NONLINEAR FEEDBACK MODELS FOR THE TUNING OF AUDITORY-NERVE FIBERS

The tuning of auditory nerve (AN) fibers is generally characterized by an increase in bandwidth and, for mid- to high-frequency fibers, a do wnward shift in the center frequency as sound level increases. Changes in bandwidth are accompanied by changes in the phase properties of th e fibers; thus the timing of neural discharges also changes as a funct ion of sound level. This study focuses on the magnitude and phase prop erties of models designed to reproduce the nonlinear properties of AN fibers that were studied electrophysiologically. The forward path of e ach model consisted of a linear second-order resonance, and each feedb ack path contained a saturating nonlinearity. In model 1, the feedback path was a simple memoryless, saturating nonlinearity. In model 2, a low-pass filter was added after the feedback nonlinearity. The ability of each model to simulate aspects of the nonlinear tuning of AN fiber s is discussed. Model 2 was able to simulate a wider range of nonlinea r behavior for different AN fibers and thus has promise for use in sim ulations of populations of fibers tuned to different frequencies.