If you want to make someone's heart beat faster, there are probably better ways than shining a flashlight into their ear.
But replace the flashlight with a laser, and the person with a zebrafish, and things take an unexpectedly interesting turn.
However, according to physicist Xiaoshuai Liu of Guangzhou University in China, the mechanism by which the light signal is transmitted from the ear to the heartbeat is somewhat more complex.
He and his colleagues did just that – and it's as incredible as it sounds.
«"When people think about light and sound, they instinctively associate light with vision and sound with hearing, which seems like a natural pairing. However, if we look at the physical principle of sound, we see that it is, in essence, vibration," Liu told ScienceAlert.
«"The idea of regulating heart rate was inspired by traditional music therapy. Given that rhythmic sound can modulate heart rate, we wondered: could we use precisely programmed light beams to create 'light music' and achieve heart rate regulation?"«
A schematic diagram summarizing the study's findings. (Xiaoshuai Liu/Guangzhou University)
The ear is a delicate, complex organ with sensitive structures capable of detecting the smallest vibrations. In most vertebrates, these structures include otoliths—tiny, stone-like calcium carbonate crystals. Sound or movement sets them in motion, stimulating sensory cells in the ear.
Sound isn't the only factor capable of physically moving a tiny object. For years, scientists have used so-called optical tweezers to manipulate tiny objects. Essentially, they use radiation pressure to apply a physical force that can push or move very, very small objects.
The otoliths found in the ears of zebrafish larvae are indeed very small.
More importantly, their movements are part of how fish perceive sound and movement.
This made them an intriguing target for a tool designed to manipulate microscopic objects. If an optical trap could cause the otolith to move without any sound reaching the ear, researchers could effectively bypass the first stage of hearing.
In the experimental setup, laser light was used to cause tiny otoliths in the zebrafish's ear to vibrate, creating a sound signal that could influence heart rate. (Liu et al., Nat. Commun. , 2026)
«"Our initial motivation was to move away from the traditional association of light with vision and sound with hearing and instead explore the possibility of making organisms 'hear' light," Liu explained.
«"Once we free ourselves from conventional thinking, any strategy capable of oscillation will be able to generate the equivalent of sound.".
And they tried. As expected.
They used optical tweezers to oscillate individual otoliths in living zebrafish larvae while simultaneously recording brain activity. The stimulation activated areas associated with hearing, suggesting that the mechanical motion was processed by the fish's auditory system.
Images of zebrafish brains showing which parts of the brain responded to otolith stimulation. (Liu et al., Nat. Commun. , 2026)
Does this mean that fish "hear" light? That question is a bit more difficult to answer.
«"The ancient Chinese philosopher Zhuangzi once said, 'You are not a fish, so how do you know the joy of a fish?'" Liu said. "Similarly, we cannot directly communicate with zebrafish to confirm whether they consciously 'hear' light.".
«"The ancient Chinese philosopher Zhuangzi once said, 'You are not a fish, so how do you know the joy of a fish?'" Liu said. "Similarly, we cannot directly communicate with zebrafish to confirm whether they consciously 'hear' light.".
Researchers were able to It was found that neural centers associated with hearing were significantly activated during optical stimulation.
But that's not all.
The researchers noticed that when the otolith moved, the fish's heart rate began to increase.
In one experiment, resting heart rate, which was about 2 beats per second, increased to about 2.7 beats per second during stimulation and then gradually returned to baseline after stimulation stopped.
Researchers have found that targeted stimulation of the otoliths can increase heart rate by approximately 50 percent.
And because optical tweezers allowed the researchers to precisely control the otolith's movement, they were able to do something even stranger—reproduce patterns corresponding to music. By varying the amplitude, frequency, and timing of the vibrations, they could simulate properties corresponding to volume, pitch, and rhythm.
These observations took research in a new direction.
«"Given that we can use light to play musical stimuli and influence heart rate, it seemed natural to wonder whether we could also apply this approach to correct abnormal heart rhythms," Liu told ScienceAlert.
«"Considerations such as these prompted us to conduct experiments to eliminate drug-induced arrhythmias.".
The researchers used drugs to induce three different types of abnormal heart rhythms in zebrafish larvae, and then applied the musical stimulation they created using optical stimulation.
In fish with abnormally slow heart rates, stimulation brought their heart rates closer to normal.
In fish in which the upper and lower chambers of the heart had separated, normal coordination was restored in six out of seven individuals.
And in fish whose heartbeat was interrupted by abnormal pauses, normal rhythm was restored in five out of seven individuals.
Even more surprisingly, some improvements persisted even after the stimulation ended.
On topic: Hits influence the brain, and this could be the future of music.
«"What was most surprising was that this approach… not only allows us to modulate heart rate but also prevent drug-induced pathological arrhythmias," Liu explained.
«"The fact that a purely optical stimulus delivered through the auditory pathway was able to restore normal heart rhythm in a disease model exceeded our initial expectations and may open up exciting possibilities for future therapeutic applications.".
Of course, these possibilities are still a long way off. Liu describes the study as a "very preliminary proof of concept"; how exactly neural signals cause changes in heart rate remains unclear, and whether this approach can be used in larger animals remains to be seen.
«"We view this work not as a definitive answer, but as an invitation to explore new horizons at the intersection of optics, auditory neurobiology, and cardiac physiology," he told ScienceAlert.
The results of the study were published in journal Nature Communications .
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