Developmental plasticity in the human auditory brainstem

Abstract

Development of the human auditory brainstem is thought to be primarily complete by the age of approximately 2 years, such that subsequent sensory plasticity is confined primarily to the cortex. However, recent findings have revealed experience-dependent developmental plasticity in the mammalian auditory brainstem in an animal model. It is not known whether the human system demonstrates similar changes and whether experience with sounds composed of acoustic elements relevant to speech may alter brainstem response characteristics. We recorded brainstem responses evoked by both click and speech syllables in children between the ages of 3 and 12 years. Here, we report a neural response discrepancy in brainstem encoding of these two sounds, observed in 3- to 4-year-old children but not in school-age children. Whereas all children exhibited identical neural activity to a click, 3- to 4-year-old children displayed delayed and less synchronous onset and sustained neural response activity when elicited by speech compared with 5- to 12-year-olds. These results suggest that the human auditory system exhibits developmental plasticity, in both frequency and time domains, for sounds that are composed of acoustic elements relevant to speech. The findings are interpreted within the contexts of stimulus-related differences and experience-dependent plasticity.

Figures

Figure 1.

Representative click-evoked waveforms from a 3-year-old (red) and a 12-year-old (black) subject. Bar graph represents mean ± 1 SD for the young and old groups. There are no significant differences in click-evoked peak V latency.

Figure 2.

A, Grand average waveform for the young group (3- to 4-year-old subjects) in red and the old group (5- to 12-year-old subjects) in black. B, Left, Enlarged VA region of the grand average waveforms. Right, The young group has significantly delayed latencies to peaks V and A compared with the old group (p < 0.001, error bars represent 1 SE). C, Individual latencies as a function of age/group for all subjects (n = 104) for peaks V and A. Red boxes represent the young group (filled symbols are the 3-year-olds), and black circles represent the old group (filled symbols are the 5-year-olds).

Figure 3.

Mean and SE for latencies of FFR peaks D, E, and F. The young group has significantly delayed peak latencies compared with the old group.

Figure 4.

Grand-averaged FFT for the young group (red) and old group (black). Thin lines represent the noise floors for each group. The young group showed significantly diminished magnitudes for the fundamental frequency (F0) and harmonics 2, 3, 9, and 10. Bar graphs represent mean and SE of significant regions for each group.