Organization of the afferent projections to the Wulst in the pigeon.

The afferent connections to the Wulst, a well-defined bulge in the forebrain roof, were studied in the pigeon. Cells of origin were identified by horseradish peroxidase retrograde tracing, after placing multiple injections in the Wulst. The results demonstrate a bilateral intratelencephalic pathway arising from the archistriatum intermedium (Ai) in the basal forebrain. Labeled cells in n. superficialis parvocellularis (SPC) and n. dorsolateralis posterior (DLP) on both sides of the brain, provide anatomical evidence for a bilateral forebrain projection of the somatosensory thalamus. A sparse ipsilateral input of unknown function from the medial thalamus originates in n. dorsomedialis anterior (DMA) and n. dorsolateralis medialis (DLM). We provide confirming evidence of the bilateral thalamofugal visual pathway ascending from nuclei of the dorsolateral thalamus (DLAmc and DLL). Projections from several brainstem structures are described, including: griseum centrale (GCt), medial and lateral reticular formation (FRM and FRL), area ventralis of Tsai (AVT), n. annularis (Anl), locus coeruleus (LoC), and the avian homologue of the raphe nucleus, n. linearis caudalis (LC). The account provides a direct anatomical demonstration of a Wulst input from the basal forebrain, the somatosensory thalamus, and the brainstem. The projection cells in the brainstem reside in structures known to contribute to ascending catecholaminergic and serotoninergic pathways.