THE DEVELOPMENT OF FREQUENCY RESOLUTION IN HUMANS AS REVEALED BY THE AUDITORY BRAIN-STEM RESPONSE RECORDED WITH NOTCHED-NOISE MASKING

Studies of tuning in infants have reported that auditory brain-stem re sponse (ABR) tuning curves generated using low-frequency probes are ad ultlike by 3 months of age while high-frequency tuning curves remain i mmature [Folsom and Wynne, J. Acoust. Soc. Am, 81, 412-417 (1987)]. Be havioral studies have similarly reported adultlike low-frequency psych oacoustic tuning curves by 3 months with high-frequency tuning curves immature until approximately 6 months of age [L. Olsho, Infant Behav. Dev. 8, 371-384 (1985); Spetner and Olsho, Child Dev. 61, 632-652 (199 0); Schneider et al., J. Exp. Psych.: Human Percept. Perform, 16, 642- 652 (1990)]. Prior to this experiment, there have been no ABR studies of the development of frequency resolution for infants older than 3 mo nths. In this study, notched-noise tuning functions were constructed f rom wave-V amplitude data for 3-month-old, 6-month-old, and adult subj ects. Tone-pip stimuli at 1000, 4000, and 8000 Hz (50 dB nHL) were pre sented simultaneously with notched-noise masking centered at frequenci es related to the tone-pip frequency (1/3-oct intervals above and belo w the probe frequency). By plotting wave-V amplitude across notched-no ise center frequency, isointensity tuning functions were generated for the three subject groups at the three probe frequencies. Auditory fil ter width (Q) and slope (dB/oct) were measured from each notched-noise tuning function in order to quantify degree of tuning. Consistent wit h previous studies, results showed that 3-month-old infants do not hav e adultlike tuning for high-frequency stimulation (8000 Hz). In contra st, by 6 months of age, tuning-function width (Q) is adultlike for bot h high- and low-frequency probes. These results, combined with previou sly reported evidence that the human cochlea is fully tuned at birth [ Abdala ct al., submitted to Hear. Res. (1995); Bargones and Burns, J. Acoust. Sec. Am, 83, 1809-1816 (1988)], suggest that immaturities in t he auditory-neural system contribute to the broad high-frequency tunin g consistently observed in 3-month-old human infants. (C) 1995 Acousti cal Society of America.