Processing of speech by the auditory nervous system.

Recordings from auditory-nerve fibers indicate that several factors are necessary to characterize the response of a single fiber to speech sounds. These are the spectrogram and level of the stimulus, the characteristic frequency and threshold of the fiber. The responses of single fibers to tones are seen as changes in the instantaneous discharge rate which can be measured in various ways. Two measures, average rate and synchronization index, have been defined for tone stimuli and have been shown to behave differently under a variety of conditions. The work with tones can lead to simplified displays of activity in model neurons, which can be used to formulate general ideas on speech coding at the nerve level.