Week 8: The auditory and vestibular systems

This week finished the sensory system part of the course by focusing on auditory and vestibular (sensing posture and acceleration) systems. There is some analogy between hearing and vision, and even though both of them are quite impressive and difficult to understand, the hearing surprised with its complex abilities.

I’m still amazed how we can understand many complex sounds (like music with many instruments) with the tonotopic structure of the basilar membrane and the hearing cortex. It somehow makes sense that the frequency-wise encoding of sound is needed, but still the sounds have a lot of fine details. Staring at a sound spectrogram does not make much sense of the sound, but heard sounds are easily recognized. Maybe as the vision is trained to recognize certain structures, the hearing is also trained to hear structures and their subtle differences in sound.

The operation of the cochlea and basilar membrane is also quite intricate. How the basilar membrane could be that stiff but still flexible enough that it could resonate easily with sounds with different frequencies? And how the inner hair cells could detect a movement with a size of an atom on the basilar membrane without vanishing into noise? And how different frequencies are separated even though one frequency probably activates several hair cells in the basilar membrane?

The sensing of sound direction is also interesting. I once did a project that utilized simulated three dimensional sounds with head related transfer function. It is relatively easy to move the sound source from left to right, but it was a bit difficult to tell the difference when the sound was from the front or behind. Maybe better simulation of that would require better head modeling or ear buds instead of the used over-ear headphones.

The operating principles of the vestibular system reminds me of inertial measurement sensors (IMUs) that detect acceleration. Of course, the vestibular system is much more biological in nature, but some IMUs are based on a weight on springs (reminding of otolith organs) and some have circular structures for angular acceleration/velocity detection (reminding of semicircular canals), like gyroscopes or rotated optical fibres measuring the time light passes the fibre.