CORTICAL EVOKED-RESPONSE TO ACOUSTIC CHANGE WITHIN A SYLLABLE

Objective: To investigate whether the evoked potential to a complex na turally produced speech syllable could be decomposed to reflect the co ntributions of the acoustic events contained in the constituent phonem es. Design: Auditory cortical evoked potentials N1 and P2 were obtaine d in eight adults with normal hearing. Three naturally produced speech stimuli were used: 1) the syllable [sei]; 2) the sibilant [s], extrac ted from the syllable; 3) the vowel [ei] extracted from the syllable. The isolated sibilant and vowel preserved the same time relationships to the sampling window as they did in the complete syllable. Evoked po tentials were collected at Fz, Ct, Pt, Al, and A2, referenced to the n ose. Results: In the group mean waveforms, clear responses were observ ed to both the sibilant and the isolated vowel. Although the response to the [s] was weaker than that to [ei], both had N1 and P2 components with latencies, in relation to sound onset, appropriate to cortical o nset potentials. The vowel onset response was preserved in the respons e to the complete syllable, though with reduced amplitude. This patter n was observable in six of the eight waveforms from individual subject s. Conclusions: It seems likely that the response to [ei] within the c omplete syllable reflects changes of cortical activation caused by amp litude or spectral change at the transition from consonant to vowel. T he change from aperiodic to periodic stimulation may also produce chan ges in cortical activation that contribute to the observed response. W hatever the mechanism, the important conclusion is that the auditory c ortical evoked potential to complex, time-varying speech waveforms can reflect features of the underlying acoustic patterns. Such potentials may have value in the evaluation of speech perception capacity in you ng hearing-impaired children.