Laminar organization of tree shrew dorsal lateral geniculate nucleus.

This study investigated the organization of the dorsal lateral geniculate nucleus (LGN) of the tree shrew (Tupaia glis) using both microelectrode recording and anatomical techniques. The tree shrew LGN contains approximately 100,000 cells, of which 20% are in layers 2 and 6. These two layers receive input from the ipsilateral eye. The topography of the tree shrew LGN was delineated by taking systematic penetrations through the structure. Examination of the organization of the LGN laminae showed the following: in layer 1 (the lamina next to the optic tract) a mixture of on-center, off-center and on-off center cells was found; the majority of these cells responded transiently to visual stimuli and they had slightly longer conduction latencies than did cells in the other laminae. On-center and off-center cells in laminae 2-6 were sharply segregated: layers 2, 3, and 4 contained off-center cells and layers 5 and 6 contained on-center cells. Most of the cells in laminae 2-6 responded in a sustained manner to visual stimuli. These results suggest that one function of the LGN lamina is to group cells into various classes. Such grouping has now been shown to occur partially or completely for 1) eye of origin, 2) cell types characterized as on-center and off-center, and 3) cell types characterized as producing transient and sustained responses. The nature and degree of laminar specificity, however, varies considerably from species to species.