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Chapter 1: Why do we feel #dizzy when turning? This is because of how out inner ear’s rotation sensors (#vestibular semi-circular canals) work, from a mechanical point of view. Watch these movies and the next for explanations.
2/ The inner ear's #vestibular semi-circular canals are liquid-filled tubes. When the head rotates, the liquid stays in place and flows in the canal. This activates hair cells (in a structure called cupula) that sense the rotation.
3/ However, when turning too much, the liquid starts to rotate with the canal and the rotation signal fades out. Furthermore, when the rotation stops, the liquid keeps flowing and creates a rotation after-effect.
4/ In fact, canals after-effects occur all the time when we make large head movements (e.g. left/right movements with 120° amplitude illustrated here). They occur all the time in real life! How comes we are not constantly dizzy?
5/ The brain can improve canals dynamics: in humans, rotation sensation lasts longer (time constant 10-20s) than canal signal (4-5s). This is enough to deal with ordinary movements (as in movie 4/) but after-effects still occur after long rotations.
6/ In #computationalneuroscience terms, the brain uses an internal model of the canals to perform dynamic #Bayesian inference. The time constant of the optimal rotation estimate is governed by the amount of noise on the canals and by a Bayesian prior.
7/ Indeed, improving canals dynamics requires an integration which amplifies sensory noise at low frequencies. To limit noise accumulation, the brain relies on a Bayesian prior towards zero velocity. The Bayesian optimal is a rotation perception with 10-20s time constant.
8/ The dynamics above represent what happens in darkness. When rotating in light, visual information can improve rotation perception somewhat, but not enough (in humans) to overcome canals dynamics. Hence the dizziness!
9/ Rotation estimation is in fact multisensory: the brain can incorporate retinal flow, efference copies, and even graviception and proprioception and weight them dynamically. This has been modelled using Kalman filters for decades. We’ll come to that!
10/ Stay tuned for the next chapter, in which we will explain that, if these rotations above make you feel dizzy, well, we can make it much worse :D! And there will be more internal models! In 3D! Here’s a preview…
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