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

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

Replace the torch with a laser, however, and the person with a zebrafish, and the result becomes remarkably intriguing.

According to Xiaoshuai Liu, a physicist at Guangzhou University in China, the route from light entering an ear to a changed heartbeat is somewhat more complicated.

That is exactly what Liu and his team have achieved – and it is as extraordinary 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?”

How laser light moves zebrafish otoliths

The ear is a finely structured, delicate organ containing sensitive features that detect minute vibrations. In most vertebrates, these include otoliths: tiny, stone-like calcium carbonate crystals. When sound or movement makes them shift, they stimulate sensory cells in the ear.

Yet sound is not the only force capable of moving something microscopic. Scientists have used optical tweezers for years to manipulate tiny objects. In essence, the technique uses radiation pressure to apply a physical force that can push or move an extremely small object.

The otoliths in zebrafish larvae’s ears are indeed exceptionally small.

Crucially, the way these structures move helps the fish perceive sound and motion.

This made them a compelling target for a technique intended to manipulate microscopic objects. If an optical trap could shift an otolith without sound ever reaching the ear, the researchers could in effect circumvent the first stage of hearing.

“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 they put the idea to the test. As one would.

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

Does this mean the 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 significantly more active under optical stimulation.

Optical stimulation and zebrafish heart rate

That was not the only finding.

The researchers observed that moving a fish’s otolith caused its heart to beat faster.

In one experiment, a resting heart rate of about 2 beats per second increased to about 2.7 beats per second while stimulation was applied, before gradually moving back towards its baseline after stimulation ended.

The researchers found that precisely oscillating an otolith could raise heart rate by around 50 percent.

Because the optical tweezers gave the team exact control over the otolith’s movements, they could achieve something stranger still: recreating patterns corresponding to music. Altering the oscillations’ amplitude, frequency and timing enabled them to imitate characteristics corresponding to loudness, pitch and rhythm.

These results took the research in a fresh 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.”

Light music for drug-induced arrhythmias

The researchers used drugs to produce three distinct forms of abnormal heart rhythm in zebrafish larvae, before applying their optically generated musical stimulation.

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

In fish where the upper and lower chambers of the heart had become unsynchronised, normal coordination was re-established in six of seven animals.

And for fish whose heartbeats contained abnormal pauses, normal rhythms resumed in five of seven.

More strikingly, some of these improvements continued even after the stimulation had stopped.

Related: Hit Songs Do Something to Your Brain, And It Could Be The Future of Music

“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.”

Naturally, such applications remain far from becoming reality. Liu calls the study a “very preliminary proof-of-concept demonstration”; precisely how neural signals bring about the changes in cardiac rhythm is still uncertain, while whether this method might work in larger animals has yet to be 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 by Fiona MacDonald and edited by Fiona MacDonald. Although we take pride in our process, we are only human. If you spot an error, please let us know.