Development of subcortical speech representation in human infants.

Previous studies have evaluated representation of the fundamental frequency (F0) in the frequency following response (FFR) of infants, but the development of other aspects of the FFR, such as timing and harmonics, has not yet been examined. Here, FFRs were recorded to a speech syllable in 28 infants, ages three to ten months. The F0 amplitude of the response was variable among individuals but was strongly represented in some infants as young as three months of age. The harmonics, however, showed a systematic increase in amplitude with age. In the time domain, onset, offset, and inter-peak latencies decreased with age. These results are consistent with neurophysiological studies indicating that (1) phase locking to lower frequency sounds emerges earlier in life than phase locking to higher frequency sounds and (2) myelination continues to increase in the first year of life. Early representation of low frequencies may reflect greater exposure to low frequency stimulation in utero. The improvement in temporal precision likely parallels an increase in the efficiency of neural transmission accompanied by exposure to speech during the first year of life.