Madrid. Neuroscientists have solved the mystery of locating sound sources underwater, absent in humans, describing the hearing mechanism of a tiny fish in the journal Nature.
When we are underwater, humans cannot determine where a sound is coming from, which travels there about five times faster than on land. This makes directional hearing, or sound localization, almost impossible because the human brain determines the origin of a noise by analyzing the time difference between its arrival at one ear and the other.
In contrast, behavioral studies have shown that fish can locate sound sources as prey or predators. But how do they do it? Danielle’s brain, a fish measuring about 12 millimeters, almost completely transparent throughout its life, native to the streams of southern Myanmar, has the smallest known vertebrate brain, but still displays a number of complex behaviors, including sound communication. That, and the fact that scientists can see directly inside its brain (the head and body are almost transparent), make it interesting for research into the organ.
Professor Benjamin Judkewitz, a neurobiologist at Charité-Universitätsmedizin Berlin, and his team use the tiny fish as a window into fundamental questions, such as communication between nerve cells.
Resonance source
His most recent work is dedicated to the development of the sense of hearing and the decades-old question of how fish can locate a resonant source underwater. The directional hearing models in previous textbooks are not sufficient when applied to underwater environments.
From the song of whales to the chirping of birds or a predator stalking its prey, when sound is emitted from a source, it propagates to the surrounding environment in the form of oscillations of movement and pressure. This can be felt even by placing a hand on the cone of a speaker.
Particle vibration occurs, adjacent air moves: this is known as particle velocity, particle density also changes as the air is compressed. This can be measured as sound pressure.
Terrestrial vertebrates, including humans, perceive the direction of sound primarily by comparing the volume and time at which the sound pressure reaches both ears. The noise sounds louder and reaches the ear closest to the sound source sooner. This strategy does not work underwater.
There the sound travels much faster and is not muffled by the skull. This means that fish should also have no directional hearing ability, since there is virtually no difference in volume and time of arrival between their ears. And yet, spatial hearing has been observed in behavioral studies of several species.
To find out if and, above all, how a fish can determine the direction of sound, we built special underwater speakers and played short, loud noises.
explained Johannes Veith, one of the two first authors of the current study.
“Next, we look at how often you avoid Danionella to the speaker, that is, it recognizes the direction from which the sound comes.” For analyses, a camera was used to film each fish from above and track its exact position. This live tracking method provided a crucial advantage: the team could now locate the echoes and suppress them.
What humans perceive through the eardrum is the pressure of sound, not the speed of particles. Fish have a completely different hearing mechanism: they can also perceive the speed of particles. How exactly does this work in Danionella was revealed by images taken with a specially designed laser scanning microscope that digitizes the structures inside the fish’s ear in a strobe pattern while a sound is played.
Near an underwater speaker, water particles move back and forth along an axis oriented toward and away from the speaker. The speed of the particles moves along the direction in which the sound propagates.
A fish near the speaker also moves with the water, but the small stones in the inner ear known as otoliths move more slowly due to inertia. This results in tiny movement detected by sensory cells in the ear. The problem is that this means that the fish can only detect the axis along which the sound is moving, but not the direction from which it comes. This is because sound is a form of oscillation, a continuous back and forth motion.
This problem is solved by analyzing the speed of the particles as a function of the current sound pressure, one of several hypotheses that have attempted to explain the mechanism involved in directional hearing in the past. It turned out to be the only theory that fit the researchers’ results.
Sound pressure sets the compressible swim bladder in motion, which in turn is recognized by the hair cells of the inner ear. Through this second indirect auditory canal, sound pressure provides fish with the reference they need for directional hearing. That’s exactly what a spatial hearing model from the 1970s predicted, and now we’ve confirmed it experimentally.
Judkewitz said.
The team was also able to show that directional hearing can be fooled by reversing sound pressure; When that happened, the fish were able to perceive that this was the only one that could be seen.