Frequency representation in the emu basilar papilla

Abstract

The frequency representation on the basilar papilla of the emu was investigated by recording from single auditory afferents and labeling them iontophoretically with horseradish peroxidase. Successfully labeled fibers covered a range of characteristic frequencies from 0.05 to 3.3 kHz, which corresponded to nearly 90% of the total papillar length. The termination sites of labeled fibers within the basilar papilla correlated with their characteristic frequency, the lowest frequencies being represented apically, the highest basally. The best mathematical description of the frequency distribution was given by an exponential regression, with a uniform mapping constant of 0.8 mm/octave. This function deviates from the more commonly observed pattern of increasing space constant towards higher frequencies. Most labeled fibers innervated only one hair cell; those contacting 4 or more hair cells were all of low characteristic frequency (⩽0.11 kHz). All labeled fibers terminated within the neural 56% of the basilar papilla’s width. This innervation pattern is consistent with that observed in other bird species. There was no conclusive evidence for a change of sensitivity and/or frequency selectivity with innervational position across the papilla’s width.