Distortion product otoacoustic emission temporal responses for magnitude and phase showing ΔLI (switched f1 stimulus timing) in one rat are shown in Figure . The f2 frequency was 10 kHz. The temporal envelope magnitudes are shown in the top row, and the corresponding phases are shown in the bottom row. Each panel, moving from left to right, shows DPOAE responses evoked by increasingly higher primary levels in 5-dB steps. The DPOAE temporal envelopes for magnitude are similar to those found by other researchers in various species (Liberman et al. 1996
; Kim et al. 2001
; McGee et al. 2000
; Sun and Kim 1999
). The DPOAE magnitude near the onset initially increased with intensity, decreased to a minimum around L2 = 70 dB SPL, then substantially increased with intensity at the highest primary levels. If one were to plot the I/O function, a “notch” would be observed at 70 dB SPL. Subsequently, ΔLI (the difference between the DPOAE magnitude at the onset and that near the end of the trace) initially increases with intensity, then decreases and reverses direction (i.e., becomes negative), then increases again.
Fig. 3 DPOAE magnitude (top row) and phase (bottom row) using the switched f1 condition and an f2 frequency of 10 kHz in rat 072701. Middle-ear muscles were intact. The f2 stimulus levels used to evoke the magnitude and phase temporal responses are indicated (more ...)
Temporal envelopes for phase also change with time and, in a manner, linked with magnitude changes. Near the onset of the stimulus, the phase changes rapidly by small amount, then there are small, slow changes during the later duration of the stimuli. The direction of phase change is linked with the direction of the magnitude change. In the quasi-steady-state part of the response, the phase increases slightly and slowly with time when the corresponding DPOAE magnitude decreases with time. When the DPOAE onset magnitude increased with time (L2 = 70 dB SPL), the corresponding phase decreased with time. At the highest intensity, phase remained as decreasing with time.
The ΔLC (measured using the f1 continuous timing) for the same rat and primary stimulus conditions is shown in Figure . The format of the figure is similar to that used in Figure . The change in magnitude and phase from approximately 120 to 320 ms is a result of the introduction of the 60 dB SPL noise presented contralaterally. The amount of ΔLC increases, then decreases, and then reverses absolute value (becomes positive) at the highest intensity level. The change in ΔLC and its reversal in absolute value exhibits a similar pattern seen for ΔLI. The phase behavior changes in a similar way as well. That is, during contralateral stimulation when magnitude decreased, ΔPC was positive (phase became less negative). The ΔPC reversed direction when the DPOAE magnitude increased during presentation of the contralateral noise. In both Figures and , it appears that the starting DPOAE phase exhibits a pattern of small, increasing phase with f2 level with a large jump in phase near the notch.
Fig. 4 DPOAE magnitude (top row) and phase (bottom row) using the continuous f1 condition for an f2 frequency of 10 kHz in the same rat whose data are shown in Figure . ΔLC changes with stimulus level, becomes small for L2 = 70 dB (more ...)
Distortion product otoacoustic emission temporal envelopes for ΔLI and ΔLC in the same rat after cutting both the middle-ear muscles are illustrated in Figures and , respectively. It can be seen that both ΔLI and ΔLC are reduced considerably after sectioning of the middle-ear muscles. When residual ΔLI and/or ΔLC were observed, corresponding phase changes were either absent or very small (Figure , L2 = 75 dB SPL). It is presumed that the residual magnitude changes are the result of activation of the MOC reflex. Although smaller in amplitude, the time courses of the residual magnitude changes appear similar to those seen when the MEMs are intact.
Fig. 5 DPOAE magnitude (top row) and phase (bottom row) using the switched f1 condition for the same rat whose data are shown in Figures and after sectioning of the MEMs. ΔLI and ΔPI are substantially reduced after MEM (more ...)
DPOAE magnitude and phase using the continuous f1 condition in rat 072701 after sectioning of the MEMs. ΔLC and ΔPC are substantially reduced.
Figure shows DPOAE I/O functions using the switched f1 stimulus paradigm. The left panel illustrates the I/O function before MEM sectioning, and the middle column depicts data after MEM sectioning. The solid lines illustrate the I/O function using the DPOAE level computed from the onset of the trace (first hatched area shown in panel 2a), and the dashed line depicts the I/O function computed from the end of the temporal response (second hatched area in panel 2a). Each row is for a different f2 frequency. Before MEM section, several observations can be made. First, the I/O functions are different depending on f2 frequency. Secondly, the I/O functions for the onset and the end of the trace are different from each other. For all but the lowest frequency, the onset and offset levels are similar at low stimulus levels, but as stimulus level is raised, the DPOAE levels at the two measurement periods are different. With increasing intensity, where there are nonmonotonicities, the pattern in the I/O function for the end of the trace appears to be shifted to the right. The marked differences in I/O functions between the beginning and end of the trace are much reduced after MEM section, as can be seen in the middle column. There are still small differences between the I/O functions, but they are quite similar, even with respect to nonmonotonicities. These findings suggest that the large differences in the I/O functions between the beginning and end of the temporal responses as shown in the left panel are due mainly to middle-ear muscles. The panel at the right shows a comparison in onset I/O functions before (thicker line) and after MEM sectioning (thinner line). One important finding is that after MEM section, DPOAE levels are reduced and not in the same manner for each frequency. The majority of functions are similar to f2 frequencies of 6, 8, 12.5, and 16 kHz, where there is a shift of the function to the right and/or decreased levels in areas of saturation. Few differences were like the function for f2 = 10 kHz, where low levels are changed and not high levels. In all cases, slopes of the I/O functions remain similar pre- and post-MEM section. Even after MEM section, notches still exist, but not always at the same stimulus levels as before MEM sectioning. Because of the nonmonotonic I/O functions, both before and after MEM sectioning, it is potentially misleading to consider the effects of cutting MEMs at any one primary level. The difficulty is compounded by the shifts in the I/O function that were observed after sectioning the muscles. The differences in the I/O functions pre- and post-MEM section were attributed to the accumulation of fluid in the middle-ear space and the change in bulla integrity after MEM section.
Fig. 7 DPOAE I/O functions for one rat using the f1 switched condition. I/O functions are shown before MEM section (left column), after MEM section (middle column), and comparison before and after MEM section (right column). Each panel is a different f2 frequency. (more ...)
The amounts of ΔLI and ΔLC, before and after sectioning of the MEMs in three rats, are shown in Figures and , respectively. Each column illustrates data for a different rat. Each panel shows either ΔLI (Fig. ) or ΔLC (Fig. ) for the five different primary frequencies studied, with the lowest-frequency primaries in the top panels and increasing frequency with successively lower panels. The dotted or gray bars represent ΔLI or ΔLC measured with intact MEMs, and the black bars represent these phenomena after the MEMs had been cut. Several trends are apparent for both figures. First, the absolute value of ΔLI or ΔLC is greater than 10 dB and, occasionally, greater than 20 dB for some f2 intensities and DPOAE frequencies prior to MEM sectioning. Second, the amount of ΔLI or ΔLC is smaller for higher frequencies. Third, for all stimulus conditions, both ΔLI and ΔLC are reduced after sectioning the MEMs and rarely do the measures exceed 5 dB. We interpret these affects as a result of the MOC reflex. These observations support the conclusion that ΔLI and ΔLC in the rat are mediated, for the most part, by the MEM reflex. Fourth, there is variability among individuals in the size of ΔLI and ΔLC, both before and after MEM sectioning. Fifth, the magnitude of ΔLC is greater than ΔLI. Finally, the amount of either ΔLI or ΔLC, as well as whether itis a reduction in DPOAE level or an enhancement,is highly dependent on the I/O function. Thisis true for before and after MEM sectioning andcan be understood best by the data illustrated in Figure .
Fig. 8 ΔLI as a function L2 for three rats. Each column illustrates data for a different rat. Each row depicts data for different f2 frequencies, starting from the lowest frequency tested at the top of the column to the highest frequency tested at the (more ...)
ΔLC as a function of L2 level for three rats. The same plotting conventions were the same as those used for Figure .
Fig. 10 DPOAE I/O functions are plotted as a function of f2 level using the f1 switched condition (solid lines) and the f1 continuous condition (dashed lines) pre-MEM section. For each f2 level, the ΔLI (black bars) and ΔLC (patterned bars) are (more ...)
The relationship between amount of ΔLI/ΔLC and the I/O function is illustrated in Figure . Input–output functions for a DPOAE measured at f2= 6 kHz before MEM sectioning were measured near stimulus onset for the switched f1 condition (black line) and the continuous f1 condition (dashed line). The vertical bars show the amount of ΔLI (black bars) and ΔLC (striped bars) at each intensity. The I/O functions are nonmonotonic, each with a pronounced notch at a mid-to-high intensity. The data plotted here are representative of the entire data set. The direction and extent of ΔLI and ΔLC are related to the slope of the DPOAE I/O function. The direction of the ΔLI, decreasing or increasing, generally depends on whether the slope of the input/output function is positive or negative, respectively. From the maximum peak of the I/O function to the minimum of the notch, where the slope is negative, is where apparently paradoxical increasing ΔLI (and increasing responses to contralateral noise) is observed. At the minimum, notch response shape can be multiphasic because of the discontinuity in the slope.
Distortion product otoacoustic emission phase from three rats is illustrated in Figure . All data are for the continuous f1 condition as it was easier to measure phase more reliably from this condition as compared with the switched f1 condition. Absolute phase was not calibrated, but the relative phases within an I/O are valid. The DPOAE phase before sectioning the MEMs is depicted by closed circles, and the DPOAE phase near the offset of the contralaterally presented noise is depicted by open triangles. Distortion product otoacoustic emission phase after MEM sectioning is depicted by closed squares, and DPOAE phase near the offset of the contralaterally presented noise after MEM sectioning is depicted by open diamonds. Solid arrows and dashed arrows indicate the levels at which a “notch” or low point in the I/O function (found at the highest primary levels we presented) was found pre- and post-MEM section, respectively. The top panel (a) shows data representative of rats having notched I/O functions. Distortion product otoacoustic emission phase before MEM and after MEM sectioning showed small changes in phase with increasing signal level. Between L2 levels of 70 and 75 dB SPL, there were large changes in phase, approximately 180°. The I/O function pre-MEM section exhibited a sharp decrease at 75 dB SPL and a “notch” at 70 dB SPL post-MEM section. The large phase difference occurred near stimulus levels at which there was a notch in the I/O function. As shown by the open triangles, the DPOAE measured during the contralateral noise is fairly flat for L2 levels less than 75 dB SPL. There is a phase lead during presentation of contralateral noise pre-MEM section below the I/O function notch. After MEM section, there is no difference in DPOAE phase during contralateral stimulation, except for a small phase lag of approximately 10° at the highest primary level, which was close to where the notch of the I/O function occurred. These data illustrate the findings for almost all of our data. For ΔPC, small (on the order of 10–20°) phase lags were noticed in the region of an I/O notch. In only two instances was this not true, and one example is shown in panel b. Data are similar to those shown in panels a, with the exception that a phase lead of approximately 25° was seen at the notch of the I/O function during the presentation of contralateral noise after MEM section. This is our only example of a ΔPC that had a phase lead, rather than a phase lag, during contralateral noise stimulation post-MEM section. Additionally, there was no measurable ΔLC, only ΔPC. Panel c (lowest panel) illustrates data from another animal at the same frequency as that shown in panel a. For this animal, there was no notch in the I/O function pre-MEM section. Only small phase changes were recorded with increasing L2 level. After MEM section, DPOAE levels were low at 75 dB SPL, and the approximate 180° phase change is noted. All our data followed these examples in panels a and c, except for the one shown in panel b and one other trace where a ΔPC of 10° was noted a low set of primary levels where there was no notch in the I/O function. ΔPI were similar to ΔPC in that no changes were noted for most of the functions after MEM section. A few, small ΔPI (phase lags) post-MEM sections were noticeable and only occurred in the region of the I/O notch.
Fig. 11 DPOAE phase as a function of L2 level for the f1 continuous condition in three rats. Circles represent DPOAE phase before MEM section. The closed symbols are DPOAE phase at the beginning of the trace, and open symbols are DPOAE phase near the end of the (more ...)
In two rats, we have observed ΔLI and ΔPI in the vicinity of I/O function notch in great detail, following similar procedures to that of Maison and Liberman (2000
) and Kujawa and Liberman (2001
). In these studies, it was shown that with a fixed f1 level, and f2 level increasing in 1-dB steps starting from 15 dB below the level of f1, onset adaptation progressed from a positive to a negative value. In addition, there was a large amount of onset adaptation near the primary level that the effect switched from positive to negative. Kujawa and Liberman (2001
) noted that this effect was seen at the DPOAE I/O function notch. Our goal was to see if responses in the rat, for which MEMs appear to be a major contributor to DPOAE reduction, were similar to those in the guinea pig, in which changes appear to be mediated by the MOC reflex. In the current study, the intensity of f1 was fixed at the level that produced the local minimum in the magnitude I/O function. The intensity of f2 was varied in 1-dB steps from a value 15 dB below that of f1 to a value that was 5 dB greater. Results for one rat are shown in Figure with magnitude being displayed in the left panel and the corresponding phase in the right panel. To compare DPOAE changes with level more clearly, each trace has been normalized to its own steady-state portion at the end of the trace. Both the magnitude and phase temporal responses exhibit nonmonotonic, complex waveforms that change dramatically with 1-dB changes in f2 level. For f2 levels between 71 and 74 dB SPL, the initial waveform is positive, followed by a negative peak that becomes more negative with increasing SPL. When f2 = 75 dB SPL, the negative peak disappears, and the waveform looks as if it is decreasing ΔLI. The phase function shows the usual relationship to magnitude; that is, it is a decreasing function when magnitude is increasing. However, it, too, has a complicated waveform. It reverses its sign, similar to that of the magnitude temporal response function, except that it changes direction between f2 levels of 72 and 73 dB SPL, and the starting phase shifts approximately 180°.