Multiple pathways from the superior colliculus to the extrageniculate visual thalamus of the cat

The projection from the cat's superior colliculus to the extrageniculate visual thalamus were examined by the anterograde and retrograde transport of WGA-HRP. An acetylthiocholinesterase (ATChE) stain was employed to facilitate the differentiation of regions within the posterior thalamus. On the basis of the distribution of terminal label as well as the laminar origin of projection neurons, four pathways were delineated. Cells in the stratum griseum superficiale (primarily sublaminae II and III) innervate two regions within the nucleus lateralis posterior (LP): the medial zone, which stains darkly for ATChE, and a restricted portion of the lateral zone, adjacent to the pulvinar. Both of these pathways were found to be topographically organized. By using the fluorescent retrograde tracers, fast blue and rhodamine labeled microspheres, it was determined that the inputs to the medial and lateral zones of LP originate primarily from separate cell populations since very few neurons were found to be double-labeled. A third pathway originates principally from cells in the stratum opticum and terminates in an area just below the cholinesterase-rich region of the LP, designated as the ventral division of the LP. The fourth projection is primarily from the stratum griseum intermedium to the suprageniculate complex. Each of these four pathways arises from a population of neurons with heterogeneous morphological characteristics, and for the most part, each pathway comprises morphologically similar cells. These results suggest that visual information from the superior colliculus is conveyed to the extrageniculate thalamus via multiple pathways that may subserve diverse functions.