Abstract
There are psychoacoustic methods thought to measure gain reduction, which may be from the medial olivocochlear reflex (MOCR), a bilateral feedback loop that adjusts cochlear gain. Although studies have used ipsilateral and contralateral elicitors and have examined strength at different signal frequencies, these factors have not been examined within a single study. Therefore, basic questions about gain reduction, such as the relative strength of ipsilateral vs contralateral elicitation and the relative strength across signal frequency, are not known. In the current study, gain reduction from ipsilateral, contralateral, and bilateral elicitors was measured at 1-, 2-, and 4-kHz signal frequencies using forward masking paradigms at a range of elicitor levels in a repeated measures design. Ipsilateral and bilateral strengths were similar and significantly larger than contralateral strength across signal frequencies. Growth of gain reduction with precursor level tended to differ with signal frequency, although not significantly. Data from previous studies are considered in light of the results of this study. Behavioral results are also considered relative to anatomical and physiological data on the MOCR. These results indicate that, in humans, cochlear gain reduction is broad across frequencies and is robust for ipsilateral and bilateral elicitation but small for contralateral elicitation.
I. INTRODUCTION
Humans can hear over an extremely wide range of sound levels [0–120 dB sound pressure level (SPL)] and discriminate changes as small as 1 dB across this range (Viemeister, 1983). We are also able to hear in different sound environments, in quiet and in background noise. There appear to be multiple mechanisms that optimize the coding of incoming sounds (Dean et al., 2005), which may support this broad perceptual dynamic range. One such mechanism is the medial olivocochlear reflex (MOCR). The MOCR is a bilateral sound-activated feedback loop at the level of the brainstem, which projects to both cochleae and synapses directly on the base of the outer hair cells (OHCs) (Guinan, 1996). OHCs are the sensory organs of the “cochlear amplifier,” which provide gain and sharpen tuning by increasing the basilar membrane (BM) response to sound. The MOCR decreases the gain produced by the OHCs in a frequency-specific manner relative to the elicitor, thereby decreasing BM movement (Murugasu and Russell, 1996; Cooper and Guinan, 2006). By reducing the gain in cochlea, it has been theorized that the MOCR can shift the dynamic range in everyday listening conditions. This is supported by the fact that MOCR activation can enhance auditory nerve responses to transient sound in a noisy background in animals (Winslow and Sachs, 1988; Kawase et al., 1993) or increase sensitivity to changes in intensity in human behavior (Almishaal et al., 2017; Strickland et al., 2018). It has also been hypothesized that it may enhance the fluctuation profile for complex sounds (Carney, 2018).
In everyday environments, the MOCR would likely be elicited by sound in both ears. However, in terms of behavior, the relative strength of the MOCR with bilateral elicitors has not been studied, and there is little direct comparison of the strength of ipsilateral vs contralateral elicitors. Other important parameters, such as strength of the MOCR as a function of signal frequency and elicitor level, are also not well characterized in behavior. Much of the research on the MOCR, both psychoacoustic and physiological, has used contralateral sound as an elicitor, to avoid activating other mechanisms when multiple signals are presented to the same ear. But there is some evidence that contralateral elicitation produces the weakest response (e.g., Lilaonitkul and Guinan, 2012). In the next paragraphs, the current understanding of the relative strength of the MOCR from different elicitors and the strength across frequency region is reviewed for physiological and behavioral evidence.
Neurophysiological data in non-human mammals have provided key insights into the innervation and physiology of the MOCR. Measurements from the olivocochlear bundle (OCB) have shown that it is comprised of fibers that respond to ipsilateral (crossed), contralateral (uncrossed), and bilateral sound stimulation (Liberman and Brown, 1986; Liberman, 1988; Brown et al., 1998). There are two to three times more ipsilateral than contralateral OCB fibers (see Warr, 1992) in these species, which is reflected in the strength of the response in the OCB on one side to ipsilateral, contralateral, or bilateral stimulation. This indicates that in many species, the MOCR has a laterality bias. The relative strengths of these pathways in humans are not known. As far as frequencies affected, neurophysiological studies in these species have shown that the OCB innervation covers a broad range of the cochlea and is most dense for areas in the high-frequency, basal portion of the cochlea (Liberman and Gao, 1995; Liberman et al., 1990; Maison et al., 2003). Recent analysis of human temporal bones has shown a similar broad frequency innervation pattern, with the density of MOCR terminals peaking in the upper basal turn of the cochlea, around ∼2–4 kHz in the youngest subjects (Liberman and Liberman, 2019; see their Fig. 5). However, caution is needed when relating neurophysiological data to behavioral data. It is not known how MOCR innervation density along the cochlea relates to behavioral gain reduction at corresponding signal frequencies. Furthermore, the contralateral vs ipsilateral innervation ratio of MOCR terminals along the cochlea cannot be determined from the data in the Liberman and Liberman study.
There are further neurophysiological and modeling data that speak to the relative strength of the MOCR to ipsilateral and contralateral elicitors. Kawase and Liberman (1993) measured compound action potentials (CAPs) in cats to tone pips presented in masking noise. They found that presenting contralateral noise increased the magnitude of the CAP, consistent with an antimasking effect from the contralateral MOCR. Comparable maximum magnitudes for this effect were observed for probe frequencies of 2–21 kHz. Cutting the crossed OCB decreased the magnitude of the CAP, consistent with an antimasking effect from the ipsilateral MOCR elicited by the masking noise. The difference between the CAP in ipsilateral noise before and after cutting the OCB was taken as a measure of the strength of the ipsilateral MOCR. This measure of the ipsilateral MOCR was larger than the effect produced by the contralateral noise. Evidence for the strength of the ipsilateral pathway comes from Bonfils et al. (1986), who measured masking of CAPs in anesthetized guinea pig before and after sectioning the crossed (ipsilateral) OCB fibers. Masking was measured for simultaneous continuous tones and for 50-ms tonal forward maskers with a 5-ms delay before the probe. They found a significant decrease in the CAP masking (from 5 to 20 dB) after sectioning, with significantly larger decreases in the CAP masking for forward masking than for simultaneous masking in the same animals. These results suggest that the ipsilateral efferent pathway plays a significant role in masking and that these ipsilateral effects are large. Kawase et al. (1993) measured auditory nerve responses to tones in noise with and without contralateral noise in cats. Some of the cats were decerebrated, which avoids the effect of anesthesia. The study found that the contralateral noise increased detectability and discriminability of the tone in ipsilateral noise, estimated from the rate-level functions. Analyses of these data using an auditory nerve model incorporating the MOCR were able to estimate the effects of the ipsilateral and the contralateral noise on the responses (Chintanpalli et al., 2012; Smalt et al., 2014). These studies estimated that the effect of the ipsilateral noise was much larger than that of the contralateral noise. Two studies have found that contralateral noise decreased the magnitude of CAPs in quiet in humans. The effect is quite small, equivalent to a decrease in approximately 2 dB (Lichtenhan et al., 2016; Smith et al., 2017).
Otoacoustic emissions (OAEs) have been used to non-invasively measure MOCR effects in humans and non-human mammals (Collet et al., 1990; Backus and Guinan, 2006; Lilaonitkul and Guinan, 2009a,b, 2012). OAEs are sounds that are produced by the amplification of the OHCs that can be recorded in the ear canal (Guinan, 2006). Sounds called elicitors can be used to activate the MOCR, which typically reduce the magnitude of the OAEs compared to a condition without an elicitor (Collet et al., 1990). In animal studies, cutting the OCB eliminates most of this effect (Liberman et al., 1996). By using elicitors of different laterality with respect to the recording ear, the relative strength of each MOCR pathway can be measured. In cats, such measurements suggest a ratio of approximately 2:1 in the strength of the effect from ipsilateral and contralateral elicitors (Liberman et al., 1996). In humans, the largest effects have occurred with bilateral elicitors (Berlin et al., 1995; Lilaonitkul and Guinan, 2012). However, studies have found that the relative strength of the response to ipsilateral and contralateral elicitors depends on both the elicitor bandwidth and how the response is measured. Previous human studies have shown that when broadband noise elicitors are used, the effects of ipsilateral and contralateral elicitors are roughly equal in magnitude for stimulus frequency otoacoustic emissions (SFOAEs) (Guinan et al., 2003; Backus and Guinan, 2006; Lilaonitkul and Guinan, 2009b) and for transient-evoked otoacoustic emissions (TEOAEs) (Boothalingam and Purcell, 2015) or that the effect of a contralateral elicitor was slightly smaller than that of an ipsilateral elicitor (Berlin et al., 1995; Philibert et al., 1998). In contrast, Lilaonitkul and Guinan (2009b) showed that the decrease in OAE magnitude from an ipsilateral elicitor was approximately twice as large as that from a contralateral elicitor for SFOAEs when narrowband noise elicitors were used. Also, SFOAE magnitude and SFOAE phase may show different effects, and it is not known which is most relevant for perception (Lilaonitkul and Guinan, 2012). The ipsilateral-to-contralateral ratio may also vary with test frequency, as the ipsilateral-to-contralateral MOCR fiber ratio is highest in the basal region and decreases toward the apex in multiple mammalian species (e.g., cat; Guinan et al., 1984). SFOAEs have also been used to estimate MOCR gain reduction as a function of probe frequency. Data from SFOAEs have shown larger elicitor effects at 0.5 and 1 kHz compared to 4 kHz for all elicitor lateralities (Lilaonitkul and Guinan, 2009a, 2012). However, in these studies, it was difficult to estimate effects at higher probe frequencies because the signal-to-noise ratio (SNR) was low at these frequencies for most subjects.
There is some perceptual evidence in animals that would be consistent with a reduction in cochlear gain by sound. Smith et al. (2000) showed an increase in quiet thresholds of up to 5 dB for frequencies from 1 to 4 kHz with contralateral noise in Japanese macaques. This change in threshold decreased in one macaque when the OCB was severed. In humans, contralateral sound has been known to elevate ipsilateral quiet thresholds by a small amount since early experiments by Wegel and Lane (1924) and has been called “central masking.” Kawase et al. (2000) examined the effect of contralateral noise with a short signal occurring at a delay from the onset of the longer noise, which would allow the contralateral MOCR to be active. They found that thresholds in quiet were elevated by a few dB for frequencies from 0.25 to 8 kHz for noise levels that should be below the levels of cross talk and also too low to elicit the middle-ear muscle reflex (MEMR). Thus, these data on contralateral elicitation are consistent with a small effect that is evident across the frequency range.
In humans, perceptual evidence that would be consistent with a reduction in cochlear gain by sound comes from a phenomenon called overshoot (Zwicker, 1965), also known as the temporal effect (Hicks and Bacon, 1992), in which a signal presented at the onset of a broadband masker may be detected at a lower signal-to-masker ratio if the signal and masker are preceded by an additional sound. This additional sound can be either an extension of the masker or a separate broadband sound. We will refer to this sound as a precursor or an elicitor, as has been done in other studies.
While there may be multiple mechanisms involved in the temporal effect, several pieces of evidence link it to cochlear gain reduction. The temporal effect is reduced with temporary cochlear hearing loss caused by aspirin (McFadden and Champlin, 1990) or noise (Champlin and McFadden, 1989). In these conditions, quiet threshold increases, but the signal-to-masker ratio at threshold for the signal at the onset of the masker decreases and becomes similar to the threshold with preceding sound. The temporal effect also decreases in a graded way with permanent cochlear hearing loss (Bacon and Takahashi, 1992; Strickland and Krishnan, 2005). In this broadband masker and precursor condition, the temporal effect is largest for midlevel signals or maskers and is larger at higher frequencies than at lower frequencies (Zwicker, 1965; Bacon and Takahashi, 1992; Strickland, 2001, 2004). It has been hypothesized that this is due to higher compression at higher frequencies than at lower frequencies [summarized in Bacon and Savel (2004)]. Presenting the precursor contralaterally produces a smaller or no temporal effect (Turner and Doherty, 1997; Bacon and Healy, 2000).
When frequency selectivity is measured by fixing the signal at a low level (using a notched noise masker), a broadband ipsilateral precursor decreases frequency selectivity for signal frequencies of 1 and 4 kHz (Strickland, 2001). For a low signal level, the precursor has no effect on the no-notch condition but decreases masker threshold in conditions with a notch. As modeled by Strickland (2001), this is consistent with gain being reduced differentially for the signal and masker within the filter centered at the signal frequency in the off-frequency condition, but not in the on-frequency condition. The effect of preceding sound on frequency selectivity has also been studied using tonal and narrowband maskers with a noise elicitor presented to the contralateral ear (Kawase et al., 2000; Quaranta et al., 2005; Vinay and Moore, 2008; Wicher and Moore, 2014). For signal frequencies of 2 and 4 kHz, the contralateral elicitor decreased frequency selectivity, by decreasing the threshold masker level for off-frequency maskers. However, for lower signal frequencies, the results varied, with a contralateral elicitor sometimes increasing masker level below the signal frequency. In general, all of these changes were fairly small. It should be noted that all of the contralateral studies used long maskers and usually long signals, which may themselves have elicited ipsilateral gain reduction. Furthermore, these simultaneous masking studies may also include the effects of two-tone suppression. Two-tone suppression also decreases gain but on a much faster time scale (nearly instantaneous; Kiang et al., 1965) than the sluggish MOCR, and thus the results may be complicated by the interaction of the two mechanisms (Strickland 2004, 2008; Hegland and Strickland, 2018). To summarize, when measured with an off-frequency masker, the temporal effect is seen across the frequency range. Ipsilateral effects appear to be larger than contralateral effects, but they have not been directly compared in a single study.
Forward masking has been used to investigate cochlear gain reduction without the possibility of two-tone suppression. Short signals and maskers may be used to measure functions hypothesized to reflect cochlear processing without the influence of gain reduction. These functions may then be measured with a precursor before the signal and masker. If the masker frequency is approximately an octave below the signal frequency, the growth of masking is hypothesized to reflect the cochlear input-output function (Oxenham and Plack, 1997). Using this paradigm, several experiments have shown that an ipsilateral precursor shifts the lower leg of the input-output function to higher signal levels, consistent with a decrease in gain (Krull and Strickland, 2008; Jennings et al., 2009; Roverud and Strickland, 2010; Jennings and Strickland, 2012; Yasin et al., 2014; DeRoy Milvae and Strickland, 2018). These forward masking techniques rely on using maskers at the signal frequency (on-frequency) and maskers approximately an octave below the signal frequency (off-frequency). It is assumed that the listener attends to the auditory filter with the best signal-to-masker ratio, which will typically be at or near the signal frequency. Gain reduction at the signal frequency place is expected for both the signal and the on-frequency masker, but not the off-frequency masker. Therefore, the change in signal threshold with an off-frequency masker following a precursor can provide an estimate of gain reduction. This differential processing between on- and off-frequency maskers is the basis for studying behavioral gain reduction and is not consistent with other mechanisms such as temporal integration of the masker and elicitor (i.e., additivity of masking) (Yasin et al., 2014; DeRoy Milvae and Strickland, 2018). If multiple masker frequencies are used to trace out a psychoacoustic tuning curve, adding a precursor decreases frequency selectivity (Jennings et al., 2009; Jennings and Strickland, 2012). Nearly all of the ipsilateral forward masking gain reduction experiments have been done at 4 kHz. DeRoy Milvae and Strickland (2018) extended ipsilateral gain reduction measurements to 1, 2, and 4 kHz with a broadband precursor and found that the magnitude of gain reduction did not significantly differ across frequency.
Psychoacoustic gain reduction effects in forward masking have also been studied using contralateral precursors. Kawase et al. (2000) and Aguilar et al. (2013) found small decreases in frequency selectivity for signals from 0.5 to 4 kHz with a contralateral precursor. Fletcher et al. (2016) estimated input-output functions using temporal masking curves for an on- and off-frequency masker for a signal frequency of 2 kHz and found a small decrease in estimated gain with a contralateral noise precursor. As for the simultaneous masking results, the magnitude of gain reduction cannot be directly compared for elicitors in the contralateral vs ipsilateral ear due to differences in paradigms and subjects. No previous studies have compared gain reduction from ipsilateral and contralateral elicitors in forward masking within the same study or using the same methodology. This is important because these studies used different methods, stimuli, and conditions to estimate gain reduction, all of which could contribute to the variability seen in the effect size (strength) and frequency patterns, as outlined in Wicher and Moore (2014). Furthermore, variability between subjects can play a large role in the findings despite each subject meeting the criteria for normal hearing (e.g., Jennings et al., 2009).
It is also important to measure gain reduction as a function of precursor level. In general, studies have tended to use only one precursor level to get the maximum effect or examined only two precursor levels (Krull and Strickland, 2008; Vinay and Moore, 2008; Wicher and Moore, 2014). However, Roverud and Strickland (2014) measured gain reduction using on- and off-frequency tonal precursors at three levels as a function of precursor duration. They found that thresholds increased with precursor durations up to 140 ms for an off-frequency precursor but plateaued or even oscillated above a duration of 50 ms for an on-frequency precursor. They were able to model the shifts in signal threshold with precursor duration for the on-frequency precursor well using compression estimates from input-output functions measured for each subject, followed by gain reduction. For the off-frequency precursor, the shift in signal threshold with precursor duration was modeled well by a function with a slope of one. Taken together, the findings were found to be consistent with gain reduction, which would affect the on-frequency precursor but not the off-frequency one. Yasin et al. (2014) measured gain reduction for a broadband noise elicitor. They found that the decrease in gain for elicitor levels from 40 to 80 dB SPL had a slope of 0.33 dB/dB, which was slightly lower than the slope of the estimated input/output functions, which was 0.5 dB/dB. These studies suggest that the growth of gain reduction may be determined by cochlear compression of the elicitor in peripheral auditory channels at or near the signal frequency.
The current study was designed to investigate several of the gaps in our understanding of psychoacoustic gain reduction effects in humans. Specifically, we compared the relative strength (or magnitude) of gain reduction as a function of elicitor laterality and signal frequency in the same subjects. We used forward masking paradigms established in previous studies (e.g., DeRoy Milvae and Strickland, 2018). Gain reduction was measured for contralateral, ipsilateral, and bilateral precursors. We measured these effects for signal frequencies of 1, 2, and 4 kHz. This repeated measures design should control for variability due to differences in methodology, stimuli, and subjects that occur when comparing gain reduction effects across studies. We also used a range of precursor levels in our conditions to determine how gain reduction grows with elicitor level across conditions.
II. METHODS
Two forward masking paradigms were used to estimate gain reduction from ipsilateral, contralateral, and bilateral precursors at signal frequencies of 1, 2, and 4 kHz for a range of precursor levels. Control tests were performed to show that the results were consistent with gain reduction and that the two paradigms produced equivalent results.
A. Subjects
Five subjects (three male and two female) completed the experiments. Their ages ranged from 20 to 29 years (median = 21 years) at the time of testing. All subjects had normal auditory function, determined through the use of a battery of audiologic measures. All subjects had clinically normal pure tone thresholds [≤15 dB hearing level (HL) at audiometric frequencies between 250 and 8000 Hz]. Distortion product otoacoustic emissions (DPOAEs) were present (Bio-logic system, Natus Medical Inc., Pleasanton, CA) from 1500 to 8500 Hz (minimum criteria of –6 dB SPL distortion product and 6 dB SNR for 10 of 12 frequencies tested with no consecutive absent responses). Tympanograms (Tympstar, Grason-Stadler, Inc.) were normal (type A), indicating normal middle-ear function. Ipsilateral and contralateral acoustic reflex thresholds were measured using broadband noise elicitors. Because the signal was always presented in the subject's right ear in the current study, the acoustic reflex thresholds measured are with respect to the probe in the right ear. All clinical acoustic reflex thresholds for white noise elicitors were measured in dB HL and converted to corresponding dB SPL units for comparison to the experimental elicitors (i.e., precursors) used in the current study. To do so, noise levels from the immittance equipment were recorded from a sound level meter attached to a Zwislocki coupler mounted in a KEMAR ear. The noise levels in dB SPL were approximately 8 dB higher than the nominal levels in dB HL. No ipsilateral acoustic reflex threshold was below 60 dB HL in any subject, and thus no subject had a clinically measured acoustic reflex threshold for white noise below 68 dB SPL as shown in Table I. All subjects were paid for their time in the study except for S1, who is the first author. Other subjects were recruited via fliers on the Purdue campus. Other than S1, no other subject had previous experience in psychoacoustic tasks before beginning in our experiments. All research was conducted under a research protocol approved by the Institutional Review Board at Purdue University to safeguard the rights, safety, and well-being of our subjects.
TABLE I.
Individual ipsilateral and contralateral acoustic reflex thresholds (dB SPL) measured with a 226-Hz probe and a white broadband noise elicitor.
| Subject | Ipsilateral | Contralateral |
|---|---|---|
| S1 | 68 | 98 |
| S2 | 78 | 93 |
| S3 | 73 | 78 |
| S4 | 93 | 103 |
| S5 | 68 | 93 |
B. Stimuli
Estimates of gain reduction were made using two forward masking techniques that rely on the timing of cochlear gain reduction via the MOCR. The technique used depended on the signal frequency tested. The technique used to measure gain reduction at 2 and 4 kHz will be explained first. Initially, quiet thresholds were measured for each signal frequency. All signals used in the current study were 10-ms sinusoids, including 5-ms cos2 onset and offset ramps. This signal duration is longer than the 6 or 8 ms used in some previous studies (e.g., Jennings et al., 2009; Roverud and Strickland, 2010; DeRoy Milvae and Strickland, 2018) to ensure that the spectral spread was within one auditory filter bandwidth even at the lowest frequency of 1 kHz (DeRoy Milvae and Strickland, 2018). Next, the masker levels needed to mask a signal fixed at 5 dB sensation level (SL) was determined. A 5-dB shift in signal threshold was desired so that the signal was fixed on the lower leg of the cochlear input-output function. Gain reduction is largest in this region of the input-output function (physiologically: Cooper and Guinan, 2006; psychoacoustically: Krull and Strickland, 2008; Roverud and Strickland, 2010). The masker duration was 20 ms (including 5-ms cos2 ramps), which should be too short to elicit the MOCR during the signal presentation (James et al., 2005; Backus and Guinan, 2006).
For the gain reduction control test, the maskers were on-frequency (at the signal frequency) and off-frequency (0.6 times the signal frequency) sinusoids. That is, 2- and 1.2-kHz maskers were used for the 2-kHz signal frequency, and 4- and 2.4-kHz maskers were used for the 4-kHz signal frequency. The signal was set at 5 dB SL, and the masker level was adjusted to find the level needed to just mask the signal. By measuring both on- and off-frequency masked threshold, we were able to test whether the effects of the precursor on signal threshold were consistent with cochlear gain reduction (explained below). To verify that the maskers were equally effective at masking the signal, the on- and off-frequency maskers were fixed at the thresholds determined earlier, and the signal was varied to check that the signal thresholds were raised to 5 dB SL. On- and off-frequency masked signal thresholds were considered similar if the difference between the two conditions was less than 3 dB. If signal threshold with the fixed maskers differed by more than this amount, the masker level was adjusted, and the signal threshold was remeasured until signal thresholds were within 3 dB of one another. The stimulus configuration for these thresholds is shown in Fig. 1(a). Signal thresholds in this reference condition were then compared to a condition where a precursor intended to elicit the MOCR preceded both the masker and the signal [Fig. 1(b)]. The precursor was a 50-ms (including 5-ms cos2 onset and offset ramps) pink broadband noise (0.25–10 kHz). Pink noise has a spectrum level that decreases by 3 dB per octave. This elicitor provides a more accurate comparison of gain reduction across frequency, as pink noise will excite auditory filters across the frequency range with approximately equal energy. Additionally, previous studies have found broadband noise stimuli to be particularly effective elicitors of cochlear gain reduction (Maison et al., 2000; Lilaonitkul and Guinan, 2009a; Wicher and Moore, 2014). The precursor duration was set to 50 ms because this duration was found to be the most effective for on-frequency tonal precursors (Roverud and Strickland, 2014). For this control experiment, we used a 60-dB SPL precursor level to maximize gain reduction while avoiding activation of the MEMR in subjects (see Table I). This control experiment was conducted for both the ipsilateral and bilateral precursor conditions.
FIG. 1.
(Color online) Schematic of the stimuli used for the masker present method [(a) and (b)] and the masker absent method [(c) and (d)]. On- and off-frequency maskers were always 20 ms, as was the delay between the precursor and signal in the masker absent condition. The precursors were presented ipsilaterally, contralaterally, or bilaterally with respect to the signal ear. Precursor levels ranged from 40 to 75 dB SPL. The double-headed arrow (red) indicates that the signal was adaptively varied, while the masker was fixed at a level that shifted the signal by 5 dB with no precursor present.
A reduction in gain by the precursor is predicted to shift signal threshold more following the off-frequency masker than the on-frequency one (Kawase et al., 2000; Jennings et al., 2009; Yasin et al., 2014). This is because the off-frequency masker is processed linearly at the signal frequency place and thus not affected by gain reduction, whereas the signal and the on-frequency masker are nearly equally affected by gain reduction. This contrasts with the prediction of temporal integration, also called additivity of masking, where adding the precursor would produce equal shifts in signal threshold in the two conditions (Penner and Shiffrin, 1980; Plack and O'Hanlon, 2003; Oxenham and Moore, 1994). For sequential forward maskers, temporal integration posits that masking occurs via a neural mechanism that integrates the energy of the stimuli within a temporal window (Oxenham and Moore, 1994; Oxenham, 2001). In this case, the intensities of the precursor, masker, and signal would be integrated at some level of the auditory system. With temporal integration (additivity of masking), it would be expected that the on- and off-frequency conditions should produce equal shifts in thresholds with the addition of the precursor. In contrast, in this study, it is proposed that masking by the masker may occur within a temporal window (because the duration is too short for gain reduction to affect the signal) but masking from the precursor occurs by gain reduction.
Following the gain reduction control experiment, signal thresholds with an off-frequency masker were used for the rest of the conditions. The difference in signal threshold between the off-frequency masked signal with [Fig. 1(b)] and without [Fig. 1(a)] the precursor will be referred to as the “masker present” gain reduction estimate used in the current study. This was measured with precursor levels from 40 to 75 dB SPL, presented ipsilaterally, contralaterally, or bilaterally with respect to the signal ear. Levels of 60 dB SPL and below should be below the MEMR for all listeners for the ipsilateral and contralateral conditions. Thus, these lower levels may be interpreted as an estimate of cochlear gain reduction (Krull and Strickland, 2008; Roverud and Strickland, 2010). At higher levels, there is a possibility of MEMR activation. These levels were included to determine whether evidence of MEMR activation was evident in the results.
A second method was used to estimate gain reduction at a signal frequency of 1 kHz. For this lower signal frequency, it cannot be assumed that an off-frequency masker will have a linear response at the signal frequency place (Ruggero et al., 1997). If the off-frequency masker is not processed linearly at the signal frequency place, as it is for 2 and 4 kHz, then it too could be affected by gain reduction, thus complicating interpretation of the results. Therefore, instead of using an off-frequency masker to fix the signal on the lower leg of the input-output function, quiet threshold at 1 kHz served as the baseline condition [Fig. 1(c)] and was compared to signal threshold with a precursor and a 20-ms delay between the precursor offset and signal onset [Fig. 1(d)]. This will be referred to as “masker absent” estimate of gain reduction.
A previous study using masker present vs masker absent conditions found no significant differences in the magnitude of the gain reduction between the two methods (DeRoy Milvae and Strickland, 2018) for signal frequencies of 2 and 4 kHz. A subset of the subjects in the present study completed an ipsilateral gain reduction control experiment for the DeRoy Milvae and Strickland study for the 1-kHz signal frequency, using the on- and off-frequency masking threshold methodology explained earlier. The signal duration for the control experiment was 8 ms to match the other signal durations in that study. These data are reported again in this study to provide a gain reduction control at 1 kHz and to compare the masker present and masker absent methods at 1 kHz. The bilateral gain reduction control experiment was only conducted at 2 and 4 kHz.
After the gain reduction control experiments were completed, the growth in signal threshold with precursor level was measured for each signal frequency. During a single session, the pink noise was presented either ipsilaterally, contralaterally, or bilaterally with respect to the masker and signal and presented at a range of intensities (40–75 dB SPL, 5-dB steps). Both the magnitude and slopes of these functions were analyzed.
C. Procedure
All psychoacoustic measures were completed in a double-walled sound-attenuating booth. Stimuli were generated with custom matlab software (Bidelman et al., 2015) with a Lynx TWO-B sound card (Lynx Studio Technology, Inc., Costa Mesa, CA). The stimuli were then passed through a headphone buffer (TDT HB6, Tucker-Davis Technologies, Alachua, FL) and delivered to both ears through Etymotic ER-2 (Etymotic Research, Inc., Elk Grove Village, IL) insert earphones. The insert earphones had a flat frequency response at the eardrum from 250 to 8000 Hz. High pass noise (from 1.2 times the signal frequency to 10 kHz) was used to reduce the possibility of off-frequency listening (Nelson et al., 2001) for all parts of the experiment except during quiet threshold measurements. The high pass noise began 50 ms before the onset of the stimuli and ended 50 ms after the signal offset and was 50 dB below the signal level.
All psychoacoustic measurements utilized a three-interval forced-choice (3IFC) task using a matlab GUI, in which only one of the choices contained the signal. Each interval was visually marked on the computer screen, and intervals were separated by 500 ms of silence. Subjects could use either a mouse or the keyboard to indicate which interval contained the signal. Visual feedback was given for correct and incorrect responses. Signal and masker levels were adjusted to estimate a response threshold of 70.7% correct (Levitt, 1971). For signal threshold measures (quiet thresholds, measures of gain reduction, and the gain reduction control test), if the subject chose correctly over two consecutive trials, the level of the signal decreased, while an incorrect response would cause the level of the signal to increase (two down, one up). For masking thresholds, if the subject chose correctly over two consecutive trials, the level of the masker increased, while an incorrect response would cause the level of the masker to decrease (two up, one down). The step size was 5 dB for the first four reversals and then decreased to 2 dB for the remaining reversals. The last eight reversals were averaged to produce a final threshold for each run.
Subjects had approximately 1 h of training before data collection began in order to help them understand the task. Each session was 1–1.5 h to prevent attentional fatigue. Each condition was tested twice per session, and thresholds are an average of the last two thresholds recorded for that condition. These final thresholds served as the data reported throughout the current study in the figures and the statistical analysis. Runs with a standard deviation (SD) greater than 5 dB were discarded from the overall averages and repeated if necessary. Data from each subject were collected for a minimum of two sessions on different days for each psychoacoustic task, and additional sessions were conducted if large variability or learning effects occurred. The order of presentation of the signal frequency and laterality of the precursor was interleaved across subjects. All statistical and post hoc analyses of gain reduction were calculated with IBM SPSS 24 statistical software. All of the data in the statistical analysis of variance (ANOVA) results reported in the current study were tested for the assumption of normality in SPSS using the Shapiro–Wilk test of normal distribution and the corresponding normal Q-Q plots. With this test, any subset of data tested for this assumption with a p-value equal to or greater than 0.05 would meet the criterion to assume normal distribution. Only one subset of data tested just missed this criterion, which was the off-frequency condition at 4 kHz (see Fig. 3, gray bar) in the ipsilateral gain reduction experiment (see Sec. III A), with a p-value of 0.049. Given that the other subsets of data in the ANOVA (as well as all other subsets of data in the study) met the criterion for assumption of normality, that data for each condition were from the same subjects (repeated measures design), and that this condition just missed the assumption by 0.001, a parametric test (ANOVA) was deemed justified for the ipsilateral gain reduction control experiment at 4 kHz.
FIG. 3.
Group average shifts in threshold at 1, 2, and 4 kHz, between the masker alone condition and a condition where the ipsilateral precursor preceded the masker. The gray bars show the average shift in threshold when the masker was off-frequency, and the white bars show the average shift in threshold when the masker was on-frequency. Subjects are coded by the same symbols as in Fig. 2, although the symbol shading was removed for visual clarity. Standard error bars are included for each average threshold shift.
III. RESULTS
A. Ipsilateral gain reduction control experiment
The first step in our analysis was to test whether the threshold shifts with an ipsilateral precursor of 60 dB SPL and on- and off-frequency maskers set to be equally effective were significantly larger with the off-frequency masker than the on-frequency masker. This precursor level should be below the MEMR threshold for the ipsilateral condition. Individual subject signal thresholds, with and without the ipsilateral precursor, can be seen in Fig. 2. Subjects are coded by the symbols. The top panel shows results for 1 kHz, the middle panel for 2 kHz, and the bottom panel for 4 kHz. Recall that only a subset of the listeners was tested in this condition at 1 kHz. First note that the thresholds with no precursor (“off” and “on”) were within 3 dB of each other, indicating that the maskers were equally effective. Shifts in the signal thresholds with the precursor are shown for the on-frequency (on+pre) and off-frequency masked signal (off+pre).
FIG. 2.
Individual signal threshold averages at 1, 2, and 4 kHz for the masker present conditions with and without a 60 dB SPL ipsilateral precursor. Masker conditions included off-frequency masked (off), on-frequency masked (on), and the same masked conditions with the pink broadband noise precursor (pre+off, pre+on). Signal thresholds were significantly higher in the precursor condition with an off-frequency masker than in the condition with an on-frequency masker. SDs are indicated by vertical bars for each threshold. Subjects are coded by symbols.
One-way repeated measures ANOVAs were conducted to determine the average change in signal threshold with a precursor as the dependent variable and the masker type (on- and off-frequency) as the independent variable. The average signal threshold shifts with the precursor can be seen in Fig. 3 as bar plots. The shaded bars indicate the average signal threshold shifts for the off-frequency conditions, while the white bars indicate the average signal threshold shifts for the on-frequency conditions. Individual subject data are shown by the same symbols as used in Fig. 2, although the shading in the symbols was removed for visual clarity. For all signal frequencies, the off-frequency conditions had significantly larger threshold shifts compared to the on-frequency conditions when a precursor was added [1 kHz: F(1,2) = 33.34, p = 0.029; 2 kHz: F(1,4) = 507.92, p < 0.001; 4 kHz: F(1,4) = 79.68, p = 0.001], which is consistent with gain reduction. For some subjects, the on-frequency threshold decreased with the addition of the precursor. That is, the threshold improved below the baseline condition with the precursor, as can be seen by the symbols on the negative portion of the y axis in Fig. 3. It has been shown previously that the addition of a precursor in on-frequency masked conditions can cause improvements in signal detection (or an increase in masker threshold) that would be consistent with gain reduction (Strickland et al., 2018). This can occur if the on-frequency masker is reduced below audibility.
One last analysis was done to compare gain reduction estimates at 1 kHz from masker present conditions in the gain reduction control experiment [detailed above; see Fig. 1(a)] and masker absent conditions done for estimates in the current study [see Figs. 1(c) and 1(d)]. As stated earlier, gain reduction at 1 kHz was measured without a masker (masker absent; 20-ms delay) for the rest of the study, and therefore it was necessary to determine whether gain reduction estimates were statistically equivalent in the masker absent and the masker present conditions at this signal frequency. DeRoy Milvae and Strickland (2018) did this analysis for 2- and 4-kHz signal frequencies and found that the two methods produce statistically equivalent shifts in threshold with the addition of a precursor for each signal frequency. However, they did not do this analysis at 1 kHz since they did not collect masker absent data at this frequency (as mentioned in Sec. II B). A one-way repeated measures ANOVA was conducted with threshold shift with a precursor as the dependent variable and listening condition (off-frequency and masker absent) as the independent variable. There were no significant differences between the mean threshold shifts in the off-frequency masker condition and the no-masker condition [F(1,2) = 0.94, p = 0.43].
B. Bilateral gain reduction experiment
On- and off-frequency threshold shifts were also analyzed for bilateral precursor conditions to determine if the results were consistent with gain reduction. As for the ipsilateral precursor conditions, the bilateral precursor was fixed at 60 dB SPL. Individual subject signal thresholds with and without the bilateral precursor can be seen in Fig. 4. Subjects are coded by the same symbols used in Fig. 2. The top panel shows results for 2 kHz and bottom panel for 4 kHz. Thresholds with no precursor (“off” and “on”) are the same in Fig. 2 since they are the baseline condition.
FIG. 4.
Individual signal threshold averages at 2 and 4 kHz for the masker present conditions with and without a 60 dB SPL bilateral precursor. The layout and symbols are the same as in Fig. 2. The baseline no-precursor conditions (“off” and “on”) are the same as in Fig. 2.
One-way repeated measures ANOVAs were conducted to determine the average change in signal threshold with a precursor as the dependent variable and the masker type (on- and off-frequency) as the independent variable. The average signal threshold shifts with the precursor are shown in Fig. 5 as bar plots. Individual subject data are shown by the same symbols used in Fig. 3. The off-frequency conditions with a precursor had significantly larger threshold shifts compared to the on-frequency conditions for both 2-kHz [F(1,4) = 60.59, p = 0.001] and 4-kHz [F(1,4) = 46.95, p = 0.002] signal frequencies, consistent with gain reduction. Similar to the ipsilateral conditions, a few subjects demonstrated improvement in signal threshold for the on-frequency conditions.
FIG. 5.
Group average shifts in threshold at 2 and 4 kHz, between the masker alone condition and a condition where the bilateral precursor preceded the masker. The layout and symbols are the same as in Fig. 3.
C. Signal threshold shifts as a function of signal frequency, precursor laterality, and precursor level
The results in Secs. II A and II B show that the precursor had little to no effect with an on-frequency masker. Therefore, only the off-frequency masker was used in the rest of the experiments. Individual subject (top) and group averaged (bottom) data are shown in Fig. 6 for each listening condition. Each row represents an individual subject, indicated by the corresponding subject number, and each column represents a signal frequency. The masker absent conditions at 1 kHz are shown in the left column, while the masker present conditions at 2 and 4 kHz are shown in the middle and right columns, respectively. The horizontal dotted line indicates signal threshold without a precursor present, which is the reference condition. For 1 kHz, this line is signal threshold in quiet, while for 2 and 4 kHz, this line is signal threshold with a fixed level off-frequency masker, which shifted the signal 5 dB above quiet threshold. For each frequency tested, signal threshold is plotted as a function of precursor level with each precursor type indicated by a different symbol. Signal thresholds with ipsilateral precursors are indicated by filled circles, contralateral by filled diamonds, and bilateral by open triangles. The difference in threshold between a condition with (symbols) and without (dotted line) a precursor is interpreted as an estimate of the magnitude of gain reduction for precursor levels of 60 dB SPL and below. The grouped data are the average of these differences for each condition, and therefore the baseline (dotted line) is set at zero. Clinical MEMR thresholds are indicated by arrows along the x axis if they were within the range of precursor levels tested and are color-coded to the condition.
FIG. 6.
(Color online) Signal thresholds as a function of precursor level for individual subjects (top) and the mean (bottom). Symbols indicate precursor laterality, while dotted lines indicate the reference condition with no precursor. SD of signal thresholds is indicated by the error bars. Clinical MEMR thresholds (from Table I) are indicated by blue arrows for an ipsilateral elicitor (single or leftmost) and green arrows (rightmost) for a contralateral elicitor and are plotted corresponding to the associated precursor level on the x axis.
D. Magnitude of gain reduction
For precursor levels below the MEMR for each condition, shifts in signal threshold with a precursor may be considered an estimate of gain reduction. The overall qualitative pattern of the data (Fig. 6) is that gain reduction is seen for very low precursor levels, and signal thresholds grow compressively with increasing precursor level across subjects. Signal thresholds with a contralateral precursor are lower than thresholds with an ipsilateral or bilateral precursor, regardless of the signal frequency tested. Thresholds with ipsilateral and bilateral precursors were similar in size. These patterns were consistent across precursor levels.
To estimate the magnitude of gain reduction, we analyzed the signal threshold shifts with 50-, 55-, and 60-dB SPL precursor levels for each precursor laterality. Average threshold shifts for each listening condition at these levels are shown in Fig. 7. Gain reduction measured with 50-dB SPL precursors is shown on the top, with 55-dB SPL precursors in the middle and 60-dB SPL precursors on the bottom. These precursor levels were chosen to maximize the estimate of gain reduction across listening conditions, while avoiding levels where activation of the MEMR is a possibility. While 60 dB SPL precursors should be too low in intensity to activate the MEMR based on the clinical acoustic reflex thresholds for our subjects (see arrows in Fig. 6), acoustic wideband immittance studies have shown that clinical estimates may underestimate acoustic reflex thresholds by approximately 14 dB (in some cases as much as 24 dB) due to the frequency analysis bands used (Feeney and Keefe, 2001; Feeney et al., 2017). Therefore, choosing lower precursor levels to analyze gain reduction effects should make it very unlikely that MEMR effects are included for ipsilateral or contralateral precursors. Clinical MEMR thresholds were not measured for bilateral elicitors.
FIG. 7.
Group average gain reduction results as a function of precursor level (50, 55, and 60 dB SPL) and precursor laterality (ipsilateral, contralateral, and bilateral). For all precursor levels and signal frequencies, contralateral effects were the smallest and ipsilateral and bilateral effects were the largest.
A two-way 3 × 3 repeated measures ANOVA was conducted to determine if the mean gain reduction (dependent variable) estimates significantly varied with the independent variables precursor laterality (ipsilateral, contralateral, and bilateral) and signal frequency (1, 2, and 4 kHz). This analysis was done for each precursor level (50, 55, and 60 dB SPL). The results can be seen in Table II.
TABLE II.
Results of a 3 × 3 repeated measures ANOVA on the magnitude of gain reduction as a function of precursor laterality (Contra, contralateral; Ipsi, ipsilateral; Bi, bilateral) and signal frequency. This analysis was conducted for each of the precursor levels (50, 55, and 60 dB SPL). The main effect of precursor laterality on gain reduction was statistically significant, while the main effect of signal frequency on gain reduction was not statistically significant. This pattern was consistent for all precursor levels. Note that the degrees of freedom are different in the 60 dB SPL precursor conditions compared to the 55 and 50 dB SPL conditions because sphericity could not be assumed for these data. Therefore, we reported the more conservative Greenhouse–Geisser critical F-value instead, which helped correct for violation of sphericity. Asterisks indicate significance.
| Precursor level | Precursor laterality | p-value | ||
|---|---|---|---|---|
| F-statistic | p-value | Paired comparisons | ||
| 60 dB SPL | F(1.041,4.165) = 72.93 | p = 0.001* | Contra vs Ipsi | p = 0.002* |
| Contra vs Bi | p = 0.004* | |||
| Ipsi vs Bi | p = 0.002* | |||
| 55 dB SPL | F(2,8) = 89.099 | p = 0.001* | Contra vs Ipsi | p = 0.002* |
| Contra vs Bi | p = 0.002* | |||
| Ipsi vs Bi | p = 0.029* | |||
| 50 dB SPL | F(2,8) = 94.36 | p = 0.001* | Contra vs Ipsi | p < 0.001* |
| Contra vs Bi | p = 0.005* | |||
| Ipsi vs Bi | p = 0.035* | |||
| Signal frequency | ||||
| Precursor level | F-statistic | p-value | ||
| 60 dB SPL | F(2,8) = 1.95 | p = 0.20 | ||
| 55 dB SPL | F(2,8) = 0.76 | p = 0.49 | ||
| 50 dB SPL | F(2,8) = 0.76 | p = 0.49 | ||
For all precursor levels, there was a significant main effect of precursor laterality. Post hoc comparisons with Bonferroni corrections revealed that gain reduction with contralateral precursors was significantly different from gain reduction with ipsilateral and bilateral precursors. Gain reduction with ipsilateral and bilateral precursors also differed significantly, with the bilateral estimates being on average 1 dB less than ipsilateral estimates across signal frequencies. However, signal frequency was not a significant main effect, and there was no significant interaction between precursor laterality and signal frequency. The lack of significance of signal frequency on gain reduction can be seen readily in Fig. 7, where the gain reduction laterality patterns are similar at all signal frequencies and precursor levels analyzed.
E. Growth of signal threshold with precursor level
One observation from the average data in Fig. 6 is that gain reduction estimates seemed to grow differently across signal frequencies as the precursor level increased. To test whether the growth of gain reduction varied across signal frequencies, we fit a line to the data in Fig. 6 using a least-squared error criterion to calculate the slopes for each listening condition. Fits included the data for precursor levels of 40–60 dB SPL. These linear functions were only fit to the ipsilateral and bilateral conditions for each signal frequency. Most of the individual contralateral growth functions were not significantly greater than the baseline across this range of precursor levels. Table III summarizes the measured slopes and the corresponding average goodness of fit (R2) for each of the listening conditions.
TABLE III.
Individual and average slopes (top) and corresponding goodness of fit (bottom) for each listening condition. Only ipsilateral (Ipsi) and bilateral (Bi) precursor conditions were considered. The units of the slope are dB/dB, whereby a positive slope indicates that an increase in the precursor level (denominator) leads to an increase in the signal threshold (numerator).
| Slopes of gain reduction | ||||||
|---|---|---|---|---|---|---|
| 1 kHz | 2 kHz | 4 kHz | ||||
| Subjects | Ipsi | Bi | Ipsi | Bi | Ipsi | Bi |
| S1 | 0.18 | 0.13 | 0.27 | 0.35 | 0.29 | 0.12 |
| S2 | 0.23 | 0.25 | 0.32 | 0.25 | 0.06 | 0.065 |
| S3 | 0.34 | 0.25 | 0.23 | 0.26 | 0.047 | 0.20 |
| S4 | 0.11 | 0.29 | 0.11 | 0.24 | 0.13 | 0.13 |
| S5 | 0.24 | 0.24 | 0.27 | 0.41 | 0.095 | 0.095 |
| Mean | 0.22 | 0.23 | 0.24 | 0.31 | 0.12 | 0.12 |
| Standard error | 0.038 | 0.027 | 0.036 | 0.034 | 0.045 | 0.024 |
| Mean Goodness of fit (R2) | 0.85 | 0.92 | 0.81 | 0.87 | 0.64 | 0.68 |
A two-way 2 × 3 repeated measures ANOVA was conducted to determine if the slopes of gain reduction (dependent variable) significantly varied with precursor laterality (ipsilateral and bilateral) and signal frequency (1, 2, and 4 kHz). The overall results are summarized in Table IV. The mean slopes of gain reduction did not statistically differ across precursor laterality, indicating that ipsilateral and bilateral slopes were similar in their overall growth rates. However, the slopes statistically differed across signal frequency. Interestingly, post hoc analysis using Bonferroni corrections indicated that the mean slopes at 1 kHz did not significantly differ from the mean slopes at 2 or 4 kHz. Additionally, the mean slope at 2 kHz was not significantly different from the mean slope at 4 kHz. While the post hoc tests did not indicate any significant differences in the mean slopes of gain reduction between signal frequencies, the slopes at 1 kHz [mean (M) = 0.22, standard error (SE) = 0.023] and 2 kHz (M = 0.27, SE = 0.030) had a tendency to be steeper than at 4 kHz (M = 0.12, SE = 0.025) (see Table III; group data, Fig. 6).
TABLE IV.
Results of the 2 × 3 repeated measures ANOVA on the slopes of gain reduction as a function of precursor laterality and signal frequency. The main effect of precursor laterality on gain reduction was not statistically significant, while the main effect of signal frequency on gain reduction was statistically significant. Asterisks indicate significance. N/A, not applicable.
| Precursor range | Precursor laterality effects | |||
|---|---|---|---|---|
| F-statistic | p-value | Bonferroni | Significance | |
| 40–60 dB SPL | F(1,4) = 0.75 | p = 0.42 | N/A | N/A |
| Signal frequency effects | ||||
| Precursor range | F-statistic | p-value | Bonferroni | Significance |
| 40–60 dB SPL | F(2,8) = 7.15 | p = 0.017* | 1 vs 2 kHz | p = 0.87 |
| 1 vs 4 kHz | p = 0.23 | |||
| 2 vs 4 kHz | p = 0.057 | |||
For precursor levels above 60 dB SPL, the pattern of results was generally the same as for lower precursor levels. However, for one listener (S5), an increase in slope was observed for a bilateral precursor at levels above 60 dB SPL when the signal was 1 kHz. At these higher levels, it is possible that the MEMR was activated by the precursor. This will be considered further in Sec. IV.
IV. DISCUSSION
A. Overview
The current study used two different forward masking paradigms and used pink broadband noise precursors to elicit gain reduction (possibly by the MOCR) at lower precursor levels and/or the MEMR at higher precursor levels in adults with normal hearing. Whereas previous psychoacoustic studies of gain reduction measured the effects of a fixed level precursor with a single laterality, and often at a single signal frequency, we measured the effects of ipsilateral, contralateral, and bilateral precursors across a range of levels, at signal frequencies of 1, 2, and 4 kHz, in the same subjects with a repeated measures design. This allows direct comparison of these conditions, which has not been possible across previous studies that utilized different methodology and stimuli.
B. Gain reduction vs additivity of masking
The precursor and forward masker used in the current study could be interpreted as sequential forward maskers, which have been used in previous experiments to probe the nonlinear nature of the peripheral auditory system. Here, we will focus on studies using sequential maskers rather than simultaneous ones, since that is most relevant for this study. Penner and Shiffrin (1980) provided the original framework for these studies.
Additivity of masking experiments typically follow a series of steps: (1) the signal level is fixed, and masking thresholds for masker 1 (M1) and masker 2 (M2) are measured individually so that each of the maskers is approximately equally effective in masking the signal; (2) then the maskers are presented together (M1+M2) fixed at the levels found in the first step, and the signal level at threshold is measured. In a linear system, if the energy of the maskers and signal is integrated, the addition of the two maskers should lead to a 3-dB increase in signal threshold compared to the signal threshold shifts produced by either masker alone from step 1. For on-frequency maskers, more than a 3-dB increase is often seen. This additional masking is termed “excess masking” and has been interpreted as evidence that signal excitation grows with a slope <1, often referred to as compression (Penner, 1980; Penner and Shiffrin, 1980).
In previous studies, we considered such “additivity of forward masking” after the fact and showed that predictions from gain reduction fit the data better than additivity of forward masking (Krull and Strickland, 2008; Jennings et al., 2009) for our own data as well as previous additivity of masking data from Plack et al. (2006). In the present study, as well as several recent studies, we devised a test where gain reduction would predict results that were clearly different from the predictions of additivity of masking. Maskers at the signal frequency and at 0.6 times the signal frequency were set to just mask a 5 dB SL signal individually. Then a precursor was presented before each masker and signal. Threshold increased significantly more following an off-frequency masker than following an on-frequency one, consistent with gain reduction. This technique differs from those typically used to measure additivity of forward masking in several ways. In additivity of masking experiments, the two maskers usually have the same spectrum, either noises, on-frequency tones, or off-frequency tones. In contrast, in the gain reduction test used in this study, the precursor is a noise, and the masker is a tone. In additivity of masking experiments, the masking produced by individual maskers is compared to the masking produced by the combined maskers, and estimates of the input-output function are derived from the results. In the gain reduction test, the change in masking by the addition of the precursor is compared across the on- and off-frequency masker. The fact that the precursor increases threshold more for the off-frequency masker than the off-frequency masker is consistent with gain reduction and not with additivity of forward masking.
These results are consistent with the hypothesis that forward masking from maskers with a long enough delay between masker onset and signal onset may arise from a different process than forward masking with a shorter delay. This is consistent with data from Bonfils and Puel (1987), who measured forward masking of CAPs in guinea pigs before and after sectioning the crossed OCB. They found that when the delay between masker onset and signal onset was more than 30 ms, forward masking decreased when the OCB was cut. They proposed that some of the masking for longer delays is due to gain reduction.
This calls into question the findings of studies that have used additivity of forward masking as an estimate of compression at the signal frequency (Plack and O'Hanlon, 2003; Plack et al., 2006, 2007, 2008) or well below the signal frequency (Plack and Arifianto, 2010). Plack and Arifianto (2010) compared the typical long (200-ms) M1 and a short (20-ms) M1 and argued that gain reduction could not play a role in the short M1 condition, where they still showed compression for the masker well below the signal frequency. However, Roverud and Strickland (2014) showed evidence of gain reduction even for precursors and maskers of these short durations.
C. Gain reduction as a function of elicitor laterality
In the present study, ipsilateral and bilateral precursors produced significantly greater gain reduction than a contralateral precursor. This pattern was consistent across signal frequency (1, 2, and 4 kHz) and precursor levels of 50, 55, and 60 dB SPL. Because the data across conditions were collected from the same subjects, they provide a framework for evaluating data from previous studies. To our knowledge, no previous study has used bilateral elicitors, and those results will be considered separately below.
To make comparisons across these studies despite differences in the stimuli and methodologies used, we estimated the magnitude of gain reduction from each study by measuring the threshold change (either signal increase or masker decrease) caused by the elicitor when the masker was approximately an octave below the signal frequency, as was done in the current study. The exception is the estimate from Yasin et al. (2014), which was taken from fits to input-output functions, since raw data were not shown. A detailed summary of these studies can be found in Table V. Signal and masker threshold changes with an elicitor for signals on the lower leg of the input-output function have been shown to be similar in magnitude in previous studies (Roverud and Strickland, 2010; Jennings et al., 2009). Data points from these studies were only included from conditions where the stimuli were contiguous or, in other words, there were no (or only very short) time delays between the stimuli (elicitor, masker, and signal), or else they were on simultaneously. The longer the time delay between the elicitor and the masker plus signal (Yasin et al., 2014) or between the elicitor plus masker and the signal (Aguilar et al., 2013; Fletcher et al., 2016), the weaker the effect of the elicitor on the signal. Additionally, gain reduction is estimated from the lowest signal level used from each study when applicable, thereby ensuring that gain changes occurred with the signal on the lower leg of the input-output function, where the largest change in gain should be seen. Some studies used simultaneous masking (Vinay and Moore, 2008; Wicher and Moore, 2014), others used forward masking (Aguilar et al., 2013; Fletcher et al., 2016; Jennings et al., 2009; Jennings and Strickland, 2012; Yasin et al., 2014), and one study used both forward and simultaneous masking (Kawase et al., 2000).
TABLE V.
Methodological details for assessing gain reduction from previous studies. Data are shown in Fig. 8. The studies are ordered by precursor laterality (Contra, contralateral; Ipsi, ipsilateral) and masking type. FDMC, fixed-duration masking curve; FM, forward masking; SL, sensation level; SM, simultaneous masking; BBN, broadband noise; NBN, narrowband noise; PTC, psychophysical tuning curve; TMC, temporal masking curve; ERB, equivalent rectangular bandwidth.
| Study | Method | Masking | Elicitor laterality | Elicitor type | Elicitor level | Masker | Signal |
|---|---|---|---|---|---|---|---|
| Kawase et al. (2000) | PTC | SM and FM | Contra | White BBN; gated with masker | 50 dB SPL | NBN; 495 ms | 2 kHz; 30 ms |
| Quaranta et al. (2005) | PTC | SM | Contra | NBN; continuous | 40 dB SL | Pure tone; continuous | 1 and 4 kHz; 250 ms |
| Vinay and Moore (2008) | PTC | SM | Contra | NBN; continuous | 50 and 60 dB SL | NBN; continuous | 0.5, 1, 2, and 4 kHz; 250 ms |
| Aguilar et al. (2013) | PTC | FM | Contra | BBN; on through stimulus | 60 dB SPL | 200 ms tonal maskers | 0.5 and 4 kHz; 10 ms |
| Wicher and Moore (2014) | PTC | SM | Contra | NBN and pink BBN | 50 and 60 dB SPL | NBN; 200 ms | 1 and 2 kHz; 200 ms |
| Fletcher et al. (2016) | TMC | FM | Contra | Equal ERB BBN; continuous | 54 dB SPL | 1.22 kHz; 25 ms | 2 kHz; 5 ms |
| Jennings et al. (2009) | PTC | FM | Ipsi | 4 kHz; 160 ms | 60 dB SPL for S1–S3; 50 dB SPL S4 | 2.2 kHz; 20 ms | 4 kHz; 6 ms |
| Jennings and Strickland (2012) | PTC | FM | Ipsi | 4 kHz; 100 ms | 50 dB SPL for S1–S3; 40 dB SPL for S4 | 2.4 kHz; 20 ms | 4 kHz; 6 ms |
| Yasin et al. (2014) | FDMC | FM | Ipsi | NBN; 500 ms; | 60 dB SPL | 1.8 kHz; | 4 kHz |
| DeRoy Milvae and Strickland (2018) | FM gain reduction; masker present or absent | FM | Ipsi | Pink BBN; 50 ms | 60 dB SPL | 1.2 and 2.4 kHz); 20 ms; no masker at 1 kHz | 1, 2, and 4 kHz; 8 ms |
To ensure that the results were consistent with gain reduction, each of these studies also included conditions with an on-frequency masker. The fact that a precursor caused a larger change in threshold for an off-frequency masker than for an on-frequency one is consistent with gain reduction, as first discussed by Kawase et al. (2000) and as described earlier in this paper. In the current study, on-frequency and off-frequency comparisons were made for precursor levels of 60 dB SPL and were consistent with gain reduction, as noted in Sec. II.
Estimates of gain reduction from the current and previous psychoacoustic studies are shown in Fig. 8. Effects of contralateral elicitors are presented in the left panel, and effects of ipsilateral elicitors are presented in the right panel. The signal frequency tested is indicated on the abscissa, and the effect of the elicitor is indicated on the ordinate. It is clear that across studies, gain reduction from an ipsilateral elicitor is larger than that from a contralateral elicitor. The ipsilateral data show that even across studies where the conditions were comparable, there is variability in the magnitude of gain reduction. This underscores the importance of comparing the different conditions within subjects. In the contralateral data, gain reduction is similar across studies that used forward masking.
FIG. 8.
A summary of gain reduction effects with contralateral (left panel) and ipsilateral (right panel) elicitors from previous studies. For each panel, signal frequencies tested are on the x axis, and the threshold shift (signal or masker) with an elicitor is on the y axis. SDs are indicated by the error bars.
The two contralateral studies that showed the largest gain reduction (Vinay and Moore, 2008; Wicher and Moore, 2014) used an elicitor that continued through the masker and signal. This may increase gain reduction, as has been shown in OAE studies (e.g., Lilaonitkul and Guinan, 2009a). These two studies also used simultaneous masking and long signals and maskers, in contrast to most of the other studies. Simultaneous masking may be the reason for the negative threshold changes found by Vinay and Moore (2008). It has been argued that the decrease in masker level with an elicitor may partially be due to “two-tone” suppression, which may play a role in the simultaneous masking conditions (Kawase et al., 2000). Gain reduction would be expected to reduce suppression as well (e.g., Hegland and Strickland, 2018), which complicates interpretation of the simultaneous masking results.
In addition to the effects of contralateral and ipsilateral elicitors, gain reduction was estimated with bilateral elicitors in the current study. It was hypothesized that gain reduction from bilateral elicitors should be as large or larger than gain reduction from ipsilateral elicitors, given that the bilateral efferent pathway incorporates both contralateral and ipsilateral efferent loops (see Warr, 1992). Given the small contralateral effects noted above, it is perhaps not surprising that bilateral elicitors were not more effective than ipsilateral elicitors. However, gain reduction from bilateral elicitors was equal to or sometimes slightly smaller (∼1 dB) than gain reduction with ipsilateral elicitors.
Because of the challenges of differentiating gain reduction from other effects of preceding sound, the few previous studies that have directly compared the effects of elicitor laterality have used OAEs. The relative strength of gain reduction in the present study is consistent with SFOAE data from Lilaonitkul and Guinan (2012). For tonal and narrowband noise elicitors, for signal frequencies of 1 kHz and above, ipsilateral and bilateral elicitors produced approximately the same change in the SFOAE magnitude, while contralateral elicitors had a much smaller effect. In contrast, however, with broadband elicitors, as used in the present study, bilateral elicitors showed the largest effects, and contralateral and ipsilateral elicitors showed nearly equal effects (Lilaonitkul and Guinan, 2009a). The latter result is consistent with TEOAE studies that have shown similar decreases in magnitude from ipsilateral and contralateral broadband elicitors (Boothalingam and Purcell, 2015).
Although other studies using neural or behavioral measures have not compared elicitor laterality directly, the evidence is generally consistent with a larger effect for ipsilateral elicitors than for contralateral ones, as was found in the present study. As noted in the Introduction, studies in cats by Kawase and Liberman (1993) and Kawase et al. (1993) and modeling studies of those data (Chintanpalli et al., 2012; Smalt et al., 2014) are consistent with much larger ipsilateral than contralateral efferent strength. Similar modeling of the same data by Chintanpalli et al. (2012) was consistent with ipsilateral gain reduction of 20 dB. This magnitude is similar to estimates from Russell and Murugasu (1997) based on neural responses with electrical stimulation of the OCB in guinea pigs. The fact that gain reduction from a contralateral elicitor is small is consistent with studies of CAP reduction in humans with contralateral noise, which also indicate shifts in response equivalent to only about 2 dB (Lichtenhan et al., 2016; Smith et al., 2017).
The fact that bilateral elicitors tended to produce slightly less threshold shift than ipsilateral elicitors in the present study may reflect more central factors. The precursor is audible to the subject, and it could be argued that the ipsilateral condition is the most difficult from an attentional or confusion standpoint. However, previous studies of confusion between the masker and signal in forward masking have found confusion effects when the masker was a narrowband noise centered at the signal frequency, but not for sinusoidal or broadband forward maskers [see summary in Neff (1986)]. In these studies, confusion was inferred by a decrease in threshold for the signal when a contralateral broadband noise was presented synchronously with the ipsilateral masker. In the present study, the masker was a sinusoid, and the precursor is a broadband noise, so substantial confusion was not expected. However, it is possible that in our bilateral conditions, the contralateral pink noise may have provided a temporal cue for the subjects to help differentiate the masker and signal from the ipsilateral precursor. The origin of this cue would be non-peripheral as the contralateral stimulus does not produce any excitatory masking in the ipsilateral ear. Again, the average difference between ipsilateral and bilateral gain reduction effects was very small (∼1 dB). Additionally, in the contralateral and bilateral conditions, the precursor may be perceived as spatially separated from the masker and signal, even though the noise was not correlated between the two ears. Consistent with this hypothesis, although the gain reduction test (both ipsilateral and bilateral), comparing on- and off-frequency maskers, was consistent for all lateralities, the change in threshold was slightly smaller for both off- and on-frequency maskers with a bilateral elicitor. Therefore, attention or spatial separation may have played a small role in the effects seen.
It also is unlikely that the results reflect order effects. The order of testing was randomized across subjects. Furthermore, each subject had multiple hours of training prior to data collection to help reduce any learning effects, and each condition was repeated at least once before final data collection to determine if the threshold changed significantly from day to day. Therefore, neither order nor practice effects seem likely to have played a role in our findings.
D. Frequency effects
The average magnitude of gain reduction did not differ significantly with signal frequency. This extends the results of DeRoy Milvae and Strickland (2018), who used an ipsilateral precursor, to all precursor lateralities. Importantly, these findings are also consistent with results from previous psychoacoustic studies shown in Fig. 8 as well as data from Smith et al. (2000). In contrast, previous human OAE data measuring gain reduction effects at different probe frequencies found a large change at 1 kHz and a much weaker effect at 4 kHz (Lilaonitkul and Guinan, 2009b, 2012). However, they noted that the method used (SFOAEs) made it challenging to estimate magnitude effects at higher probe frequencies, where the SNRs tended to be low. Therefore, this result may not reflect a genuine difference in MOCR strength across frequency, but instead may be more of a methodological limitation. As mentioned in the Introduction, recent data from human cadavers showed broad frequency innervation of MOCR terminals across the audiometric frequency range (250–10 kHz), with a peak in the upper basal turn of the cochlea, equivalent to CFs of approximately 2–4 kHz in the specimens from the youngest cadavers (Liberman and Liberman, 2019; see their Fig. 5). These data suggest that the efferent innervation across frequency is similar to that for other animals when the audible frequency range is adjusted for (cat: Liberman et al., 1990; guinea pig: Liberman and Gao, 1995; mouse: Maison et al., 2003). The Liberman and Liberman (2019) data also suggest that MOCR innervation density is broadly distributed across the audibility frequency range in humans, which is consistent with the broad frequency range of the behavioral gain reduction results in the current study.
E. Growth of gain reduction with precursor level
As noted in the Introduction, the growth of gain reduction with precursor level has been examined in two previous studies. Roverud and Strickland (2014) examined gain reduction as a function of precursor duration and used three levels for tonal on-frequency and off-frequency precursors. The results were modeled well by using input-output functions measured in the same subjects to estimate compression applied to the on-frequency precursors and by assuming that the growth for off-frequency precursors was linear. Yasin et al. (2014) measured ipsilateral gain reduction for a wide range of broadband noise precursor levels. To do so, they measured fixed-duration masking curves (FDMCs) for on- and off-frequency maskers relative to a 4-kHz signal, varying the signal and masker durations while keeping the total duration at 25 ms. BM input-output functions were then derived from the functions for the on- and off-frequency masker. They also found compressive growth as a function of precursor level, which was in the range of compression measured in the derived input-output functions. This was extended in the current study to determine whether growth of gain reduction with precursor level differed across signal frequency and precursor laterality. Overall, we found that the average slopes were compressive (0.12–0.30 dB/dB; Table III) and that there were no significant differences between ipsilateral and bilateral elicitors. Interestingly, there was a difference in the slopes across signal frequency. While pairwise comparisons for each signal frequency did not reach significance, there was a tendency for the average slopes to be steeper for 1 and 2 kHz compared to 4 kHz. If it is assumed that gain reduction is driven mainly by sound coming through peripheral channels near the signal frequency, the compressive slopes may reflect the compression in each frequency region. The difference in slopes with frequency region is consistent with results from Rosengard et al. (2005), which found that input-output functions measured using psychoacoustic techniques were more compressive at high than low frequencies. The implication that cochlear compression may play a role in efferent gain control across frequency could be important for signal processing strategies trying to incorporate efferent effects in into their devices, such as in hearing aids (Jürgens et al., 2016) and cochlear implants (Lopez-Poveda et al., 2016).
In addition to the data that were analyzed for precursor levels ≤60 dB SPL, we also collected threshold responses with 65-, 70-, and 75-dB SPL precursor levels (see Fig. 6). The reasoning was just to see what happened at these higher levels and whether there was any evidence of activation of the MEMR. Yasin et al. (2014) used broadband precursor levels as high as 80 dB SPL. The clinical thresholds for the ipsilateral and contralateral MEMR are marked on Fig. 6. For most subjects, the contralateral thresholds were above the highest precursor level. For these subjects, signal threshold continues to grow compressively or plateau with precursor level. This indicates that measurements with a 60 dB SPL precursor are close to the maximum gain reduction. For one subject, S5, there was a sharp change in the growth of threshold with precursor level in one of the conditions. The pattern of this change could be consistent with MEMR activation in this subject. The steeper slope is seen with a bilateral precursor, which would be expected to have the lowest MEMR reflex (Møller, 1962). The effect is seen at 1 kHz, consistent with the low-frequency effect of the MEMR (Borg et al., 1984). In a few listeners, thresholds decreased at the highest precursor levels in the contralateral conditions. This could also be consistent with MEMR activation, since the off-frequency masker is lower in frequency than the signal. Perhaps a more sensitive physiological measurement could be employed in future gain reduction studies to detect MEMR activation at a range of precursor levels such as that in wideband acoustic immittance (WAI) tympanometry. This would have the benefit of determining the frequencies affected by the MEMR as a function of the precursor level. Additionally, WAI tympanometry is on average a 12–13 dB more sensitive measure than typical clinical measures (Keefe et al., 2010; Feeney et al., 2017). However, it appears that in this behavioral study it is possible to go to fairly high precursor levels without obvious MEMR effects in general.
F. Final thoughts
The techniques used in this paper are designed to obtain a measure of gain change that is free of complicating effects of suppression of the signal by the masker or elicitor. However, in everyday life, gain reduction would typically be elicited by bilateral ongoing sound. It is likely that the amount of gain reduction may be larger during the elicitor (Lilaonitkul and Guinan, 2012) and that suppression also plays a role. In addition, the measures in this study are for signals near threshold, where gain reduction increases threshold. However, benefits for gain reduction typically occur at higher levels, where a decrease in gain would increase the slope of the input-ouput function and magnify changes in signal level. May and McQuone (1995) showed that intensity discrimination in cats for an 8-kHz signal in background noise worsened after the ipsilateral and contralateral OCBs were severed. Almishaal et al. (2017) and Jennings et al. (2018) found an improvement in detection of amplitude modulation in short carriers of 2 and 4 kHz following a precursor, for mid-level carriers, that decreased with cochlear hearing impairment. Strickland et al. (2018), using the same forward masking paradigm used in the present study, showed that increases in threshold for a 5-dB SL, 4-kHz signal with an off-frequency masker following a precursor were similar in magnitude to decreases in masking for a 15–20 dB SL signal with an on-frequency masker following a precursor. The implication is that the increases in signal threshold measured in the current study would translate to a benefit in perception at higher levels, as for example in listening to speech in noise (Clark et al., 2012).
In terms of the magnitude of gain reduction with precursor laterality, the present study is clear in showing that the effects of contralateral elicitors are quite small, while ipsilateral and bilateral elicitors show clear effects. While contralateral elicitors are useful in not eliciting other mechanisms that may occur when stimuli are in the same ear, it should be recognized that they produce the smallest effect. It is interesting that the elicitor laterality results in the current study are consistent with previous results from neural and behavioral studies and from SFOAE studies using narrowband elicitors. However, some previous OAE measures using broadband elicitors show nearly equal effects for ipsilateral and contralateral elicitors. It is important to consider that in OAE measurements, a change in the response to a fixed signal is measured with a precursor, while in behavioral experiments, typically the signal or masker is adjusted to measure a threshold after a precursor. Stimuli used in OAE measurements also must be intense enough to elicit a measurable response, and thus the signal may be higher than that used in behavioral experiments. Last, the understanding of cochlear amplification is changing rapidly as new types of measurements are made. The latest understanding is that the motion of the reticular lamina is greater than that of the BM (Ren et al., 2016). Data from Dewey et al. (2019) in mice show that the cochlear amplifier is responsible for the peak of the reticular lamina response. This means the on- vs off-frequency analysis in the present paper should still be valid, even though the exact mechanism is different. It is not clear what part of the cochlear response OAEs reflect (Guinan, 2018), nor is it completely understood if MOCR effects in OAEs reflect the same processes as those in MOCR behavioral paradigms using similar stimuli [see Jennings (2021) for review]. Therefore, it is important to use behavioral, neural, and OAE measures in fully understanding gain reduction.
Finally, care must be taken in extrapolating the results of this paper. While the no-masker condition appears to be a reasonable estimate of gain reduction in listeners with hearing thresholds within the normal range, this does not mean that this will be true for all listeners. Since the MOCR acts to reduce neural adaptation (Kawase et al., 1993), listeners without a MOCR could show forward masking from other mechanisms. Therefore, researchers measuring the effects of gain reduction should use a control test, such as the on- vs off-frequency masker test used in this study and in DeRoy Milvae and Strickland (2018).
G. Conclusions
In the measure used in this study, which is assumed to measure gain reduction near threshold:
-
(1)
Gain reduction was largest for ipsilateral and bilateral elicitors and was much smaller for a contralateral elicitor.
-
(2)
The magnitude of gain reduction did not differ with signal frequency.
-
(3)
The growth of gain reduction with precursor level tended to be higher for signal frequencies of 1 and 2 kHz than 4 kHz, although this did not reach statistical significance.
ACKNOWLEDGMENTS
This research was supported by National Institutes of Health [National Institute on Deafness and Other Communication Disorders (NIDCD)] Grant Nos. T32 DC016853 and R01 DC008327.
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