Dáibhid Ó Maoiléidigh
Laboratory of Sensory Neuroscience
The Rockefeller University
January 28, 2014
The Babel Fish: Active Signal Detection in the Ear
The auditory and balance systems of vertebrates utilize detectors known as hair bundles to transduce mechanical input into electrical signals. A simple model of hair-bundle mechanics predicts that a bundle's dynamics and the response of a bundle to external signals may be controlled by the bundle's mechanical load , which differs between organisms, among receptor organs, and with the position within individual auditory organs. A loaded bundle's state diagram resembles a fish with a loop of Hopf bifurcations attached to a wedge of fold bifurcations. A hair bundle can operate as a low-pass filter, step detector, quiescent resonator, amplifier, active oscillator, or bistable switch depending on how it is mechanically loaded. To test this notion, we employed a feedback-based clamp system to change the loads confronting individual hair bundles. The observed fish diagrams and responses of bundles to stimuli confirm the model's predictions. These responses resembled those of hair bundles from the amphibian balance system, the reptilian hearing organ, or the mammalian auditory system, demonstrating the essential similarity of bundles from different species and receptor organs. A bundle exhibited features similar to the auditory system as a whole when its operating point was chosen to be near the loop of Hopf bifurcations, namely great sensitivity, sharp frequency tuning, broad dynamical range, and the tendency to oscillate spontaneously. Unlike an artificial device that is designed to respond in a stereotyped manner to a specific signal, a hair bundle can function in different mechanical contexts as various types of signal detector.
 Ó Maoiléidigh D, Nicola EM, Hudspeth AJ (2012) The diverse effects of mechanical loading on active hair bundles. Proc Natl Acad Sci USA 109:1943-48.