity were presented monaurally via calibrated TDH-49 headphones at a level of 105 dB nHL (normalized hearing level)/140 dB peak SPL (sound pressure level). A 0.1 ms click delivered via TDH 49 headphones contains a broad frequency spectrum with dominant energy in the 1–4 kHz range (Hall, 1992). Bone-conducted (BC) stimuli were generated in response to a 1 ms square-wave pulse (condensation polarity, 20 V amplitude) and delivered binaurally via a hand-held bone-vibrator (Bruel and Kjaer 4810 minishaker) at Fz. We have adopted the terminology ‘‘BC stimuli’’ as a label to describe stimulation by both whole-head translation, which is dominant below 150–400 Hz, and elastic wave propagation induced at higher frequencies (Stenfelt, 2011). Calibration of the minishaker, achieved using a Bruel and Kjaer sound level meter and an artificial mastoid (Bruel and Kjaer 4930), confirmed an intensity of 147 dB force level (24 N). 2.3. Procedure Subjects were studied while seated in a 360-degree Epley Omniax Rotator (Vesticon™, Oregon St Portland, USA). They were secured comfortably by means of two shoulder straps and harnesses across the hips and thighs. A horizontal headband made of nylon was further applied approximately 2 cm above the superior orbital margins to ensure the head remained firmly fixed to the headrest. The skin in the infra-orbital region was then prepared with 70% isopropyl alcohol swabs. Active (inverting) electrodes were placed over the infra-orbital region beneath each eye, close to the lower lid margin. Reference leads (non-inverting electrodes) were placed 2 cm below and a ground electrode was placed over the sternum. At the beginning of each experiment, subjects were asked to activate their extraocular muscles by gazing superiorly to a point corresponding to maximum upgaze. The distance between this point and the superior orbital margin was measured and a visual target was created for each subject. The target was mounted to a bite-bar, which was worn by the subject to ensure consistency of up-gaze across each angle of orientation. This method was chosen because subjects had difficulty maintaining ‘‘upward gaze’’ along their own mid-sagittal plane when tilted. Subjects were instructed to fixate on the target from the commencement of each recording until the stimulus was extinguished. Subjects were moved to one of 8 predetermined roll tilt angles (which were at 45° separations: see Fig. 1) in random order. The
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Fig. 1. oVEMP reflexes recorded from 20 normal subjects at 8 orientations in the roll plane. The traces represent the grand average data from raw unrectified infraorbital EMG responses to air and bone-conducted stimuli. The responses to right and left ear stimulation are represented by black and light grey traces, respectively.
zero roll–tilt angle represented earth-vertical. Angles were measured clockwise from the perspective of the experimenter facing the subject. Thus, a tilt to the subjects left shoulder was considered 45°. oVEMPs were recorded at each position for AC and BC stimuli. For BC stimulation, particular care was taken to ensure the abutment of the bone-vibrator was always positioned normal to the skull surface by the same experimenter. Once both AC and BC oVEMP recordings were complete, the subject was returned to the upright position for a 1–2 min rest period. For the more arduous conditions (135°, 180° and 225°), a rest was given between single recording runs for each ear. 2.4. EMG recording and analysis Unrectified elctromyographic activity (EMG) was sampled at 10,000 Hz, bandpass filtered (3–500 Hz) and averaged in response to 125 AC and 50 BC stimuli. Reflex latencies and amplitudes for monaural AC stimuli were identified from the contralateral trace only, while responses from both traces were analyzed for bilateral BC stimulation. Latencies (in milliseconds) were recorded from the
onset of the stimulus to the peak of the early negative (excitatory) component (n10) of the response. Peak-to-peak amplitudes were calculated as the difference in microvolts, between the first negative (n10) and positive peaks. The percentage difference in amplitudes recorded from each ear was expressed as an asymmetry ratio (AR) using the modified Jonkees formula: AR = 100 ⁄ (right Amplitude left Amplitude)/(right Amplitude + left Amplitude).
2.5. Accelerometry For BC stimuli, concurrent measurements of the three-dimensional (3D) stimulus-skull response were taken from the mastoids of four subjects at four angles of orientation (0°, 90°, 180°, 270°) using tri-axial accelerometers (TMS international). The accelerometers were secured firmly over the mastoid process using adhesive tape and a tight elastic headband. For each angle of orientation, the unfiltered acceleration response was amplified and sampled at 2500 Hz over 120 ms (20 ms pre- and 100 ms post-stimulus onset) using custom Labview-based software.
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3. Statistical methods
25.0
20.0
BC
Amplitude (µV)
All data were analyzed using SPSS version 18. For BC stimuli, the effects of orientation and ear (left vs right) on reflex amplitudes, latencies and symmetry were investigated using a 2 8 repeated measures ANOVA. As data for AC stimuli were not from a normal distribution, the effects of orientation were analyzed using Friedman tests. For post hoc analysis we chose to examine six paired comparisons (0–180°; 0–90°; 0–270°; 90–270°; 90–180°; 180– 270°) using paired t-tests (BC) and wilcoxon signed rank tests (AC). Bonferroni corrections were applied to control for familywise error. Descriptive statistics are represented in the text as the mean ± standard deviation (SD).
15.0
BC AC
10.0
5.0
4. Results All twenty subjects had recordable oVEMPs in response to BC stimuli in all eight orientations tested. While AC oVEMPs were also reliably recorded in the upright position, five subjects demonstrated absent AC reflexes at one or more roll–tilt angles; 45° (n = 1), 90° (n = 2), 135° (n = 3), 180° (n = 4), 225° (n = 3), 270° (n = 3), 315° (n = 2). Descriptive statistics are summarized in Table 1. 4.1. Reflex amplitudes Head orientation had a significant effect on oVEMP reflex amplitudes for both AC (Friedman tests left and right ears; p < 0.001) and BC (F(7,133) = 8.066; p < 0.001) stimuli. As indicated in Fig. 2, amplitudes decreased with increasing angular departure from the upright position. 4.1.1. Bone-conduction vibration For BC stimuli, amplitudes recorded in the ‘‘upside down’’ condition (180°) were significantly smaller than those recorded in the upright position and when compared with Left (90°) and right (270°) ear down positions (p-value range = < 0.001–0.015). When upside down, there was a 23.2% (right) and 25.5% (left) decrease in mean amplitudes compared to the upright; amplitudes decreased from 19.2 ± 8.9 and 17.7 ± 9.9 lV to 14.3 ± 5.7 and 13.6 ± 5.8 lV for the right and left ears, respectively. There was no significant difference in amplitudes recorded at 90° and 270° compared to the upright position, or when left (90°) and right (270°) ear down positions were compared with each other (p 6 0.05). Asymmetry ratio’s for BC stimuli (Fig. 3a) were unaffected by roll–tilt angle (F(7,133) = 0.297, p = 0.912) and there were no ear effects on ANOVA (F(1,19) = 1.464; p = 0.241). As shown for the four subjects in Fig. 4, the directions of acceleration as defined in the X-, Y-, and Z-axes were the same at 0, 90°, 270°, and 180° angles. Although the magnitude of acceleration var-
Right Left 5
0 27
31
5 22
0 18
5 13
90
45
0
0.0
Roll-tilt angle (degrees) Fig. 2. Average (mean ± SE) peak-to-peak oVEMP amplitudes to air (A) and bone (B) conducted stimuli. For bone-conducted stimulation at Fz, which produces bilateral vestibular activation, we have taken the surface recording beneath each eye to represent activation of the contralateral ear. Thus, averaged reflex amplitudes for right infraorbital recordings represent the left ear and left infraorbital recordings represent the right ear.
ied across subjects, it was always greatest for the X-axis component and there was no evidence of any obvious trend in the magnitude of acceleration across orientation. In other words, whereas all four subjects (8 ears) demonstrated a lower amplitude oVEMP response at 180° on comparison with the upright (0°) position (mean amplitude decrease = 7.3 ± 3.5 lV), the magnitude of the dominant X-axis component was larger in exactly 50% of the recordings and smaller in the other 50% with average values of 0.15 ± 0.05 and 0.17 ± 0.07 g for the upright and inverted positions. (Fig. 4, Table 2). Fourier analysis of the dominant (X) acceleration component, performed using a Hanning window on the first 10 ms of the response, confirmed a predominantly low frequency (250 Hz) were evident in the spectrum of the lower magnitude Y- and Z- axis acceleration components.
4.1.2. Air-conducted sound Significantly lower amplitudes were also recorded for AC stimuli at orientations of 180° when compared with left (270°), right (90°), and upright (0°) positions (Wilcoxon p-value range:
Table 1 Descriptive statistics representing the mean and standard deviation (SD) for amplitude, amplitude asymmetry (AR) and n10 latency of oVEMPs recorded at each orientation in the roll-plane. Absent AC oVEMP responses recorded from 5 subjects were excluded from the calculation of asymmetry ratios. Roll–tilt
0°
45°
90°
135°
180°
225°
270°
315°
Reflex amplitudes AC R L AR BC R L AR
10.8(5.7) 9.5(6.7) 7.5(26.9) 19.2(8.9) 17.7(9.9) 4.9(20.3)
10.7(6.3) 8.7(7.3) 12.1(29.1) 20.8(10.7) 19.1(10.2) 5.6(22.4)
9.5(5.4) 7.0(5.2) 16.4(22.1) 19.0(7.7) 17.5(7.7) 5.0(18.2)
8.0(5.1) 4.7(2.9) 26.5(16.1) 16.0(6.2) 14.4(6.7) 7.3(19.5)
6.2(5.0) 4.1(3.5) 17.1(26.2) 14.3(5.7) 13.6(5.8) 2.7(16.3)
7.0(4.9) 6.9(5.4) 5.2(23.8) 15.9(7.4) 14.7(7.2) 3.7(17.9)
9.4(5.9) 9.2(6.1) 5.2(24.3) 18.1(6.6) 16.9(7.0) 5.0(17.3)
10.6(5.9) 8.7(5.8) 10.7(21.9) 18.5(8.3) 17.7(9.4) 3.9(18.3)
Reflex latencies AC R L BC R L
8.6(0.4) 8.8(0.5) 9.0(0.3) 9.1(0.4)
8.6(0.3) 9.0(0.6) 9.2(0.6) 9.5(0.9)
8.6(0.5) 9.1(0.9) 9.2(0.3) 9.3(0.5)
8.7(0.5) 9.3(0.9) 9.1(0.4) 9.4(0.8)
8.7(0.6) 8.9(0.5) 9.5(1.4) 9.4(0.7)
8.7(0.4) 9.0(0.6) 9.2(0.4) 9.4(0.7)
8.8(0.8) 8.9(0.5) 9.2(0.4) 9.2(0.5)
8.9(0.9) 9.0(0.6) 9.1(0.4) 9.1(0.4)
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(A)
(B)
631
(C)
Fig. 3. Effects of orientation on reflex symmetry. (A) Mean ± SE asymmetry ratios across roll–tilt angle indicate a significant effect of orientation for AC, but not BC stimuli. The mean change in AC oVEMP amplitudes relative to the upright (0°) condition is compared for the right and left ears in response to AC (B) and BC (C) stimuli. For AC stimuli there is a trend for larger differences (an amplitude decrease) for the left ear for left roll plane tilts, whereas the right ear shows a greater decrease for right roll plane tilt. For BC stimuli, the trend is the same for both ears.
Fig. 4. Relationship between head/body orientation, acceleration of the left (black) and right (grey) mastoid and oVEMP amplitudes averaged across the four subjects. Acceleration is represented in three dimensions defined by naso-occipital (X), inter-aural (Y) and vertical (Z) components. The arrows on the figure head indicate the direction of acceleration corresponding to a positive (upward) peak for each acceleration trace. At each angle of orientation, the direction of the initial skull acceleration was backward (X) outward (Y), and downward (Z). The magnitude of the initial acceleration was always greatest for the X component and smallest for the z component. Fourier analysis performed on the first 10 ms of the dominant (X) component indicates a predominantly low frequency power spectrum. oVEMP waveforms, averaged across the four subjects, are shown for left ear/right eye (black) and right ear/left eye (grey) recordings in the far right panel. Mean amplitudes were largest in the upright position (left ear = 31.8 lV; right ear = 34.6 lV) and smallest in the 180° position (left ear = 23.8 lV; right ear = 27.9 lV).
L) as the head/body was tilted beyond 0° to the left, with the AR being maximal at 135°. The magnitude of the asymmetry then decreased for rotation beyond 180°, once the right became
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Table 2 Acclerometry measurements from four subjects at four angles of orientation. The magnitude of the initial acceleration peak (g) is shown for each X, Y and Z axis component along with the corresponding oVEMP amplitude (abbreviated ‘‘AMP’’). Each row represents measurements relating to the right (R) and left (L) ear for subjects 1–4. 0°
1 2 3 4 Mean
R L R L R L R L
90°
180°
270°
X
Y
Z
AMP
X
Y
Z
AMP
X
Y
Z
AMP
X
Y
Z
AMP
0.25 0.20 0.13 0.15 0.10 0.17 0.10 0.12 0.15
0.05 0.11 0.06 0.06 0.10 0.13 0.13 0.15 0.10
0.08 0.04 0.05 0.03 0.04 0.03 0.05 0.02 0.04
46.1 56.4 53.2 43.5 24.8 15.3 14.2 12.1 33.2
0.32 0.32 0.21 0.32 0.13 0.25 0.14 0.1 0.22
0.05 0.13 0.02 0.09 0.13 0.15 0.12 0.13 0.10
0.1 0.03 0.08 0.08 0.04 0.01 0.06 0.03 0.05
36.7 31.7 53.9 42.4 24.5 12.7 11.6 11.6 28.1
0.32 0.19 0.16 0.19 0.13 0.16 0.09 0.11 0.17
0.07 0.15 0.04 0.1 0.11 0.13 0.11 0.1 0.10
0.1 0.06 0.13 0.08 0.05 0.03 0.03 0.02 0.06
36.9 42.6 48.6 34.3 17.7 8.5 8.5 9.7 25.8
0.21 0.23 0.2 0.18 0.14 0.23 0.1 0.1 0.17
0.05 0.12 0.05 0.06 0.08 0.14 0.06 0.16 0.09
0.04 0.07 0.04 0.04 0.08 0.06 0.05 0.04 0.05
39.5 48.4 52.5 43.3 21.2 17.4 8.2 6.4 29.6
dependent. Fig. 3b, compares the change in amplitudes recorded for each ear, across each roll–tilt angle relative to the upright position. With the left ear down at 135°, there was a 4.8 lV reduction in amplitudes for the left ear, whereas amplitudes for the right ear decreased by only 2.9 lV. Conversely, with the right ear down (225°), amplitudes recorded from the left ear were reduced by 2.6 lV, while the right ear showed a greater reduction of 3.8 lV. These results contrast with those in Fig. 3c for BC stimuli, which shows an approximately equal change in amplitudes for each ear. 4.1.3. Ear effects for AC stimuli Overall, left ear amplitudes to AC stimuli were smaller than those recorded from the right ear. Given the tolerance limits for calibration of supra-aural earphones (±3 dB), it is possible that there was a slight difference in the output of the headphones that may have been sufficient to produce a difference in amplitudes recorded from each ear. However, as we analyzed AC amplitudes for each ear separately, this does not detract from the findings of the study. 4.2. Reflex Latencies On comparing reflex latencies with ANOVA, neither ear (F1,19 = 1.824; p = 0.193) nor orientation (F7,133 = 2.007; p = 0.103) had any effect on n10 latencies to BC stimuli. For AC stimuli, there was an effect of orientation on n10 latencies for the left ear, with a trend for longer latencies in the upside down (180°) orientation. Examination or the raw data indicated prolonged latencies were typically associated with low amplitude, poorly formed responses. 5. Discussion Overall, our results indicated a significant effect of head and body orientation on oVEMPs recorded in response to air- and bone-conducted stimuli. For both stimuli there was a progressive decrease in oVEMP amplitudes with increasing angular departure from the upright position, which was most significant for head/ body positions below the horizontal plane. Additionally, for AC stimuli, there was a significant effect of head orientation on amplitude asymmetry ratios. Asymmetry ratios for BC stimuli were unaffected by head and body orientation. We now consider the possible mechanisms that could account for the observed results. 5.1. The effects of head and body orientation on amplitude AR The effect of orientation on asymmetry ratios (AR) for the AC oVEMP indicated a greater degree of amplitude reduction from the inverted ear (‘‘down ear’’). Could this be due to the sensitivity bias of the utricle for laterally directed sheer forces? (Dai et al., 1989). Given that a greater degree of excitation is expected for ipsi-
versive head tilt, we could hypothesize that an increased resting discharge of irregular afferents from the dependent ear could render them less sensitive to non-physiological stimuli, resulting in asymmetric oVEMPs. However, if this were true, such an asymmetry should have been amplified in the BC oVEMPs, which are proposed to represent a more robust utriculo-ocular reflex (Curthoys and Vulovic, 2011). Yet we did not demonstrate BC oVEMP asymmetry pointing to the dependent ear. A more plausible explanation for the dissociated stimulus effects on AR is that a change in head position imposes mechanical constraints that selectively affect responses to AC sound. For example, increased intra-cochlear pressure, secondary to increased intracranial pressure may impede sound transmission through the middle ear (Voss et al., 2010). These effects are likely to be more pronounced for the lower ear, where pressure on the cochlear windows is greatest and stapes motion is directed against gravity. Shojaku et al. (2008) further suggested VEMP amplitudes may be affected by the weight of the otoconial membrane. A head tilt ±90°, brings the utricular macula into the plane of the gravitational axis where the mechanical loading on utricular hair cells is maximal. In this position, the capacity for further hair cell displacement by stapes motion may be limited. Vestibular activation by BC is independent of the middle ear system and as a much more intense stimulus, is less likely to be affected by minor mechanical constraints.
5.2. Effects of 180° tilt on reflex amplitudes When positioned at angles equal to, or above the horizontal, the effects of orientation on oVEMP amplitudes were small. This is consistent with Ito et al. (2007) results for the cVEMP. However, upon lowering the head below the horizontal, statistically significant differences were recorded for both stimuli. For BC stimuli, these effects were not due to differences in skull acceleration magnitude or direction across orientation. In the 180° position, gravitational forces are aligned with the polarization vectors of saccular not utricular hair cells. Could this mean the oVEMP responses recorded in our study are not primarily due to utricular activation? Recent studies have indicated that the AC oVEMP comprises two separate tuning peaks (Zhang et al., 2011). Based on the predicted resonance of the utricle and saccule (Todd et al., 2009), the authors proposed a saccular origin for the high frequency tuning peak, with the lower frequency peak representing a stronger utricular component (Zhang et al., 2011, 2012). Given that our AC oVEMPs were recorded in response a broad spectrum click, a greater contribution from saccular afferents might therefore be expected. However, the effect on amplitudes was also observed for the BC stimulus, which produced a predominantly low frequency stimulus-skull response in the axis of the largest (X) acceleration component. Further, contrasting with the utricle, strong projections from the
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saccule to the extraocular muscles have not been demonstrated (Isu et al., 2000). 5.3. Saccular/utricular convergence One means by which saccular activation could affect oVEMP amplitudes is through central convergence. Afferent inputs from all divisions of the vestibular labyrinth are known to converge at the level of the vestibular nucleus (Goldberg 2000; Kushiro et al., 2000; Dickman and Angelaki, 2002; Uchino et al., 2005). Second order projections to the ocular-motor nuclei therefore reflect a combination of irregular and regular afferent inputs from the utricle and saccule (Goldberg, 2000; Kushiro et al., 2000). Irregular afferents are highly sensitive to changes in linear acceleration, sound and vibration (Curthoys et al., 2006; Curthoys and Vulovic, 2011), but demonstrate adaptation to static tilt (Fernandez and Goldberg, 1976; Goldberg et al., 1990; Zhou et al., 2006). Regular afferents, while less sensitive, exhibit sustained responses static forces. In the 180° (inverted) position, activation of static tilt-sensitive (regular) saccular afferents may serve to modulate the input from AC and BC sensitive (irregular) utricular afferents and affect the oVEMP amplitudes. A final important consideration concerns the nature of the experimental procedure. A 180° tilt was not without some discomfort for the subject. Typically, subjects experienced a sensation of fullness in the head, reflecting increased intracranial pressure. This may have affected their ability to concentrate and maintain fixation on the target. The recording of a well-formed oVEMP response is also dependent on the positioning of the electrodes (Rosengren et al., 2005). When upside-down there is a tendency for the skin to sag under the influence of gravity, potentially altering the proximity of the electrodes relative to the extraocular muscles. Thus, at least some of the observed changes in this study, particularly those recorded during 180° head/body tilt, could reflect procedural factors. Our study yielded distinctly different results from Iwasaki et al. (2012). Unlike these authors, we did not find a selective modulation of the BC oVEMP asymmetry by ±90° head tilt. Differences in methodology, such as the use of a bite-bar mounted fixation target, which ensured gaze along the subjects own mid-saggital plane, could have minimized inter ocular asymmetries in the direction of gaze. A further point of difference between their study and ours concerns the driving voltage of the bone-vibrator. Iwasaki et al. (2012) used a 4 ms 500 Hz sine wave, which was associated with a Y- head acceleration component in the order of 0.4 g. Our stimulus was generated in response to a 1 ms square wave pulse, which produced a Y axis component less than half the magnitude reported in Iwasaki’s study. Temporal and spectral differences in the stimulus-skull response can also be expected for the different stimuli, but were not reported for comparison. In further contrast to our study, the heads of Iwasaki et al. subjects were not restrained. These differences may have resulted in different patterns of skull-vibration across head/body orientation and more unequal acceleration at the mastoids for positions at ±90°. 6. Conclusions The effects of head and body orientation on oVEMP could be explained by a number of factors including mechanical constraints imposed by gravity, increased intracranial pressure, and central convergence of otolith afferents with differing discharge properties. The oVEMP amplitude asymmetry to BC was unaffected by lateral head tilt. It is therefore unlikely that gravity produces any significant alteration in the sensitivity of irregular utricular afferents to BC stimuli.
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Financial support MSW is funded by the Garnett Passe and Rodney Williams Memorial Foundation and the National Health and Medical Research Council of Australia.
Conflict of interest None. Acknowledgements The authors Mr. Satendra Pratap, Michael Todd and Leigh McGarvie for Technical Support. References Colebatch JG, Halmagyi GM, Skuse NF. Myogenic potentials generated by a clickevoked vestibule-collic reflex. J Neurol Neurosurg Psychiatry 1994;57:190–7. Curthoys IS, Markham CH. Convergence of labyrinthine input on units in the vestibular nuclei of the cat. I. Natural stimulation. Brain Res 1971;35:469–90. Curthoys IS, Vulovic V. Vestibular primary afferent responses to sound and vibration in the guinea pig. Exp Brain Res 2011;210:347–52. Curthoys IS, Kim J, McPhedran A, Camp S. Bone conducted vibration selectively activates irregular primary otolithic vestibular neurons in the guinea pig. Exp Brain Res 2006;175:256–67. Dai MJ, Curthoys IS, Halmagyi GM. Linear acceleration perception in the roll-plane before and after unilateral vestibular neurectomy. Exp Brain Res 1989;77:315–28. Dickman JD, Angelaki DE. Vestibular convergence patterns in vestibular nuclei neurons of alert primates. J Neurophysiol 2002;88:3518–33. Fernandez C, Goldberg JM. Physiology of peripheral neurons innervating otolith organs in the squirrel monkey. I. Response to static tilt and to long-duration centrifugal force. J Neurophysiol 1976;39:970–84. Fernandez C, Goldberg JM, Abend WK. Response to static tilts of peripheral neurons innervating otolith organs of the squirrel monkey. J Neurophysiol 1972;35:978–87. Goldberg JM, Desmadryl G, Baird RA, Fernandez C. The vestibular nerve of the chinchilla IV. Discharge properties of utricular afferents. J Neurophysiol 1990;64:781–90. Goldberg JM. Afferent diversity and the organization of central vestibular pathways. Exp Brain Res 2000;130:277–97. Hall JW. Effect of stimulus factors. In: Hall JW, editor. Handbook of auditory evoked responses. Massachusetts: Allyn and Bacon; 1992. p. 104–76. Isu N, Graf W, Sato H, Kushiro K, Zakir M, Imagawa M, et al. Sacculo-ocular reflex connectivity in cats. Exp Brain Res 2000;131:262–8. Ito K, Kjarino S, Murofushi T. Effect of head position on vestibular evoked myogenic potentials with toneburst stimuli. Acta Oto-Laryngol 2007;127:57–61. Iwasaki S, McGarvie LA, Halmagyi GM, Burgess AM, Kim J, Colebatch JG, et al. Headtaps evoke a crossed vestibulo-ocular reflex. Neurol 2007;68:1227–9. Iwasaki S, Chihara Y, Smulders YE, Burgess AM, Halmagyi GM, Curthoys IS, et al. The role of the superior vestibular nerve in generating ocular vestibular-evoked myogenic potentials to bone conducted vibration at Fz. Clin Neurophysiol 2009;120:588–93. Iwasaki S, Chihara Y, Egami N, Fujimoto C, Murofushi T, Yamasoba T. Different effects of head tilt on ocular vestibular-evoked myogenic potentials to boneconducted vibration and air-conducted sound. Exp Brain Res 2012;223:389–96. Jombik P, Spodniak P, Bahyl V. Direction-dependent excitatory and inibitory ocular vestibular-evoked myogenic potentials (oVEMPs) produced by oppositely directed accelerations along the midsaggital axis of the head. Exp Brain Res 2011;211:251–63. Kushiro K, Zakir M, Sato H, Ono S, Ogawa Y, Meng H, et al. Saccular and utricular inputs to single vestibular neurons in cats. Exp Brain Res 2000;131:406–15. Lin CM, Wang SJ, Young YH. Ocular vestibular evoked myogenic potentials via boneconducted vibrations applied to various midsagittal cranial sites. Otol Neurotol 2010;31:157–61. Lorente de No R. Anatomy of the eighth nerve: I. The central projection of nerve endings of the internal ear. Laryngoscope 1933;43:1–38. Manzari L, Tedesco A, Burgess AM, Curthoys IS. Ocular vestibular-evoked myogenic potentials to bone-conducted vibration in superior vestibular neuritis show utricular function. Otolaryngol Head Neck Surg 2010;143:274–80. McCue MP, Guinan Jr JJ. Acoustically responsive fibers in the vestibular nerve of the cat. J Neurosci 1994;14:6058–70. Murofushi T, Curthoys IS. Physiological and anatomical study of click-sensitive primary vestibular afferents in the guinea pig. Acta Otolaryngol 1997;117:66–72. Rosengren SM, Todd NPM, Colebatch JG. Vestibular-evoked extraocular potentials produced by stimulation with bone-conducted sound. Clin Neurophysiol 2005;116:1938–48.
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Shin BS, Oh SY, Kim JS, Kim TW, Seo MW, Lee H, et al. Cervical and ocular vestibularevoked myogenic potentials in acute vestibular neuritis. Clin Neurophysiol 2012;123:369–75. Shojaku H, Watanabe Y, Tsubota M, Katayama N. Evaluation of the vestibular evoked myogenic potential during parabolic flight in humans. Exp Brain Res 2008;187:477–81. Stenfelt S. Acoustic and physiologic aspects of bone conduction hearing. Adv Otorhinolaryngol 2011;71:10–21. Todd NPM, Rosengren SM, Aw ST, Colebatch JG. Ocular vestibular evoked myogenic potentials (OVEMPs) produced by air- and bone-conducted sound. Clin Neurophysiol 2007;118:381–90. Todd NPM, Rosengren SM, Colebatch JG. A Utricular origin of frequency tuning in low frequency vibration in the human vestibular system? Neurosci Lett 2009;451:175–80. Uchino Y, Sasaki M, Sato H, Bai R, Kawamoto E. Otolith and canal integration on single vestibular neurons in cats. Exp Brain Res 2005;164:271–85. Voss SE, Adegoke MF, Horton NJ, Sheth KN, Rosand J, Shera CA. Posture systemati cally alters ear-canal reflectance and DPOAE properties. Hear Res 2010;263: 43–51.
Xu Y, Simpson I, Tang X, Zhou W. Acoustic clicks activate both the canal andotolith vestibulo-ocular reflex pathways in behaving monkeys. J Assoc Res Otolaryngol 2009;10:569–77. Young ED, Fernandez C, Goldberg JM. Responses of squirrel monkey vestibular neurons to audio-frequency sound and head vibration. Acta Oto-laryngol 1977;84:352–60. Zhang AS, Govender S, Colebatch JG. Tuning of the ocular vestibular evoked myogenic potential (oVEMP) to AC sound shows two separate peaks. Exp Brain Res 2011;213:111–6. Zhang AS, Govender S, Colebatch JG. Tuning of the ocular vestibular evoked myogenic potential to bone-conducted sound stimulation. J Appl Physiol 2012;112:1279–90. Zhou W, Tang BF, Newlands SD, King WM. Responses of monkey vestibular-only neurons to translation and angular rotation. J Neurophysiol 2006;96: 2915–30. Zhu H, Tang X, Wei W, Xu Y, Mustain W, Zhou W. Click-evoked responses in vestibular afferents in rats. J Neurophysiol 2011;106:754–63.
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Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Ocular vestibular evoked myogenic potentials: The effect of head and body tilt in the roll plane Rachael L. Taylor a,1, Minzhi Xing a,1, Deborah A. Black b, G. Michael Halmagyi a, Miriam S. Welgampola a,⇑ a b
Institute of Clinical Neurosciences, Royal Prince Alfred Hospital, Central Clinical School, University of Sydney, Sydney, NSW, Australia Faculty of Health Sciences, University of Sydney, NSW, Australia
See Editorial, pages 439–441
a r t i c l e
i n f o
Article history: Available online 19 November 2013 Keywords: oVEMP Otolith organs Static head tilt Gravitational acceleration
h i g h l i g h t s Head and body tilt below the horizontal plane results in a significant decrease in the amplitudes of
oVEMPs evoked by both air-conducted (AC) and bone-conducted (BC) stimulation. The asymmetry ratio of the BC oVEMP was unaffected by head tilt in the roll plane, implying that BC
stimuli and static tilt each activate different populations of afferents. oVEMPs are optimally recorded in the upright position.
a b s t r a c t Objective: To explore effects of whole-head/body tilt in the roll plane on ocular-vestibular evoked myogenic potentials (oVEMP). Methods: Twenty healthy subjects were randomly tilted in an Eply Omniax rotator across a series of eight angles from 0° to 360° (at 45° separations) in the roll plane. At each position, oVEMPs to air-conducted (AC) and bone-conducted (BC) stimulation were recorded from unrectified infra-orbital surface electromyography during upward gaze. oVEMP amplitudes, latencies and amplitude asymmetry were compared across each angle of orientation. Results: Head orientation had a significant effect on oVEMP reflex amplitudes for both AC and BC stimulation (p < 0.001). For both stimuli there was a trend for lower amplitudes with increasing angular departure from the upright position. Mean amplitudes decreased by 42.6–56.8% (AC) and 23.2–25.5% (BC) when tilted 180°. Roll-plane tilt had a significant effect on amplitude asymmetry ratios recorded in response to AC stimuli (p < 0.001), indicating a trend for lower amplitudes from the dependent (down) ear. Amplitude asymmetry ratios for BC stimuli were unaffected by head and body orientation. Conclusions: The results confirm an effect of head and body orientation on oVEMP reflexes recorded in response to air- and bone-conducted stimuli. Significance: The upright position yields an optimal oVEMP response. Ó 2013 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction The utricle and saccule contain the sensory hair cells for detecting linear head acceleration and head orientation with respect to gravity. Deflection of hair cell stereocilia occurs during head movement due to the inertia of the otoconial membrane, and during
⇑ Corresponding author. Tel.: +61 295158820; fax: +61 295158347. 1
E-mail address: [email protected] (M.S. Welgampola). Equal contribution.
head tilt as a result of gravitational shearing forces. For the saccule, the macula is aligned in the sagittal plane. Therefore hair cells are maximally sensitive to vertical head acceleration and gravity when the head is upright or at 180° from upright. Conversely, utricular hair cells are oriented approximately orthogonal to the saccule and are responsive to horizontal head acceleration and head tilt in roll- and pitch-planes. However, while each utricle is responsive to roll plane tilt in both directions, the degree of afferent excitation is asymmetrical. Single unit studies in cats and squirrel monkeys indicate a greater proportion of afferents are excited by ipsilateral head tilt (when the ear is tilted toward the ground) (Curthoys and
1388-2457/$36.00 Ó 2013 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clinph.2013.08.031
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Markham, 1971; Fernandez et al., 1972; Fernandez and Goldberg, 1976). Some otolith afferents are also sensitive to non-physiological stimuli (Young et al., 1977; McCue and Guinan, 1994; Murofushi and Curthoys, 1997; Curthoys et al., 2006; Xu et al., 2009; Zhu et al., 2011; Curthoys and Vulovic, 2011). Loud air-conducted (AC) sound and bone-conducted (BC) stimulation give rise to a phase-locked increase in the firing rate of irregular afferents innervating type I hair cells of the utricle and saccule (Curthoys et al., 2006; Curthoys and Vulovic, 2011). A recently developed test of the otolith-ocular reflex pathways exploits the use of AC and BC stimulation to elicit a recordable response from the extraocular eye muscles, coined the ocular vestibular evoked myogenic potential, or oVEMP (Todd et al., 2007; Iwasaki et al., 2007). When recorded infraorbitally in response to AC sound and positive polarity BC stimulation at Fz, the oVEMP represents an excitatory, contralaterally dominant surface potential (Todd et al., 2007; Iwasaki et al., 2007). Whether the oVEMP to AC or BC stimulation represents predominantly saccular or utricular activation is controversal. Given the marked asymmetry in responses recorded from patients with superior vestibular neuritis, the oVEMP is thought to be mediated by the superior vestibular nerve (Iwasaki et al., 2009; Manzari et al., 2010; Shin et al., 2012). The superior nerve contains all afferents originating from the utricle as well as a small proportion of afferents from the anterior region of the saccule (Lorente de No, 1933). Based on this pattern of innervation, it is proposed that oVEMPs are predominantly utricular in origin (Manzari et al., 2010; Curthoys and Vulovic, 2011; Shin et al., 2012). However, other studies provide evidence that the stimulus frequency (Todd et al., 2009; Zhang et al., 2011, 2012), modality (AC or BC) and site of BC application (Rosengren et al., 2005; Lin et al. 2010, Jombik et al., 2011) may all be important factors in determining which otolith hair cells are preferentially stimulated. A contribution from the canal afferents is also possible, given that some studies have demonstrated their acoustic sensitivity (Young et al., 1977; Xu et al., 2009; Zhu et al., 2011). The proportion of afferents from each end organ that are activated by different types of AC and BC stimuli is yet to be determined. Given the dual sensitivity of the otolith organs to both physiological (gravitational acceleration) and non-physiological (AC and BC) stimuli, we hypothesized the oVEMP may be modulated by head and body orientation. Previous researchers have used AC stimuli to investigate the effects of head position on the cervical Vestibular Evoked Myogenic Potential (cVEMP; Colebatch et al., 1994). Ito et al. (2007) studied subjects in upright, supine, prone, left and right lateral positions, but reported no significant change in corrected cVEMP amplitudes. However, Shojaku et al. (2008) found that under the condition of microgravity imposed during parabolic flight, cVEMP amplitudes were significantly greater than in normal gravity or hypergravity. They suggested that dislocation of the saccular otoconia from the sensory epithelium could increase the mobility of the otolith hair cells, giving rise to an augmented response. During the course of our manuscript preparation a study was published by Iwasaki et al. (2012), which specifically examined oVEMP symmetry across five head orientations (upright and tilted at 45° and 90° to the left or right). Smaller amplitudes were recorded from the lower ear in response to bone-conducted stimulation, yet amplitudes to air-conducted sound were symmetrical across all angles of orientation. The authors suggested that activation of different populations of otolith afferents might account for the different results obtained for the two stimuli. In the present study we investigate the effects of gravity on oVEMP amplitude, latency and symmetry over an extended (360°) range of head and body rotation in the roll plane. If the
oVEMP indeed represents significant utricular activation, we expected the greatest effect on oVEMP amplitudes to occur when the head is oriented orthogonal to the gravitational acceleration vector (90° roll plane tilts to the left and right). 2. Methods 2.1. Subjects Participants were selected following a pilot recording of oVEMPs in the upright position to ensure peak–peak amplitudes of at least 2 lV in response to AC sound. A total of 20 healthy volunteers aged 21–52 (32.1 ± 9.1) with no history of hearing loss or vertigo were recruited and studied in accordance with the guidelines of the Helsinki declaration. 2.2. Stimuli Stimuli were generated at a rate of 5/s by the evoked potential stimulator of a Medelec Synergy EMG/EP system (Viasys Health Care Systems, Old Woking, UK). Air-conducted (AC) 0.1 ms clicks of alternating polarity were presented monaurally via calibrated TDH-49 headphones at a level of 105 dB nHL (normalized hearing level)/140 dB peak SPL (sound pressure level). A 0.1 ms click delivered via TDH 49 headphones contains a broad frequency spectrum with dominant energy in the 1–4 kHz range (Hall, 1992). Bone-conducted (BC) stimuli were generated in response to a 1 ms square-wave pulse (condensation polarity, 20 V amplitude) and delivered binaurally via a hand-held bone-vibrator (Bruel and Kjaer 4810 minishaker) at Fz. We have adopted the terminology ‘‘BC stimuli’’ as a label to describe stimulation by both whole-head translation, which is dominant below 150–400 Hz, and elastic wave propagation induced at higher frequencies (Stenfelt, 2011). Calibration of the minishaker, achieved using a Bruel and Kjaer sound level meter and an artificial mastoid (Bruel and Kjaer 4930), confirmed an intensity of 147 dB force level (24 N). 2.3. Procedure Subjects were studied while seated in a 360-degree Epley Omniax Rotator (Vesticon™, Oregon St Portland, USA). They were secured comfortably by means of two shoulder straps and harnesses across the hips and thighs. A horizontal headband made of nylon was further applied approximately 2 cm above the superior orbital margins to ensure the head remained firmly fixed to the headrest. The skin in the infra-orbital region was then prepared with 70% isopropyl alcohol swabs. Active (inverting) electrodes were placed over the infra-orbital region beneath each eye, close to the lower lid margin. Reference leads (non-inverting electrodes) were placed 2 cm below and a ground electrode was placed over the sternum. At the beginning of each experiment, subjects were asked to activate their extraocular muscles by gazing superiorly to a point corresponding to maximum upgaze. The distance between this point and the superior orbital margin was measured and a visual target was created for each subject. The target was mounted to a bite-bar, which was worn by the subject to ensure consistency of up-gaze across each angle of orientation. This method was chosen because subjects had difficulty maintaining ‘‘upward gaze’’ along their own mid-sagittal plane when tilted. Subjects were instructed to fixate on the target from the commencement of each recording until the stimulus was extinguished. Subjects were moved to one of 8 predetermined roll tilt angles (which were at 45° separations: see Fig. 1) in random order. The
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Fig. 1. oVEMP reflexes recorded from 20 normal subjects at 8 orientations in the roll plane. The traces represent the grand average data from raw unrectified infraorbital EMG responses to air and bone-conducted stimuli. The responses to right and left ear stimulation are represented by black and light grey traces, respectively.
zero roll–tilt angle represented earth-vertical. Angles were measured clockwise from the perspective of the experimenter facing the subject. Thus, a tilt to the subjects left shoulder was considered 45°. oVEMPs were recorded at each position for AC and BC stimuli. For BC stimulation, particular care was taken to ensure the abutment of the bone-vibrator was always positioned normal to the skull surface by the same experimenter. Once both AC and BC oVEMP recordings were complete, the subject was returned to the upright position for a 1–2 min rest period. For the more arduous conditions (135°, 180° and 225°), a rest was given between single recording runs for each ear. 2.4. EMG recording and analysis Unrectified elctromyographic activity (EMG) was sampled at 10,000 Hz, bandpass filtered (3–500 Hz) and averaged in response to 125 AC and 50 BC stimuli. Reflex latencies and amplitudes for monaural AC stimuli were identified from the contralateral trace only, while responses from both traces were analyzed for bilateral BC stimulation. Latencies (in milliseconds) were recorded from the
onset of the stimulus to the peak of the early negative (excitatory) component (n10) of the response. Peak-to-peak amplitudes were calculated as the difference in microvolts, between the first negative (n10) and positive peaks. The percentage difference in amplitudes recorded from each ear was expressed as an asymmetry ratio (AR) using the modified Jonkees formula: AR = 100 ⁄ (right Amplitude left Amplitude)/(right Amplitude + left Amplitude).
2.5. Accelerometry For BC stimuli, concurrent measurements of the three-dimensional (3D) stimulus-skull response were taken from the mastoids of four subjects at four angles of orientation (0°, 90°, 180°, 270°) using tri-axial accelerometers (TMS international). The accelerometers were secured firmly over the mastoid process using adhesive tape and a tight elastic headband. For each angle of orientation, the unfiltered acceleration response was amplified and sampled at 2500 Hz over 120 ms (20 ms pre- and 100 ms post-stimulus onset) using custom Labview-based software.
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3. Statistical methods
25.0
20.0
BC
Amplitude (µV)
All data were analyzed using SPSS version 18. For BC stimuli, the effects of orientation and ear (left vs right) on reflex amplitudes, latencies and symmetry were investigated using a 2 8 repeated measures ANOVA. As data for AC stimuli were not from a normal distribution, the effects of orientation were analyzed using Friedman tests. For post hoc analysis we chose to examine six paired comparisons (0–180°; 0–90°; 0–270°; 90–270°; 90–180°; 180– 270°) using paired t-tests (BC) and wilcoxon signed rank tests (AC). Bonferroni corrections were applied to control for familywise error. Descriptive statistics are represented in the text as the mean ± standard deviation (SD).
15.0
BC AC
10.0
5.0
4. Results All twenty subjects had recordable oVEMPs in response to BC stimuli in all eight orientations tested. While AC oVEMPs were also reliably recorded in the upright position, five subjects demonstrated absent AC reflexes at one or more roll–tilt angles; 45° (n = 1), 90° (n = 2), 135° (n = 3), 180° (n = 4), 225° (n = 3), 270° (n = 3), 315° (n = 2). Descriptive statistics are summarized in Table 1. 4.1. Reflex amplitudes Head orientation had a significant effect on oVEMP reflex amplitudes for both AC (Friedman tests left and right ears; p < 0.001) and BC (F(7,133) = 8.066; p < 0.001) stimuli. As indicated in Fig. 2, amplitudes decreased with increasing angular departure from the upright position. 4.1.1. Bone-conduction vibration For BC stimuli, amplitudes recorded in the ‘‘upside down’’ condition (180°) were significantly smaller than those recorded in the upright position and when compared with Left (90°) and right (270°) ear down positions (p-value range = < 0.001–0.015). When upside down, there was a 23.2% (right) and 25.5% (left) decrease in mean amplitudes compared to the upright; amplitudes decreased from 19.2 ± 8.9 and 17.7 ± 9.9 lV to 14.3 ± 5.7 and 13.6 ± 5.8 lV for the right and left ears, respectively. There was no significant difference in amplitudes recorded at 90° and 270° compared to the upright position, or when left (90°) and right (270°) ear down positions were compared with each other (p 6 0.05). Asymmetry ratio’s for BC stimuli (Fig. 3a) were unaffected by roll–tilt angle (F(7,133) = 0.297, p = 0.912) and there were no ear effects on ANOVA (F(1,19) = 1.464; p = 0.241). As shown for the four subjects in Fig. 4, the directions of acceleration as defined in the X-, Y-, and Z-axes were the same at 0, 90°, 270°, and 180° angles. Although the magnitude of acceleration var-
Right Left 5
0 27
31
5 22
0 18
5 13
90
45
0
0.0
Roll-tilt angle (degrees) Fig. 2. Average (mean ± SE) peak-to-peak oVEMP amplitudes to air (A) and bone (B) conducted stimuli. For bone-conducted stimulation at Fz, which produces bilateral vestibular activation, we have taken the surface recording beneath each eye to represent activation of the contralateral ear. Thus, averaged reflex amplitudes for right infraorbital recordings represent the left ear and left infraorbital recordings represent the right ear.
ied across subjects, it was always greatest for the X-axis component and there was no evidence of any obvious trend in the magnitude of acceleration across orientation. In other words, whereas all four subjects (8 ears) demonstrated a lower amplitude oVEMP response at 180° on comparison with the upright (0°) position (mean amplitude decrease = 7.3 ± 3.5 lV), the magnitude of the dominant X-axis component was larger in exactly 50% of the recordings and smaller in the other 50% with average values of 0.15 ± 0.05 and 0.17 ± 0.07 g for the upright and inverted positions. (Fig. 4, Table 2). Fourier analysis of the dominant (X) acceleration component, performed using a Hanning window on the first 10 ms of the response, confirmed a predominantly low frequency (250 Hz) were evident in the spectrum of the lower magnitude Y- and Z- axis acceleration components.
4.1.2. Air-conducted sound Significantly lower amplitudes were also recorded for AC stimuli at orientations of 180° when compared with left (270°), right (90°), and upright (0°) positions (Wilcoxon p-value range:
Table 1 Descriptive statistics representing the mean and standard deviation (SD) for amplitude, amplitude asymmetry (AR) and n10 latency of oVEMPs recorded at each orientation in the roll-plane. Absent AC oVEMP responses recorded from 5 subjects were excluded from the calculation of asymmetry ratios. Roll–tilt
0°
45°
90°
135°
180°
225°
270°
315°
Reflex amplitudes AC R L AR BC R L AR
10.8(5.7) 9.5(6.7) 7.5(26.9) 19.2(8.9) 17.7(9.9) 4.9(20.3)
10.7(6.3) 8.7(7.3) 12.1(29.1) 20.8(10.7) 19.1(10.2) 5.6(22.4)
9.5(5.4) 7.0(5.2) 16.4(22.1) 19.0(7.7) 17.5(7.7) 5.0(18.2)
8.0(5.1) 4.7(2.9) 26.5(16.1) 16.0(6.2) 14.4(6.7) 7.3(19.5)
6.2(5.0) 4.1(3.5) 17.1(26.2) 14.3(5.7) 13.6(5.8) 2.7(16.3)
7.0(4.9) 6.9(5.4) 5.2(23.8) 15.9(7.4) 14.7(7.2) 3.7(17.9)
9.4(5.9) 9.2(6.1) 5.2(24.3) 18.1(6.6) 16.9(7.0) 5.0(17.3)
10.6(5.9) 8.7(5.8) 10.7(21.9) 18.5(8.3) 17.7(9.4) 3.9(18.3)
Reflex latencies AC R L BC R L
8.6(0.4) 8.8(0.5) 9.0(0.3) 9.1(0.4)
8.6(0.3) 9.0(0.6) 9.2(0.6) 9.5(0.9)
8.6(0.5) 9.1(0.9) 9.2(0.3) 9.3(0.5)
8.7(0.5) 9.3(0.9) 9.1(0.4) 9.4(0.8)
8.7(0.6) 8.9(0.5) 9.5(1.4) 9.4(0.7)
8.7(0.4) 9.0(0.6) 9.2(0.4) 9.4(0.7)
8.8(0.8) 8.9(0.5) 9.2(0.4) 9.2(0.5)
8.9(0.9) 9.0(0.6) 9.1(0.4) 9.1(0.4)
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(A)
(B)
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(C)
Fig. 3. Effects of orientation on reflex symmetry. (A) Mean ± SE asymmetry ratios across roll–tilt angle indicate a significant effect of orientation for AC, but not BC stimuli. The mean change in AC oVEMP amplitudes relative to the upright (0°) condition is compared for the right and left ears in response to AC (B) and BC (C) stimuli. For AC stimuli there is a trend for larger differences (an amplitude decrease) for the left ear for left roll plane tilts, whereas the right ear shows a greater decrease for right roll plane tilt. For BC stimuli, the trend is the same for both ears.
Fig. 4. Relationship between head/body orientation, acceleration of the left (black) and right (grey) mastoid and oVEMP amplitudes averaged across the four subjects. Acceleration is represented in three dimensions defined by naso-occipital (X), inter-aural (Y) and vertical (Z) components. The arrows on the figure head indicate the direction of acceleration corresponding to a positive (upward) peak for each acceleration trace. At each angle of orientation, the direction of the initial skull acceleration was backward (X) outward (Y), and downward (Z). The magnitude of the initial acceleration was always greatest for the X component and smallest for the z component. Fourier analysis performed on the first 10 ms of the dominant (X) component indicates a predominantly low frequency power spectrum. oVEMP waveforms, averaged across the four subjects, are shown for left ear/right eye (black) and right ear/left eye (grey) recordings in the far right panel. Mean amplitudes were largest in the upright position (left ear = 31.8 lV; right ear = 34.6 lV) and smallest in the 180° position (left ear = 23.8 lV; right ear = 27.9 lV).
L) as the head/body was tilted beyond 0° to the left, with the AR being maximal at 135°. The magnitude of the asymmetry then decreased for rotation beyond 180°, once the right became
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Table 2 Acclerometry measurements from four subjects at four angles of orientation. The magnitude of the initial acceleration peak (g) is shown for each X, Y and Z axis component along with the corresponding oVEMP amplitude (abbreviated ‘‘AMP’’). Each row represents measurements relating to the right (R) and left (L) ear for subjects 1–4. 0°
1 2 3 4 Mean
R L R L R L R L
90°
180°
270°
X
Y
Z
AMP
X
Y
Z
AMP
X
Y
Z
AMP
X
Y
Z
AMP
0.25 0.20 0.13 0.15 0.10 0.17 0.10 0.12 0.15
0.05 0.11 0.06 0.06 0.10 0.13 0.13 0.15 0.10
0.08 0.04 0.05 0.03 0.04 0.03 0.05 0.02 0.04
46.1 56.4 53.2 43.5 24.8 15.3 14.2 12.1 33.2
0.32 0.32 0.21 0.32 0.13 0.25 0.14 0.1 0.22
0.05 0.13 0.02 0.09 0.13 0.15 0.12 0.13 0.10
0.1 0.03 0.08 0.08 0.04 0.01 0.06 0.03 0.05
36.7 31.7 53.9 42.4 24.5 12.7 11.6 11.6 28.1
0.32 0.19 0.16 0.19 0.13 0.16 0.09 0.11 0.17
0.07 0.15 0.04 0.1 0.11 0.13 0.11 0.1 0.10
0.1 0.06 0.13 0.08 0.05 0.03 0.03 0.02 0.06
36.9 42.6 48.6 34.3 17.7 8.5 8.5 9.7 25.8
0.21 0.23 0.2 0.18 0.14 0.23 0.1 0.1 0.17
0.05 0.12 0.05 0.06 0.08 0.14 0.06 0.16 0.09
0.04 0.07 0.04 0.04 0.08 0.06 0.05 0.04 0.05
39.5 48.4 52.5 43.3 21.2 17.4 8.2 6.4 29.6
dependent. Fig. 3b, compares the change in amplitudes recorded for each ear, across each roll–tilt angle relative to the upright position. With the left ear down at 135°, there was a 4.8 lV reduction in amplitudes for the left ear, whereas amplitudes for the right ear decreased by only 2.9 lV. Conversely, with the right ear down (225°), amplitudes recorded from the left ear were reduced by 2.6 lV, while the right ear showed a greater reduction of 3.8 lV. These results contrast with those in Fig. 3c for BC stimuli, which shows an approximately equal change in amplitudes for each ear. 4.1.3. Ear effects for AC stimuli Overall, left ear amplitudes to AC stimuli were smaller than those recorded from the right ear. Given the tolerance limits for calibration of supra-aural earphones (±3 dB), it is possible that there was a slight difference in the output of the headphones that may have been sufficient to produce a difference in amplitudes recorded from each ear. However, as we analyzed AC amplitudes for each ear separately, this does not detract from the findings of the study. 4.2. Reflex Latencies On comparing reflex latencies with ANOVA, neither ear (F1,19 = 1.824; p = 0.193) nor orientation (F7,133 = 2.007; p = 0.103) had any effect on n10 latencies to BC stimuli. For AC stimuli, there was an effect of orientation on n10 latencies for the left ear, with a trend for longer latencies in the upside down (180°) orientation. Examination or the raw data indicated prolonged latencies were typically associated with low amplitude, poorly formed responses. 5. Discussion Overall, our results indicated a significant effect of head and body orientation on oVEMPs recorded in response to air- and bone-conducted stimuli. For both stimuli there was a progressive decrease in oVEMP amplitudes with increasing angular departure from the upright position, which was most significant for head/ body positions below the horizontal plane. Additionally, for AC stimuli, there was a significant effect of head orientation on amplitude asymmetry ratios. Asymmetry ratios for BC stimuli were unaffected by head and body orientation. We now consider the possible mechanisms that could account for the observed results. 5.1. The effects of head and body orientation on amplitude AR The effect of orientation on asymmetry ratios (AR) for the AC oVEMP indicated a greater degree of amplitude reduction from the inverted ear (‘‘down ear’’). Could this be due to the sensitivity bias of the utricle for laterally directed sheer forces? (Dai et al., 1989). Given that a greater degree of excitation is expected for ipsi-
versive head tilt, we could hypothesize that an increased resting discharge of irregular afferents from the dependent ear could render them less sensitive to non-physiological stimuli, resulting in asymmetric oVEMPs. However, if this were true, such an asymmetry should have been amplified in the BC oVEMPs, which are proposed to represent a more robust utriculo-ocular reflex (Curthoys and Vulovic, 2011). Yet we did not demonstrate BC oVEMP asymmetry pointing to the dependent ear. A more plausible explanation for the dissociated stimulus effects on AR is that a change in head position imposes mechanical constraints that selectively affect responses to AC sound. For example, increased intra-cochlear pressure, secondary to increased intracranial pressure may impede sound transmission through the middle ear (Voss et al., 2010). These effects are likely to be more pronounced for the lower ear, where pressure on the cochlear windows is greatest and stapes motion is directed against gravity. Shojaku et al. (2008) further suggested VEMP amplitudes may be affected by the weight of the otoconial membrane. A head tilt ±90°, brings the utricular macula into the plane of the gravitational axis where the mechanical loading on utricular hair cells is maximal. In this position, the capacity for further hair cell displacement by stapes motion may be limited. Vestibular activation by BC is independent of the middle ear system and as a much more intense stimulus, is less likely to be affected by minor mechanical constraints.
5.2. Effects of 180° tilt on reflex amplitudes When positioned at angles equal to, or above the horizontal, the effects of orientation on oVEMP amplitudes were small. This is consistent with Ito et al. (2007) results for the cVEMP. However, upon lowering the head below the horizontal, statistically significant differences were recorded for both stimuli. For BC stimuli, these effects were not due to differences in skull acceleration magnitude or direction across orientation. In the 180° position, gravitational forces are aligned with the polarization vectors of saccular not utricular hair cells. Could this mean the oVEMP responses recorded in our study are not primarily due to utricular activation? Recent studies have indicated that the AC oVEMP comprises two separate tuning peaks (Zhang et al., 2011). Based on the predicted resonance of the utricle and saccule (Todd et al., 2009), the authors proposed a saccular origin for the high frequency tuning peak, with the lower frequency peak representing a stronger utricular component (Zhang et al., 2011, 2012). Given that our AC oVEMPs were recorded in response a broad spectrum click, a greater contribution from saccular afferents might therefore be expected. However, the effect on amplitudes was also observed for the BC stimulus, which produced a predominantly low frequency stimulus-skull response in the axis of the largest (X) acceleration component. Further, contrasting with the utricle, strong projections from the
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saccule to the extraocular muscles have not been demonstrated (Isu et al., 2000). 5.3. Saccular/utricular convergence One means by which saccular activation could affect oVEMP amplitudes is through central convergence. Afferent inputs from all divisions of the vestibular labyrinth are known to converge at the level of the vestibular nucleus (Goldberg 2000; Kushiro et al., 2000; Dickman and Angelaki, 2002; Uchino et al., 2005). Second order projections to the ocular-motor nuclei therefore reflect a combination of irregular and regular afferent inputs from the utricle and saccule (Goldberg, 2000; Kushiro et al., 2000). Irregular afferents are highly sensitive to changes in linear acceleration, sound and vibration (Curthoys et al., 2006; Curthoys and Vulovic, 2011), but demonstrate adaptation to static tilt (Fernandez and Goldberg, 1976; Goldberg et al., 1990; Zhou et al., 2006). Regular afferents, while less sensitive, exhibit sustained responses static forces. In the 180° (inverted) position, activation of static tilt-sensitive (regular) saccular afferents may serve to modulate the input from AC and BC sensitive (irregular) utricular afferents and affect the oVEMP amplitudes. A final important consideration concerns the nature of the experimental procedure. A 180° tilt was not without some discomfort for the subject. Typically, subjects experienced a sensation of fullness in the head, reflecting increased intracranial pressure. This may have affected their ability to concentrate and maintain fixation on the target. The recording of a well-formed oVEMP response is also dependent on the positioning of the electrodes (Rosengren et al., 2005). When upside-down there is a tendency for the skin to sag under the influence of gravity, potentially altering the proximity of the electrodes relative to the extraocular muscles. Thus, at least some of the observed changes in this study, particularly those recorded during 180° head/body tilt, could reflect procedural factors. Our study yielded distinctly different results from Iwasaki et al. (2012). Unlike these authors, we did not find a selective modulation of the BC oVEMP asymmetry by ±90° head tilt. Differences in methodology, such as the use of a bite-bar mounted fixation target, which ensured gaze along the subjects own mid-saggital plane, could have minimized inter ocular asymmetries in the direction of gaze. A further point of difference between their study and ours concerns the driving voltage of the bone-vibrator. Iwasaki et al. (2012) used a 4 ms 500 Hz sine wave, which was associated with a Y- head acceleration component in the order of 0.4 g. Our stimulus was generated in response to a 1 ms square wave pulse, which produced a Y axis component less than half the magnitude reported in Iwasaki’s study. Temporal and spectral differences in the stimulus-skull response can also be expected for the different stimuli, but were not reported for comparison. In further contrast to our study, the heads of Iwasaki et al. subjects were not restrained. These differences may have resulted in different patterns of skull-vibration across head/body orientation and more unequal acceleration at the mastoids for positions at ±90°. 6. Conclusions The effects of head and body orientation on oVEMP could be explained by a number of factors including mechanical constraints imposed by gravity, increased intracranial pressure, and central convergence of otolith afferents with differing discharge properties. The oVEMP amplitude asymmetry to BC was unaffected by lateral head tilt. It is therefore unlikely that gravity produces any significant alteration in the sensitivity of irregular utricular afferents to BC stimuli.
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Financial support MSW is funded by the Garnett Passe and Rodney Williams Memorial Foundation and the National Health and Medical Research Council of Australia.
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