Segregation of receptive field properties in the lateral geniculate nucleus of a New-World monkey, the marmoset...

The lateral geniculate nucleus (LGN) in humans and Old-World monkeys is dominated by the representation of the fovea in the parvocellular (PC) layers, and most PC cells in the foveal representation have red-green cone opponent receptive field properties. It is not known whether these features are both unique to trichromatic primates. Here we measured receptive field properties and the visuotopic organization of cells in the LGN of a New-World monkey, the marmoset Callithrix jacchus. The marmoset displays a polymorphism of cone opsins in the medium-long wavelength (ML) range, which allows the LGN of dichromatic ("red-green color blind") and trichromatic individuals to be compared. Furthermore, the koniocellular-interlaminar layers are segregated from the main PC layers in marmoset, allowing the functional role of this subdivision of the LGN to be assessed. We show that the representation of the visual field in the LGN is quantitatively similar in dichromatic and trichromatic marmosets and is similar to that reported for macaque; the vast majority of LGN volume is devoted to the central visual field. ON- and OFF-type responses are partially segregated in the PC layers so that responses are more commonly encountered near the external border of each layer. The red-green (ML) opponent cells in trichromatic animals were all located in the PC layers, and their receptive fields were within 16 degrees of the fovea. The koniocellular zone between the PC and magnocellular layers contained cells that receive excitatory input from short wavelength sensitive cones ("blue- cells") as well as other nonopponent cells. These results suggest that the basic organization of the LGN is common to dichromatic and trichromatic primates and provide further evidence that ML and SWS opponent signals are carried in distinct subdivisions of the retinogeniculocortical pathway.