Frank Musiek, PhD
Background
Masking level differences (MLDs), also termed bilateral masking level differences (BMLDs), were introduced to the scientific community by Ira Hirsh and J.C.R. Licklider in independent reports in 1948. Hirsh reported on MLDs for tonal stimuli and Licklider for speech stimuli. Though it is not the intent of this brief communication to address methodological facets of MLDs some basic comments about what the procedure entails seems in order to orient the reader.
The MLD procedure requires the individual being tested to detect (at threshold) usually tonal signals (most often 500 Hz) or speech signals (usually spondees) embedded in noise (usually broadband). In order to obtain a masking level difference,testing is administered under essentially two main conditions termed homophasic and antiphasic. The homophasic condition requires that the tones or speech, presented to each ear simultaneously, are in phase with each other. This is also the case for bilaterally presented noise signals – this stimuli condition is denoted as SoNo. The antiphasic condition is identified as such when the tones or speech are out of phase with each other or the noise signals are out of phase with each other. These conditions are denoted as SpNo and SoNp, respectively. The SpNo and SoNp will yield better thresholds than the homophasic conditions in normal hearers—this difference is known as the “release from masking” and is computed in dB.There are a variety of other conditions (though less commonly used) that can be used to obtain MLDs and the interested reader is referred to Brown and Musiek (2013) for more information in this regard. Masking level differences are greater for low frequency signals, therefore low frequency tones and speech signals are favored as stimuli (see McCullagh and Bamiou 2014 for review).
Clinical Populations
Early on in the research of MLDs, inter-aural timing was considered an important aspect as time differences at the two ears was a factor in discriminating phase differentials. This in turn garnered attention to the low auditory brainstem, and specifically the superior olivary complex (SOC) as a majorphysiological contributor to MLDs. This view was supported, at least to some degree, by one of the early clinical studies conducted by Noffsinger and colleagues (1972). This study looked at MLDs in individuals with multiple sclerosis and revealed that nearly 50% of these patients showed abnormal 500 Hz MLDs and almost ¾ of this population had abnormal MLDs for spondees as the stimuli. A number of years later, Hannley et al. 1983 and Hendler et al. 1990 also showed reduced MLDs for multiple sclerosis populations. In addition, a relatively recent publication has shown significantly reduced MLDs in individuals with multiple sclerosis compared to a control group(Faghihzadeh et al. 2012).This latter study advocated the use of MLDs for investigating clinical populations with central auditory involvement. Clearly, MLDs appear to be a valuable procedure for detecting auditory dysfunction related to multiple sclerosis. However, based on these studies it was difficult to know where in the central auditory nervous system (CANS) the dysfunction was mediated. Though most investigatorsconsidered the lower brainstem as the critical locus, there was little supporting site–of–lesion data . This issue was resolved by a couple of studies that added to both our clinical and basic science knowledge of brainstem function and MLDs. One study by Olsen et al. (1976) showed that individuals with confirmed lesions of the brainstem revealed reduced MLDs but those with cortical lesions did not. This was an important finding and was followed by perhaps one of the most definitive site of lesion studies in regard to MLDs. Lynn et al. (1981) measured speech MLDs (they employed CVs) in patients with low brainstem,high brainstem, and cortical lesions. Included also was a control (normal) group of subjects. The cortical and high brainstemlesion group revealed normal MLDs, similar to the control group. However, the low brainstem group revealed markedly reduced MLDs. In fact all patients with low brainstem lesions yielded abnormal MLDs. This clinical study’s findings were consistent with the basic science concepts of the SOC being a key mediator of the release from masking effect. Lynn et al. (1981)research also increased the specificity of the site of lesion information pertinent to MLDs.
Regarding a slightly different neuroauditory population, military individuals who were exposed to blast injuries have demonstrated abnormal MLDs (Gallun et al. 2012). However, there remains a need for more studies with this population. Similarly, children with learning disorders (dyslexia) have been shown to reveal reduced MLDs but again there is a paucity of data for this clinical group (Puter-Katz, et al., 2011).
Of special interest is the work that has shown a relationship between ABR and MLD findings in patients with brainstem involvement (see Noffsinger et al. 1984, McCullagh and Bamiou, 2014). There appears to be a correspondence between an abnormal or missing wave III (and beyond) on the ABR and reduced MLDs. This relationship is a reminder of the clinical utility of MLDs. Audiologist who do not have access to ABR to assess brainstem integrity could employ MLDs as an able substitute. Quick responding neurons in the low auditory brainstem are likely to be involved in ABR as well as MLD responses, therefore similar findings for pontine lesions should make perfect sense. This MLD – ABR relationship further solidified the diagnostic value of MLDs.
In summary, MLDs have a proven track record in regard to a measurement tool for auditory brainstem integrity. It is inexpensive, easy to administer and requires little time to complete. In the opinion of this writer MLDs are sorely under utilized for all the valuable auditory information it can provide.
References
Brown, M. and Musiek, F. (2013) The Fundamentals of Masking Level Differences for assessing auditory function…The Hearing Journal, 66, 1, 16-17
Burnham, Maria Noelle, “Normal Masking Level Difference Parameters For Use in the Clinical Evaluation of Auditory Processing Disorders” (2010). All Theses and Dissertations. 2139
Faghihzadeh,S., Majidi, H. Sedaie, M. et al. (2012) Comparison of Masking Level Difference in Patients with Multiple Sclerosis and Healthy Control Group, , 21(1):1-7
Gallun, F.J., Diedesch, A.C., Kubli, L.R., Walden, T.C., Folmer,R.L., Lewis, M.S., McDermott, D.J.,
Fausti, S.A., & Leek, M.R.. Performance on tests of central auditory processing by individuals exposed to high-intensity blasts. J of RehabilRes Dev. 2012;49(7):1005–25.
Hannley, M., Jerger, J., Rivera, V. (1983) Relationships among auditory brainstem responses, masking level differences and the acoustic reflex in multiple sclerosis, Audiology, 22, 20-33.
Hendler, T., Squires, N., & Emmerich, D. (1990) Psychphysicalmeasures of central auditory dysfunction in multiple sclerosis: Neurophysiological and neuroanatomical correlates. Ear & Hearing, 11, 403 – 416.
Hirsh, I.J. (1948). The influence of interaural phase on interauralsummation and inhibition. Journal of the Acoustical Society of America, 20, 536–544.
Licklider, J.D. (1948). The influence of interaural phase relations upon the masking of speech by white noise. Journal of the Acoustical Society of America, 20, 150–159.
Lynn, G.E. , Gilroy, J. , Taylor, P.C. & Leiser, R.P. (1981). Binaural masking level differences in neurological disorders. Archives of Otolaryngology, 107, 357–362.
McCullagh, J. and Bamiou, D. (2014) Measures of binaural interaction. In: Editors, F. Musiek and G. Chermak, Handbook of Central Auditory Disorder: Auditory neuroscience and diagnosis. San Diego, Plural Publishers
Noffsinger, D., Schaefer, A. B., & Martinez, C. D. (1984). Behavioral and objective estimates of auditory brainstem integrity. Seminars in Hearing, 5, 337-349.
Noffsinger, D., Olsen, W.O., Carhart, R., Hart, C.W., Sahgal, V.(1972) Auditory and vestibular aberrations in multiple sclerosis. Acta Otolaryngol Suppl., 303:1-63
Olsen, W.O. , Noffsinger, D. & Carhart, R. (1976). Masking level differences found in clinical populations . Audiology, 15, 287–301.
Putter-Katz, H., Feldman, I., and Hildesheimer, M. (2011) Binaural masking level difference in skilled reading children and children with dyslexia. Journal of Basic and Clinical Physiology and Pharmacology, 22, 59-63.
Sweetow, R. & Reddell, R. (1978). The use of masking level differences in the identification of children with perceptual problems. Journal of the American Auditory Society, 4, 52–56.
