A neural substrate for unique hues has also eluded investigators. The receptive fields of retinal ganglion cells (Sun, Smithson, Zaidi, & Lee,
2006) and lateral geniculate nucleus (LGN) cells (Derrington, Krauskopf, & Lennie,
1984) combine L-, M-, and S-cone outputs to correspond to the cardinal axes (L–M) and (S) (Krauskopf, Williams, & Heeley,
1982), but not to unique-hue axes (Webster, Miyahara, Malkoc, & Raker,
2000). Recordings from neurons in macaque striate cortex (Lennie, Krauskopf, & Sclar,
1990) and extrastriate cortex (Kiper, Fenstemaker, & Gegenfurtner,
1997) complicate the picture by revealing cells preferentially tuned to many other colors beyond those characteristic of LGN cells. Cells with narrow hue sensitivities spread over the color circle have also been reported in anterior and posterior inferotemporal (PIT) cortex (Conway, Moeller, & Tsao,
2007; Komatsu, Ideura, Kaji, & Yamane,
1992). This indicates a high-dimensional neural substrate for color percepts, far beyond the two dimensions of Hering's color-opponent mechanisms. Stoughton and Conway (
2008) made a case for an overrepresentation of cells tuned to the unique hues in posterior inferior temporal cortex, but this inference was challenged by Mollon (
2009).