Single thalamic neurons project to both lateral suprasylvian visual cortex and area 17: a retrograde fluorescent double-labeling study.

Area 17 and the posteromedial lateral suprasylvian (PMLS) visual cortex receive inputs from three thalamic nuclei in common: the lateral division of the lateral posterior nucleus (LPl), the C-laminae of the lateral geniculate nucleus (LGNd), and the medial interlaminar nucleus (MIN). The present study determined whether these projections originate from the same cells via bifurcating axons or from separate populations of cells. Double-label retrograde transport techniques were used to label cells projecting to area 17 with one fluorescent dye and to label cells projecting to PMLS cortex with a different dye. The two dyes used were fast blue and Evans blue. Following injections into the two cortical areas, some cells were double labeled and some were single labeled in all three thalamic nuclei studied. However, the relative number of double- and single-labeled cells, as well as the relative number of cells single-labeled following injections into each cortical area, differed among the three thalamic nuclei. In both MIN and the C-laminae of the LGNd, the number of double-labeled cells was small. Similarly, the number of cells single labeled with the dye placed in PMLS cortex was small in these two nuclei. In contrast, a relatively large number of cells were single labeled with the dye placed in area 17, especially in the C-laminae of the LGNd. These results suggest that in both MIN and the C-laminae of the LGNd, few cells project to both area 17 and the PMLS cortex, few cells project only to PMLS cortex, and a relatively greater number of cells project only to area 17. In LPl, many cells were labeled after the cortical injections. In fact, when the areas of densest labeling for both dyes overlapped, almost every labeled cell in LPl was double labeled. This indicates that almost all LPl cells that project to one cortical area also project to the other via a bifurcating axon.