Central organization of wave localization in the clawed frog, Xenopus laevis. II. Midbrain topology for wave directions.

The organization of water wave localization within the midbrain of the clawed frog Xenopus was investigated by performing behavioral tests on frogs that had partial midbrain ablations. The criterion of localization was the orientation of turns toward the origin of impinging waves. All lesion effects became apparent as localization failure within an angular sector of wave direction. The sectors were contralateral to the lesion and of various sizes, some comprising the complete hemifield. Localization outside of the sectors was not affected. Thus, wave localization is topologically organized with respect to wave direction. Two topological projections were found. Lesions of the first projection resulted in unoriented responses to the affected wave directions. After lesions of the other projection, the frog responded to waves from the affected directions by lunges without turns. It is suggested that the two types of localization failures are due to impairment of the sensory wave direction detection and of the sensorimotor transfer, respectively. The essential midbrain areas are presumably the magnocellular nucleus of the torus magnocellularis and the ventrolateral tectum, but a considerable part of the localization might be done in the medulla.