(Inside Science) – Scientists don't fully understand how we detect faint sounds, because they should be drowned out by the background noise that the ear itself produces. Now, however, researchers at UCLA have produced clues to the process that allows us to hear a pin drop, or understand a whispered comment. They did so using hair cells taken from bullfrogs that they studied in laboratory glassware.
The UCLA team used an optical microscope and a high-speed camera to detect how the relationship between signals from faint sounds and bundles of the frogs' ear hairs differs from that between signals from louder sounds and the hair bundles.
Researchers in this field already knew that the hair cells synchronize with strong sound signals. They oscillate in phase with the incoming sounds; the louder the sound, the greater the degree of synchronization.
But in the case of the softest sounds, the UCLA team found, the cells intermittently lose and then regain synchronization in a process called "phase slip."
It is those slips that permit the cells to detect the faint sounds through the ambient noise.
"We show that phase slips occur," said Dolores Bozovic, an associate professor of physics and astronomy at UCLA who led the team. "What was surprising was their intermittent occurrence. That’s potentially more powerful than having synchronization all the time."
Why did the team carry out the study on bullfrogs' hair cells rather than those of humans or other mammals?
"We need to open up the organ to access the probes and get precise measurements but not damage the fine machinery of the hair cells itself," Bozovic explained. "Bullfrog cells are very robust organs. Mammalian cells are much more fragile."
In humans and other mammals, the sound-processing system lies in the cochlea, the spiral-shaped cavity in the inner ear that contains the hair cells bathed in fluid. Thousands of tiny hair cells in the ear convert the vibrations of incoming sound waves into electrical signals that the brain processes.
The sound vibrations compete with others caused by the temperature in the inner ear. "At room temperature, the 'thermal jitter' means that the hair bundles will show fluctuations in their positions comparable to those caused by incoming signals," Bozovic said.
Bullfrogs do not possess cochleas. Instead, an organ called the sacculus carries out the cochlea's duties, which include hosting the hair cells.
Nevertheless, the frogs' hearing systems are similar to those of mammals and just as sensitive to faint sounds. The sacculus is "one of the common organs used to study the mechanics of hearing," Bozovic said.
Despite their robustness, frogs' hair cells can't be studied inside the ear. Current techniques do not allow scientists to image them there with the necessary precision.
So the Bozovic group, like others, worked with bundles of hair cells in a container that resembles a slightly modified glass microscope slide – a process technically called in vitro.
Because they had removed the hairs from the frogs, the team couldn't use sounds to stimulate them.
"We applied a mechanical stimulus using flexible glass fibers attached to the tips of the hair bundles," Bozovic said. The fibers were attached to a machine that created the necessary vibrations.
"We imaged the hair cells on an optical microscope and recorded their movements with a high-speed camera," she added.
The images showed that the phase slips occurred near an area of dynamic instability, called a bifurcation, Bifurcations are points at which the behavior of the system changes – in this case from the usual synchronization between hair cells and strong sounds.
The team found that the occurrence of phase slips depended on the strength, or amplitude, of the signal. "The rate of phase slips is reduced as the amplitude of the signal increases," Bozovic said.
However, the team found no definitive stimulus level below which full synchronization between the stimulus and vibrations of hair cells gives way to phase slips.
"The rate of phase slips is reduced as the amplitude of the signal increases, but there is no threshold," Bozovic noted.
Bozovic's team includes physics professor Robijn Bruinsma and graduate students Yuttana Roongthumskul and Roie Shlomovitz. Roongthumskul, who carried out much of the detailed study, headed the report on the research in the journal Physical Review Letters.
"The paper adds to the substantial literature showing that hair cells, the sensory receptors of the inner ear, operate near one or more dynamical bifurcations that confer specific properties on hearing," said A. James Hudspeth, professor of neuroscience at Rockefeller University, in New York. He added, "I would rate the reputation of the UCLA group highly."
The results of the current study present opportunities for further research. "We’re now looking at how multiple cells connected to each other react to the signals," Bozovic said. "We’re asking the question: How does the synchronization between cells work?"