The architecture of an inhibitory sidepath within the avian tectofugal system.

The tectofugal system dominates vision in most avian species. A key component of this pathway is the projection from the optic tectum onto the nucleus rotundus and the nucleus subpretectalis. Since subpretectalis has inhibitory projections onto rotundus, it constitutes a modulatory tectofugal sidepath to the tectorotundal system. We analyzed the connections and the immunocytochemical pattern of the subpretectalis in pigeons and show that it receives afferents from some tectal celltypes and from the nucleus pretectalis. Subpretectalis-neurons project non-topographically onto pretectalis and the rostrolateral rotundus. In addition, our immunocytochemical data make it likely that the cells of the subpretectalis receive glutamatergic and GABAergic input. These data provide evidence that the tectofugal sidepath over the subpretectalis could be involved in two major functions: The first is a modulation of attentional shifts from one eye to the other, while the second is a temporal fine-tuning of rotundal units.