In his early 30s, Liszt toured around Europe, thrilling audiences with furiously fast tempos and large leaps across the keyboard. Not only can the human ear keep up with Liszt's flying fingers, but it can also split the complex chords and prestissimo phrases into individual frequencies in real time—thanks in large part to a high-speed motor that turns the ear's sensory cells into tiny “muscles” that “push back” on sound-induced vibrations (Dallos et al., 2008; Johnson et al., 2011).

Membranes of outer hair cells are packed with a modified anion transporter, called prestin. When chloride ions bind to prestin's intracellular surface, depolarization produces a conformational change that shrinks the protein's volume and shortens the cell. A few years ago, Dallos et al. (2008) engineered a mouse to express a mutant prestin that properly localized to the membrane but couldn't shrink the cell's height. The mice displayed decreased sensitivity to sound and a reduced ability to split sound waves into their individual vibrational frequencies.