Two-Tone Rate Suppression in Auditory Nerve Fibres: Time Course of Suppression and Excitation in Peristimulus Time Histograms

Abstract

An explanation for two tone rate suppression of responses in cochlear nerve fibres is central to understanding the cochlear mechanisms that determine sensory coding in the ear. Recently, it has been proposed that the origin of the suppression lies in the fundamental concept of cochlear gain, i.e. active electro-mechanical feedback to the displacement response of the basilar membrane (BM), which determines the level of synaptic drive from inner hair cells (IRCs) to afferent nerve fibres (Geisler et al., 1990). In this “saturation feedback model”, the amplitude of the electrical response of outer hair cells (OHCs) determines the amount of high gain feedback, which is applied to a restricted longitudinal extent of the BM. This conception conforms with a saturating nonlinearity (receptor current of ORCs) and narrow band pass- filtered feedback. In response to appropriate stimuli, the saturating non-linearity provides suppression of individual frequency components (interference - Engebretson and Eldridge, 1968) and a relative attenuation of the displacement response at that BM location, specific to the attenuated frequency component. In this paper, we consider the time course of suppression and excitation of neural rate driven by characteristic frequency (CF), that results from the superposition of a test tone (suppressor) on a continuous probe tone at CF. The data suggest that factors other than suppressive effects at the IHC are involved in rate suppression.