The responses of parvocellular and magnocellular cells in the monkey lateral geniculate nucleus (LGN) were assessed to red-green color substitution at various luminance ratios. Parvocellular LGN cells lacking well defined color selectivity could be rendered unresponsive to red-green light exchange at specific luminance ratios. By contrast, magnocellular LGN cells could not be silenced at any luminance ratio. The difference between these two types of cells is attributable to the greater sensitivity of magnocellular cells. The results suggest that magnocellular cells are well suited for the detection of any spatially localized change, be it the result of luminance or wavelength change.