> the vestibular system still won’t think you are going forward
Why not? Movement is relative, of course, and I'm moving relative to the floor and relative to what I see. What does the vestibular system detect that isn't happening?
Why not? Movement is relative, of course, and I'm moving relative to the floor and relative to what I see. What does the vestibular system detect that isn't happening?
But a sufficiently-clever mechanical VR system could blow air at you as you move, so let's ignore that one.
Your inner ear is literally an accelerometer. When you accelerate (or rotate your body around relative to a gravitational pull), the liquid in your cochlea sloshes around, and the hairs in it (which aren't just on the bottom surface, but all around the sides and top as well) feel the liquid as it passes by them and build an image of your position and acceleration in space. (It's oddly similar to how the wires in an old-school mercury tilt-switch accelerometer interpret the mercury sloshing around. Just with a lot more pairs of wires!)
Also, you've got an interoceptive sense: your organs are somewhat free to move inside your body; and so, as you experience g-forces, your internal organs continue to move a bit after your body is stopped — and you experience this relative displacement of your organs as a familiar and intuitive "lurching" sensation.
When your inner-ear accelerometer signal fails to cohere with your overall computed sense of motion, you get the feverish-dizzy, lacking-air, mentally-induced-nausea feeling of carsickness.
When your interoceptive signal fails to cohere with your overall computed sense of motion, you get the bad kind of "lurching sensation" — which, if it happens enough times, causes a unique kind of acute "body load" nausea purely localized within the gut (something common among riders of rollercoasters with negative-g-force moments.)
And I don't think there's any way of actually telling whether you're accelerating or tilting, as long as you keep the forces within a certain limited range?
Which immediately makes me wonder -- what if you put this HoloTile floor on top of a platform that can tilt on both axes? What if the moment it starts to detect you walking forwards, it tilts "uphill" in the direction you're walking? When you suddenly stop walking, it tilts backwards slightly for a split second, then resets to level.
Because this actually matches what our body experiences on a sidewalk -- when we walk forwards we actually tilt our body slightly forwards as we accelerate, and tilt it backwards briefly to stop. (Same as a Segway does, if that's easier to picture.)
So if the floor tilts... would that be good enough to complete the illusion, and send the right signals to the inner ear? Because the liquid in your cochlea is now sloshing around in the right way? The interoception is matching as well?
The part that makes Star Tour work is that your entire body is moved around when the ship is tilted (because you're strapped to your seat). That wouldn't work with a tilting floor, I think.
> the liquid in your cochlea sloshes around
The vestibules are not the cochlea. The cochlea is responsible for sound. The vestibules are just about the sensation of movement, the structures are called Semicircular canals. You have one is each axis of rotation, more or less (x-y-z)
> feel the liquid as it passes by them
The hair cells are embedded in a crystal matrix, the otolith. The inertia of the 'heavy' crystal pulls the hair cells (which only look like hair and are not actually hair) and that sends a signal to the brain. Not the liquid. The Spins from drunkenness are due to the change in the density of the liquid in the vestibules (from the additional alcohol) causing a different buoyancy of the otoliths and thereby making the hair cells change resting position.
> build an image of your position and acceleration
Only the acceleration, not the position. You can only feel, via these organs, the 2nd derivative of position (0th derivative - position, 1st derivative - velocity, 2nd derivative - acceleration)
> When your inner-ear accelerometer signal fails to cohere with your overall computed sense of motion, you get the feverish-dizzy, lacking-air, mentally-induced-nausea feeling of carsickness.
This feeling is very unique to people and their lived experiences. Seasickness will subside for most people, and sailors/figure skaters/ballerinas/skateboarders/etc generally have a higher tolerance to motion sickness.
Overall, very good overview! Thank you!
> You can only feel, via these organs, the 2nd derivative of position (0th derivative - position, 1st derivative - velocity, 2nd derivative - acceleration)
Well, sure, but the thing that I meant by "builds a picture" isn't just what you're feeling (which, indeed, is acceleration, not position), but also the information the brain has available to predict with.
IIRC, I've read about "hardwired neural logic" (i.e. a brain function outside of our conscious awareness that we have from birth) for flow-state obstacle avoidance in known environments, which involves the brain doing internal dead reckoning based on integrating over its acceleration-picture of the body — thus allowing a person to e.g. make their way home through a forest they've walked through a million times, with their eyes closed, yet stepping over tree roots at just the right time.
Dead reckoning over an integration of the acceleration-picture also seems to come into play with proprioception. Thus why people who are given a local nerve block in an arm, might end up smacking themselves in the face with said arm when trying to e.g. adjust their glasses. They're missing the feedback signals† from the arm that would allow them to compute the acceleration of the arm, and therefore the moment-to-moment position of the arm.
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† I'd love to know how the brain figures out what the acceleration forces being applied to the extremities are. I'm guessing you'd probably be almost the right person to know the answer to that question! (The perfect person would be one of those "prosthetic arm you can feel" researchers.)
I know the brain is getting signals back from the muscles in the extremity — how flexed they are, how much work they're having to do to keep in position — and that's probably a majority of the information needed... presuming you are flexing the muscle.
But we still know where our limbs are in space even when we have them "loose." So there's likely an additional component — the one that allows people without numbness in their extremities, to avoid being smacked in the face by their own hand when someone picks their arm up and drops it above their face while they're asleep.
My naive guess for what that data source could be: pressure on blood vessels. As you move the extremity, the blood drags behind the limb, and so some blood vessels feel an temporary increase in pressure, and others a temporary decrease in pressure. The brain then takes the derivative of the blood pressure data; builds a map of blood pressure deltas; and then "recognizes", in each extremity's picture, the accelerations upon the extremity.
The propioceptive positioning does occur in muscles, but, as far as I remember, it's mostly in the tendons. In that, your brain/spine knows the stretch of all the tendons and then back calculates that to a position based on experience. So, people with very recent tendon injuries or differential loads on the muscles will have a somewhat hard time knowing the position of their body (by very recent, I mean within a few minutes/seconds). Blood pressure would be an interesting way to do the math, but as blood pressure changes a lot, it would be doing a lot of work here. Also, most of these systems are inherited from very ancient times when blood was a bit different and before hearts like ours. Animals want to know where they are more than they care to have much of a heartbeat.
'Fun' little ways that we have discovered these things are mostly when someone has a disease that causes them to do wacky stuff. Phantom limb diseases are a bit like that. There are some other ones, but I can't remember then, sorry!
If you'd like to learn more about propioception and all the little biological fun times, I'd suggest just about any intro neuroscience textbook. They're kinda fun once you get into them. For grad level learning, Kandel's Principals of Neural Science is the go-to book. Lots of case studies in there.
Something like this would happen to me sometimes when I was intoxicated on alcohol and cannabis together (getting crunk, the kids used to say). After achieving a substantial buzz, if I closed my eyes and leaned back deeply in a chair or lay down on my back, I could get this vivid sensation of falling rapidly. Just like going down a steep rollercoaster. As long as I stayed in the sweet spot intoxication-wise, it was a thrilling and pleasurable sensation.
From what I recall, after a few brief moments of exhilaration (that I could maybe relax and focus into in order to prolong), a subconscious awareness that I'm not actually moving takes over, which turns the sensation into pure nausea. Right in the pit of my stomach, like you describe. Sometimes if the nausea passed quickly enough I could open my eyes and sit up, re-center myself, and then induce the effect again. Anyway, kind of relevant in a weird way. The things we do for cheap thrills...
And on that note, I could see potential for some people to get acclimated to this effect of the technology and actually enjoy it as part of the experience.
What you can and should do is to tilt it a bit just to make it more like running, but its still far from regular experience. You body knows, and trust me its not 1 or 2 things that are out of place, its everything.
You can and will get used to it, but never ever it will feel like regular running.
That's also true on a treadmill. I think the bigger problem would be that the world appears to be moving (unlike a treadmill) and you don't feel any acceleration.
The treadmill shows that performing the walking movements without acceleration doesn't generally cause motion sickness.
If you actually use a treadmill, or watch motion capture, you’ll see that this isn’t the case. It’s a lot of slight accelerations and decelerations. The variation change from person to person, and more experienced runners are able to minimize them, but they’re still present.
Edit: this study is peripherally related https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8160945/
Isn't that always true? If I pay attention to it, I notice a lot of tiny movements of my head, as I just sit here typing.
"You don't have acceleration" is just talking about the big accelerations from 0 mph to roughly 3 mph to 0 mph, not implying that you're completely motionless as if frozen in ice.
(change in momentum, however, is another story.)
Think of it like a plumb bob hung inside your skull. Would it be canted forward or back? I don’t think relative to floor matters for that kind of mechanical detection of movement.