AbstractReconstruction of serial sections through the nucleus isthmi in the frogRana pipiensshows that the cortex of the nucleus consists of a sheet of cells which is folded back on itself to form two more or less parallel faces. Cyto‐architectural features allow three regions to be distinguished: a 7“rim region,” which previous work had suggested may be involved in the physiological pathway from one eye to the ipsilateral tectal lobe, and two additional regions which are here termed the anterior and posterior “nonrim cortex,” re‐spectively. The cytoarchitectural features which distinguish the rim region from the other two regions are largely absent in the tadpole.Analysis of retrograde transport of horseradish peroxidase (HRP) following widespread injections into one tectal lobe indicates that the anterior nonrim cortex projects ipsilaterally while the rim cortex and posterior nonrim cortex both project contralaterally. The differing projections of the three regions are paralleled by the pattern of retrograde degeneration observed after unilateral tectal lesions.We have studied the topographic organization of the connections between the nucleus isthmi and the two tectal lobes by making small injections of HRP at tectal loci with known visual field input. Patterns of retrograde cellular labelling show that the anterior nonrim cortex projects topographically to the entirety of the tectal lobe on the same side of the brain. The rim region projects topographically to the binocularly activated part of the opposite tectal lobe; the posterior nonrim cortex projects to the monocularly activated part. There is a discontinuity in the mapping from the nucleus to the opposite tectal lobe which corresponds to the border between binocular and monocular tectum. Patterns of anterograde labelling indicate that the afferent projection from the ipsilateral tectal lobe is topographic and organized so that afferents from a given tectal locus contact cells in the rostral part of the nucleus which project back to that locus. Afferents from tectal re‐gions representing binocular visual field in addition continue across the nucleus to terminate within the rim region. The organization of the afferent and efferent projections of this region is such as to link tectal loci which relate to the same direction in visual space.Our findings provide new evidence that the nucleus isthmi is involved in the pathway from one eye to the ipsilateral tectal lobe. They also show how the nucleus is organized to distribute information from a given locus in one tectal lobe to cells which project to the visually corresponding but in general anatomically different pairs of loci in the two tectal lobes. Finally, our findings confirm the existence of a projection from the nucleus isthmi to the opposite monocular tectum and suggest that this may be part of an intertectal circuit linking anatomically corresponding rather than visually corresponding pairs o
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