Evidence for sublaminar organization of the cells of origin of the geniculo-striate pathway in the cat: a novel method for cortical deposition of HRP.

A novel technique for retrograde labelling was used to trace the axonal projections from the lateral geniculate nucleus (LGN) to the striate cortex in the cat. Cortical deposits of horseradish peroxidase were made in the form of long straight lines oblique to layer IV. The projections along the lengths of these linear deposits were inferred, by retinotopic correspondence, from the distributions of the retrogradely labelled cells in the LGN. The results are consistent with an earlier report, by Humphrey et al., that the depths of axon terminations in layer IV reflect the depths of the parent cell bodies in layers A and A1 of the LGN. The pattern observed in the present experiments--border cells in the A layers project mainly to cortical layers IVa and centre cells to IVb--suggests that: (1) the sublaminar segregation of the ON and OFF pathways that occurs in the cat LGN is reversed by convergent projections to IVa from the dorsal (ON-rich) and ventral (OFF-rich) borders of the A layers, and (2) the paucity of Y input to IVb is due to the near absence of Y cells in the narrow central sublaminae of the A layers that supply the main input to IVb.