Superimposed visual maps in the isthmotectal system of the leopard frog, Rana pipiens

The retina of the leopard frog projects topographically to the entire contralateral tectum. In the rostromedial tectum, there is a map of the binocular region of the visual field seen from the ipsilateral eye that is in register with the map of the binocular region of the visual field seen from the contralateral eye. The ipsilateral eye projects indirectly to the tectum through nucleus isthmi, a midbrain structure. Nucleus isthmi receives input from the ipsilateral optic tectum and sends projections bilaterally that cover both tectal lobes. I show electrophysiologically that across the entire extent of n. isthmi there are two superimposed maps, one map representing the entire visual field of the contralateral eye, the other map representing the binocular visual field of the ipsilateral eye. The behavioral consequences of localized lesions to n. isthmi were studied and compared to the behavioral consequences of localized lesions to the tectum representing equivalent areas of the visual field. Lesions to the tectum produce scotomas in the corresponding portion of the visual field. Lesions to n. isthmi, even medial n. isthmi representing the superior visual field, lead to scotomas in the temporal-most portion of the contralateral ground level visual field. Thus, the representation of visual space in n. isthmi is not a simple copy of the tectal representation of visual space.; The crossed isthmotectal projection terminates not only in the rostromedial tectum but also in the caudolateral tectum, yet there has been no indication that it is functional. By extracellular recording in layer 7 of the caudolateral tectum, visual activity driven by stimulation of the ipsilateral eye in the monocular visual field was discovered. At individual layer 7 recording sites in the caudolateral tectum, the multiunit receptive field evoked from the ipsilateral eye is located at the mirror image position to the multiunit receptive field driven by the contralateral eye. Multiple penetrations into the tectum in individual animals revealed that the crossed isthmotectal projection to the caudolateral tectum forms a topographic map of the ipsilateral monocular visual field. Information about the ipsilateral eye visual field arrives in the caudolateral tectum via the contralateral nucleus isthmi.