Directionality of midbrain auditory neuronal responses in two frogs that communicate with calls of different temporal...

Studies in ranid frogs have shown clear patterns of responses to sound direction in midbrain auditory neurons, in consonance with the ability of males and females to locate sound sources. To explore the diversity and extent of such neuronal mechanisms, we recorded directional single-unit responses of torus semicircularis auditory neurons in the leptodactylid frogs Pleurodema thaul and Eupsophus calcaratus which produce amplitude modulated and tonal advertisement calls, respectively. In both species the largest proportion of neurons respond preferentially to contralateral sound source locations. Directionality proportions were calculated from differences in spike counts produced by individual neurons to stimuli broadcast from different azimuth angles. E. calcaratus, but not P. thaul, gives higher directional responses to stimuli built with noise carrier relative to stimuli built with tone carrier. In addition, E. calcaratus gives higher directional responses to tonal and noise stimuli relative to P. thaul, however no difference in directionality occur in response to stimuli designed after the conspecific advertisement calls. The larger directionality of responses of E. calcaratus to noisy relative to tonal stimuli aligns with a general ability of vertebrates to localize better sounds containing broad spectra. The larger directionality of neuronal responses of E. calcaratus relative to P. thaul could relate to the sharper contrast between the noisy stimuli and the tonal structure of the advertisement call of this frog. These results expand the knowledge on neuronal sound localization ability of frogs, as studies on this issue had so far been restricted to a few ranid species.