Unmasked Thresholds and Minimum Masking in Infants and Adults: Separating Sensory from Nonsensory Contributions to Infant-Adult Differences in Behavioral Thresholds

Abstract

The purposes were to determine the degree to which infant unmasked thresholds differed from those of adults, to determine the effect of signal frequency on those differences, and to determine whether there is a difference in minimum effective masking as a function of age and frequency. The data were applied to a model to address the question of the origin (sensory versus nonsensory) of infant-adult differences in behavioral unmasked thresholds. Infant behavioral thresholds in quiet and in noise for pure tones of 0.5 and 2 kHz were measured using computer-based visual reinforcement audiometry. Adult thresholds were measured using the same apparatus and a modified conventional audiometric procedure. Twenty-three infants and 24 adults were included in the analyses. The masked thresholds versus masker intensity data were used to estimate minimum masking levels (MMLs). Group mean unmasked thresholds of infants were greater than those of adults at both frequencies and the difference was greater at 0.5 kHz (14.2 dB) than at 2 kHz (5.5 dB). Infant MMLs were greater than those of adults by 10.1 dB at 0.5 kHz and by 1.2 dB at 2 kHz. The fact that a masker had to be 10.1 dB greater in intensity for infants than for adults at 0.5 kHz and 1.2 dB greater at 2 kHz suggests that there is a difference in sensitivity between the groups of about 10 and 1 dB for the two frequencies, respectively. According to a proposed model, the difference between infants and adults in MMLs is less than the difference in unmasked thresholds owing to the influence of nonsensory factors that bias infant unmasked thresholds. Such factors, therefore, contribute 4.1 dB (i.e., 14.2 dB-10.1 dB) at 0.5 kHz and 4.3 dB (i.e., 5.5 dB-1.2 dB) at 2 kHz based on the data. The infant-adult differences in unmasked thresholds, with greater difference at 0.5 kHz than at 2 kHz, are consistent with earlier research on infant hearing. The data reveal that the greater difference between infants and adults at 0.5 kHz relative to 2 kHz is not a result of nonsensory factors alone, but is largely due to a difference in sensory processing. Only about 4 dB of the difference between infants and adults in unmasked thresholds can be attributed to nonsensory factors and the effect is not frequency specific according to the proposed model. Consideration of establishing audiometric reference values for infants that are different than those based on adults, owing to the elevated behavioral unmasked thresholds of normal-hearing infants, is suggested.