Sensitivity in the cochlea through local tuning rules

The auditory range of the human ear spans over a trillion-fold range in sound intensity and ten octaves of frequency. Sound is converted into a surface wave along the basilar membrane whose mechanical properties lead to a frequency-dependent resonant position. Along this basilar membrane, embedded hair cells sense small displacements and transduce them into electrical signals. It is commonly believed that these hair cells operate near a so-called Hopf bifurcation crucial to their enormous dynamic range. However, hair cells are only a small perturbation to the mechanical properties of the basilar membrane. While the state of an individual hair cell influences the mechanics of every frequency mode, they can only sense local displacements of the basilar membrane dominated by a narrow frequency band. How these hair cells, acting individually, can tune the entire cochlea to be sensitive to all frequencies remains an open question. Here we present a model where hair cells use only local information, their mean squared displacement in response to thermal noise, to tune the entire cochlea to an array of critical modes. This feedback scheme, agnostic to precise molecular details, can robustly tune the cochlea’s modes to criticality even in the presence of perturbations. This work illuminates how individual hair cells can collectively create a critical cochlea rather than being independently tuned.