Tuning between Central Auditory Pathways and the Ear

Emphasis in research on auditory signal reception has shifted in recent years from studies of the peripheral area alone to include more and more the central portion of the auditory pathways. The findings are characterized by timing mechanisms well balanced between excitatory and inhibitory processes on the one hand and both the selection of auditory information and the concept of two channels of the auditory transmission system on the other. Data on two types of optimizing processes will be presented: the process involved in selecting auditory information and the system responsible for the synergism of frequency (pitch)-and periodicity-analysis. The human CNS selects the 100 bits/sec processed for conscious perception from the 109 bits/sec offered from all sensory receptors in two principal ways: 1) "Specific auditory information" is modulated by "unspecific" information processed through the recticular formation of the brain stem; 2) The descending fiber systems alter selectively the information-flow on every level of the auditory pathway. The filtered information perceived in turn triggers a set of inborn and learned behavioral responses such as speech, mimicry and motor, altogether representing approximately 107 bits/sec. The system for frequency-and periodicity-analysis of the sensory excitation uses space and time attributes, respectively. Psychophysiological as well as electrophysiological data allow the relating of these two attributes to the two sets of sensory sources, the space on the basilar membrane (inner hair cells) and the time dependence of excitation (predominantly outer hair cells).