Auditory selectivity for acoustic features that confer species recognition in the túngara frog

In anurans, recognition of species-specific acoustic signals is essential to finding a mate. In many species, behavioral tests have elucidated which acoustic features contribute to species recognition, but the mechanisms by which the brain encodes these species-specific signal components are less well understood. The túngara frog produces a `whine' mating call that is characterized by a descending frequency sweep. However, much of the signal is unnecessary for recognition, as recognition behavior can be triggered by a descending two-tone step that mimics the frequency change in a portion of the whine. To identify the brain regions that contribute to species recognition in the túngara frog, we exposed females to a full-spectrum whine, a descending two-tone step that elicits recognition, the reversed two-tone step that does not elicit recognition, or no sound, and we measured expression of the neural activity-dependent gene egr-1 in the auditory brainstem and thalamus. We found that the behavioral relevance of the stimuli was the best predictor of egr-1 expression in the laminar nucleus of the torus semicircularis but not elsewhere. That is, the laminar nucleus responded more to the whine and the two-tone step that elicits recognition than to the reversed two-tone step. In contrast, in other brainstem and thalamic nuclei, whines induced egr-1 expression but tones did not. These data demonstrate that neural responses in the laminar nucleus correspond to behavioral responses of females and they suggest that the laminar nucleus may act as a feature detector for the descending frequencies characteristic of conspecific calls.