Development of pitch processing: infants' discrimination of iterated rippled noise stimuli with unresolved spectral content.

Sound frequency is extracted at the level of the cochlea, and is represented by two neural codes: a spectral (place) code that is maintained by tonotopic maps extending into primary auditory cortex, and a temporal code based on the periodicity of action potentials in auditory nerve fibers. To date, little work has examined infants' ability to perceive pitch when spectral content cannot be resolved by cochlear filters; the present experiments do so using high-pass filtered iterated rippled noise (IRN) stimuli. Using a conditioned head-turn paradigm, most 8-month-old infants showed above-chance discrimination of a change from 167 to 200 Hz in the fundamental frequency (F0) of such high-passed filtered IRN stimuli, but only when first exposed to a training target stimulus that emphasized pitch through the addition of a sine wave tone to the IRN stimulus at the F0. However, even after this period of pitch priming, performance was quite poor relative to that found in previous studies using stimuli with resolved spectral content. These results support the idea that 8-month-olds can perceive pitch when only unresolved spectral content is present in the stimulus, but that such processing is not yet robust.