Areas of Research - Neurobiological Studies of Hearing - Communication Engineering Laboratory

We know that the inner ear separates the frequency components of complex sounds by mechanical means, but exactly how is this feat accomplished?

The cochlea is the part of the inner ear that converts acoustic signals into the neural code that conveys auditory information to the brain. Modeling the function of the cochlea has been an active area of research for many years, yet several mysteries remain.

First is the problem of tuning. How does one explain, for example, the auditory frequency selectivity seen in auditory nerve fibers? That is, how does the cochlea deliver to each nerve fiber only a specific, narrow range of frequencies? We know from direct measurements within the cochlea that the tuning of auditory nerve fibers is entirely mechanical. However, there is disagreement about how this mechanical tuning is accomplished.

The second major mystery is the question of cochlear nonlinearities. Somehow, the cochlea compresses the large dynamic range of acoustic pressure variations that enter the ear into the much smaller dynamic range that can be processed by the sensory hair cells that detect these signals in the cochlea. The dynamic range of the of the hair cells (between thermal noise and signal saturation) is about 1,000, whereas the range of audible sound pressure levels is about 100,000. The cochlea is a nonlinear signal processing device that, in addition to separating frequency components, is able to compress the dynamic range of input signals without significant degradation of the signal content.