Coding of amplitude modulation in the auditory midbrain of the bullfrog (Rana catesbeiana) across metamorphosis

The functional development of the auditory system across metamorphosis was examined by recording neural activity from the torus semicircularis of larv al and postmetamorphic bullfrog froglets in response to amplitude-modulated sound. Multiunit activity in the torus semicircularis during early larval stages showed significant phase-locking to the envelopes of amplitude-modul ated noise bursts, up to modulation rates as high as 250 Hz. Beginning at m etamorphic climax and continuing into the froglet period, phase locking was restricted to the more limited frequency range characteristic of adult fro gs. The onset of operation of the tympanic pathway does not reinstate the h ighly synchronous neural activity characteristic of the operation of the fe nestral pathway. Modulation transfer functions based on spike count did not show tuning for modulation rate in early stage tadpoles, but a greater var iety of shapes of these functions emerged as development proceeded. Most of the different kinds of modulation transfer functions seen in adult frogs w ere also observed in froglets, but band-pass functions were not as sharply peaked. These data suggest that different neural codes for processing of th e periodicity of complex signals operate in early stage tadpoles than in po stmetamorphic froglets.