Effects of the Periodicity and Vowelness of Sounds on Auditory Cortex Responses in Children

The mechanisms of the human brain decoding speech sounds are of fundamental and applied interest in many areas of neuroscience. This study addresses the roles of periodicity and the speech nature (fixed formant structure) of vowel sounds in modulating auditory cortex activity in typically developing children. We proposed that both of these characteristics are typical of the vowel sounds of speech and that they are processed by different neural networks in the auditory cortex. To test this hypothesis, we constructed a set of acoustic stimuli by manipulating their periodicity and vowelness separately and used magnetoencephalography combined with individual models of the cortical surface to evaluate the cortical topography of the sources of auditory cortex responses and their strengths. The cohort consisted of nine typically developing children aged 7–12 years. We found that early auditory cortex responses (50–150 msec after stimulus onset) were highly sensitive to both the periodicity and vowelness of sounds, with independent tuning of neural networks to each of these properties of speech sounds. Differences in the locations, time dynamics, and hemisphere asymmetry of these differential responses indicated that “sound vowelness zones” in the temporal cortex constitute the earliest level in the speech information processing hierarchy at which processing of the acoustic properties of a periodic signal is transformed into decoding of speech sounds. These results allow the specific features and roles of possible impairments to the processing of the low-level properties of speech sounds and difficulties in speech perception in children with pervasive developmental disorders to be evaluated.