Masking-level differences in the elderly: a comparison of antiphasic and time-delay dichotic conditions.

Four dichotic thresholds (S pi N0, S0 N pi, S0 N tau, and S pi N pi tau) were measured for young and old subjects using both burst and continuous broadband maskers. Masking-level differences (MLDs) were determined by subtracting dichotic thresholds from homophasic thresholds (S0 N0 or S pi N pi). The S0 N0 thresholds for the old subjects did not differ significantly from those for the young subjects in either the continuous or the burst masking noise conditions. The S pi N pi thresholds for the old subjects did not differ significantly from those for the young subjects in the continuous masking noise condition, but there was a significant age effect (3 dB) when burst masking noise was used. Both young and old subjects obtained larger MLDs in continuous masking noise than in burst masking noise. MLDs for old subjects were smaller than MLDs for young subjects by 4.3, 5.0, 2.7, and 1.6 dB in burst masking noise and by 4.9, 3.5, 2.5, and 1.4 dB in continuous masking noise, respectively in the S pi N0, S pi N pi tau, S0 N tau, and S0 N pi conditions. Four young subjects there was a hierarchy in the size of the MLD obtained in the four dichotic conditions, with the MLD being significantly larger in the S pi N0 and S pi N pi tau conditions; however, the size of the MLD was the same in all four conditions for the old subjects. Compared to young subjects, the performance of the old subjects was characterized by decreased ability to use homophasic cues in burst masking noise and decreased ability to use interaural difference cues. These findings were observed at four signal frequencies.