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A Theory-based treatment of the potential contribution to anti-masking by inhibition from type-II neurons in the dorsal cochlear nucleus: modeling and simulation
The medial olivocochlear (MOC) reflex pathway, which receives excitatory input from the cochlear nucleus and gives inhibitory feedback to the cochlea, has been said to modify the cochlear amplification gain and enhance the audibility of non-stationary tones in noise. This enhancement of audibility is referred to as anti-masking. In the present research, we simulate the potential role of inhibition from dorsal cochlear nucleus (DCN) to ventral cochlear nucleus (VCN) in anti-masking. The tuberculoventral (TUB) cells in the DCN, being categorized as Type II, are known to be insensitive to noise stimuli; therefore, we hypothesize that their frequency-specific inhibition to the T-stellate (TS) cells would reduce the firing rate of MOC interneurons when a tone is present. In contrast, when only broadband noise is present, this TUB inhibition network is not activated so it does not affect the function of the MOC pathway. Based on these assumptions, an integrated computer model is built, comprised of (i) a nonlinear model of cochlear mechanics, (ii) the excitatory projection from the auditory nerve fibers (ANFs) and the inhibitory projection from the D-stellate (DS) cells to both the TS and the TUB cells, (iii) the inhibitive projection from the TUB to the TS cells, and (iv) a heuristic equation describing the dynamics of OHC gain modification. Simulations of the neural networks were conducted by the leaky integrate-and-fire method. When the system is stimulated with a stationary tone in noise, the spatial excitation pattern of ANFs exhibits higher contrast between the characteristic-frequency place and adjacent places when the TUB-TS inhibition is present. The results may suggest two things: first, the perceptibility of low-level tones in noise is enhanced. Secondly, because of the inhibition from TUB to TS, anti-masking is possible even when the tone is stationary.