Virtual cells vindicate hearing theory

A collaboration led by Pascal Martin of the Curie Institute in Paris yoked live and virtual cells to tackle the question, How does the ear amplify faint signals by factors of up to 1000? Hearing relies on converting mechanical vibrations to electrical impulses. The transduction takes place in the cochlea and is carried out by micron-scale hairs that sprout from specialized hair cells. To the tiny hairs, the watery liquid that surrounds them is viscous, just like honey would be to a tuning fork. An active feedback mechanism in the hair cells not only overcomes the viscosity, but also amplifies faint signals. But the feedback can’t provide all, or even most, of the ear’s prodigious amplification. Two years ago, theorists proposed that elastic coupling among groups of hair cells could make up the shortfall. Coupling, they argued, lowers the detection threshold by averaging out each cell’s noise fluctuations. Hair cells in frogs, humans, and other terrestrial vertebrates are indeed coupled to each other by various flexible structures. To prove that coupling boosts amplification, Martin’s team extracted a vibration-sensing organ from a bullfrog and attached a flexible whisker to one of the hair cells (see schematic). Through sophisticated computer control, the whisker was made to vibrate in a way that mimicked the net force on the live cell (green) exerted by two neighboring hair cells (gray). The expected enhancement to the amplification showed up. (J. Barral et al., Proc. Natl. Acad. Sci. USA, in press. )—Charles Day