INFANTS INTENSITY DISCRIMINATION - SPECTRAL PROFILES

Seven-month-old infants' detection of a change in the spectral shape o f an auditory stimulus, a task termed profile analysis, was assessed. Such changes are usually described as changes in the quality or timbre of a sound, and it has been suggested that listeners discriminate the quality of vowels and musical instruments by analyzing spectral shape s. In a visually reinforced operant head-turn procedure, infants first learned to discriminate two harmonic complexes that contained the sam e frequencies. Variations in the relative intensities of those frequen cies distinguished the two stimuli: The intensity of successive harmon ics either increased (rising spectrum) or decreased (falling spectrum) . These sounds were then presented at randomly varying sound pressure levels. This roving stimulus level minimized the usefulness of intensi ty cues at specific frequencies and forced infants to attend to the en tire spectral profile. Although infants' performance on the profile an alysis task deteriorated relative to that on the discrimination task, they successfully grouped the stimuli in accordance with their spectra l profiles. These results provide evidence that infants, like adults, can perform a profile analysis task but that it is more difficult than a simple discrimination task.