Cortical processing of change detection: Dissociation between natural vowels and two-frequency complex tones

We compared magnetoencephalographic responses for natural vowels and for so unds consisting of two pure tones that represent the two lowest formant fre quencies of these vowels. Our aim was to determine whether spectral changes in successive stimuli are detected differently for speech and nonspeech so unds. The stimuli were presented in four blocks applying an oddball paradig m (20% deviants, 80% standards): (i) \a\ tokens as deviants vs. /i/ tokens as standards; (ii) /e/ vs. \i\; (iii) complex tones representing \a\ forman ts vs. /i/ formants: and (iv) complex tones representing \e\ formants vs. \ i\ formants. Mismatch fields (MMFs) were calculated by subtracting the sour ce waveform produced by standards from that produced by deviants. As expect ed, MMF amplitudes for the complex tones reflected acoustic deviation: the amplitudes were stronger for the complex tones representing /a/ than \e\ fo rmants, i.e.. when the spectral difference between standards and deviants w as larger. In contrast, MMF amplitudes for the vowels were similar despite their different spectral composition, whereas the MMF onset time was longer for \e\ than for /a/. Thus the degree of spectral difference between stand ards and deviants was reflected by the MMF amplitude for the nonspeech soun ds and by the MMF latency for the vowels.