[omershapira]

Every "Perception for VR" class I give focuses on understanding proprioception. Designers ignore it until they experience spatial planning just with proprioception:

• Start by closing your eyes and touching your nose. Why can you do that?

• Continue by trying to touch two fingers on opposite hands behind your head. Why can't you do that? (because your proprioceptors are saturated).

• Now, you can build an environment with post-it notes and a blindfold: https://omershapira.com/blog/2016/04/the-painful-introductio...

[jackhalford]

I just touched two fingers behind my head with eyes closed (although does that really matter, hands being out of sight?). So I still don’t get the point about proprioception

[atomicnumber3]

I tested this out too. I noticed a few things where GPs explanation is probably abbreviated:

I noticed that finger-to-nose, I was able to do the first time, every time. 100% accuracy, and with no "do a little circle to figure out the last 0.5in". As opposed to touch-fingers-behind-head, I could "feel" the uncertainty and successfully touching fingers the first time, while not impossible, was probably 10-20% success rate. I was able to improve it by doing a little (radius of perhaps 5mm?) circle with both fingers once I knew I was about in the right place, since I was usually within about a fingers-breadth. But the whole process was just a lot less bulletproof than finger-to-nose. Which I think was GPs point.

[johnchristopher]

N=2, nose is a hit 100% of the time with pinpoint accuracy but fingers behind the head only hit if I repeat the movement. Going much slower increase accuracy but it's never as good as hitting the nose anyway. At normal or increased speed I usually miss by a finger width.

[chrisweekly]

same

[anigbrowl]

Continue by trying to touch two fingers on opposite hands behind your head. Why can't you do that?

I can, easily, every time, any pair of fingers, and also behind my back and many other positions. Really, I don't care for this assumptive style of explication.

I can readily believe it is an issue for many or even most people (you have investigated and know far more about this topic than I). I am able to do this from a combination of spacing out on this sort of thing since childhood and a lot of athletic training, but surely this isn't that rare.

[maroonblazer]

Not only am I unable to touch two fingers behind my head with my eyes closed, I'm not able to do it in front of my head. I can easily touch my nose with my eyes closed. Is there something going on where there's one less degree of freedom with the nose, which makes it easier?

[manwe150]

Seems a reasonable hypothesis to me. I noticed I can improve or reduce accuracy in many different positions by bending arm/finger/hand at an unusual angle until it seemed that my mind was a bit less certain exactly how twisted or crooked was the final configuration, and the uncertainty would lie most strongly in the axis I expected (the most bent angle) and most accurate in the most direct axis