The vessels in the eye are a window into the brain. Blood in them pulses in time with the heart, and this pulsation resembles a musical chord: it has a fundamental tone and many overtones. When the brain is actively working, the chord changes.
Laser holography allows us to see blood movement in the tiniest retinal vessels without a single incision. Scientists measured pulse waves in the arteries of a person's eye while they looked at a regular screen and then at flickering light. The waves were recorded over many heartbeats and decomposed into simple components, just as a musician breaks a chord into notes.
This is like an orchestra that starts playing out of tune: each instrument on its own, no common melody. The scientists believe that this discord in the pulse reflects how the brain's vessels redistribute blood flow under load. If we learn to read this "chord," we could detect disruptions in brain function at the earliest stages — long before symptoms appear.
The retina is the only place in the body where vessels are visible directly, without dissection. That's why the eye is called the "mirror of the brain."