Prosodic processing in the developing brain.

Speech prosody is considered to be one of the most important sources of information for infants in acquiring their native language. Using multi-channel near-infrared spectroscopy in 10-month-old infants, we examined cortical activation when normal and flattened speech sounds were presented to the infants. The flattened speech sound was generated by eliminating changes in the pitch contours of the original utterance. We found bilateral activation under both speech conditions. In a direct comparison between the two conditions, the right temporal and temporoparietal regions, and bilateral prefrontal regions showed more prominent activation in response to flattened speech than to normal speech. These results demonstrate that the unfamiliar pitch contours of flattened speech induce additional processing in the cortical regions of 10-month-old infants, suggesting that 10-month-old infants already have neural mechanisms for the processing of at least a part of the prosodic structures in their native language. To investigate developmental changes in cortical activation patterns, we compared the present results with those of our previous study using the same paradigm with 3-month-old infants. We propose that speech processing in the infant brain develops from analyzing pitch information per se, to comparing and integrating information in input speech sounds with acquired prosodic structures.