Objective: 1) To determine whether the N1-P2 acoustic change complex is elicited by a change of periodicity in the middle of an ongoing stimulus, in the absence of changes of spectral envelope or rms intensity. 2) To compare the N1-P2 acoustic change complex with the mismatch negativity elicited by the same stimuli in terms of amplitude and signal to noise ratio.
Design: The signals used in this study were a tonal complex and a band of noise having the same spectral envelope and rms intensity. For elicitation of the acoustic change complex, the signals were concatenated to produce two stimuli that changed in the middle (noise-tone, tone-noise). Two control stimuli were created by concatenating two copies of the noise and two copies of the tone (noise-only, tone-only). The stimuli were presented using an onset-to-onset interstimulus interval of 3 sec. For elicitation of the mismatch negativity, the tonal complex and noise band stimuli were presented using an oddball paradigm (deviant probability = 0.14) with an onset-to-onset interstimulus interval of 600 msec. The stimuli were presented via headphones at 80 dB SPL to 10 adults with normal hearing. Subjects watched a silent video during testing.
Results: The responses to the noise-only and tone-only stimuli showed a clear N1-P2 complex to the onset of stimulation followed by a sustained potential that continued until the offset of stimulation. The noise-tone and tone-noise stimuli elicited an additional N1-P2 acoustic change complex in response to the change in periodicity occurring in the middle. The acoustic change complex was larger for the tone-noise stimulus than for the noise-tone stimulus. A clear mismatch negativity was elicited by both the noise band and tonal complex stimuli. In contrast to the acoustic change complex, there was no significant difference in amplitude across the two stimuli. The acoustic change complex was a more sensitive index of peripheral discrimination capacity than the mismatch negativity, primarily because its average amplitude was 2.5 times as large.
Conclusions: These findings indicate that both the acoustic change complex and the mismatch negativity are sensitive indexes of the neural processing of changes in periodicity, though the acoustic change complex has an advantage in terms of amplitude. The results support the possible utility of the acoustic change complex as a clinical tool in the assessment of peripheral speech perception capacity.