Representation of speech-like sounds in the discharge patterns of auditory-nerve fibers.

It is now possible to study how acoustic characteristics important for speech discrimination are represented in the discharge patterns of auditory-nerve fibers. The electrical activity of single auditory-nerve fibers in response to speech-like sounds (single and two formant synthetic stimuli and "fricative" noise bursts) was recorded with microelectrodes in anesthetized cats. Results demonstrate that a conceptualization of some basic properties of responses to simple acoustic stimuli is useful in interpreting qualitatively how certain characteristics of speech-like sounds can be coded. Specific examples are given for (1) the rapid changes in amplitude and spectrum that occur for each syllable, (2) the fundamental frequency of voiced sounds, and (3) the spectral envelope of fricative consonants. It is also shown that the presence of background noise can have qualitatively different effects on how various characteristics of speech-like sounds are represented.