Masking level difference: another tool for the evaluation of peripheral and cortical defects

Area of Science:

• Neuroscience
• Audiology
• Speech-Language Pathology

Background:

• Binaural unmasking, quantified by masking level difference (MLD), is crucial for understanding auditory processing in complex listening environments.
• Central nervous system (CNS) disorders can subtly affect auditory perception, even with normal audiograms, necessitating specialized testing.
• Previous research has explored MLD in various hearing pathologies, but its specific relationship with unilateral vascular CNS lesions requires further investigation.

Purpose of the Study:

• To measure the masking level difference (MLD) for speech in noise in patients with unilateral cerebral vascular lesions (CNS patients) and compare it to various hearing-impaired groups and normal-hearing controls.
• To investigate the influence of hemispheric dominance on binaural unmasking in CNS patients.
• To evaluate the utility of sensitized speech testing methods for detecting auditory processing deficits in CNS disorders.

Main Methods:

• Masking level difference (MLD) was measured for speech signals (meaningful sentences) masked by broadband noise across different signal-to-noise ratios (S/N).
• Testing included various listening conditions: SmNm, Sdelta tNo, and SmNu (correlated noise).
• Speech signal intensity was normalized for pathological groups using binaural loudness balance and localization procedures to ensure subjective suprathreshold perception.

Main Results:

• A statistically significant tendency was observed in CNS patients: MLD was greater when the leading ear (in time) was ipsilateral to the normal cerebral hemisphere.
• Control and other pathological groups (conductive hearing loss, presbyacusis, sudden deafness, Menière's disease) were also tested for comparison.
• The findings suggest that the integrity of the cerebral hemisphere influences binaural unmasking capabilities.

Conclusions:

• The results highlight a specific deficit in binaural unmasking related to hemispheric lateralization in CNS patients with vascular lesions.
• The study underscores the importance of specialized auditory tests, like sensitized speech testing, for identifying subtle auditory processing issues in CNS disorders.
• Understanding these binaural processing differences can inform diagnostic and rehabilitative strategies for patients with neurological conditions affecting hearing.