Telencephalic connections of the visual system of the chicken: tracing the interrelation of the efferents of the visual Wulst and the hyperstriatum ventrale.

In the present study the telencephalic connections between the centres of the tectofugal and thalamofugal pathways were investigated with Phaseolus vulgaris leucoagglutinin and biotinylated dextran-amine anterograde tracers in chicken using light and electron microscopy. No direct connection was found between the visual Wulst and the ectostriatum or the telencephalic centres of the tectofugal and thalamofugal pathways. Besides other projections, the visual Wulst emitted fibres also to the middle and lateral parts of the hyperstriatum ventrale. Further experiments revealed that the middle part of the hyperstriatum ventrale projected to the ectostriatum centrale and periphericum and established an indirect connection between the visual Wulst and the ectostriatum. The lateral part of the hyperstriatum ventrale sent a few efferent fibres toward the diencephalon and brainstem, but projected massively to the ectostriatum periphericum, neostriatum intermedium pars laterale, the ventral part of the neostriatum caudale and the archistriatum dorsale. Considering that these areas are structures of the tectofugal circuitry as well, the presence of these connections may result in a more elaborate visual processing. The neostriatum may be an associative visual center and possibly a modulatory area toward the archistriatum intermedium dorsale, by which visual information may mediate, modulate and control the movements.