Shapes of cat auditory nerve fiber tuning curves

Abstract

Tuning curves of auditory nerve fibers in normal-hearing cats were fitted by a computational model comprising four processes. One process accounts for sensitivity in tuning curve tails and consists of an approximation to bandpass filtering by extracochlear structures. The second and third processes describe passive and active components of basilar membrane (BM) mechanics, respectively. The former consists of a lowpass filter function, which provides baseline threshold sensitivity and filtering above characteristic frequency (CF), and the latter consists of a Gaussian that accounts for sharp tuning and high sensitivity around CF. A fourth process, modeled as a high-pass filter, was needed in many fits to account for breaks and plateaus in threshold sensitivity at frequencies above CF. The latter three processes operated on cochlear spatial coordinates rather than stimulus frequency. The four-process description closely accounted for shapes of most tuning curves. Tuning curve tails possessed minima at 40–80 dB SPL, and minima increased with fiber CF. High-frequency cutoffs of tail filters tended to increase with CF, but low-frequency cutoffs were generally constant across CF. Functions describing tails varied from ear to ear but behaved in a similar manner for fibers from a single ear. Passive components of BM resonances possessed baselines with sensitivities that decreased with CF and cutoff slopes that increased with CF. The magnitude of the active component increased smoothly with CF over an 80 + dB range, and its spatial extent was essentially constant at 1.5 mm or 6% of cochlear length regardless of gain magnitude, fiber CF, or threshold sensitivity. Tuning curves from fibers with high and medium spontaneous rates (SRs) and similar CFs had nearly identical shapes, with the sole difference being essentially constant differences in sensitivity across the entire excitatory frequency range. Tuning curve shapes from fibers with low SRs were more variable. These could either resemble those obtained from similarly-tuned fibers with higher SRs, or they could exhibit lower tip-to-tail ratios and reduced active component magnitudes. The latter were typically associated with low maximum discharge rates.