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

Masking level difference (MLD) due to binaural unmasking was measured for speech signals (5-word meaningful sentences) masked by broad-band noise. Tests were carried out in a group of patients with unilateral cerebral lesions of vascular origin and apparently normal pure-tone audiograms (CNS patients), in a control group (normally hearing young adults) and in 5 other groups of patients (conductive symmetrical hearing loss, conductive asymmetrical hearing loss, bilateral presbyacusis, unilateral sudden deafness, Menière's disease). Testing pattern implied three or more S/N ratios in the listening conditions, of SmNm, Sdelta tNo and SmNu (noise correlated), and the speech signal intensity was 70 dB SPL re 20 muPa for the control group, whilst for the pathological cases, speech level intensities were established by means of alternate binaural loudness balance and simultaneous balancing median-plane localization procedures to assess subjective suprathreshold sound image localization at the midline. The results obtained in the control group and in the patients are discussed. With specific reference to the CNS patients, a statistically significant tendency was evident for the binaural condition to produce more MLD when the ear leading in time was ipsilateral to the normal hemisphere. The importance of this and of sensitized speech testing methods in CNS disorders is discussed on the basis of the results obtained in the CNS patients.