Electrophysiology of infant speech feature detection and discrimination.

Abstract

Cortical auditory evoked potentials (CAEPs) were obtained from awake, typically developing, normally-hearing infants, aged 4–10 months, in response to speech sound tokens. In experiment 1, 50-ms tokens of “Ling sounds” (/a/, /i/, /u/, /s/, /∫/, /m/) and 50-ms tone bursts at 500–4000 Hz were used as stimuli and presented at 10 dB level increments to develop CAEP latency and amplitude input-output functions. Observer-based psychophysical methods were used to determine detection thresholds for the tone bursts and speech tokens. Infant thresholds were elevated by more than 20 dB for tone bursts and by more than 30 dB for speech tokens with respect to adults. CAEP input-output functions in infants had significantly steeper slopes than those for adults. In experiment 2, 500-ms synthesized vowel tokens (/a/, /i/, /o/, /u/) were used to evoke CAEP components P1-N1-P2 and the acoustic change complex (ACC). Observer-based psychophysical methods were used to determine the infants’ ability to discriminate between these vowel sounds. Robust ACCs were obtained for all vowel contrasts even when behavioral tests of discrimination were not successful. The results of these experiments will be discussed with respect to the development of the neural generators responsible for CAEP and perceptual abilities.