Parallels between frequency selectivity measured psychophysically and in cochlear mechanics.

One of the most important features of the auditory system is its action as a frequency analyser. The frequency analysis appears to have its basis in the mechanical patterns of vibration on the basilar membrane (BM). Its properties can be measured psychophysically using masking experiments and the results explained using the concept of the auditory filter (critical bandwidth). A method of measuring the auditory filter shape at a particular centre frequency is described. This method is based upon the power-spectrum model of masking which assumes: 1) when detecting a signal in a masker the observer uses the single filter giving the highest signal-to-masker ratio; 2) threshold corresponds to a fixed signal-to-masker ratio at the output of that filter. The variation of the auditory filter bandwidth with centre frequency is described and related to measurements of the frequency-position map on the BM in man. The equivalent rectangular bandwidth (ERB) of the auditory filter corresponds approximately to a constant distance of 0.9 mm on the BM. Changes in the auditory filter shape with level are described and are shown to correspond, at least qualitatively, to input-output functions measured on the BM and in single neurones of the auditory nerve. Finally, a method is described for deriving the excitation pattern of a sound from its power spectrum, using the results of auditory-filter measurements. The excitation pattern derived in this way probably corresponds to the distribution of excitation along the BM.