It is already known that the local treatment with Alphagan eye drops (a drug commonly used by glaucoma patients) reduces ocular blood flow. In-vitro studies suggest that the vasoconstrictive effect of Alphagan is more pronounced when this drug is combined with a prostaglandin analogue (eg. Xalatan eye drops). The investigators also question whether this effect is more pronounced in subjects suffering from a primary vascular dysregulation (PVD). To test for the effect of these drugs on PVD and non-PVD subjects the investigators will take measurements of corneal temperature as surrogate for ocular blood flow.