 Recently the investigators could show that flicker induced vasodilatation is blunted in patients with open angle glaucoma. However, the investigators results are limited by the fact that only data about retinal vessel diameters, not blood flow per se, are available. The further development of the investigators current flicker stimulation technique now allows us to determine blood flow velocity during flicker stimulation. Thus, in the current study, the investigators set out to determine whether this blood flow response is impaired in patients with glaucoma as compared to those in healthy volunteers and whether this is related to altered neural activity. 