Read on Twitter
Chapter 5: today let's philosophize about the #vestibular system and think out of the box. You heard of the "velocity storage" which improves the perception of long rotations? But what is this good for in real life? I will show that it is actually more useful for short rotations!
2/ Background: canals in the inner ear sense head rotation, but their signal decays over time (time constant ~4s). It means they make error during long rotations (their signal decays) and after the end of these rotations (after-effect).
3/ The velocity storage (VS) is a central process that increases the time constant of rotation sensing to 10-30s (here 16s), thus reducing errors. Nice, but most natural rotations are short (<1s). So what is the use of the VS? How significant are canal error in real life?
4/ In fact, the VS is multisensory and integrates visual signals, gravity, efference copies. It can bring final errors to ~0 (see chapter 4). But the question remains: why not just use raw canals signals? Can significant canals error occur during real-life fast movements?
5/ To answer, let us think differently. In vestibular experiments, we typically think is term of rotation velocity and duration. In everyday life, our movements aim at rotating (quickly) by a certain amount, say 90° or 180°. What about we think in term of amplitude and duration?
6/ For instance rotate 180°. You may rotate slowly, (10s at 18°/s). This is long and canals decrease to ~0, so peak error is 17°/s. But surprise: simulate a 5x faster rotation (90°/s for 2s). Absolute canal error increases to 35°/s!
7/ What? Did I mess up the math? No: it is easy to show this is correct with a couple of equations. Is this an artifact from the rectangle velocity I used? No; here is a similar result with a more natural Gaussian velocity profile.
8/ So, for a given rotation amplitude, a faster rotation will produce more canal errors. It will asymptote to a max error for an infinite velocity (=zero duration): the asymptote is 45°/s for a 180° rotation, 22.5°/s for a 90°.
9/ In practice, rotating 90° means looking from left to right. Do it rapidly and you’ll get ~20°/s of canal error. To put things into perspective, canal sensation threshold is about 1°/s. Turning around rapidly produces ~40°/s of canals error!
10/ In summary, we think that the VS and multisensory integration improve rotation perception and VOR during "long" movements. In reality, it is movement amplitude that matters, and the "multisensory" VS corrects for significant errors (e.g. 20-40°/s) during natural movements.
11/ This was the fifth chapter of my series of threads on fundamental mechanisms of self-motion perception. Links to other chapters and info, including where to download videos, are here: https://twitter.com/JeanLaurensLab/status/1302869103766634496
