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 Table of Contents  
ORIGINAL ARTICLE
Year : 2014  |  Volume : 28  |  Issue : 1  |  Page : 1-9

Characterizing the Effects of Frequency on Parameters of Short Tone-bursts Induced Ocular Vestibular Evoked Myogenic Potentials


Department of Audiology, All India Institute of Speech and Hearing, Manasagangothri, Mysore, Karnataka, India

Date of Web Publication31-Dec-2014

Correspondence Address:
Niraj Kumar Singh
Department of Audiology, All India Institute of Speech and Hearing, Manasagangothri, Mysore - 570 006, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0974-2131.127385

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  Abstract 

Stimulus is the essence of any audiovestibular investigation and ocular vestibular-evoked myogenic potential (oVEMP) would be no different. Several investigations have examined the effect of frequency of stimulus on oVEMP parameters with prime reports concentrated around amplitude and to a lesser extent threshold. Effects of stimulus frequency on latency-related parameters have been sparingly explored with equivocal results. Thus, the aim of this study was to investigate the effects of air-conducted frequency-specific short tone-bursts on latency, amplitude, and threshold-related parameters of various peaks of oVEMP. A normative study was conducted to obtain oVEMP responses from 50 healthy individuals in the age range of 18 - 30 years. Tone-bursts at octave and midoctave frequencies from 250 to 2000 Hz were used to acquire responses from the inferior oblique muscle using contralateral electrode placement. oVEMPs were present in 100% of the individuals at or below the frequency of 1000 Hz. The largest amplitudes and the lowest threshold corresponded to 500 Hz tone-burst, whereas 250 Hz produced largest absolute latencies as well as interpeak latency intervals (P < 0.05). Frequency had no effect on interaural latency difference as well as interaural amplitude ratio. Owing to largest amplitudes and best thresholds, 500 Hz appears better stimuli for clinical recording of oVEMPs. This is true irrespective of the peak complex being assessed is n1p1 or p1n2.

Keywords: Amplitude, frequency, latency, oVEMP, threshold


How to cite this article:
Singh NK, Barman A. Characterizing the Effects of Frequency on Parameters of Short Tone-bursts Induced Ocular Vestibular Evoked Myogenic Potentials. J Indian Speech Language Hearing Assoc 2014;28:1-9

How to cite this URL:
Singh NK, Barman A. Characterizing the Effects of Frequency on Parameters of Short Tone-bursts Induced Ocular Vestibular Evoked Myogenic Potentials. J Indian Speech Language Hearing Assoc [serial online] 2014 [cited 2018 Jan 19];28:1-9. Available from: http://www.jisha.org/text.asp?2014/28/1/1/127385


  Introduction Top


The ocular vestibular-evoked myogenic potential (oVEMP) is an otolith response that is believed to travel via the vestibuloocular reflex (VOR) pathway and can be recorded from the inferior oblique muscle of the eye when the eye is in upward centre gaze. [1],[2],[3],[4],[5] Though mostly referred as a biphasic component owing to frequent reports of only an initial negative peak at around 10 − 12 ms (n10 or n1) and a subsequent positive peak at around 15 − 20 ms (p15 or p1) poststimulus onset [1],[6],[3],[7],[8] other peaks (such as n2, p2, and n3) are also reported to occur in oVEMP recordings of healthy individuals. [9],[10] Despite the reports of oVEMP being produced along the VOR pathway, it is one of the few tests that can reveal the status of otolith organs. Due to the involvement of VOR pathway that originates at the utricle (utriculoocular pathway), oVEMP is found to aid in the diagnosis of several vestibular pathologies that are not necessarily associated with abnormalities of the semicircular canals. [1],[2],[6]

oVEMP can be recorded in response to galvanic, vibratory (mechanical) as well as acoustic stimuli. [1],[6],[3],[7],[8] The concurrent research in the area of oVEMP, however, has mainly concentrated around the acoustic stimulation and; therefore, it is of paramount importance that effects of variations in various stimulus parameters of air-conduction-evoked oVEMP be investigated.

Stimulus is the essence of any audiovestibular investigation and oVEMP would be no different. The effects of a number of stimulus parameters, such as stimulus type, [7],[11] stimulus mode, [1],[7],[8] presentation type, [5],[12] rise/fall time, [13],[14] intensity [15],[16] and frequency [17],[18],[16],[2],[19],[20],[21],[22],[24] on the air-conduction-evoked oVEMP have been examined by several of the studies previously. While tone-bursts have been shown to produce larger amplitudes and better thresholds than clicks, [2],[11] binaural and monaural stimulation produce interchangeable results [5],[12] and decreasing intensity produces reduction in amplitude without affecting the latencies. [16] The other important parameters (especially rise-plateau-fall time and frequency) have shown equivocal results, with some casting disagreements over others' results. [13],[17],[18],[16],[19],[14],[20],[21],[22],[23]

Several investigations have examined the effect of frequency of stimulus on oVEMP parameters. [18],[17],[19],[21],[23],[22],[20] Most of these have concentrated on the effect of frequency on amplitude, reporting largest amplitudes in response to 500 Hz tone bursts and thereby confirming the tuning of oVEMP to 500 Hz among healthy individuals. However, others have obtained results against this obligation. Todd et al., reported largest amplitude to fall in the range of 400−800 Hz as against 1000 Hz reported by some others. [24] Some of them have also investigated its impact on threshold and reported best thresholds also to coincide with the frequency corresponding to largest amplitude which is 500 Hz. However, only a few have explored the effects on latency-related parameters. The results regarding effect of frequency on latency appears to be equivocal. While some have reported a trend toward reduction in latency with increasing frequency, [17] others have shown no such effect. [20] The use of small sample size and variability in stimulus parameters used between most of these studies might account for some of the inconsistencies and therefore calls for more detailed examination. In addition to the above-mentioned inconsistencies observed for effect of frequency on oVEMP parameters, the need for exploration is further emphasized by the lack of a single investigation reporting about the effects of frequency on all the parameters using the same set of subjects. Additionally, there is also dearth of reports regarding the effects of frequency of stimulus on later peaks of oVEMP (n2, p2, and n3) that were shown to occur by previous investigations on oVEMP [9],[10] but not well-explored subsequently by other studies. Thus, the aim of this study was to investigate the effects of air-conducted frequency-specific short tone-bursts on latency, amplitude, and threshold-related parameters of various peaks of oVEMP.


  Materials and Methods Top


Participants

The study included 50 adults (25 males and 25 females; age range: 18−30 years; mean age: 25.2 ± 5.7 years) with normal audiovestibular functioning as shown by normal results in audiological evaluation (pure-tone audiometry, speech audiometry, immittance evaluation, and auditory brainstem response). The vestibular health was ensured through administration of detailed case history using neurootological questionnaire (unpublished) developed by the Department of Otolaryngology, All India Institute of Speech and Hearing, Mysore, India. This questionnaire has subsections which focus on giddiness/dizziness (swaying, blackouts, falling, and light-headed), vegetative symptoms, duration of symptoms, and trigger mechanisms. Questions related to other eye, ear, taste, and neurological disorders, which can lead to dizziness, are also included. The participants were further screened to rule out the presence of any balance system abnormalities using behavioural measures such as Romberg test, Fukuda stepping test, Tandem gait test, and Past pointing test. They were included in the study after obtaining informed written consents for their participation. Further, their enrolment to the study was completely voluntary and on a nonpayment basis. The study was approved by the institutional review board and received ethical committee clearance as a part of the approval for doctoral thesis synopsis.

Procedure

After compliance with the subject selection criteria, the participants underwent oVEMP recording. They were seated comfortably in an upright position and gold-plated cup-shaped electrodes were placed at the respective electrode sites after scrubbing the skin overlying the inferior oblique muscle and forehead with a commercially available skin preparing gel. The electrode configuration involved the placement of the noninverting electrode directly beneath the eye over the inferior oblique muscle of the eye, inverting electrode 2 cm below the active electrode over the cheek and the ground electrode on the forehead, as this configuration is reported to be optimal by previous studies on oVEMP. [1],[2],[3] The absolute electrode impedance and interelectrode impedance values were ensured below 5 kΩ and 2 kΩ respectively. The standard Etymotic ER-3A insert earphones of the Biologic Navigator Pro evoked potential system (version 7.0.0) was placed in the contralateral ear canal as contralateral stimulation is reported to result in higher response prevalence, larger amplitudes, and lower variability of the oVEMP response resulting from the crossed nature of the utriculoocular pathway. [25],[3],[6] The participants were required to maintain eye gaze in the superomedial direction throughout the recording by fixing the gaze on a point at an elevation of 30°, which is found appropriate by previous studies. The oVEMP were obtained in all the participants beginning with 125 dB SPL alternating polarity tone-bursts (Blackman window, 2 ms rise/fall time, 0 ms plateau time) at the octave and midoctave frequencies from 250 Hz to 2000 Hz. The order of presentation of each of the frequencies was randomized to counter the order effect. The tone-bursts were presented at a rate of 5.1 Hz to obtain averaged waveforms for 200 sweeps per recording. The intensity was reduced in 10 dB sound pressure level steps to reach the threshold at each frequency. An analysis window of 64 ms (including a prestimulus recording of 11 ms for obtaining baseline) was used. The responses were band-pass-filtered between 10 and 1000 Hz and amplified by a factor of 5000.

Measures and statistics

The waveforms were analyzed by two experienced audiologists independently. After ensuring high interjudge reliability (α ≥ 0.8 on Chronbach's alpha test) and high correlation (r ≥ 0.8), the markings by only one of the judges was used for further statistical procedures. Of the five peaks that have been reported to occur among healthy individuals, first negative (n1), first positive (p1), and second negative (n2) were found to consistently occur among all the participants and hence parameters related to only these peaks were documented for further statistics. The parameters analyzed and documented were response rates, absolute latency, interpeak latency difference (ILPD), interaural latency difference (IALD), peak-to-peak amplitude, interaural amplitude ratio (IAAR) (asymmetry ratio), and threshold. The IAAR was calculated using Jonkee's formula earlier derived for use with cervical vestibular evoked myogenic potential (cVEMP). [26] As per this formula, IAAR is the percentage of ratio obtained from dividing the difference between the peak-to-peak amplitude of one side by peak-to-peak amplitude of the other side's response.

The statistical analysis was done for the data obtained from all the participants of the study using commercially available Statistical Package for Social Sciences (SPSS) software version 17.0. The parameters of ocular vestibular-evoked myogenic potential responses were subjected to statistical analysis using two-way repeated measures analysis f variance (ANOVA), repeated measures ANOVA, and Bonferroni adjusted multiple comparisons, in addition to obtaining mean and standard deviation.


  Results Top


The responses were recorded across six frequencies from both ears of all the 50 participants and waveforms were marked for n1, p1, and n2. [Figure 1] shows the waveforms obtained from one of the participants of the study.
Figure 1: Representative waveforms from one ear of one of the participants showing variations in parameters of ocular vestibular-evoked myogenic potential with changes in frequency

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The responses were present in both ears of all 50 participants for tone-burst frequencies from 250 to 1000 Hz and declined with increasing frequency thereafter. The response rate was 85% (six individuals with binaural absence and three with monaural absence) and 69% (12 with binaural absence and 7 with monaural absence) at stimulus frequency of 1500 and 2000 Hz, respectively. There was a trend toward increase in absolute latencies of all the peaks with increasing frequency of the stimulus. Such a specific trend was not observed for interpeak latency difference (IPLD), IALD, and IAAR. However, the amplitude and threshold demonstrated an interesting trend. The largest amplitudes and best (smallest) thresholds were observed at 500 Hz. While amplitude reduced, the thresholds increased (worsened) gradually on changing frequency in either direction of this frequency. [Table 1]a and b show mean and standard deviation of these parameters across frequencies.

Latency

The effect of frequency on latency of n1, p1, and n2 was evaluated using two-way repeated measures ANOVA for frequencies and ears. Separate two-way repeated measures ANOVA were administered for each peak as the study did not aim at interpeak comparisons for latency. The results revealed no significant main effect of ear on latency of n1 [F (1,67) = 0.372, P > 0.05], p1 [F (1,67) = 0.378, P > 0.05] as well as n2 [F (1,67) = 0.149, P > 0.05]. However, there was a significant main effect of frequency on latency of n1 [F (5,335) = 54.999, P < 0.001], p1 [F (5,335) = 55.494, P < 0.001], and also n2 [F (5,335) = 188.962, P < 0.001]. However, a significant interaction effect between ears and frequencies was not observed for latency of n1 [F (5,335) = 0.358, P > 0.05], p1 [F (5,335) = 0.915, P > 0.05], or n2 [F (5,335) = 0.630, P > 0.05]. The Bonferroni adjusted multiple comparisons for each of the above peaks revealed significantly longer latency at 250 Hz compared with all other frequencies (P < 0.05) and no significant difference was observed between any of the other pairs of frequencies (P > 0.05).

IPLD

The ILPD was calculated between two pairs of peaks viz. n1and p1 (n1p1) and p1 and n2 (p1n2). The ILPD for both the pairs of peaks appeared to show larger values at 250 Hz compared with other frequencies; however, no noticeable difference was observed between the frequencies thereafter. [Figure 2] shows the changes in ILPD for n1p1 as well as p1n2 across frequencies. In order to evaluate the statistical significance of this observation, separate two-way repeated measures ANOVA were used for each peak pair of ILPD. The results revealed a significant main effect of frequency on ILPD of n1p1 [F (5,335) = 6.009, P < 0.001] as well as p1n2 [F (5,335) = 45.597, P < 0.001]. However, there was neither a significant main effect of ear on n1p1 [F (1,67) = 0.268, P > 0.05] and p1n2 [F (1,67) = 0.067, P > 0.05] nor a significant interaction between frequency and ear for n1p1 [F (5,335) = 2.098, P > 0.05] or p1n2 [F (5,335) = 1.377, P > 0.05] ILPD. The Bonferroni adjusted multiple comparisons revealed the ILPD at 250 Hz to be significantly longer than all other frequencies for both the pairs of IPLD (P < 0.05). There was no significant difference between any of the other pairs of frequencies (P > 0.05).
Figure 2: Mean and 95% confi dence intervals of absolute latencies and interpeak latency difference of n1, p1, and n2 peaks ocular vestibular-evoked myogenic potential across frequencies. ''Star'' indicates statistically significant difference (P < 0.05)

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IALD

The IALD was obtained for n1, p1 as well as n2 peaks of the oVEMP waveform. The IALD for each peak was compared across frequencies using separate repeated measures ANOVA for each peak. The results revealed no significant main effect of frequency on IPLD of n1 [F (5,105) = 2.055, P > 0.05], p1 [F (5,105) = 1.573, P > 0.05], and n2 [F (5,105) = 1.508, P > 0.05]. Hence, further analysis using post hoc test was not required.

Amplitude

The peak-to-peak amplitudes for both the peak complexes (n1p1 and p1n2) were observed to be largest for a frequency of 500 Hz and declined subsequently on either side thereafter. [Figure 3] shows the amplitude across frequencies for the peak complexes. The statistical significance of this trend was investigated using two-way repeated measures ANOVA. The results of the statistical analysis indicated a significant main effect of frequency on amplitudes of n1p1 complex [F (5,335) = 128.357, P < 0.001] as well as p1n2 complex [F (5,335) = 148.376, P < 0.001]. Nonetheless, there was no significant main effect of ear for either n1p1 complex [F (1,67) = 0.278, P > 0.05] or p1n2 complex [F (1,67) = 0.135, P > 0.05]. Further, there was also no significant interaction between ear and frequency for amplitude of n1p1 complex [F (5,335) = 0.390, P > 0.05] as well as p1n2 complex [F (5,335) = 0.239, P > 0.05]. The Bonferroni adjusted multiple comparisons revealed the amplitude of n1p1 complex to be significantly different between all the possible pairs of frequencies (P< 0.001) except between 250 Hz and 750 Hz and also between 1500 Hz and 2000 Hz which were not significantly different (P> 0.05). The Bonferroni adjusted multiple comparisons revealed significant difference between all the possible pairs of frequencies (P< 0.001) for amplitude of p1n2 complex.
Figure 3: Mean and 95% confi dence intervals for n1p1 and p1n2 complex amplitude of oVEMP across frequencies

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IAAR

Repeated measures ANOVA was done to compare IAAR across frequencies. The results revealed a significant main effect of frequencies on IAAR for n1p1 complex of oVEMP [F (5,105) = 7.014, P < 0.001]. The Bonferroni adjusted multiple comparisons revealed a significant difference between 250 Hz and 2000 Hz (P < 0.05) and 500 Hz and 2000 Hz (P < 0.01). There was no significant difference between any of the other possible pairs of frequencies. Further, repeated measures ANOVA revealed no significant main effect of frequency on IAAR for p1n2 complex. Hence further post hoc tests were not necessitated. [Figure 4] shows the effect of frequency on IAAR of n1p1 as well as p1n2 complex of oVEMP.
Figure 4: Mean and 95% confidence intervals for interaural amplitude ratio (asymmetry ratio) of ocular vestibular-evoked myogenic potential across frequencies. ''Star'' indicates statistically significant difference(P < 0.05)

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Threshold

Threshold was defined as the lowest intensity level in dB SPL at which a particular peak complex could be reliably identified. The thresholds at 500 Hz were observed to be smallest (best) and subsequently worsened (increased) on either direction of 500 Hz. [Figure 5] shows this trend in threshold for both the peak complexes. Separate two-way repeated measures ANOVA for threshold of n1p1 and p1n2 complex were accomplished to evaluate the effect of ear as well as frequency. The results revealed no significant main effect of ear on threshold of n1p1 complex [F (1,67) = 3.172, P > 0.05] as well as p1n2 complex [F (1,67) = 0.085, P > 0.05]. However, there was a significant main effect of frequency on threshold of n1p1 complex [F (5,335) = 208.134, P < 0.001] and also p1n2 complex [F (5,335) = 205.322, P < 0.001]. Further, there was no a significant interaction between ear and frequency for threshold of n1p1 complex [F (5,335) = 2.221, P > 0.05] and p1n2 complex [F (5,335) = 0.689, P > 0.05]. The Bonferroni adjusted multiple comparisons revealed the threshold of both the peak complexes to be significantly different between all the possible pairs of frequencies (P < 0.01). The only exception to this was threshold of n1p1 at 750 Hz which was not significantly different from the threshold at 250 Hz and 1000 Hz (P > 0.05).
Figure 5: Mean and 95% confidence intervals for threshold of n1p1 and p1n2 peak complexes across frequencies of ocular vestibular-evoked myogenic potential

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  Discussion Top


During the course of the study, the contralateral oVEMP were recorded from 50 individuals with normal audiovestibular system. The response rate was found to be 100% for frequencies between 250 and 1000 Hz and deteriorated sharply to 85% and 69% at 1500 and 2000 Hz. This is in agreement with the previous studies which also reported reduction in response prevalence for higher frequencies. [16] Higher response rate (response prevalence) might be attributed to low frequency responsivity of the otolith organs. [27]

Latency

The absolute latencies were compared across frequencies for each of the three peaks (n1, p1, and n2). The results revealed significantly longer latency for 250 Hz compared with all the other frequencies. For the other frequencies, although there was a trend toward reduction in latency with increasing frequency, the results of statistical analysis confirmed a lack of statistical significance of this trend. The studies in the oVEMP literature have shown equivocal findings in this regard. While the results of the present study shows agreement with some of the studies, [17],[16] there is slight disagreement with the findings. [20] Though Chihara et al., did not mention about this observation, the ''[Table 1]'' of their results tends to demonstrate agreement with the findings of the present study. In their study, the latency of n1 for air-conducted tone-bursts was shown to vary from 11.6 ± 0.7 ms at 250 Hz to 10.2 ± 0.7 ms at 1000 Hz. Similarly, the latency of p1 was observed to decrease from 17.8 ± 0.7 ms at 250 Hz to 15.5 ± 0.7 ms at 1000 Hz. This pattern was evident both for air-conduction as well as bone-conduction stimuli. The finding of significantly longer latency for 250 Hz stimuli compared with other frequencies in Murnane et al., has been attributed to a software fault which produced longer (double) rise/fall times for this frequency compared with others. However, the present study had exercised control in terms of rise/fall and plateau time and thereby the stimulus duration. The stimulus duration used in the present study was 4 ms (2 ms rise/fall time and 0 ms plateau time) irrespective of the frequency. However, the findings remained pretty much the same as the one reported by Murnane et al. Therefore, it appears unlikely that the rise/fall time could have produced the difference observed between the latency corresponding to 250 Hz and all others. The results of Piker also revealed, although not statistically significant, a tendency for n1 latencies to decrease with increasing tone-burst frequency ((Figure 17). [20] Thus, there appears a consensus that latencies show a tendency for reduction with increasing stimulus frequency. However, the reason behind such an occurrence is not yet clear. More physiological studies are required to unveil the reason behind such an occurrence only for the latency of 250 Hz.
Table 1:

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The results regarding comparison of IPLD among the frequencies portrayed an identical picture to the results of absolute latencies. The IPLD corresponding to a tone-burst frequency of 250 Hz was found to be significantly longer than those at other frequencies. This was true not only for n1p1 but also for p1n2 IPLD. This was again true despite the control on the other stimulus parameters like rise/fall time, plateau time, and stimulus duration across the frequencies. None of the other studies exploring the effect of frequency on oVEMP parameters have touched upon this aspect and hence this may be considered a novel finding.

The latencies of each of the peaks were compared between the ears in order to arrive at the IALD. The results of statistical analysis revealed no significant difference in IALD across frequencies. Although previous studies have not looked in to this aspect of latency, this might be a crucial parameter when evaluating the pathologies affecting latencies, especially unilateral pathologies like vestibular schwannomas.

Amplitude

The comparison of amplitudes of n1p1 as well as p1n2 complex across frequencies revealed maximum amplitude at 500 Hz and a subsequent reduction in the amplitudes on either side of the frequency thereafter. Though there are no reports regarding the effect of frequency on the amplitude of p1n2 complex, the reports with regard to n1p1 complex are in sync with the findings of those reported previously. [19],[21] The findings of the present study are, however, not in consonance with some of the other studies reported previously in this context. [18],[28] While Todd et al., reported the maximum amplitude of oVEMP for frequencies between 400 and 800 Hz, Lewis et al., confirmed it to be associated with a frequency centred at 1000 Hz. Todd et al., did not exclusively record oVEMP at 500Hz, which has been reported as the tuning frequency by several others including ours. Nonetheless, the results of present study fall within the range of frequencies reported by them. However, the results of present study were in stark contrast to those reported by some of the others. [22] The differences from the findings of Lewis et al., might be attributed to the use of 10 ms plateau time and 1 ms rise/fall time by these authors as against the used of 0 ms plateau time and 2 ms rise/fall time in the present study. The study assessing the effect of rise/fall and plateau time on oVEMP amplitude has shown a no significant effect of variations in these parameters on amplitude. [14] However, the plateau times used in the above study were very limited (2 ms and 4 ms) and hence may not explain the differences observed. As opposed to this, Lee et al., had shown largest amplitudes and lowest thresholds for 0.5 and 1 ms rise/fall times when used in conjunction with 2 ms plateau time among a host of other rise/fall and plateau times. The impact of variations in rise/fall time and plateau time might be more clearly illustrated by the reports regarding the impact of these parameters on cVEMP, a close associate of oVEMP. While exploring the impact of variations in these parameters, the studies have shown smaller amplitudes for smaller rise/fall [29],[30] and plateau times [31] Such a large difference in these stimulus parameters and thereby the corresponding increase in stimulus duration might have resulted in larger differences in energy between 500 Hz and 1000 Hz in Lewis et al., which might have caused higher amplitude for 1000 Hz compared with 500 Hz. In the present study, the difference was relatively smaller between the two sets of stimuli which probably did not enhance 1000 Hz considerably. Owing to cVEMPs and oVEMPs being otolith responses and hence closely related, a similar effect of rise/fall and plateau time may be expected for oVEMPs. In addition to the differences in stimulus parameters observed between the two sets of studies, the differences might also be brought about by the large difference in sample size (12 subjects in Lewis et al., as against 50 subjects in the present study). The differences from Taylor et al., might be attributed to the use of decibel normalized hearing level by them as against dB SPL in the present study.

The reasons behind maximum amplitude at 500 Hz have been shown to derive majorly from two schools of thoughts. While Welgampola and Colebatch [32] associated the origin of low frequency resonance to electrical resonance of the hair cells, Todd et al., attributed this to the mass-spring damping properties of the otolith organs. The latter explanation appears more viable and has received support from the studies on pathologies which cause changes in frequency tuning properties of the otolith responses. [33] The resonance frequency for human otolith organs has been reported to be in the vicinity of 400−500 Hz [27],[34] and thereby conforms to the finding of maximum amplitude at 500 Hz which is closest to 400 Hz among the frequencies used in the present study. In addition to the resonance of the middle ear, the contribution from the middle ear filtering, which was not accounted for in the present study, might have also had some influence on the finding, especially at 250 Hz.

IAAR

The IAAR, which is also frequently referred as asymmetry ratio, was compared between the frequencies and the results revealed no difference in this aspect between the frequencies. The findings of the present study are in concurrence with those reported previously. [21] While evaluating the frequency tuning properties of oVEMP, they also reported a lack of difference in IAAR between the frequencies in healthy individuals. A lack of difference in this parameter between the frequencies, even though amplitudes are affected by frequency, could be explained on the basis of the way it is calculated. The IAAR is the percentage ratio of differences in amplitude between the ears and their sum for a particular stimulus. Thus, it is a comparative value of interear comparison for the same stimulus. Also, the two ears are likely to maintain the aspect ratio across the stimuli. For example, the difference in amplitude between two frequencies in one ear is likely to be proportional to the difference between amplitudes of the same two frequencies in the other ear of the same healthy individual. This would thus contribute to near sameness in the asymmetry ratio for both the stimuli.

Threshold

The findings of the present study revealed that best (lowest) thresholds of oVEMP were obtained for a frequency of 500 Hz in the healthy individuals. This is in agreement with the previous research reports in this context. [19],[23] The reasons for this frequency bias could again be attributed to the mechanical resonance of the otolith organs which was reported to lie in the vicinity of 400 Hz [27],[28],[29],[30],[31],[32],[33],[34] in addition to the middle ear filtering that is likely to impact the mow frequencies more.


  Conclusion Top


The largest amplitudes and the lowest threshold corresponded to 500 Hz tone-burst, whereas 250 Hz produced largest absolute latencies as well as interpeak latency intervals. Frequency had no effect on both the interaural parameters (IALD as well as IAAR). Owing to largest amplitudes and best thresholds, 500 Hz appears better stimuli for clinical recording of oVEMPs. This is true irrespective of whether the peak complex being assessed is n1p1 or p1n2.

 
  References Top

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