Stimulus-frequency Otoacoustic Emissions Research Articles

Medial olivocochlear influence on stimulus-frequency otoacoustic emission input-output functions

Modulation of cochlear mechanics by the medial olivocochlear efferent system is characterized by a reduction in active, outer hair cell-mediated amplification of basilar membrane motion. This increases cochlear thresholds and linearizes basilar membrane input-output functions for low-to-moderate stimulus levels. Significant efferent effects have also been observed for responses to higher stimulus levels, potentially reflecting changes in the mechanical properties of the cochlear partition. In humans, sound activated changes in stimulus-frequency otoacoustic emissions have been used as a tool for investigating the dynamics of the medial olivocochlear reflex. However, the degree to which the amplitude and phase of otoacoustic emissions are related to those of basilar membrane motion is not entirely clear. For the purposes of comparison with invasive physiological measurements in animals, stimulus-frequency otoacoustic emission input-output functions were obtained from human subjects in the presence and absence of contralateral acoustic stimulation. Medial olivocochlear effects were quantified in terms of the absolute change in emission level as well as the vector change, which incorporates changes in emission phase. The extent to which efferent modulation of emissions in humans reflects that observed in previous reports of basilar membrane motion will be discussed.

Suppression tuning of distortion product otoacoustic emissions in humans: results from cochlear mechanics simulation

Is human hearing more sharply tuned than other mammals? This has been a heavily debated subject in the field of cochlear mechanics. The debate continues partially due to lack of non-invasive methods to estimate human cochlear tuning accurately. Recently, Gorga et al. (2011, JASA) derived tuning curves from suppression of distortion product (DP) otoacoustic emissions (OAEs) in normal-hearing human ears. Frequencies of the primary tones were varied from 0.5 to 8 kHz. Sharpness of tuning was analyzed in terms of the Q-value of equivalent rectangular bandwith, which ranged from 4 to 10 at the lowest stimulus level tested. The Q-values were similar to that of psychoacoustic tuning but lower than inferred from latencies of stimulus-frequency OAEs (Shera et al. 2002, PNAS). In the present work, we simulate DPOAE suppression based on a computer model of cochlear mechanics (Liu and Neely, 2010, JASA). The simulated DPOAE suppression tuning curves (STCs) resemble those obtained in experiments but discrepancies remain. At high frequencies, the simulated DPOAE STCs are not as sharply tuned as the magnitude response of traveling waves in the model. Confounding factors and interpretation of results will be discussed. (Supported by Taiwan's NSC and NTHU)

Interrelationships between spontaneous and low-level stimulus-frequency otoacoustic emissions in humans

It has been proposed that OAEs be classified not on the basis of the stimuli used to evoke them, but on the mechanisms that produce them (Shera and Guinan, 1999). One branch of this taxonomy focuses on a coherent reflection model and explicitly describes interrelationships between spontaneous emissions (SOAEs) and stimulus-frequency emissions (SFOAEs). The present study empirically examines SOAEs and SFOAEs from individual ears within the context of model predictions, using a low stimulus level (20 dB SPL) to evoke SFOAEs. Emissions were recorded from ears of normal-hearing young adults, both with and without prominent SOAE activity. When spontaneous activity was observed, SFOAEs demonstrated a localized increase about the SOAE peaks. The converse was not necessarily true though, i.e., robust SFOAEs could be measured where no SOAE peaks were observed. There was no significant difference in SFOAE phase-gradient delays between those with and without observable SOAE activity. However, delays were larger for a 20 dB SPL stimulus level than those previously reported for 40 dB SPL. The total amount of SFOAE phase accumulation occurring between adjacent SOAE peaks tended to cluster about an integral number of cycles. Overall, the present data appear congruous with predictions stemming from the coherent reflection model and support the notion that such comparisons ideally are made with emissions evoked using relatively lower stimulus levels.

Influence of acute hyperglycemia on otoacoustic emissions and the medial olivocochlear reflex

Stimulus-frequency (SF) otoacoustic emission (OAE) amplitude and the amplitude of medial olivocochlear (MOC) inhibition of SF OAEs for ipsilateral, contralateral and bilateral MOC reflex elicitors were recorded in six subjects with type 2 diabetes during a glucose tolerance test (GTT). Five of the six subjects were tested twice for a total of 11 trials and three subjects were tested in a control experiment. During the GTT experiment, the subjects' blood glucose was elevated from a euglycemic level below 150 mg/dL to a hyperglycemic level above 160 mg/dL following the consumption of a bolus of 80 g of sugar. A subset of three subjects were tested in a control experiment during which SF OAE and MOC reflex measurements were made while blood sugar levels remained constant within the euglycemic region. Mean SF OAE amplitudes were elevated following glucose consumption. A statistically significant increase in MOC inhibition amplitude was observed during elevated sugar levels for the 11 GTT trials. Maximum inhibition occurred about an hour after glucose consumption when blood glucose levels peaked. Results indicate that acute hyperglycemia influences efferent control of the cochlea in people with type 2 diabetes.

Frequency tuning of medial-olivocochlear-efferent acoustic reflexes in humans as functions of probe frequency

The medial-olivocochlear (MOC) acoustic reflex is thought to provide frequency-specific feedback that adjusts the gain of cochlear amplification, but little is known about how frequency specific the reflex actually is. We measured human MOC tuning through changes in stimulus frequency otoacoustic emissions (SFOAEs) from 40-dB-SPL tones at probe frequencies (f(p)s) near 0.5, 1.0, and 4.0 kHz. MOC activity was elicited by 60-dB-SPL ipsilateral, contralateral, or bilateral tones or half-octave noise bands, with elicitor frequency (f(e)) varied in half-octave steps. Tone and noise elicitors produced similar results. At all probe frequencies, SFOAE changes were produced by a wide range of elicitor frequencies with elicitor frequencies near 0.7-2.0 kHz being particularly effective. MOC-induced changes in SFOAE magnitude and SFOAE phase were surprisingly different functions of f(e): magnitude inhibition largest for f(e) close to f(p), phase change largest for f(e) remote from f(p). The metric ΔSFOAE, which combines both magnitude and phase changes, provided the best match to reported (cat) MOC neural inhibition. Ipsilateral and contralateral MOC reflexes often showed dramatic differences in plots of MOC effect vs. elicitor frequency, indicating that the contralateral reflex does not give an accurate picture of ipsilateral-reflex properties. These differences in MOC effects appear to imply that ipsilateral and contralateral reflexes have different actions in the cochlea. The implication of these results for MOC function, cochlear mechanics, and the production of SFOAEs are discussed.

Effects of Low-Frequency Biasing on Otoacoustic and Neural Measures Suggest that Stimulus-Frequency Otoacoustic Emissions Originate Near the Peak Region of the Traveling Wave

Stimulus-frequency otoacoustic emissions (SFOAEs) have been used to study a variety of topics in cochlear mechanics, although a current topic of debate is where in the cochlea these emissions are generated. One hypothesis is that SFOAE generation is predominately near the peak region of the traveling wave. An opposing hypothesis is that SFOAE generation near the peak region is deemphasized compared to generation in the tail region of the traveling wave. A comparison was made between the effect of low-frequency biasing on both SFOAEs and a physiologic measure that arises from the peak region of the traveling wave--the compound action potential (CAP). SFOAE biasing was measured as the amplitude of spectral sidebands from varying bias tone levels. CAP biasing was measured as the suppression of CAP amplitude from varying bias tone levels. Measures of biasing effects were made throughout the cochlea. Results from cats show that the level of bias tone needed for maximum SFOAE sidebands and for 50% CAP reduction increased as probe frequency increased. Results from guinea pigs show an irregular bias effect as a function of probe frequency. In both species, there was a strong and positive relationship between the bias level needed for maximum SFOAE sidebands and for 50% CAP suppression. This relationship is consistent with the hypothesis that the majority of SFOAE is generated near the peak region of the traveling wave.

Human cochlear tuning estimates from stimulus-frequency otoacoustic emissions

Two objective measures of human cochlear tuning, using stimulus-frequency otoacoustic emissions (SFOAE), have been proposed. One measure used SFOAE phase-gradient delay and the other two-tone suppression (2TS) tuning curves. Here, it is hypothesized that the two measures lead to different frequency functions in the same listener. Two experiments were conducted in ten young adult normal-hearing listeners in three frequency bands (1-2 kHz, 3-4 kHz and 5-6 kHz). Experiment 1 recorded SFOAE latency as a function of stimulus frequency, and experiment 2 recorded 2TS iso-input tuning curves. In both cases, the output was converted into a sharpness-of-tuning factor based on the equivalent rectangular bandwidth. In both experiments, sharpness-of-tuning curves were shown to be frequency dependent, yielding sharper relative tuning with increasing frequency. Only a weak frequency dependence of the sharpness-of-tuning curves was observed for experiment 2, consistent with objective and behavioural estimates from the literature. Most importantly, the absolute difference between the two tuning estimates was very large and statistically significant. It is argued that the 2TS estimates of cochlear tuning likely represents the underlying properties of the suppression mechanism, and not necessarily cochlear tuning. Thus the phase-gradient delay estimate is the most likely one to reflect cochlear tuning.

Cochlear measures of selective auditory attention.

The possibility that selective auditory attention can affect the responses of the cochlea via the medial olivocochlear (MOC) pathway was investigated in human listeners using a nonlinear version of the stimulus-frequency otoacoustic emission (SFOAE), called the nSFOAE [Walsh et al. (2010)]. During nSFOAE recording, listeners attended to one of two competing speech streams, each composed of seven randomly-selected spoken digits that were interleaved with the nSFOAE stimuli. The talker was female in one ear and male in the other (randomized across trials). The task of the listener was to match a subset of the digits spoken by the female talker to one of two choices presented visually on a computer screen. The nSFOAE stimulus was a tone (4.0 kHz, 200 ms) presented simultaneously to the two ears, either in quiet (tone-alone) or in noise (tone-plus-noise). When nSFOAEs were measured during periods of selective attention, the tone-plus-noise responses exhibited larger maximum magnitudes, and larger changes from the tone-alone magnitudes, compared to the responses measured during a no-attention condition, in which the competing speech streams were presented, but with no digit-matching required. The differences in magnitudes across conditions were as much as 4–5 dB. [Work supported by the NIDCD.]

Specification of absorbed-sound power in the ear canal: Application to suppression of stimulus frequency otoacoustic emissions

An insert ear-canal probe including sound source and microphone can deliver a calibrated sound power level to the ear. The aural power absorbed is proportional to the product of mean-squared forward pressure, ear-canal area, and absorbance, in which the sound field is represented using forward (reverse) waves traveling toward (away from) the eardrum. Forward pressure is composed of incident pressure and its multiple internal reflections between eardrum and probe. Based on a database of measurements in normal-hearing adults from 0.22 to 8 kHz, the transfer-function level of forward relative to incident pressure is boosted below 0.7 kHz and within 4 dB above. The level of forward relative to total pressure is maximal close to 4 kHz with wide variability across ears. A spectrally flat incident-pressure level across frequency produces a nearly flat absorbed power level, in contrast to 19 dB changes in pressure level. Calibrating an ear-canal sound source based on absorbed power may be useful in audiological and research applications. Specifying the tip-to-tail level difference of the suppression tuning curve of stimulus frequency otoacoustic emissions in terms of absorbed power reveals increased cochlear gain at 8 kHz relative to the level difference measured using total pressure.

Modeling cochlear dynamics: Interrelation between cochlea mechanics and psychoacoustics

A model of the cochlea was used to bridge the gap between model approaches commonly used to investigate phenomena related to otoacoustic emissions and more filter-based model approaches often used in psychoacoustics. In the present study, a nonlinear and active one-dimensional transmission line model was developed that accounts for several aspects of physiological data with a single fixed parameter set. The model shows plausible excitation patterns and an input-output function similar to the linear-compressive-linear function as hypothesized in psychoacoustics. The model shows realistic results in a two-tone suppression paradigm and a plausible growth function of the 2f(1)-f(2) component of distortion product otoacoustic emissions. Finestructure was found in simulated stimulus-frequency otoacoustic emissions (SFOAE) with realistic levels and rapid phase rotation. A plausible "threshold in quiet" including finestructure and spontaneous otoacoustic emissions (SOAE) could be simulated. It is further shown that psychoacoustical data of modulation detection near threshold can be explained by the mechanical dynamics of the modeled healthy cochlea. It is discussed that such a model can be used to investigate the representation of acoustic signals in healthy and impaired cochleae at this early stage of the auditory pathway for both, physiological as well as psychoacoustical paradigms.

Tectorial Membrane Morphological Variation: Effects upon Stimulus Frequency Otoacoustic Emissions

The tectorial membrane (TM) is widely believed to play an important role in determining the ear's ability to detect and resolve incoming acoustic information. While it is still unclear precisely what that role is, the TM has been hypothesized to help overcome viscous forces and thereby sharpen mechanical tuning of the sensory cells. Lizards present a unique opportunity to further study the role of the TM given the diverse inner-ear morphological differences across species. Furthermore, stimulus-frequency otoacoustic emissions (SFOAEs), sounds emitted by the ear in response to a tone, noninvasively probe the frequency selectivity of the ear. We report estimates of auditory tuning derived from SFOAEs for 12 different species of lizards with widely varying TM morphology. Despite gross anatomical differences across the species examined herein, low-level SFOAEs were readily measurable in all ears tested, even in non-TM species whose basilar papilla contained as few as 50–60 hair cells. Our measurements generally support theoretical predictions: longer delays/sharper tuning features are found in species with a TM relative to those without. However, SFOAEs from at least one non-TM species (Anolis) with long delays suggest there are likely additional micromechanical factors at play that can directly affect tuning. Additionally, in the one species examined with a continuous TM (Aspidoscelis) where cell-to-cell coupling is presumably relatively stronger, delays were intermediate. This observation appears consistent with recent reports that suggest the TM may play a more complex macromechanical role in the mammalian cochlea via longitudinal energy distribution (and thereby affect tuning). Although significant differences exist between reptilian and mammalian auditory biophysics, understanding lizard OAE generation mechanisms yields significant insight into fundamental principles at work in all vertebrate ears.

Otoacoustic Estimation of Cochlear Tuning: Validation in the Chinchilla

We analyze published auditory-nerve and otoacoustic measurements in chinchilla to test a network of hypothesized relationships between cochlear tuning, cochlear traveling-wave delay, and stimulus-frequency otoacoustic emissions (SFOAEs). We find that the physiological data generally corroborate the network of relationships, including predictions from filter theory and the coherent-reflection model of OAE generation, at locations throughout the cochlea. The results support the use of otoacoustic emissions as noninvasive probes of cochlear tuning. Developing this application, we find that tuning ratios-defined as the ratio of tuning sharpness to SFOAE phase-gradient delay in periods-have a nearly species-invariant form in cat, guinea pig, and chinchilla. Analysis of the tuning ratios identifies a species-dependent parameter that locates a transition between "apical-like" and "basal-like" behavior involving multiple aspects of cochlear physiology. Approximate invariance of the tuning ratio allows determination of cochlear tuning from SFOAE delays. We quantify the procedure and show that otoacoustic estimates of chinchilla cochlear tuning match direct measures obtained from the auditory nerve. By assuming that invariance of the tuning ratio extends to humans, we derive new otoacoustic estimates of human cochlear tuning that remain mutually consistent with independent behavioral measurements obtained using different rationales, methodologies, and analysis procedures. The results confirm that at any given characteristic frequency (CF) human cochlear tuning appears sharper than that in the other animals studied, but varies similarly with CF. We show, however, that the exceptionality of human tuning can be exaggerated by the ways in which species are conventionally compared, which take no account of evident differences between the base and apex of the cochlea. Finally, our estimates of human tuning suggest that the spatial spread of excitation of a pure tone along the human basilar membrane is comparable to that in other common laboratory animals.

Overshoot measured physiologically and psychophysically in the same human ears

A nonlinear version of the stimulus-frequency otoacoustic emission (SFOAE) was measured using stimulus waveforms similar to those used for behavioral overshoot. Behaviorally, the seven listeners were as much as 11 dB worse at detecting a brief tonal signal (4.0 kHz, 10 ms in duration) when it occurred soon after the onset of a wideband masking noise (0.1–6.0 kHz; 400 ms in duration) than when it was delayed by about 200 ms, and the nonlinear SFOAE measure exhibited a similar effect. When either lowpass (0.1–3.8 kHz) or bandpass noise (3.8–4.2 kHz) was used instead of the wideband noise, the physiological and behavioral measures again were similar. When a highpass noise (4.2–6.0 kHz) was used, the physiological and behavioral measures both showed no overshoot-like effect for five of the subjects. The physiological response to the tone decayed slowly after the termination of the noise, much like the time course of resetting for behavioral overshoot. One subject exhibited no overshoot behaviorally even though his cochlear responses were like those of the other subjects. Overall, the evidence suggests that some basic characteristics of overshoot are obligatory consequences of cochlear function, as modulated by the olivocochlear efferent system.

Coherent reflection without traveling waves: On the origin of long-latency otoacoustic emissions in lizards

Lizard ears produce otoacoustic emissions with characteristics often strikingly reminiscent of those measured in mammals. The similarity of their emissions is surprising, given that lizards and mammals manifest major differences in aspects of inner ear morphology and function believed to be relevant to emission generation. For example, lizards such as the gecko evidently lack traveling waves along their basilar membrane. Despite the absence of traveling waves, the phase-gradient delays of gecko stimulus-frequency otoacoustic emissions (SFOAEs) are comparable to those measured in many mammals. This paper describes a model of emission generation inspired by the gecko inner ear. The model consists of an array of coupled harmonic oscillators whose effective damping manifests a small degree of irregularity. Model delays increase with the assumed sharpness of tuning, reflecting the build-up time associated with mechanical resonance. When tuning bandwidths are chosen to match those of gecko auditory-nerve fibers, the model reproduces the major features of gecko SFOAEs, including their spectral structure and the magnitude and frequency dependence of their phase-gradient delays. The same model with appropriately modified parameters reproduces the features of SFOAEs in alligator lizards. Analysis of the model demonstrates that the basic mechanisms operating in the model are similar to those of the coherent-reflection model developed to describe mammalian emissions. These results support the notion that SFOAE delays provide a noninvasive measure of the sharpness of cochlear tuning.

Cochlear Sources and Otoacoustic Emissions

Current understanding suggests that there are two different mechanisms by which otoacoustic emissions (OAEs) are generated in the cochlea. These mechanisms include a nonlinear-distortion mechanism and a coherent-reflection mechanism. Distortion product OAEs (DPOAEs) are believed to include contributions from both mechanisms, while stimulus frequency OAEs (SFOAES), at least at low and moderate levels, are believed to be generated primarily by the coherent-reflection mechanism. In the case of DPOAEs, the interaction of the two mechanisms produces a series of alternating peaks and valleys in the response level when recorded in small frequency increments. This pattern of peaks and valleys typically is referred to as fine structure. There has been much speculation that the interaction of the two mechanisms and the resulting fine structure limits the clinical test performance of DPOAEs. There are few data to address this speculation. Here, we review the literature that describes the cochlear source mechanisms and their potential relationship to clinical applications. We then present results for preliminary data collected in a group of 10 normal-hearing subjects where we explore the influence of common approaches to setting DPOAE stimulus parameters on the resulting fine structure. These preliminary results suggest that, at the moderate stimulus levels used in clinical applications, each of the different stimulus parameters results in a similar amount of fine structure and, therefore, fine structure cannot be eliminated through manipulation of stimulus parameters. We also review the results of some preliminary efforts to identify stimulus parameters that can be used to record SFOAEs (OAEs generated by the reflection mechanism). The potential clinical applications of SFOAEs have received little attention in the literature. By identifying stimulus parameters producing robust responses in normal-hearing ears, it may be possible to more fully evaluate clinical applications of SFOAEs.

Mechanics of the frog ear

The frog inner ear contains three regions that are sensitive to airborne sound and which are functionally distinct. (1) The responses of nerve fibres innervating the low-frequency, rostral part of the amphibian papilla (AP) are complex. Electrical tuning of hair cells presumably contributes to the frequency selectivity of these responses. (2) The caudal part of the AP covers the mid-frequency portion of the frog’s auditory range. It shares the ability to generate both evoked and spontaneous otoacoustic emissions with the mammalian cochlea and other vertebrate ears. (3) The basilar papilla functions mainly as a single auditory filter. Its simple anatomy and function provide a model system for testing hypotheses concerning emission generation. Group delays of stimulus-frequency otoacoustic emissions (SFOAEs) from the basilar papilla are accounted for by assuming that they result from forward and reverse transmission through the middle ear, a mechanical delay due to tectorial membrane filtering and a rapid forward and reverse propagation through the inner ear fluids, with negligible delay.

Properties of a nonlinear version of the stimulus-frequency otoacoustic emission.

A procedure for extracting the nonlinear component of the stimulus-frequency otoacoustic emission (SFOAE) is described. This nSFOAE measures the amount by which the cochlear response deviates from linear additivity when the input stimulus is doubled in amplitude. When a 4.0-kHz tone was presented alone, the magnitude of the nSFOAE response remained essentially constant throughout the 400-ms duration of the tone; response magnitude did increase monotonically with increasing tone level. When a wideband noise was presented alone, nSFOAE magnitude increased over the initial 100- to 200-ms portion of the 400-ms duration of the noise. When the tone and the wideband noise were presented simultaneously, nSFOAE magnitude decreased momentarily, then increased substantially for about the first 100 ms and then remained strong for the remainder of the presentation. Manipulations of the noise bandwidth revealed that the low-frequency components were primarily responsible for this rising, dynamic response; no rising segment was seen with bandpass or highpass noise. The rising, dynamic nSFOAE response is likely attributable to activation of the medial olivocochlear efferent system. This perstimulatory emission appears to have the potential to provide information about the earliest stages of auditory processing for stimuli commonly used in psychoacoustical tasks.

Frequency regions responsible for activating the efferent response in human ears.

The nSFOAE is a nonlinear version of the stimulus-frequency otoacoustic emission (SFOAE). In past research, when a wideband noise was presented simultaneously with an ipsilateral 4.0-kHz tone, the nSFOAE to the tone showed a brief hesitation and then a gradual rise that asymptoted after about 100 ms. A similar dynamic response was also seen when the noise was low-pass filtered at 3.8 kHz. However, no dynamic response was seen when the noise was band-passed (400 Hz wide, centered at 4.0 kHz) or high-passed above 4.2 kHz. The interpretation is that the medial olivocochlear efferent system is activated by the wideband and low-pass noises. To identify the frequency components of the noise that are most relevant for activating the rising dynamic response, the nSFOAE was measured for the 4.0-kHz tone in the presence of 400-Hz bands of noise having a spectrum level of about 30-dB SPL and centered at 3.6, 3.2, 2.8, 2.4, and 2.0 kHz. Further exploration was done with noise bands 200 Hz in width and of 33-dB SPL spectrum level, as well as with additional frequencies of the tone. There were individual differences in the frequency regions yielding the maximum dynamic nSFOAE response. [Work supported by NIDCD.]

Evidence for dynamic cochlear processing in otoacoustic emissions and behavior.

Psychophysical research suggests that active cochlear processing may partially explain temporal effects observed in certain auditory masking tasks [Strickland (2001), (2004)]. To investigate this possibility, this study examined the relationship between subjects’ psychophysical performance and the responses of their cochleas to the same stimulus waveforms. Stimulus-frequency otoacoustic emissions (SFOAEs) were recorded in the ear canal using a nonlinear procedure. The results showed that the nonlinear SFOAE to a brief tonal signal (4 kHz, 10 ms, 60 dB SPL) in a background noise (100–6000 Hz, 400 ms, 25 dB/Hz) increased in magnitude (with signal delay) over a similar time course to subjects’ improvement psychophysically in detecting the same signal. Manipulation of the noise bandwidth revealed that the increases in SFOAE magnitude and the improvement in psychophysical detection both were caused primarily by off-frequency components of the noise: lowpass components for SFOAEs, and lowpass or highpass components for psychophysics. These findings are consistent with a change in the slope of the cochlear input-output function from highly compressive to less compressive over the duration of the background noise, a change that might be attributable to negative feedback from the efferent system. [Work supported by NIDCD.]

Human Medial Olivocochlear Reflex: Effects as Functions of Contralateral, Ipsilateral, and Bilateral Elicitor Bandwidths

Animal studies have led to the view that the acoustic medial olivocochlear (MOC) efferent reflex provides sharply tuned frequency-specific feedback that inhibits cochlear amplification. To determine if MOC activation is indeed narrow band, we measured the MOC effects in humans elicited by 60-dB sound pressure level (SPL) contralateral, ipsilateral, and bilateral noise bands as a function of noise bandwidth from 1/2 to 6.7 octaves. MOC effects were quantified by the change in stimulus frequency otoacoustic emissions from 40 dB SPL probe tones near 0.5, 1, and 4 kHz. In a second experiment, the noise bands were centered 2 octaves below probe frequencies near 1 and 4 kHz. In all cases, the MOC effects increased as elicitor bandwidth increased, with the effect saturating at about 4 octaves. Generally, the MOC effects increased as the probe frequency decreased, opposite expectations based on MOC innervation density in the cochlea. Bilateral-elicitor effects were always the largest. The ratio of ipsilateral/contralateral effects depended on elicitor bandwidth; the ratio was large for narrow-band probe-centered elicitors and approximately unity for wide-band elicitors. In another experiment, the MOC effects from increasing elicitor bandwidths were calculated from measurements of the MOC effects from adjacent half-octave noise bands. The predicted bandwidth function agreed well with the measured bandwidth function for contralateral elicitors, but overestimated it for ipsilateral and bilateral elicitors. Overall, the results indicate that (1) the MOC reflexes integrate excitation from almost the entire cochlear length, (2) as elicitor bandwidth is increased, the excitation from newly stimulated cochlear regions more than overcomes the reduced excitation at frequencies in the center of the elicitor band, and (3) contralateral, ipsilateral, and bilateral elicitors show MOC reflex spatial summation over most of the cochlea, but ipsilateral spatial summation is less additive than contralateral.