Coupling of hair cell oscillations in the inner ear determines certain properties of hearing

The hair bundles in our inner ears are powerful detectors that allow us to distinguish oscillations smaller than thermal fluctuations, with a time resolution of tens of microseconds and an amplitude range spanning more than six orders of magnitude.

“It is not fully understood how this system achieves these properties, or how it is even physically possible for a biological system to perform this well in the presence of the thermal fluctuations in the surrounding fluid,” said Justin Faber, an author on a recent paper aiming to understand the synchronization that occurs within inner ear hair bundles. “These properties have puzzled physicists, mathematicians and biologists for decades.”

By artificially coupling inner ear hair bundles, Faber and his co-author, Dolores Bozovic, identified two different types of partial synchronization that occurs in the bundles’ oscillation – one in which a subset of the oscillators synchronize while the rest are incoherent, and another in which a number of different clusters of synchronization form within the larger system.

They developed a simple model to describe this behavior and found a tradeoff occurs between the strength of coupling and the system’s sensitivity to different frequencies. While stronger coupling reduces the impact results in a greater resilience to noise, a weaker coupling is required for achieving a larger range of detectable frequencies.

“Because of this tradeoff, the ideal regime for signal detection occurs at intermediate levels of coupling strength,” Faber said. “It is at these intermediate levels of coupling that we find states of partial synchronization.”

However, how these hair cells communicate with neurons remains poorly understood. Faber and Bozovic hope to optically measure this interaction to further elucidate the mechanics of hearing.

Source: “Chimera states and frequency clustering in systems of coupled inner-ear hair cells,” by Justin Faber and Dolores Bozovic, Chaos (2021). The article can be accessed at https://doi.org/10.1063/5.0056848 .