The postnatal development of frequency-place code and tuning characteristics in the auditory midbrain of the phyllostomid bat, Carollia perspicillata.

This report describes the postnatal development of hearing range, auditory sensitivity and tonotopy within the inferior colliculus (IC) of a mammal specialized for ultrasonic hearing. The experimental animal, Carollia perspicillata, has an adult hearing range of 7-110 kHz (characteristic frequencies) but lack any significant overrepresentation of a limited frequency band as known for rhinolophoid bats and Pteronotus. The audiogram of the newborn Carollia includes characteristic frequencies from 8 to 76 kHz, which is about 65% of the adult hearing range. As in adults, low frequencies are represented in the dorsolateral portion of the IC. However, at birth the ventromedial IC is non-responsive to acoustic stimulation up to intensities of 90 dB SPL. During development there is a progressive conversion of non-responsive IC areas into acoustically responsive slabs with characteristic frequencies above 76 kHz along the dorsolateral to ventromedial (low-to-high frequency) IC axis. This development is superimposed by a non-uniform shift of characteristic frequency: a decrease of CFs in dorsolateral regions, and an increase of CFs in ventromedial areas. The results suggest a bidirectional shift of frequency representation along the cochlear tonotopic axis.