A direct connection between visual Wulst and Tectum opticum in the pigeon (Columba livia) demonstrated by horseradish peroxidase.

Labelled neurons were observed bilaterally in part of the visual Wulst (Hyperstriatum accessorium and Hyperstriatum intercalatum superior) following unilateral injection of the enzyme horseradish peroxidase into the Tectum opticum of pigeons. No labelling was observed in Hyperstriatum dorsale. Horseradish peroxidase positive fibres running through the Tractus septomensencephalicus and ending bilaterally in the Tectum opticum were found following injection of the enzyme into Hyperstriatum accessorium. Labelled fibres were also found to end in the Hyperstriatum ventrale ventro-ventrale. These findings provide evidence for a bilateral projection of the telencephalic station of the thalamofugal pathway onto the tectofugal system, as suggested by electrophysiological data. The functional organization of these projections is discussed.