Telencephalic Neuronal Activation Associated with Spatial Memory in the Terrestrial Toad Rhinella arenarum: Participation of the Medial Pallium during Navigation by Geometry

Amphibians are central to discussions of vertebrate evolution because they represent the transition from aquatic to terrestrial life, a transition with profound consequences for the selective pressures shaping brain evolution. Spatial navigation is one class of behavior that has attracted the interest of comparative neurobiologists because of the relevance of the medial pallium/hippocampus, yet, surprisingly, in this regard amphibians have been sparsely investigated. In the current study, we trained toads to locate a water goal relying on the boundary geometry of a test environment (Geometry-Only) or boundary geometry coupled with a prominent, visual feature cue (Geometry-Feature). Once learning had been achieved, the animals were given one last training session and their telencephali were processed for c-Fos activation. Compared to control toads exposed to the test environment for the first time, geometry-only toads were found to have increased neuronal labeling in the medial pallium, the presumptive hippocampal homologue, while geometry-feature toads were found to have increased neuronal labeling in the medial, dorsal, and lateral pallia. The data indicate medial pallial participation in guiding navigation by environmental geometry and lateral, and to a lesser extent dorsal, pallial participation in guiding navigation by a prominent visual feature. As such, participation of the medial pallium/hippocampus in spatial cognition appears to be a conserved feature of terrestrial vertebrates even if their life history is still tied to water, a brain-behavior feature seemingly at least as ancient as the evolutionary transition to life on land.