Neurological Disorders

Some of the most prominent applications of the MLD to clinical populations have concerned patients with lesions affecting the auditory nerve and auditory brainstem. The rationale for using the MLD in such cases was based on the assumption that the critical stages of auditory processing underlying the MLD occur in the low or mid-brainstem. It was reasoned that lesions affecting the transmission of fine timing information within this region would be associated with reduced MLDs. The results from several audiological investigations have supported this assumption. For example, reduced MLDs have been reported in listeners with tumors of the auditory nerve and low brainstem, and in listeners with multiple sclerosis (Quaranta and Cervellera, 1974; Olsen and Noffsinger, 1976; Olsen, Noffsinger, and Carhart, 1976; Lynn et al., 1981). Poor binaural performance in such cases has been attributed to gross changes in the temporal discharge patterns in the peripheral auditory nervous system, due to either physical pressure on the nerve or, in the case of multiple sclerosis, demyelination of low brainstem neural tissue. In additional support of the idea that the MLD is determined by relatively peripheral auditory processes, several studies have indicated that the MLD is usually not reduced in listeners having specifically cortical auditory lesions (e.g., Bocca and Antonelli, 1976).

Binaural tests other than the MLD have also been used to probe for the existence of peripheral auditory neural disorder. For example, a test of interaural time discrimination termed phase response audiometry has been applied to patients having neural lesions in the auditory periphery (Nilsson and Liden, 1976; Almqvist, Almqvist, and Johnson, 1989). In general, such patients have been found to have a reduced ability to discriminate changes in interaural time differences.

Hearing Dysfunction Related to Aging

Presbyacusis refers not only to the cochlea-based losses of threshold sensitivity that typically accompany the normal aging process but also to possible auditory neural dysfunction that may coexist with (or exist independently of) cochlear loss. In general, results from studies of the MLD in the elderly indicate reduced MLDs with advancing listener age. MLDs are often reduced in presbycusic listeners, particularly when hearing loss is present at the frequencies of the test stimulus. Of greater interest is the fact that MLDs are sometimes reduced in elderly listeners (Fig. 1) even when the audiograms of the listeners do not indicate an age-related hearing loss (e.g., Grose, Poth, and Peters, 1994). Such findings are usually interpreted in terms of abnormal auditory neural processing in the aging auditory system. The nature of the underlying neural abnormality accounting for the reduced MLDs in elderly listeners is unknown. It is possible that such a dysfunction could make a significant contribution to the overall hearing disability associated with aging, as the MLD measures the kinds of auditory function that underlie, at least in part, our abilities to localize sound sources and to hear desired signals in noise backgrounds.

Figure 1..  

MLDs for a 500-Hz pure tone presented in a 58 dB/Hz, 100-Hz wide band of noise centered on 500 Hz. The open circles represent data from young adults and the filled triangles represent data from elderly adults. All listeners had normal hearing thresholds. Data are adapted from Grose et al. (1994).

Cochlear Hearing Loss

As reviewed above, the MLD has potential relevance to site of lesion clinical audiological testing because of its sensitivity to neural auditory dysfunction. Unfortunately, the MLD is affected by a wide range of hearing pathologies, making the clinical specificity of this test relatively poor. For example, the MLD is often reduced in listeners with cochlear hearing loss (e.g., Olsen and Noffsinger, 1976; Hall, Tyler, and Fernandes, 1984; Jerger, Brown, and Smith, 1984). MLDs are particularly likely to be reduced in cases of asymmetrical cochlear hearing loss, but reduced MLDs are also quite common in cases of symmetrical hearing loss. Although reduced MLDs in cochlear hearing loss may sometimes be accounted for in terms of a relatively low sensation level of stimulation or in terms of stimulation asymmetry, in some studies MLDs in listeners with cochlear hearing loss (particularly Ménière's disease) are reduced more than would be expected from stimulus level and asymmetry factors (Schoeny and Carhart, 1971; Staffel et al., 1990). Whereas such findings undermine the MLD as a test that can differentiate between cochlear and retrocochlear sites of lesion, they point to the potential of this test for understanding the effects of cochlear hearing loss on the processing of binaural information.

One general finding associated with cochlear hearing loss is variability of results among listeners. This clearly holds true for the MLD. It is not presently obvious what accounts for the variability in the size of the MLD across listeners with cochlear hearing loss. Some possible factors include the cause of the hearing loss, whether a particular case of hearing loss is associated with a substantial reduction in the number of nerve fibers contributing information for binaural analysis, and whether the cochlear disease state may affect the symmetry of the frequency or place encodings at the two cochleae. Although there is controversy on this point, it has been speculated that some forms of cochlear hearing impairment may be associated with a reduced ability of the auditory nerve to phase lock (Woolf, Ryan, and Bone, 1981). This could contribute to reduced MLDs, and could also account for the poor interaural time discrimination found in many cochlear-impaired listeners. It is likely that several causes contribute to reduced MLDs among cochlear-impaired listeners. The challenge of determining the specific mechanisms underlying the results in particular patients remains.

Conductive Hearing Loss

In some listening conditions, conductive hearing loss can be considered in terms of a simple attenuation of sound. In this sense, performance in an ear with conductive hearing loss would be expected to be similar to that in a normal ear stimulated at lower level. The situation for some aspects of binaural hearing is more complicated. If the conductive loss is different in the two ears, the associated attenuation will be asymmetrical. This asymmetry could reduce the efficiency of binaural hearing. Colburn and Hausler (1980) also pointed out that another possible source of poor binaural hearing in conductive impairment is related to bone conduction. For sound presented via headphones, both the air conduction route of stimulation and the bone conduction route of stimulation are theoretically relevant. In normal-hearing listeners, the influence of bone-conducted sound on the stimuli reaching the cochleae is probably of no material consequence. In cases of conductive hearing loss, however, the bone-conducted sound could have a significant effect on the composite waveforms reaching the cochleae and could materially affect the distribution of interaural difference cues. It is therefore possible that MLDs could be substantially reduced because of this factor in cases of conductive hearing loss.