Employing the Acoustic Change Complex for Vowel Discrimination

Abstract

A change occurring within an acoustic stream evokes the cortical potential known as the acoustic change complex (ACC). The present study addressed the effects of level and vowel contrast type on ACC amplitudes and latencies. Nineteen normally hearing adults were tested with vowel tokens, /a/, /i/, /o/, and /u/, of 500 ms duration presented at 2/s, in an oddball paradigm at 40 and 70 dBA in the sound field. The ACC was present for all vowel contrasts. The ACC for vowel contrasts are robust even at low levels (40 dBA), i.e., 25-30 dB HL, as evidenced by the high prevalence of responses in the contrast (vowel change) conditions that were significantly greater in amplitude than in the control (no vowel change) conditions. ACC absolute amplitudes were significantly larger at 70 dBA and amplitudes for control conditions were 4-6 times smaller than contrast conditions, but latencies were not sensitive to level or vowel type. ACC amplitude ratios were then calculated as: amplitude of contrast responses ÷ amplitude of control responses. The ACC amplitude ratios varied with vowel pair and by level. The distribution of amplitude ratios obtained indicated that over 90% of ratios are greater than 1.0, and over 84% are greater than 2.0, suggesting that this metric is a sensitive measure of cortical speech feature discrimination. These data set a baseline for a larger study of the ACC in infants, children, and adults with hearing loss.