Speech coding in the auditory nerve: IV. Sounds with consonant-like dynamic characteristics.
Discharge patterns of auditory-nerve fibers in anesthetized cats were obtained for two stimulus levels in response to synthetic stimuli with dynamic characteristics appropriate for selected consonants. A set of stimuli was constructed by preceding a signal that was identified as /da/by another sound that was systematically manipulated so that the entire complex would sound like either /da/, /ada/, /na/, /sa/, /sa/, or others. Discharge rates of auditory-nerve fibers in response to the common /da/-like formant transitions depended on the preceding context. Average discharge rates during these transitions decreased most for fibers whose CFs were in frequency regions where the context had considerable energy. Some effect of the preceding context on fine time patterns of response to the transitions was also found, but the identity of the largest response components (which often corresponded to the formant frequencies) was in general unaffected. Thus the response patterns during the formant transitions contain cues about both the nature of the transitions and the preceding context. A second set of stimuli sounding like /s/ and /c/ was obtained by varying the duration of the rise in amplitude at the onset of a filtered noise burst. At both 45 and 60 dB SPL, there were fibers which showed a more prominent peak in discharge rate at stimulus onset for /c/ than for /s/, but the CF regions that reflected the clearest distinctions depended on stimulus level. The peaks in discharge rate that occur in response to rapid changes in amplitude or spectrum might be used by the central processor as pointers to portions of speech signals that are rich in phonetic information.