BASILAR-MEMBRANE MOTION IN RELATION TO 2-TONE SUPPRESSION

It is proposed that two-tone suppression of rate responses in auditory -nerve fibres by a low-side suppressor cannot be explained in terms of basilar membrane motion. In a model, the amplitude of the mechanical response, either to the tone at characteristic frequency (CF), or to t he CF tone combined with a second, lower frequency lone (a suppressor) , is taken as the effective stimulus to inner hair cells (IHC), the vo ltage response of which is considered responsible fbr excitatory drive to auditory-nerve fibres. Many empirical mechanical and physiological effects are simulated accurately by the model, particularly phenomena observed in two-tone experiments using low-side suppressor tones, tha t authors have described as two-tone suppression. It is argued in this paper, however, that such phenomena strictly do not constitute suppre ssion in the cochlear response and provide no explanation for rate sup pression in nerve fibres. According to the model presented here and co nsistent with experimental data, suppression of the spike response to a CF tone in an auditory-nerve fibre by a low-side suppressor cannot b e explained in terms of the mechanics of the BM. Conclusions by others that experiments support a mechanical explanation for low-side rate s uppression are shown to be questionable. It is concluded that low-side suppression of neural responses is explicable only in terms of a non- mechanical factor derived from the response to the low frequency tone, that depresses responsiveness in fibres at the CF location. Adherence to the model of low-side neural rate suppression depending on reduced net mechanical response of the BM is contrary to experimental evidenc e, furthermore it overlooks a profound influence additional to synapti c drive, that is implied in the shaping of responses in auditory-nerve fibres. (C) 1998 Elsevier Science B.V.