Abstract:

Acoustic limitations on propagation of low-frequency complex communication sounds make the amplitude modulation (AM) rate an important auditory feature for both aquatic larval (tadpole) and partly terrestrial (adult) bullfrogs. Phase-locked responses to AM signals were recorded from neurons in the auditory midbrain during metamorphic and postmetamorphic development. The best modulation frequency (BMF) and maximal significant AM rate are significantly negatively correlated with the developmental stage. Pre- and early prometamorphic tadpoles show phase locking to higher AM rates than postmetamorphic animals. Late prometamorphic (deaf period) tadpoles show a substantial reduction in auditory sensitivity, lower BMFs, reduced bandwidths of modulation transfer functions (MTF), and loss of significant phase locking. By the onset of the metamorphic climax, there is a recovery of auditory sensitivity and phase-locked responding, with late climax tadpoles showing auditory sensitivity similar to that of postmetamorphic frogs. The changes in neural responding across metamorphosis are correlated with morphological changes in the auditory periphery and central auditory nuclei, and match differences in the acoustic environment as the animal shifts from an aquatic to a more terrestrial lifestyle. [Research supported in part by an NIH research Grant NS28565 (AMS) and an NSF Graduate Fellowship (SBH).]