The classical oblique effect appears in flicker thresholds for luminous-contrast gratings at all temporal frequencies, but is absent in flicker thresholds for chromatic (red/green) gratings that are coarse enough to be fully resolved by the opponent-color pathways. This result is consistent with the physiological finding that luminosity cells in the striate cortex are generally orientation selective, whereas opponentcolor cells are not.
D. H. Kelly, "No oblique effect in chromatic pathways," J. Opt. Soc. Am. 65, 1512-1514 (1975)