D. H. Marshall
A. R. Palmer
J. R. Foster
MRC Inst. of Hear. Res., University Park, Nottingham NG7 2RD, UK
The time course of the development of the BILD was measured in a cross-sectional study of 113 subjects with normal hearing who were approximately evenly distributed in age on a logarithmic scale between 3 and 30 years. An adaptive procedure was used to measure the minimal level at which subjects could distinguish the names of 14 familiar toys in the NoSo and NoS(pi) conditions in pink noise (60 dB A) and in quiet. Between age 3 and adulthood, thresholds fell by 5 dB (NoSo), 11 dB (NoS(pi)), and 13 dB (quiet), and the BILD (NoSo--NoS(pi)) increased from 2.5 to 8.5 dB. The changes in thresholds followed the same time course in the three conditions, suggesting a common cause. If it is assumed that the within-subjects standard deviation of thresholds can be taken as an index of the variability of the internal representations of speech and noise, the data can be described adequately by a model in which the internal signal-to-noise ratio required for correct identification declines with age as subjects become more familiar with the acoustic differences between the test words. Thus, it may be possible to account for the development of the BILD without invoking the maturation of binaural analysis.