Unilateral light–dark transitions affect choroidal blood flow in both eyes

There is recent evidence that the perfusion of the choroid changes during dark–light transitions. We set out to investigate this response in more detail and to elucidate possible mechanisms involved in this process. For this purpose, the effect of dark–light transitions on choroidal perfusion was studied in healthy subjects. Choroidal blood flow and ocular fundus pulsation amplitude were measured as indices of choroidal perfusion during dark–light transitions using laser Doppler flowmetry and laser interferometry, respectively. In the first experiment, subjects were first kept in room light for 20 min, then light conditions were changed to darkness for 20 min, and thereafter, subjects were exposed to room light again. Both choroidal parameters decreased (−12% to −14%) during darkness but returned to baseline after the final room light period. In the second experiment, the index eye underwent the same procedure, whereas the contralateral eye was kept in light throughout the experiment. Choroidal haemodynamic parameters in the index eye reacted in a way comparable to that seen in the first experiment. The eye that was kept in light also reacted, but the effect tended to be less pronounced than that seen in the index eye (−8% to −10%). The observation that choroidal blood flow in both eyes reacts during unilateral light–dark transitions indicates that choroidal perfusion rate is adapted to retinal illumination conditions by neural control mechanisms.