Square-wave gratings with particular spatial characteristics in visual illusions. Patients with migraine are particularly susceptible to these illusions and report disc. it. Their discomfort tends to be greater when the gratings are illuminated by red light, a tendency 1 known by controls. Gratings that induce illusions have been found to impair the recognition of opt superimposed targets in headache-free control subjects. We measured the impairment of target detection under illuminants of various chromaticities in migraineurs with and without aura and in mat controls. Migraineurs with aura had significantly higher thresholds for target detection than either migraineurs without aura or controls; in addition, the effect of chromaticity was slightly more pronounced in both migraine groups than in the control group. These findings are consistent with a recent suggestion that migraine with aura might give rise to subclinical damage to the primary visual cortex.