Cortical projections from the suprasylvian gyrus to the reticular thalamic nucleus in the cat.

The cat's suprasylvian gyrus was injected iontophoretically with either 4% wheat germ agglutinin-horseradish peroxidase, 4% dextran-fluororuby or 4% dextran-biotin. The locations of labelled fibres, presumed terminals and cell bodies were determined with the aid of a camera lucida attachment and computer aided stereometry. Cells from the crown of the suprasylvian gyrus project to the dorsal-most portion of the rostral half of the reticular nucleus. The region or 'sector' is distinct, albeit with some overlap, from the visual sector of the reticular nucleus defined by projections from adjacent extrastriate visual cortices. The projection from the suprasylvian gyrus to the reticular nucleus has a rough topography such that the caudal areas project to the more caudal aspects of the sector and rostral areas project to the more rostral areas of the reticular nucleus. There is a large degree of overlap of rostrocaudal projections from the suprasylvian gyrus within the sector, however, the projections originating from rostral sites are situated in a more ventral location compared to the projection originating from the caudal suprasylvian gyrus. Analysis of the distribution of biotin labelled presumptive terminals did not support the notion of 'slabs' or regional variation in terminal density across the mediolateral thickness of the reticular nucleus. In addition, a number of presumptive terminals were found within the internal capsule which coincided with the position of retrogradely labelled cells in the internal capsule following thalamic injections and appears to be part of the perireticular nucleus. The results suggest that the reticular nucleus may be segregated into sectors connected with modality specific cortical areas (e.g. striate and extrastriate visual areas) and nonspecific sectors connected with polymodal (e.g. area 7) cortical regions. The reticular nucleus and its connections with the suprasylvian gyrus may form an important link in binding eye movements to sensory integrative process through visuomotor and auditory thalamic connections.