Scientists Used Light to Wiggle a Fish's Ear Stones, And You'll Never Guess Why

If you are trying to make somebody's heart race, shining a torch into their ear is probably not the best method.

Replace the torch with a laser and the person with a zebrafish, however, and the result becomes unexpectedly compelling.

According to Xiaoshuai Liu, a physicist at Guangzhou University in China, the route by which light entering an ear changes the pace of a heartbeat is rather more complicated.

That is exactly what Liu and his team have achieved – and it is as remarkable as it sounds.

"When people think of light and sound, they instinctively associate light with vision and sound with hearing, a seemingly natural pairing. However, if we trace back to the physical principle of sound, it is fundamentally vibration," Liu told ScienceAlert.

"The idea of controlling heart rate was inspired by traditional music therapy. Given that rhythmic sound can modulate cardiac rhythm, we wondered: Could we use precisely programmed light beams to compose 'light music' and achieve heart rate regulation?"

A schematic diagram summarising the research. (Xiaoshuai Liu/Guangzhou University)

How light moves zebrafish ear stones

The ear is a finely intricate organ containing sensitive features that can detect extremely small vibrations. In most vertebrates, these include otoliths: minute, stone-like calcium carbonate crystals. When sound or movement makes these crystals shift, sensory cells within the ear are stimulated.

Yet sound is not the only force capable of making a tiny object move. Scientists have long used optical tweezers to manipulate microscopic items. In essence, the technique uses radiation pressure to apply a physical force that can push or shift something exceedingly small.

Otoliths in the ears of zebrafish larvae are certainly small enough for this purpose.

More significantly, their movement helps the fish sense both sound and motion.

This made them a promising subject for an instrument built to manipulate microscopic objects. If an optical trap could shift an otolith without sound ever entering the ear, the scientists would in effect circumvent the first stage of hearing.

The experimental arrangement used laser light to oscillate minute otoliths in a zebrafish ear, creating an auditory signal capable of affecting its heart rate. (Liu et al., Nat. Commun. 2026)

"Our initial motivation was to break away from the conventional association of light with vision and sound with hearing, and instead explore the possibility of making organisms 'hear' light," Liu explained.

"Once we break free from conventional thinking, any strategy capable of oscillating can generate the equivalent of sound."

So the team tested the idea. Naturally.

Using optical tweezers, they oscillated individual otoliths in living zebrafish larvae while recording brain activity. The stimulus activated regions linked to hearing, indicating that the fish's auditory circuitry was processing the mechanical movement.

Images of zebrafish brains showing the areas that responded to otolith stimulation. (Liu et al., Nat. Commun. 2026)

Does this show that a fish is actually ‘hearing’ light? That question is more difficult to resolve.

"The ancient Chinese philosopher Zhuangzi once said, 'You are not the fish, how do you know the fish's joy?'" Liu said. "Similarly, we cannot directly communicate with zebrafish to confirm whether they are consciously 'hearing' the light."

"The ancient Chinese philosopher Zhuangzi once said, 'You are not the fish, how do you know the fish's joy?'" Liu said. "Similarly, we cannot directly communicate with zebrafish to confirm whether they are consciously 'hearing' the light."

What the team was able to establish was that neural centres associated with hearing became markedly more active during optical stimulation.

Light music and zebrafish heart rate

There was another finding, too.

The scientists saw that shifting an otolith made a fish's heart beat faster.

In one test, a resting rate of roughly 2 beats per second increased to around 2.7 beats per second while the stimulation was applied, before gradually moving back towards its baseline once it stopped.

The researchers determined that precisely oscillating an otolith could raise heart rate by approximately 50 percent.

Because optical tweezers gave the researchers exact control over an otolith's movement, they could achieve something stranger still: recreate patterns associated with music. Altering the oscillations' amplitude, frequency and timing allowed them to imitate characteristics corresponding to loudness, pitch and rhythm.

Those results took the research in a different direction.

"Given that we could use light to reproduce musical stimuli and influence heart rate, it seemed like a natural progression to ask whether we could also apply this approach to correct abnormal cardiac rhythms," Liu told ScienceAlert.

"This reasoning led us to conduct experiments on rescuing drug-induced arrhythmias."

The team used drugs to trigger three distinct types of abnormal cardiac rhythm in zebrafish larvae, then delivered its optically produced musical stimulation.

For fish with unusually slow heartbeats, the stimulation returned their heart rates close to normal.

In six of seven fish whose upper and lower heart chambers had become unsynchronised, normal coordination was restored.

For fish whose heartbeats were broken up by abnormal pauses, normal rhythms reappeared in five of seven cases.

More strikingly, certain improvements continued even after the stimulation was switched off.

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"The result that surprised us most was that this approach … could not only modulate heart rate but also rescue drug-induced pathological arrhythmias," Liu explained.

"The fact that a purely optical stimulus, delivered through the auditory pathway, could restore normal cardiac rhythm in a disease model exceeded our initial expectations and might open up exciting possibilities for future therapeutic applications."

Limits of the optical stimulation research

Naturally, such possibilities remain far from being realised. Liu calls the work a "very preliminary proof-of-concept demonstration"; it is still unclear exactly how neural signals bring about the changes in heart rhythm, while whether this approach could work in larger animals has not yet been examined.

"We view this work not as a definitive answer, but as an invitation to explore a new frontier at the intersection of optics, auditory neuroscience, and cardiac physiology," he told ScienceAlert.

The findings were published in Nature Communications.

This article was fact-checked and edited by Fiona MacDonald. Although we take pride in our process, we are only human. If you find an error, please let us know.