Estimates of the strength of repetition pitch in infants

To measure the strength of the pitch of iterated rippled noise (IRN), 24 7- to 8-month-olds and 24 adults were tested in an operant conditioning procedure. To generate IRN, a 500-ms Gaussian noise was delayed by 5 or 6 ms (pitches of 200 and 166 Hz) and added to the original noise for 16 iterations. IRN stimuli having one delay were presented repeatedly, and on signal trials the delay changed for 6 s. Overall stimulus level roved from 63–67 dBA. Infants learned to turn their heads toward the sound, and adults learned to press a button when the delay of the stimulus changed. Testing started with IRN stimuli having 0 dB attenuation (i.e., maximal pitch strength). Then, stimuli having weaker pitches (i.e., progressively greater attenuation applied to the delayed noise) were presented. Strength of pitch can be quantified as the maximum attenuation for which pitch can be discerned. For each subject, threshold attenuation for pitch strength was extrapolated as the 71% point on a psychometric function depicting percent-correct performance as a function of attenuation. Mean thresholds revealed that the pitch percept was significantly weaker for infants (6.9 dB) than for adults (19.1 dB).