Right ear advantage for vocal communication in frogs results from both structural asymmetry and attention modulation.
Right-ear/left-hemisphere advantage (REA) in processing species-specific vocalizations has been demonstrated in mammals including humans. Two models for REA are typically proposed, a structural model and an attentional model. These hypotheses were tested in an anuran species, the Emei music frog (Babina daunchina) in which females strongly prefer male calls produced from inside mud-retuse burrows (high sexual attractiveness or HSA calls) to those produced in open fields (low sexual attractiveness or LSA calls). Isochronic playbacks were used to control for attention to stimuli presented to either the left or right sides of female subjects while electroencephalogram (EEG) signals were recorded from the left and right midbrain and telencephalon. The results show that relative EEG power in the delta band declined while those of the alpha and beta bands increased with time in the left but not the right midbrain. Since the anuran midbrain receives auditory information derived primarily from the contralateral auditory nerve, these results support the idea that REA occurs in frogs because communication sounds are processed preferentially in the left midbrain. Furthermore, though differences in the dynamic changes of the delta, alpha and beta bands in the left midbrain between acoustic stimuli were not statistically significant, these changes were stronger during the playback of HSA calls toward which females tend to allocate greater attentional resources. These results imply that REA in frogs results from the combined effects of structural asymmetry and attention modulation.