Two-tone suppression with relation to cochlear processing

Under appropriate conditions of stimulation, the response of an auditory nerve fiber to an excitatory tone (f1) can be reduced by introducing a second or suppressor tone (f2). Suppression may be studied using two techniques: When f2 is non or weakly excitatory, suppression magnitude can be measured using reduction in overall discharge rate as an indicator. And when f2 is strongly excitatory, suppression can be studied by employing low-frequency tones for f1 and measuring reductions in discharge synchronization at the f1 frequency caused by the introduction of f2. When it occurs, suppression behaves in a manner which is both orderly and independent of the ability of f2 to elicit spikes. Suppression exists throughout a fiber's response area, and its magnitude depends only on the parameters of the suppressor tone. Suppression at a given f2 intensity is maximal when f2 = CF, and its threshold is close to the excitation threshold for f2. “Iso-suppression contours,” which show the frequencies and intensities required to suppress any f1 by a given amount, indicate that the suppression bandwidth at threshold is greater than the excitation bandwidth at threshold, and that the isosuppression bandwidth narrows as more suppression is induced. These and other lines of evidence support the notion that suppression effects are governed by mechanical aspects of the cochlear response. The implications of these findings for cochlear processing and the “second filter” will be discussed, and recent data on the spatial extent of suppression will be presented. [Work supported by NIH.]