3.1. Middle-ear pressures
The tympanic peak pressure (TPP), assumed equal to the middle-ear pressure (MEP), was measured before each DPOAE and reflectance measurement session. reports these MEPs at each of the two postural positions. The MEP from 11 of the 12 ears was always within ±25 daPa of zero with the subject in the upright position; the exception was Subject 9, whose MEP ranged from −66 to −30 daPa in the upright position. When the subjects were tilted, 5 of these 11 ears remained within ±25 daPa of zero for all sessions. The seven ears that were not always within ±25 daPa of zero tended to show increases in MEP between the upright position and the tilted position. The average change in MEP when a subject went from upright to tilted was calculated for each subject, and then averaged across all subjects, resulting in an average increase of 32 daPa.
Figure 1 Middle-ear pressures measured as tympanic peak pressure (TPP) via tympanometry. Measurements are offset slightly on the x-axis in the order they were taken; for a given ear, the upright measurement made first is furthest to the left and the corresponding (more ...)
There were a total of 54 pressure measurements in the upright position and 54 measurements in the tilted position (excluding those that correspond to high distortion measurements, see methods section 2.6); 53 of these measurements are common to measurements made consecutively in the upright and tilted position, resulting in the ability to calculate 53 changes in pressure between the upright and tilted position. If we define ΔMEP as ΔMEP
, then of the 53 measurements of ΔMEP, 31 cases have −6 ≤ ΔMEP ≤ 25 daPa, 8 cases have 25 < ΔMEP < 50 daPa, 7 cases have 50 < ΔMEP < 75 daPa, and 7 cases have 75 < ΔMEP ≤ 90 daPa.
3.2. Upright and Tilted Measurements
Distortion product otoacoustic emissions (DPOAE) and reflectance were measured on the left ears from 12 subjects in both the upright and tilted positions. shows an example of these measurements from Subjects 4, 9, and 11. Subjects 4 and 11 were chosen at random from the 12 subjects, using the “rand” function in Matlab, and subject 9 was chosen as the ear that showed some of the smaller changes in DPOAE measures with tilting and had the least repeatable reflectance measures. The upper row of plots the DPOAE magnitudes for each ear with the subject in the upright position (black) and the tilted position (gray), the middle row plots the corresponding DPOAE angles, and the lower row plots the power reflectances. The results from these example subjects have several features in common with the results from the other nine subjects (not shown). In general, tilting the subject reduced the DPOAE magnitudes the most at the lower frequencies, with reductions in magnitude up to 1500 to 2000 Hz. Above 2000 Hz, the changes in DPOAE magnitude between upright and tilted were generally less than 5 dB.
Figure 2 DPOAE magnitudes (upper row), DPOAE angles (middle row) and power reflectance (lower row) from the left ears of Subject 4 (left column), Subject 9 (middle column), and Subject 11 (right column). Measurements plotted in black correspond to the upright (more ...)
The DPOAE angles showed systematic changes between the upright and tilted conditions, but unlike the DPOAE magnitudes, the changes in angle are larger for frequencies above about 1000 Hz. For most frequencies the tilted position led to increases in the angle, which varied from a small fraction of a cycle to more than a quarter of a cycle. The change in angle depends on both frequency and the specific ear. This systematic change in angle between the upright and tilted positions documented here for the example subjects is consistent with the measurements on all other ears. In some of the other ears, the change in angle was a reduction in angle instead of an increase in angle when tilted; however, the repeatable and steady change between the two positions occurred across all ears.
The power reflectance generally increases with tilting for the lower frequencies, with smaller changes above 1000 to 2000 Hz. While some ears show repeatable power reflectance from session-to-session (e.g., Subject 4), others show both more intrasubject variability across sessions that include either clear separation between upright and tilted measurements (e.g., Subject 11) or some overlap between the upright and tilted measurements (e.g., Subject 9 is one of three ears that fit this latter category).
3.3. Changes in DPOAE and reflectance measurements with changes in posture
For each measurement session on each ear, the difference between the DPOAE magnitude, DPOAE angle, and power reflectance in the upright and tilted positions was calculated, and the mean difference for each ear is plotted ( upper row). The upper-left plot shows that the group mean of the individual mean DPOAE magnitude differences increases from about 10 dB at 500 Hz to 13 dB at 1000 Hz, decreases to 7 dB at 1400 Hz, and then decreases to nearly zero above 2000 Hz. All measurements on all ears show this general pattern of larger low-frequency differences and small to nearly no differences above about 2000 Hz. Thus, DPOAE magnitudes are systematically reduced at frequencies below 1500 Hz when a subject is tilted.
Figure 3 Mean changes (upper row) and the corresponding p values (lower row) for each individual ear in DPOAE magnitudes (left), DPOAE angles (middle), and power reflectance (right) between the upright and tilted positions. The DPOAE magnitudes and angles and (more ...)
The middle plot () shows that the group mean of the individual mean DPOAE angle differences is systematically different from zero. For most subjects at most frequencies, the angle difference is on the order of −0.10 to −0.25 cycles. Thus, DPOAE angles appear to be systematically changed from normal at all frequencies in the 500 to 4000 Hz range when a subject is tilted.
The right plot shows that the group mean of the individual mean power reflectance differences is systematically different from zero at lower frequencies; below 1500 Hz, most ears show mean changes ranging from −0.05 to −0.25. Above 1500 Hz, the changes are smaller and are both positive and negative, with a mean that hovers near zero. In summary, changes in reflectance measures between the upright and tilted positions appear to be systematically different from zero at frequencies below 1000 to 1500 Hz.
(lower row) plots p values computed for each individual subject to test whether or not the data collected at the two positions (upright and tilted) are different. The DPOAE magnitudes show their strongest changes at the lowest frequencies, with a median p value below 0.01 for frequencies from 600 to about 1500 Hz. Above about 2000 Hz, the DPOAE magnitudes become more similar between the two conditions, and the p values associated with DPOAE magnitudes measured at the two positions increase above 0.05. In contrast to the DPOAE magnitudes, the DPOAE angles have a median p value below 0.01 from 600 to 3000 Hz; above 3000 Hz the 25 to 75% range approaches 0.5, but below 3000 Hz the p values associated with all DPOAE angles are generally below 0.05. The power reflectances show their smallest p values at the lowest frequencies. Below about 1000 Hz, the median p values associated with changes in power reflectance are below p = 0.01, and above 1000 Hz the median p value exceeds 0.05.
The dashed lines in all of the plots of correspond to three ears that include 11 of the 14 largest changes in middle-ear pressure (|ΔMEP| > 50daPa). There is no evidence that the results from these ears with larger ΔMEP changes differ systematically from ears with smaller changes in ΔMEP.
The repeated measures regression model results are shown in . This model includes all data taken on all ears and predicts significant changes between positions for all three measurements at the p < 0.001 level up to 2016 Hz. The DPOAE magnitudes difference between upright and tilted increases with frequency up to a maximum of 14.3 dB at 844 Hz and then decreases with increasing frequency at most of the measured frequencies. There is not a systematic trend with frequency in the DPOAE angle difference, but the value itself ranges from −0.08 to −0.22 cycles for the 516 to 2016 Hz frequency range. The effect of frequency on the power reflectance difference is similar to that for the DPOAE magnitudes; the difference increases with frequency to a maximum of 0.21 at 844 Hz and then generally decreases with frequency to 0.05 at 2016 Hz. Above 2000 Hz, the DPOAE angle continues to be a significant predictor at the p < 0.001 level up to the highest frequency measured of 3984 Hz. The DPOAE magnitudes and power reflectance are significant at some frequencies above 2000 Hz.
Table 1 Differences in the predicted least-squares means of DPOAE magnitudes (dB), DPOAE angles (cycles), and power reflectance between the upright (90°) and tilted (−45°) positions.
3.4. Intra-subject variability of measurements
Multiple measurements of DPOAE magnitudes, DPOAE angles, and reflectance were made on each subject. plots the standard deviations of the multiple measurements for each individual ear for each condition. In the upright position, DPOAE magnitudes have standard deviations between about 1 and 2 dB SPL (25 to 75% range) for the frequency range of 500 to 4000 Hz (, upper left). The tilted condition (, upper middle) has a somewhat larger 25 to 75% range of about 1.5 to 3 dB SPL for frequencies below 2000 Hz, with a similar range to the upright condition above 2000 Hz. The standard deviations for the DPOAE angles ( middle row) are smaller in the upright position than tilted, with a 25 to 75% range generally within about 0.01 to 0.04 cycles for upright and about 0.02 to 0.06 cycles when tilted. The standard deviations for the power reflectance (, lower row) have means that hover near 0.05 for both conditions, with a 25 to 75% range that is smaller for the upright position than for the tilted one. For frequencies below about 2000 Hz, the tilted subjects have a larger range of standard deviations that approach 0.10 for some frequencies.
Figure 4 Standard deviations calculated for DPOAE magnitudes (upper row), angles (middle row), and power reflectance (lower row) for upright (left column) and tilted (right column) postures. For each subject and each posture, a standard deviation was calculated (more ...)