Suppression Of Distortion Product Otoacoustic Emissions Research Articles

Inferior colliculus stimulation and changes in 2f 1−f 2 distortion product otoacoustic emissions in the rat

The external, central and dorsal subnuclei of the inferior colliculus (ICX, ICC, ICD respectively) have different patterns of descending projections to the periolivary region. We found that electrical stimulation of these subnuclei in anesthetized rats causes suppression of 2f 1−f 2 distortion product otoacoustic emissions (DPOAE). The responses in DPOAEs to stimulation of ICX and ICC are characterized by a large (7–25 dB), initial suppression which lasted 25–30 s (early phase) followed by a sustained smaller (3–15 dB) suppression (late phase). ICX stimulation produces the largest suppressions of DPOAEs in both contralateral and ipsilateral ears equally. Stimulation of the ICC causes large suppressions in the contralateral ear DPOAEs but much smaller than the suppressions of DPOAEs caused by ICX stimulation. Stimulation of the ICC causes a much smaller suppression in the ipsilateral ear DPOAEs than in the contralateral ear DPOAEs. ICD stimulation, however, produces little or no suppression of DPOAEs in either ear when compared to other subnuclei. No frequency-specific changes in DPOAEs were seen with stimulation of any of the subnuclei of the inferior colliculus (IC). Sectioning of middle ear muscles did not negate the suppressive effects of IC stimulation on DPOAEs. These findings verify that the IC has a subnucleus-specific influence over cochlear micromechanics.

Modulation of distortion product otoacoustic emissions (DPOAE) by low-frequency masker tones in human ears

Biasing of the basilar membrane displacement by a loud low-frequency tone causes changes of the DPOAEs depending on the phase of the masker. A corresponding model helps to understand the nonlinear behavior of the cochlea and to improve the diagnosis of inner ear diseases. Up to now, short time Fourier transform has been used to measure the DPOAE suppression pattern. This method has inherent time and frequency resolution limits. Therefore a new approach is presented, where the primaries and all frequencies generated by the process are harmonics of the masker. A rectangular time window with length according to the masker period is used. The part of the spectrum representing the DPOAE signal is regarded as modulated tone, consisting of a DPOAE frequency (2f1−f2) and spectral side frequencies. The time course of the DPOAE suppression pattern is computed by superposition of these complex components. The results are compared with a simulation of the distortions of the outer hair cells’ mechano-electrical transfer function. In addition, a cochlear impedance model is verified. Variation of masker frequency results in the expected phase shift of the DPOAE modulation pattern and allows the assessment of the lower cutoff frequency of the human cochlear impedance.

A developmental study of distortion product otoacoustic emission (2f1-f2) suppression in humans

Suppression of the 2f1-f2 distortion product otoacoustic emission (DPOAE) provides an effective paradigm for the study of functional cochlear maturation in humans. DPOAE iso-suppression tuning curves (STCs) represent some aspect of peripheral filtering, probably related to the boundaries of distortion generation. Studies conducted thus far suggest that the cochlear tuning assessed by this technique is adult-like in humans by term birth (Abdala et al., Hear. Res. 98 (1996) 38–53; Abdala and Sininger, Ear Hear. 17 (1996) 374–385). However, there have been no studies of cochlear tuning in premature human neonates. DPOAE STCs and suppression growth functions were measured from 14 normal-hearing adults, 33 term and 85 premature neonates to investigate the developmental time course of cochlear frequency resolution and non-linearity. Premature neonates showed non-adult-like DPOAE suppression at f2 of 1500 and 6000 Hz: (1) STCs were narrower in width (Q10) and steeper in slope on the low-frequency flank of the tuning curve; (2) suppressor tones lower in frequency than f2 produced atypically shallow growth of DPOAE suppression. The influence of immature conductive pathways cannot be entirely ruled out as a factor contributing to these results. However, findings may indicate that an immaturity exists in cochlear frequency resolution and non-linearity just prior to term birth. The bases of this immaturity are hypothesized to be outer hair cell in origin.

Distortion product otoacoustic emission suppression tuning curves in human adults and neonates

Distortion product otoacoustic emission (DPOAE) iso-suppression tuning curves (STC) were generated in 15 normal-hearing adults and 16 healthy term-born neonates for three f 2 frequencies. The 2f 1-f 2 DPOAE was elicited using f 2 f 1 = 1.2 , L1 = 65 and L2 = 50 dB SPL. A suppressor tone was presented at frequencies ranging from 1 octave below to 1/4 octave above f, and varied in level until DPOAE amplitude was reduced by 6 dB. The suppressor level required for 6 dB suppression was plotted as function of suppressor frequency to generate a DPOAE STC. Forward-masked psychoacoustic tuning curves (PTO were obtained for three of the adult subjects. Results indicate that DPOAE STCs are stable and show minimal inter- and intea-subject variability. The tip of the STC is consistently centered around the f, region and STCs are similar in shape, width ( Q 10) and slope to VIIlth-nerve TCs. PTCs and STCs measured in the same subject showed similar trends, although PTCs had narrower width and steeper slope. Neonatal STCs were recorded at 3000 and 6000 Hz only and were comparable in shape, width and slope to adult STCs. Results suggest: (1) suppression of the 2f 1-f 2 DPOAE may provide an indirect measure of cochlear frequency resolution in humans and (2) cochlear tuning, and associated active processes in the cochlea, are mature by term birth for at least mid- and high-frequencies. These results provide significant impetus for continued study of DPOAE suppression as a means of evaluating cochlear frequency resolution in humans.

Effects of noise and exercise on distortion product otoacoustic emissions

The effect of physical exercise on both distortion product otoacoustic emissions (DPOAE), and on the temporary effects of noise on human cochlear function was examined. Changes in DPOAEs were compared to changes in behavioural thresholds and the possible relation between contralateral suppression of DPOAEs and susceptibility to noise exposure was investigated. Békésy audiometry, tympanometry and DPOAEs were measured in 8 subjects on 3 separate occasions: before and after noise exposure; before and after exercise at 60% of maximal oxygen uptake ( VO 2max); and before and after a combination of noise exposure and exercise, all of 10 min duration. Noise exposure was third-octave band noise of 102 dB SPL centered at 2 kHz. The magnitude of the effect of noise exposure on DPOAE amplitude averaged in the 2–4 kHz range was comparable but weakly correlated to the magnitude of the temporary threshold shift (TTS) measured as the change in Békésy threshold at 3 kHz. There was no effect of physical exercise without noise exposure on either the Békésy threshold or the DPOAE amplitude. Physical exercise significantly increased the noise-induced TTS and the effect of noise exposure on DPOAE amplitude. A positive correlation was found between the temporary effect of noise exposure and the contralateral suppression effect on DPOAE amplitude.

The efferent-mediated suppression of otoacoustic emissions in awake guinea pigs and its reversible blockage by gentamicin.

The physiology of the medial efferent olivocochlear system involves suppressive interactions of contralateral sounds on ipsilateral sound-evoked responses, but its role is largely unknown to date. Medial efferents act at the level of cochlear outer hair cells via cholinergic synapses and might affect their mechanical activity, thereby modulating auditory sensitivity. The aim of the present work was to obtain noninvasive measurements of distortion-product otoacoustic emissions (DPOEs), which reflect outer hair cell function, in order to establish the characteristics of medial efferent-induced suppression in awake, restrained guinea pigs. A clear suppression of DPOEs was induced by continuous contralateral white noise presented at 20-70 dB SPL, in the absence of any confounding effect of anesthesia, middle-ear muscles, or acoustic cross talk. Recently, acute injection of a high dose of the aminoglycoside antibiotic gentamicin (150 mg/kg) was reported to alter the suppressive effect of contralateral noise on eighth nerve-compound action potentials, presumably by blocking efferent synapses to outer hair cells. This hypothesis was confirmed with DPOEs for which a single injection of gentamicin at the same dose abolished suppression after about 1-2 h, whereas no change in basal levels was observed. Complete recovery was obtained after 48 h. This experiment may provide an easy, noninvasive tool for studying auditory function with and without functioning efferents.

Effect of anticholinergic drugs on human spontaneous and click-evoked otoacoustic emissions, frequency discrimination, and lateralization using interaural intensity differences

The primary neurotransmitter for the cochlear efferent system is widely believed to be acetylcholine. If anticholinergic drugs reduce neurotransmitter activity in the efferent pathway, they should produce effects in accord with those seen in animal studies where the olivocochlear bundle (OCB) was severed. Experiments were conducted to measure effects of three common anticholinergic drugs, diphenhydramine (BenadrylR), hyoscyamine (LevsinR), and scopolamine (Transderm-ScopR) on two psychophysical tasks and two physiological measures. There was no evidence of impaired performance in a complex frequency-discrimination task or in a lateralization task using interaural intensity differences. Also, no compelling evidence was found to indicate that either spontaneous or click-evoked otoacoustic emissions were enhanced by these drugs. After the beginning of these experiments, a report appeared [S. G. Kujawa etal., Hear. Res. 74, 122–134 (1994)] indicating that, in guinea pigs, suppression of distortion-product otoacoustic emissions was reversed by anticholinergics having antinicotinic action, but less so by those having antimuscarinic action. The three anticholinergic drugs used here are primarily antimuscarinic in their action, which may explain the failure to observe the predicted effects. [Work supported by NIDCD.]

A nicotinic-like receptor mediates suppression of distortion product otoacoustic emissions by contralateral sound

The purpose of this investigation was to provide in vivo pharmacologic characterization of a cholinergic receptor mediating the suppressive effects of medial olivocochlear (MOC) efferent activation. MOC neurons were activated by contralateral sound and the resulting suppression of ipsilateral distortion product otoacoustic emissions (DPOAEs) was monitored before and after intracochlear perfusions of cholinergic antagonists. Results revealed a dose-dependent blockade of contralateral suppression of DPOAEs by a wide variety of nicotinic and muscarinic cholinergic receptor antagonists, as well as by non-traditional antagonists of cholinergic activity. The nicotinic antagonists, α-bungarotoxin, curare and k-bungarotoxin, and the glycine antagonist, strychnine, blocked contralateral suppression at nanomolar concentrations and demonstrated similar potencies. IC 50 values were 2.38 × 10 −7, 2.79 × 10 −7, 3.81 × 10 −7 and 2.96 × 10 −7 M, respectively. These agents were followed in potency by the nicotinic antagonist, trimethaphan (1.75 × 10 −6 M), the M 3 muscarinic antagonist, 4-DAMP (1.88 × 10 −6 M) and the GABA A antagonist, bicuculline (2.39 × 10 −6 M). Increasingly greater concentrations of the muscarinic antagonists, atropine (9.52 × 10 −6 M), AF-DX 116 (2.72 × 10 −5 M) and pirenzepine (8.24 × 10 −4 M) were necessary to block contralateral suppression of DPOAEs. The in vivo pharmacology of this putative outer hair cell cholinergic receptor suggests that it may be a member of the nicotinic family of receptors.

Suppression tuning characteristics of 2f1-f2 distortion product otoacoustic emissions.

The 2f1-f2 distortion product otoacoustic emission (DPOAE) is suppressed by a third tone following a frequency selective pattern and outlining tuning curves which are generally similar to neural and psychophysical findings. The most effective suppressor tone lies between f1 and f2; less suppressive effects are produced by an added tone that lies in the 2f1-f2 site. The slope of iso-suppression curves is much steeper on the higher flank than on the lower flank of the suppression curve (respectively 100-115 dB/octave and 25-35 dB/octave). A suppression fine structure can be observed in the region of the tip especially for higher DPs probably connected with the fine structure of distortion products and with the instability of cochlear activity. A saturation point, around 70 dB or more has been evidenced on the growth rate functions. Continuous and interrupted suppressor presentation can induce some differences in the responses. Suppression of DPOAEs can provide a wider knowledge on active non-linear mechanisms in the cochlea, and on frequency selectivity also in a clinical context.