The eye's vascular network is not just a peripheral part of the circulatory system; it is a 'mirror' of the brain's hemodynamics, anatomically and functionally connected to it. Blood flow in retinal vessels pulses in sync with the cardiac rhythm, and this pulsation can be thought of as an acoustic chord: it features a dominant frequency, the fundamental tone corresponding to the heart's rhythm, along with numerous harmonic components, or overtones, that reflect the mechanical properties of the vessel wall and the microcirculatory bed. When the brain shifts into a state of active information processing, the spectral composition of this 'chord' changes.
Laser holography, an interferometric imaging technique that allows contactless recording of tissue micro-movements with nanometer precision, enables observation of the pulsations of the smallest retinal arterioles without surgical intervention. The researchers recorded pulse waves in retinal arteries of subjects under two conditions: while passively viewing a uniformly lit screen and while exposed to a flickering light stimulus. The signal was recorded over many cardiac cycles and then subjected to spectral decomposition — a procedure akin to a musician breaking a complex chord into individual notes, extracting the amplitude and phase of each frequency component.
This picture can be likened to an orchestra that has lost its conductor's synchronization: each instrument continues to play, but the overall rhythmic structure falls apart. According to the researchers, this disorganization of the pulse pattern reflects the process of neurovascular coupling — the restructuring of local blood flow in the brain's vessels in response to the increased metabolic demand of active neurons. Retinal vessels, embryologically and anatomically similar to cerebral ones, reproduce this dynamics. If we can decipher the informational content of such a 'vascular chord'—that is, link specific spectral changes to particular states of the neurovascular system: it would pave the way for early, preclinical diagnosis of cerebral circulatory disorders and neurodegenerative processes long before overt symptoms appear.
The retina is the only place in the body where blood vessels are visible directly, without dissection. That's why the eye is called the 'mirror of the brain'.