High-Frequency Sensitivity in Infants

Auditory thresholds were determined for infants and adults to half-octave bands of noise centered at 10,000 and 19,000 hertz. Adults were significantly more sensitive than infants at 10,000 hertz, but at 19,000 hertz, adults and infants had comparable thresholds. In 1876, Sir Francis Galton reported his invention of a brass whistle for the purpose of testing the audibility of "shrill notes" (1). Although Galton noted that "young people hear shriller sounds than older people," he did not specify what he meant by young or old. In any case, it is clear that Galton did not try his whistle on infants or young children. To our knowledge there have been no subsequent studies of age-related changes in high-frequency sensitivity in infancy and early childhood largely because of technical difficulties in determining auditory thresholds in this population (2). We have recently developed a signal-detection technique, however, which can be used to determine auditory thresholds for infants as young as 6 months of age (3). This technique is a refinement of the visual reinforcement audiometry procedures developed by Moore et al. (4). A mother is seated in the corner of a double-walled, sound-attenuating chamber with the infant on her lap facing away from her. On either side of the infant at angles of 450 are loudspeakers for presenting auditory signals. Directly on top of each speaker is a smoked glass enclosure containing a mechanical toy that can be illuminated and activated for a 4-second period. An experimenter, who is seated in the booth facing the mother and infant, waits until the child looks directly at her before pressing a button to begin a trial. A sound is then presented on one of the two speakers (selected at random) and remains on until the infant makes a head turn of 45° or more toward either side. (A head turn is typically elicited quickly on the first trial.) The experimenter then presses one of two buttons to indicate the direction of the head turn. If the head turn is in the direction of the speaker producing the signal, the signal is turned off and the toy above that speaker is illuminated and activated for 4 seconds. If the turn is in the opposite direction, a silent intertrial interval of 4 seconds occurs. During the entire session, the mother and experimenter wear earphones over which a masking noise is presented to prevent them from detecting the location of the sound. We used two test signals-a half-octave band of noise centered at 10,000 Hz SCIENCE, VOL. 207, 29 FEBRUARY 1980 and another centered at 19,000 Hz. The half-octave bands were produced by filtering the output of a white-noise generator (General Radio) with a universal band-pass filter (General Radio). Signals had a rise and decay time of 25 msec (and a 30-dB falloff per octave on either side of the half-octave) and were presented over tweeters (ESS-Heil HT 400). Other circuit details and calibration procedures were identical to those reported earlier (3). Wherever possible, infants were tested in two sessions with the order of test stimuli counterbalanced and with a 10minute break between sessions. To ensure that an infant could perform the task, he or she had to meet a training criterion consisting first of four successive correct responses at 72-dB sound-pressure level (SPL). The intensity was then reduced 10 dB and the same criterion employed. Four different test levels were then presented five times each in a randomized order, constrained as in our earlier study (3). The test levels were 7, 17, 27, and 37 dB (SPL) for 10,000 Hz and 17, 27, 37, and 47 dB (SPL) for 19,000 Hz. At the beginning of the second ses-