Tectal connectivity in the frog Rana pipiens: tectotegmental projections and a general analysis of topographic organization.

Recent studies of visually elicited orienting in the frog Rana pipiens suggest that tectofugal signals important in this behavior relay in the midbrain tegmentum before descending to the spinal cord. They also suggest that the high degree of topographic organization displayed by the retinotectal projection may be less characteristic of other tectal afferent and efferent pathways. To explore these possibilities, we have studied patterns of retrograde and anterograde labelling following multiple and single injections of horseradish peroxidase into the tectum. We have found that the midbrain tegmentum is a major terminal zone for tectal efferent projections. Our material also provided a description of the boundaries of other structures which project to and receive input from the tectum. With this background, we studied topographic organization by analyzing for each structure the distribution of labelling following multiple injections, and comparing it with the label distribution following single injections at tectal loci with known visual field input. Multiple injections produced patchy anterograde and retrograde labelling in the nucleus isthmi, with the number of patches corresponding to the number of tectal sites injected. Single injections produced labelling in restricted regions of the nucleus isthmi, the location of which varied systematically with the location of the tectal injection site. In all other structures studied, labelling was more evenly distributed following multiple injections. In none of these structures could we detect systematic variations in the location of labelling associated with variations in the location of single tectal injection sites, and the labelling following single injections was frequently coextensive with that following multiple injections. We also found no evidence that there exist structures which project to or receive input from particular tectal regions and not others. We conclude that there exist adequate neuroanatomical substrates for a tectotegmentospinal pathway believed to be important for visually elicited orienting in the frog. We also conclude that a high degree of topographic organization is more the exception than the rule in considering tectal connections generally in the frog. Topographic organization was readily apparent in connections related to the nucleus isthmi but not in connections related to any other nonretinal structure.