Thread by @analogist_net

In honor of @Foone's thread on the *visual* quirks of our brains, let's talk about "how bullshit insane our brains are": sensory and motor systems edition.

https://twitter.com/Foone/status/1014267515696922624

Just like with the visual system, "hack your time perception" is trick is one that our brains employ a whole lot, to try make actions *appear* as if they're happening at the same time. This probably makes cause-and-effect easier to associate.

For example, try scratching your leg. You can feel your hand move; you can feel force in your fingers. You can feel your nail on your skin. Feels like they're all happening at about the same time, right?

Wrong.

Your motor commands zips down from your brain through α-motor neurons to your fingers at around 100 m/s. They move; your legs then feel that scratch slowly at 40 m/s through type II sensory fibers.

That signal's roundtrip takes 40-50 ms to just travel through your body.

After the sensory signals process, 100-200 ms has passed - from intent to conscious sensation. For reference, that would be the network roundtrip time from the US west coast, all the way to Europe, *and back*.

Crazy thing is, if that happened in a game you're playing, you would complain about lag.

But for scratching, you *don't feel it at all* - your brain hacked time perception backwards to make you think the movement intent, the movement, and the sensation occurred at the same time.

This "hack sensations back in time" trick is also related to another effect - cancelling the sensations of your own actions.

In movement neuroscience (and controls), this is the "efference copy". You expect the movement to feel a certain way, then you subtract that expectation.

Subtracting the expected sensation from the actual one -- that is, paying attention to what *wasn't* unexpected -- is a great strategy for a lot of reasons, chief among them being that the brain doesn't have to waste as much energy paying attention when things are *going right*.

Most people can't tickle themselves. Even using a tool, like a pen, it's very hard. Ever wondered why? One of the dominant reasons is that our movement efference copy is being canceled out, muting the feeling.

But! That cancellation only works within about 150-200 milliseconds.

In a famous experiment in 1999, Dan Wolpert's lab built a robot. You moved the robot's arm, and another arm mirrored your movements, after an adjustable short delay.

You could now in fact tickle yourself - as long as the robot copying your movements was delayed by about 200 ms.

This cause-and-effect hack works in other ways too. One of the ways it does so is in sensory integration.

As we now know, if you see, hear, and feel what you think is same event, these signals are probably all hitting your brain at *very* different times.

So the brain tries its hardest to try to mix all of these senses together into what it thinks is the same event.

One of the more famous illusions is the rubber hand illusion. Stare at a realistic rubber hand, and have someone touch IT - and your own hand - at the same time.

If your hand is hidden well enough, and the rubber hand is placed in a plausible position, and the two touches are at around the same time, many people start to feel like the rubber hand is part of their own body.

The word generally used is "ownership". It feels like it is part of you, because the sight of the touch, and the feel of the (separate) touch, are associated in your brain. If someone threatens the rubber hand with a knife, you flinch. the-scientist.com/infographics/i…

It shouldn't be surprising to you know that this illusion starts fading if the two touches become separated by more than 200-300ms. journals.plos.org/plosone/articl…

And if you're curious, our lab was looking into if this illusion works if you're stimulating the brain directly as well, by passing currents into the sensory cortex in patients with implanted electrodes. (Yes, it does.) pnas.org/content/114/1/…

Just as @Foone pointed that the visual system isn't like a camera streaming high-res video back at you, but is full of inconsistencies like blind spots and and selectively high-res (foveal) regions, the same thing is true of the skin on your body.

A classic example of "tactile acuity" is 2-point discrimination, whether you can tell that 2 pinpricks are, in fact, from 2 different pins.

Well, you might know that this is about a millimeter of acuity in your fingertip, and many centimeters on your torso. That's not bullshit.

What is less well known is that this is affected by whether or not those pins touch simultaneously, or one after another. onlinelibrary.wiley.com/doi/full/10.10…

It's also affected by whether or not the pins cause pain. It's even affected by whether you have chronic pain.

What is probably most bullshit is that tactile resolution is improved by whether or not you're *looking toward* the general direction of the touch.

EVEN IF YOU CAN'T SEE ANYTHING BECAUSE YOUR ARM IS IN COMPLETE DARKNESS.

sciencedirect.com/science/articl…

(A lot of this background -- and a lot of the ongoing work in neural touch perception at the @NeuralE_Ctr -- I learn from my colleagues @croninja1 and @djcald. Look for work from them.)

(Aside: while your 2-point acuity is ~1 mm, you can feel down to *nanometers* -- if you're allowed to stroke the texture.

This is *awesome insane*, not *bullshit insane*. Basically, your brain turns the texture into a symphony of vibrating frequencies. pnas.org/content/110/42…)

Another way your brain tries to blend vision and feel together is the size-weight and material-weight illusions.

Basically, if a larger object weighs the same as a smaller object, the larger (less dense) object also *feels lighter*. As long as you get to look at it first.

Remember, the two objects weigh the same. And you can lift them the same way - by a handle on a string, say - and the bigger looking one still FEELS lighter.

Other bullshit insane effects include:
- Lighter color feels lighter
- More metallic feels lighter
- Colder feels heavier

link.springer.com/article/10.100…

A lot of this is from how the brain weighs (heh) prior experience. A lot of work has been done in trying to figure out how experience is used -

but the short story is that through extensive training you can be tricked into thinking the reverse too, that smaller feels heavier.