Does the ACC have potential as an index of early speech discrimination ability? A preliminary study in 4-month-old infants with normal hearing.

OBJECTIVE: The acoustic change complex (ACC), an auditory evoked potential (AEP) comprises overlapping slow cortical responses, which reflects discrimination capacity in the absence of attention, has not yet been recorded in infants. Because the ACC is a large response, it may be useful as an index of discrimination for infants at both the individual and group level. This is an advantage compared with mismatch negativity, another AEP that reflects discrimination of a change in stimulus, because mismatch negativity is based on difference waves and is most sensitive to group effects. The two objectives of this study were to determine whether: (1) the ACC can be elicited to a change in the English consonants /da/ and /ba/ in young infants and adults whose native language is English, and (2) the ACC can also be elicited to changes in Hindi consonant contrasts reflecting the predicted patterns of discrimination for young infants reported in previous studies.
DESIGN: Participants were six adults and twenty-five 4-month-old infants whose native language was English, and were at low risk for hearing loss. Stimuli were concatenated consonant pairs comprised from a dental /da/, plus either /ba/, Hindi retroflex /Da/, a second /da/ or a silent period (i.e., /dada/, /daba/, /daDa/ and /da_/). It was predicted that adults would show the largest ACC to /daba/, similar responses to /dada/ and /daDa/, and no ACC to /da_/, whereas, it was predicted that infants would show a similar ACC to /daba/ and /daDa/, a smaller ACC to /dada/ and no ACC to /da_/. The stimuli were a total of 564 msec in duration and were presented at 86 dB peak SPL with an interstimulus interval of 2200 msec. At least 100 accepted trials for each participant were required in the final waveform to be included in the study. Individual peak amplitudes and latencies were measured for the P1, N1, P2, and N2 components of the response to the initial /da/ and the ACC. Grand mean waveforms were averaged for each stimulus condition.
RESULTS: ACCs were elicited in adults to /dada/, /daba/, and /daDa/ with a trend toward a larger grand mean ACC for /daba/ compared with the other stimulus conditions. For infants, cortical responses to /da_/ resembled the adult P1-N1-P2 complex in morphology but had much longer latencies; /daba/ was the only stimulus that consistently elicited ACCs in infants. The ACC to /daba/ had a more distinct and less variable morphology compared with both /dada/ and /daDa/, which might reflect that the infants detected a greater change from /da/ to /ba/ than from /da/ to either /da/ or /Da/. It may also be the case that the ACC could not be detected for these other stimuli because the stimulus duration and interstimulus intervals used in this study were not optimal for eliciting ACCs for a range of stimuli. The pattern of speech discrimination, as reflected by the ACC, only loosely parallels the behavioral discrimination patterns reported in previous studies.
CONCLUSIONS: These preliminary findings show that it is possible to record an ACC in young infants and provide a starting point for further investigation of the infant ACC and its utility as an index of discrimination.