Topographic projections between the nucleus isthmi and the tectum of the frog rana pipiens

The connections between the nucleus isthmi and the tectum in the frog have been determined by several anatomical techniques: iontophoresis of horseradish peroxidase into the tectum, iontophoresis of 3H‐proline into the nucleus isthmi and the tectum, and Fink‐Heimer degeneration staining after lesions of the nucleus isthmi. The results show that the nucleus isthmi projects bilaterally to the tectal lobes. The ipsilateral isthmio‐tectal fibers are distributed in the superficial layers of the tectum, coincident with the retinotectal terminals. The contralateral isthmio‐tectal fibers travel anteriorly adjacent to the lateral optic tract and cross the midline in the supraoptic ventral decussation, where they turn dorsally and caudally; upon reaching the tectum, the fibers end in two discrete layers, layers 8 and A of Potter. The tectum projects to the ipsilateral nucleus isthmi and there is a reciprocal topographic relationship between the two structures. Thus, a retino‐tecto‐isthmio‐tectal route exists which may contribute to the indirect ipsilateral retinotectal projection which is observed electrophysiologically. The connections between the nucleus isthmi and the tectum in the frog are strikingly similar to the connections between the parabigeminal nucleus and the superior colliculus of mammals.