Localization of blue-on cells in the lateral geniculate nucleus of the macaque monkey using a tungsten marking technique

Abstract The lateral geniculate nucleus (LGN) relays color information from the retina to the cerebral cortex via anatomically segregated pathways. Although accumulating evidence suggests that neurons conveying signals from short-wavelength-sensitive (S) cones reside in the koniocellular (K) layers, it has been technically challenging to definitively correlate physiological recordings with their histological locations in macaques, which are routine trichromatic primate. In this study, we investigated the distribution of blue-responsive (S+) cells in the macaque LGN using extracellular single-unit recordings combined with a novel, high-resolution, and minimally invasive technique for marking recording sites. Recorded units were classified using cone-isolating stimuli to identify their cone inputs, and the recording site of S+ units were marked for histological verification. We found that the majority of cells responsive to S+ stimulation were located either within the K-layers or at the border between the K- and parvocellular (P) layers. We also found one instance located in the pregeniculate nucleus. Our results provide strong evidence that both New World and Old World monkeys share this color-segregating pathway in the LGN.