Olivocochlear System - Physiology of The Olivocochlear System - Proposed Functions of The MOCS - Detection and Discrimination of Sounds in Noise

Detection and Discrimination of Sounds in Noise

The MOC-induced effects discussed thus far have all been observed in experiments conducted in silence (generally in sound-attenuated booths or rooms). However, measuring the cochlea’s response to sounds in these conditions may not reveal the true biological function of the MOCS, since evolving mammals are rarely in silent situations, and the MOCS is particularly responsive to noise (Guinan et al., 2003). The first experiments investigating the effects of MOC stimulation in the presence of noise were conducted on guinea pigs by Nieder and Nieder (1970a, 1970b, 1970c), who measured cochlear output evoked by click stimuli presented in constant background noise (BGN). In this condition, they found that the N1 potential evoked by click stimuli was enhanced during a period of MOC stimulation. This finding has been confirmed using both electrical stimulation (Dolan and Nuttall, 1988; Winslow and Sachs, 1987) and acoustic activation (Kawase et al., 1993, Kawase and Liberman, 1993) of the mammalian MOCS. Winslow and Sachs (1987) found that stimulating the OCB:

“...enables auditory nerve fibres to signal changes in tone level with changes in discharge rate at lower signal-to-noise ratios than would be possible otherwise.” (Page 2002)

One interpretation of these findings is that MOC stimulation selectively reduces the auditory nerve’s response to constant background noise, allowing a greater response to a transient sound (Guinan, 1996). In this way, MOC stimulation would reduce the effect of both suppressive and adaptive masking, and for this reason, the process has been referred to as “unmasking” or “antimasking” (Kawase et al., 1993, Kawase and Liberman, 1993). Antimasking has been suggested to occur in a similar fashion in humans (Kawase and Takasaka, 1995), and has implications for selective listening since the rapid unmasking of a sound resulting from MOC activation would increase the overall signal-to-noise ratio (SNR), thus facilitating better detection of a target sound.

In humans, psychophysical experiments conducted in constant BGN have also implicated the OCB in selective listening. The research perhaps most relevant to this thesis has been performed by Scharf and his colleagues. In 1993, Scharf et al. presented data from eight patients who had undergone unilateral vestibular neurectomy to treat Ménière’s disease, a procedure which severs the OCB (presumably both the MOCS and the LOCS). Scharf et al. (1993) did not find any clear differences in subjects’ thresholds to tones in noise before and after surgery. Shortly after this finding, Scharf et al. (1994, 1997) performed a comprehensive set of psychophysical experiments from a total of sixteen patients who had undergone unilateral vestibular neurectomy (including the original eight subjects). They measured performance in the psychophysical listening tasks before and after surgery, and found no significant difference in performance for (i) detection of tones, (ii) intensity discrimination of tones, (iii) frequency discrimination of tones, (iv) loudness adaptation, and (v) detection of tones in notched-noise. Their only positive finding was that most patients detected unexpected sounds in the operated ear better than in the healthy ear, or the same ear before surgery. This result was obtained using a truncated probe-signal procedure which led the patient to expect a certain frequency on each trial. Twelve subjects completed this experiment. Their procedure was similar to that of Greenberg and Larkin (1968), except only 50% of trials (not 77%) contained a target whose frequency matched that of the auditory cue. The other 50% of trials containing a probe whose frequency differed from that of the cue. Also, only two probe frequencies were used, one whose frequency was higher than the target, and one whose frequency was lower than the target. All trials contained an auditory cue (at the target frequency) prior to the first observation interval. The results were used to construct a basic attentional filter, which displayed detection level of the expected (and cued) target frequency and the two unexpected probe frequencies. From the two published reports (Scharf et al., 1994, 1997), ears for which the OCB has been lesioned showed an attentional filter with an average depth of about 15%-correct less than those ears for which the OCB was intact. Although there is no way to empirically convert this value to dB, a rough estimate based on psychometric functions presented by Green and Swets (1966) yields a value of 2-3 dB. Their results have been summarised in the inset figure.

Scharf and his colleagues argued that sectioning the OCB in these patients released suppression of unexpected frequencies. This effect was not present in all subjects, and large variation between subjects was observed. Nevertheless, no other psychophysical characteristics of hearing were affected following sectioning of the OCB. Scharf et al. (1997) concluded that OCB-mediated suppression of sounds in the cochlea was responsible for the suppression of unexpected sounds, and thus plays a role in selective attention in normal hearing. In contrast to Scharf's theory, Tan et al. (2008) argued that the OCB's role in selective listening pertains to the enhancement of a cued, or expected tone. This enhancement may be caused by the activity of the MOCS on the outer hair cells resulting in antimasking.

Although Scharf et al.’s (1993, 1994, 1997) experiments failed to produce any clear differences in the basic psychophysical characteristics of hearing (other than the detection of unexpected sounds), many other studies using both animals and humans have implicated the OCB in listening-in-noise tasks using more complex stimuli. In constant BGN, rhesus monkeys with intact OCBs have been observed to perform better in vowel discrimination tasks than those without (Dewson, 1968). In cats, an intact OCB is associated with better vowel identification (Heinz et al., 1998), sound localisation (May et al., 2004), and intensity discrimination (May and McQuone, 1995). All of these studies were performed in constant BGN. In humans, speech-in-noise discrimination measurements have been performed on individuals who had undergone unilateral vestibular neurectomy (resulting in OCB sectioning). Giraud et al. (1997) observed a small advantage in the healthy ear over the operated ear for phoneme recognition and speech intelligibility in BGN. Scharf et al. (1988) had previously investigated the role of auditory attention during speech perception, and suggested that speech-in-noise discrimination is assisted by attentional focus on frequency regions. In 2000, Zeng et al., reported that vestibular neurectomy did not directly affect pure-tone thresholds or intensity discrimination (confirming earlier findings of Scharf et al. 1994; 1997). For the listening-in-noise tasks, they observed a number of discrepancies between the healthy and operated ear. Consistent with the earlier findings of May and McQuone (1995), intensity discrimination in noise was observed to be slightly worse in the ear without OCB input. However, Zeng et al.’s main finding related to the “overshoot” effect, which was found to be significantly reduced (~50%) in the operated ears. This effect was first observed by Zwicker (1965), and was characterised as an increased detection threshold of a tone when it is presented at the onset of the noise compared to when it is presented in constant, steady-state noise. Zeng et al. proposed that this finding is consistent with MOCS-evoked antimasking; that is, MOCS-evoked antimasking being absent at the onset of noise however becoming active during steady-state noise. This theory was supported by the time course of MOC activation (Liberman and Brown, 1986; Backus and Guinan, 2006) being similar to the time course of the overshoot effect (Zwicker, 1965), as well as the overshoot effect being disrupted in subjects with sensorineural hearing loss, for whom the MOCS would be most likely ineffectual (Bacon and Takahashi, 1992).

Read more about this topic:  Olivocochlear System, Physiology of The Olivocochlear System, Proposed Functions of The MOCS

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