The relationship between MOC reflex and masked threshold

Abstract

Otoacoustic emission (OAE) amplitude can be reduced by acoustic stimulation. This effect is produced by the medial olivocochlear (MOC) reflex. Past studies have shown that the MOC reflex is related to listening in noise and attention. In the present study, the relationship between strength of the contralateral MOC reflex and masked threshold was investigated in 19 adults. Detection thresholds were determined for 1000-Hz, 300-ms tone presented simultaneously with one repetition of a 300-ms masker in an ongoing train of masker bursts. Three masking conditions were tested: 1) broadband noise 2) a fixed-frequency 4-tone complex masker and 3) a random-frequency 4-tone complex masker. Broadband noise was expected to produce energetic masking and the tonal maskers were expected to produce informational masking in some listeners. DPOAEs were recorded at fine frequency intervals from 500 to 4000 Hz, with and without contralateral acoustic stimulation. MOC reflex strength was estimated as a reduction in baseline level and a shift in frequency of DPOAE fine-structure maxima near 1000-Hz. MOC reflex and psychophysical testing were completed in separate sessions. Individuals with poorer thresholds in broadband noise and in random-frequency maskers were found to have stronger MOC reflexes.

Introduction

The medial olivocochlear (MOC) efferent loop is thought to have a direct effect on cochlear function (e.g., Fex, 1967, Francis and Nadol, 1993, Mountain, 1980; reviewed by Thiers et al., 2002). Neurons in the superior olivary complex receive afferent input via the cochlear nucleus and project to both the ipsilateral and contralateral ears through the crossed and uncrossed olivocochlear bundle, respectively (e.g., Guinan, 2010). By hyperpolarizing outer hair cells, the MOC system reduces cochlear amplifier gain, decreasing the output of the cochlea.

In humans, MOC effects on the cochlear response can be investigated using otoacoustic emissions (OAE). Early studies showed that contralateral acoustic stimulation (CAS) presented simultaneously with the OAE-evoking stimulus, altered the level of the OAE (e.g., Collet et al., 1990, Puel and Rebillard, 1990). An ipsilateral MOC elicitor can also alter the OAE but introduces confounds (Guinan, 2006). The contralateral MOC (MOC) reflex is mediated by the uncrossed efferent fibers from the medial olivary complex (Liberman and Brown, 1986, Robertson and Gummer, 1985).

It is thought that the MOC reflex increases the effective signal-to-noise ratio in the auditory nerve response, thus improving perception in noise. This effect is known as “MOC unmasking” (Guinan, 2006, Guinan, 2010, Kujawa and Liberman, 2001). Physiological evidence in animals (e.g., Kawase et al., 1993, Liberman, 1988, Winslow and Sachs, 1988) has shown that MOC activity improves the auditory nerve’s response to signals by reducing the response to a noisy background, effectively shifting the dynamic range of hearing (e.g., Dolan and Nuttall, 1988, Kawase et al., 1993, Kujawa and Liberman, 2001).

Psychophysical studies have linked the MOC reflex to “overshoot”, the elevated threshold observed when the probe is presented at the onset of a simultaneous masker, compared to the case when the probe is presented some time after the onset of a simultaneous masker (Keefe et al., 2010, Schmidt and Zwicker, 1991, Zwicker, 1965). The magnitude of the MOC reflex has also been linked to attention (Froehlich et al., 1993, Garinis et al., in press, Giard et al., 1994, Harkrider and Bowers, 2009, Perrot et al., 2005) and auditory training (de Boer and Thornton, 2008). However, these studies only indirectly address the question of whether this reflex improves hearing in noise generally.

Three approaches have been used to show a more direct connection between hearing in noise and the MOC effect. One approach has been to show that presenting a sound that would be expected to activate the MOC reflex also improves hearing in noise (Giraud et al., 1997, Kumar and Vanaja, 2004, Micheyl and Collet, 1996, Zeng et al., 2000). These studies demonstrated that the magnitude of improvement in perception with CAS was correlated with the magnitude of the MOC reflex; individuals who showed greater CAS-induced improvements in hearing, also had stronger MOC reflexes. One problem of interpretation in such studies is the possibility of centrally mediated unmasking effects. Although the cited experiments used uncorrelated noise to avoid a binaural masking level difference, interaural level differences can shift the perceived intracranial position of the image evoked by binaural uncorrelated noise (Hartmann and Constan, 2002). This could lead to a release from masking.

The role of the MOC reflex in auditory perception has also been tested by examining the psychoacoustical performance of patients who have had their efferent fibers severed during vestibular neurectomy (VNT) (Giraud et al., 1997, Scharf et al., 1994, Scharf et al., 1997, Zeng et al., 2000). Two of these studies reported that speech recognition in noise was poorer in the operated ear than in the unoperated ear of VNT patients (Giraud et al., 1997, Zeng et al., 2000), although Zeng et al. noted that peripheral hearing loss in the operated ear might have contributed to this effect. Scharf et al. reported that neither tone detection in quiet nor tone-in-noise detection was affected by VNT (but see Tan et al., 2008). Forward masked intensity discrimination at mid levels, overshoot, intensity discrimination in noise and the detection of unexpected frequencies were affected by the VNT surgery, but other psychoacoustical measures appeared to be unaffected (Scharf et al., 1997, Zeng et al., 2000, Zwicker, 1965). Giraud et al. (1997) also found that speech recognition did not improve with CAS nor was OAE level altered. Thus, these studies provide somewhat inconsistent evidence that disruption of the MOC reflex is associated with hearing-in-noise deficits. It is possible that the OCB had only partially been transected in some cases.

Finally, several studies of adults with normal hearing have examined the relationship between the absolute level of psychoacoustic performance in noise and the magnitude of the MOC reflex, reasoning that people with stronger reflexes should be better at processing signals in noise. The results have also been inconsistent. Micheyl and Collet (1996) reported that individuals with stronger MOC reflexes had better thresholds for a 2000-Hz tone in dichotic noise; no relationship was found for a 1000-Hz tone. Bhagat and Carter (2010) reported stronger MOC reflexes in individuals who had better 1000-Hz thresholds; no relationship was found for a 2000-Hz tone. The opposite result has also been reported, as Micheyl et al. (1995) found that individuals with stronger MOC reflexes had poorer thresholds for a three-tone complex in noise. Recently Wagner et al. (2008) failed to detect any relationship between MOC reflex strength and speech recognition in noise.

The present study sought to clarify the relationship between the contralaterally evoked MOC reflex and masked sensitivity in normal-hearing adult listeners. As in the previous studies, the relationship between MOC reflex strength and threshold for a tone in noise was examined. However, threshold was also examined in several masking conditions. Masking of a tone by a broadband noise results from the interaction of the tone and masker at the auditory periphery. This is referred to as energetic masking. Masking also occurs in conditions where alterations in cochlear gain would have little effect on threshold. For example, for many listeners masking occurs when the frequencies in a masker change randomly from presentation to presentation even when there is little or no spectral overlap between the signal and masker. This is called informational masking (Neff and Green, 1987, Pollack, 1975). Recent studies indicate that informational masking of a tone by a tonal complex—with no components near the target-tone frequency—is possible, even when the masker frequencies do not vary (Bonino and Leibold, 2008, Leibold and Werner, 2006; Leibold and Bonino, 2009).

Informational masking reflects a failure of selective listening, rather than limited spectral resolution. As noted above, Scharf et al. (1997) found that, unlike normal-hearing subjects, surgical VNT patients did not detect expected frequencies better than unexpected frequencies in the operated ear. This suggests a deficit in selective listening. Tan et al. (2008) argued that the advantage of expected over unexpected sounds in normal-hearing listeners (tested in the typical psychoacoustic “probe-signal” paradigm) results from an enhancement of activity at the expected signal frequency, mediated by MOC activity. If the MOC reflex plays a role in selective listening, then it may be associated with informational masking; hence, variation in the strength of the MOC reflex could at least partially account for the observed between-subject variability in this type of masking. To address this possibility, the present study assessed the relationship between several indices of MOC reflex strength and masked sensitivity in both energetic and informational masking conditions.