When a musical instrument plays a note, the sound wave produced is not a single frequency but a spectrum of harmonics. The composition of that spectrum determines the note’s timbre, the distinguishing factor between the sounds of, for example, a flute and a trumpet. Even when the fundamental frequency is absent and only the higher harmonics are present, the adult human brain can fill in the missing fundamental and perceive the note’s pitch. Now Bonnie Lau and Lynne Werner of the University of Washington have tested whether infants in different stages of development can do the same. The researchers had their infant participants listen to sequences of notes drawn from a set of 10 high-harmonic complexes, five for each of two fundamental frequencies. When the infants were conditioned to associate changes in fundamental frequency with the activation of a mechanical toy, they responded behaviorally whenever the missing fundamental was changed. Curiously, three-month-old and seven-month-old infants performed equally well at the task, even though the sensory pathways in the brain change radically between those ages: At three months, the auditory cortex is not yet mature, and sounds are processed mostly in the brain stem. Another surprise was that both groups of infants performed better than adults who had no musical training. The researchers hypothesize that adults are more easily distracted than infants by changes in timbre rather than pitch, but they have yet to explain why. (B. K. Lau, L. A. Werner, J. Acoust. Soc. Am. 136, 760, 2014.)
