Stimulus presentation strategies for eliciting the acoustic change complex: increasing efficiency.

Objective: The purpose of this study was to compare four strategies for stimulus presentation in terms of their efficiency when generating a speech-evoked cortical acoustic change complex (ACC) in adults and children.

Design: Ten normally hearing adults (aged 22 to 31 yrs) and nine normally hearing children (aged 6 to 9 yrs) served as participants. The ACC was elicited using a 75-dB SPL synthetic vowel containing 1000 Hz changes of second formant frequency, creating a change of perceived vowel between /u/ and /i/. The ACC was recorded from Cz using four stimulus formats:ACC magnitude was expressed as the standard deviation of the voltage waveform within a window believed to span the ACC. Noise magnitude was estimated from the variances at each sampling point in the same window. Efficiency was expressed in terms of the ACC to noise magnitude ratio divided by testing time.

Results: ACC magnitude was not significantly different for the two directions of second formant change. Reducing interonset interval from 2 to 1 sec increased efficiency by a factor close to two. Combining data from the two directions of change increased efficiency further, by a factor approximating the square root of 2.

Conclusion: Continuous alternating stimulus presentation is more efficient than interrupted stimulus presentation in eliciting the ACC. The benefits of eliminating silent periods and doubling the number of acoustic changes presented in a given time period are not seriously offset by a reduction in root mean square response amplitude, at least in young adults and in children as young as 6 yrs.