M-cone stimulation induces a pupil constriction to stimulus offset, whereas, with L cones, the pupil responds conventionally with a constriction to onset. To test the possibility that this paradox is linked to the LVMratio, we measured the strength of the effect by injecting a variable amount of positive or negative luminance contamination on either side of M-cone isolation and identifying a balance point at which the pupil responded equally to onset and offset.Nineteen individualswere recruited. In observers with lowLVMratio, the paradoxical effect wasweak. There was a significant relationship (r2 =0.561) between the balance point and LVMratio. The effect is likely to be linked to strong inhibitory signals associated with cone-opponent pathways.