Neurovascular coupling or functional hyperemia is defined as an essential physiologic mechanism in the brain, which is necessary for the local adaption of blood flow to altered metabolic demands of the tissue. It has been shown that also in the eye, blood flow is considerably coupled to retinal neural activity. The current concept of functional hyperemia is that visual stimulation, as flicker light, effectuates increasing neural activity in the retina, which elevates the metabolic needs of the retinal tissue for oxygen and glucose and consequently induces dilatation and augmented blood flow in the retinal vasculature. In several studies, stimulation with flicker light has been shown to induce an increase of blood flow in major retinal arteries and veins as well as an increase of optic nerve head blood flow. Up until now, flicker induced changes in blood flow were measured solely in the major retinal arteries and veins with systems such as the commercially available dynamic vessel analyzer (DVA) by Imedos and with laser Doppler velocimetry (LDV).

In the present study, the investigators propose to measure the response of total retinal blood flow to diffuse luminance flicker stimulation with bi-directional Fourier Domain Doppler Optical Coherence Tomography (FDOCT) as well as with Laser Speckle Flowgraphy (LSFG) in healthy subjects by assessing vessel diameter, blood velocity and blood flow of all retinal vessels. For comparative reasons, the investigators will furthermore assess the blood flow of major retinal arteries and veins with the dynamic vessel analyzer (DVA) and laser Doppler velocimetry (LDV).

Inclusion Criteria:

• Informed consent for participation
• Men and women aged between 18 and 45 years
• Non-smokers
• Normal findings in the medical history and physical examination unless the investigator considers an abnormality to be clinically irrelevant
• Systolic blood pressure < 140 mmHg, diastolic blood pressure < 90 mmHg
• Normal ophthalmic findings, ametropia less than 3 diopters

Exclusion Criteria:

• History or presence of ocular disease
• Ametropy ≥ 3 dpt
• Treatment with any drug in the 3 weeks preceding the first study day
• Symptoms of a clinically relevant illness in the 3 weeks before the first study day
• Participation in a clinical trial in the 3 weeks preceding the first study day
• Blood donation during the 3 weeks preceding the first study day
• History of family history of epilepsy
• Abuse of alcoholic beverages
• Pregnancy