Development of the N1-P2 auditory evoked response to amplitude rise time and rate of formant transition of speech sounds.

We investigated the development of weighting strategies for acoustic cues by examining the morphology of the N1-P2 auditory evoked potential (AEP) to changes in amplitude rise time (ART) and rate of formant transition (RFT) of consonant-vowel (CV) pairs in 4-6-year olds and adults. In the AEP session, individuals listened passively to the CVs /ba/, /wa/, and a /ba/ with a superimposed slower-rising /wa/ envelope (/ba/(wa)). In the behavioral session, individuals listened to the same stimuli and judged whether they heard a /ba/ or /wa/. We hypothesized that a developmental shift in weighting strategies should be reflected in a change in the morphology of the N1-P2 AEP. In 6-year olds and adults, the N1-P2 amplitude at the vertex reflected a change in RFT but not in ART. In contrast, in the 4-5-year olds, the vertex N1-P2 did not show specificity to changes in ART or RFT. In all groups, the N1-P2 amplitude at channel C4 (right hemisphere) reflected a change in ART but not in RFT. Behaviorally, 6-year olds and adults predominately utilized RFT cues (classified /ba/(wa) as /ba/) during phonetic judgments, as opposed to 4-5-year olds which utilized both cues equally. Our findings suggest that both ART and RFT are encoded in the auditory cortex, but an N1-P2 shift toward the vertex following age 4-5 indicates a shift toward an adult-like weighting strategy, such that, to utilize RFT to a greater extent.