Thalamo-tectal projections in the frog

The optic tectum of the leopard frog was examined for evidence of terminal degeneration following lesions in forebrain structures. No evidence was found of a telencephalic tectal projection. Degeneration following lesions in forebrain sites which give rise to tectal afferents are located in the dorsal thalamus, specifically the corpus geniculatum laterale in the rostral thalamus and the nucleus posterolateralis in the caudal thalamus. Corpus geniculatum projects to the ipsilateral tectum where it distributes in both the superficial and deep laminae over the rostromedial one-third of that structure. The rostral portion of the nucleus posterolateralis projects rather sparsely to the ipsilateral tectum. In contrast, the posterior portion of the posterolateral nucleus projects to both the superficial and deep laminae of the rostral two-thirds of the ipsi- and contralateral tectum. The significance of these and reciprocal tectothalamic projections for behavioral expression is considered.