Select Page

A bit on visual illusions

There’s an optical illusion I’ve seen in one of Ramachandran’s books (I think it was Phantoms), which I’ve found very interesting for a long time. Both because, like many of the illusions he shows, it’s very easy to do… and because I could never, bloody ever get it to work for me. It worked great for others – but never for me.

The illusion is a fairly simple one. Stick out your index finger in front of your nose, pointing up, about 30 centimeters/1 foot in front of your face.  Then pick an object at the back of the room, and turn so that it gets to be directly behind the finger. If you concentrate on the object, you should get two fingers. If you concentrate your sight on the finger, you should see two objects. This is because of how your visual fields cross depending on where you concentrate your sight.

Everyone I’ve tried it with could easily get this illusion. In fact it’s so common, no-one was really moved by it. “OK, yes, I see two things, it’s fairly obvious, that’s what happens when the eyes refocus. Where’s the illusion?”

Thing is, I could never ever manage it.

I always get one finger and one object, no matter how I concentrate, what distance the finger or the object. For some reason or other, my brain overrides the normal split image and keeps a whole singular image generated, no matter where I concentrate on.

This, I think, might be one of the reasons I don’t get stereograms – you know, the jumbled pictures that seem like visual noise until you look at them just right and suddenly “hey, it’s an elephant!” They don’t work for me, and I do guess it might have something to do with the finger illusion. Although, on the other hand, they don’t work for many other people, so it might not be this issue.

But there is something that does work for me, that I’ve not heard anyone experience anything like this.

And that’s high FPS movies.

Usually you get movies at 24 or 30 fps. But sometimes you get more – this can be done automatically by some TVs, you can get a special plug-in for your computer, etc. and sometimes movies are simply shot like that from the start. (The Hobbit, for example, was 48fps.)

Now for me 60 fps looks really weird, like I’m watching a theater play.  The depth is suddenly increased tremendously. Noone I’ve talked with about this has ever had a similar experience – 60fps looks unusual to them, because they’re accustomed to 24, but there is no mention about the depth perception issue. (You can check 60fps versions of many movie scenes on youtube for comparison. In fact, try looking for 120 fps as well, as I’ll get to it.) Interestingly, it works far better on real life bits – people, real backgrounds, then on virtual ones – greenscreen backgrounds, CGI, etc.

Now the really interesting part? Give me about 5-10 minutes of watching 60 fps movies, and my real-life depth perception goes haywire. It’s as if my body map, my sense of space goes into overdrive, as if I can literally feel the space far better, not just know or recognize it, but have it filled, in a way. It’s as if my proprioception is reaching beyond my body and sensing the space. The experience tends to disappear after about half an hour or so, although I can get flashbacks of it later on. Also, it’s purely visual, despite branching out into proprioception – when I close my eyes, it dissapears.

I’ve recently tried it on a 120fps clip and the results were even more profound, both in watching the movie and in real life. Meanwhile I find no reduction of depth in going from 30 fps to 15 or 10 fps, so it doesn’t work the other way round.

I have never heard anyone else have this experience, nor have I found any mentions of it in literature. I’ve found a little piece on how 240 fps increases depth perception more than 120 or 60, but little beyond that, and nothing about the aftereffects. I’ve tried googling it, but found not only no research, but even no mention of such weird effects on quora and the like, so I assume these are not all too common. 

So I figured I’d share them, as something interesting, and something to test for in yourself – since the test itself is fairly easy and will only take you about 5-10 minutes. I’d be especially interested in hearing from people who do get the finger illusion, but also get the fps depth illusion in real life, as currently I have a hunch they might be connected/mutually exclusive.