THE FREQUENCY CODING IN THE AUDITORY-SYST EM

From micromechanical and neurophysiological point of view, the cochlea performs a spectral analysis where frequencies are represented in a b idimentional space. One of the first theories of frequency coding in t he cochlea was made by von Bekesy, who showed that maximum displacemen t on a specific region of the basilar membrane of post-mortem cochlea was dependent on the stimulating frequency. The mismatch of this theor y with recent data on the frequency selectivity, obtained from auditor y nerve fibers and from the basilar membrane of well preserved physiol ogical preparations, stimulated the rise of new theories in relation t o the discovered properties of the outer hair cells. Results of spectr al analysis by the cochlea are conveyed to the central nervous system by the auditory nerve that links auditory receptors to the brainstem. In the central nervous system, the frequency representation shows a th ree-dimensional organization in all stages of the ascending auditory s ystem. At the macroscopic level, frequency discrimination is made up o f tonotopic maps which come from complex and precise cytoarchitectural arrangements. At the cellular level, tuning curves present various sh apes resulting from complex integration and inhibition mechanisms. The se mechanisms may improve the frequency selectivity over a large range of intensities. Frequency coding at cellular level and tonotopic maps can be modified and even improved in the central auditory system. How ever, auditory thresholds depend directly on cochlear properties and i s not improved by the central auditory system.