THE FREQUENCY CODING IN THE AUDITORY-SYSTEM

From micromechanical and neurophysiological point of view, the cochlea performs a spectral analysis where frequencies are represented in a bidimentional space. One of the first theories of frequency coding in the cochlea was made by von Bekesy, who showed that maximum displacement on a specific region of the basilar membrane of post-mortem cochlea was dependent on the stimulating frequency. The mismatch of this theory with recent data on the frequency selectivity, obtained from auditory nerve fibers and from the basilar membrane of well preserved physiological preparations, stimulated the rise of new theories in relation to the discovered properties of the outer hair cells. Results of spectral 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 three-dimensional organization in all stages of the ascending auditory system. At the macroscopic level, frequency discrimination is made up of tonotopic maps which come from complex and precise cytoarchitectural arrangements. At the cellular level, tuning curves present various shapes resulting from complex integration and inhibition mechanisms. These 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. However, auditory thresholds depend directly on cochlear properties and is not improved by the central auditory system.