Awesome natural optical illusion/effect

June 9, 2012 2:15 PM

Is there a name for the strong optical effect/illusion that we experienced today, when looking elsewhere after watching a waterfall (165m height) from about 100m distance for about 20 seconds: that afterwards everything we looked at seemed to strongly contract vertically?

It was very surprising, and worked repeatedly; the area of vision affected pretty much corresponded to the body of falling/expanding water previously focussed-on. My guess is this has something to do with saccades(?). Also: the vertical shrinking was bi-directional, towards the center of vision - which would indicate there's some compensation at play, coming off watching something that, as it moved, also stretched out vertically? (But how/where/why would this compensation be taking place?)

I can't imagine this hasn't been experienced/described before, so I'm expecting there's a precise name for the effect - I just don't know how I would go about finding it.

posted by progosk to science & nature (11 answers total)

Yep, it's called motion aftereffect.
posted by Specklet at 2:19 PM on June 9, 2012

Rock Band is a good way to replicate this.
posted by cmoj at 2:28 PM on June 9, 2012 [2 favorites]

Bingo.

So the compensation is neural, it would seem.

Is there a way to put the explanation of neural adaptation in layman's terms?

(On preview: Rock Band?)
posted by progosk at 2:33 PM on June 9, 2012

I get this with Rock Band all the time. The constant horizontal motion throughout the song triggers the same aftereffect after the song is over. It's one of the reasons I have a hard time playing Rock Band anymore because it nauseates me a little bit. The brain is a wacky organ.
posted by altopower at 2:36 PM on June 9, 2012

If you've not played Rock Band before, here's a sample of what the display looks like. (The actual song starts around 30s in.) Note the constant vertical scrolling that you would be staring at as you played.
posted by Johnny Assay at 2:42 PM on June 9, 2012

Basically, visual neurones adapt and respond selectively to moving contours in the waterfall. When you look away and no longer see the motion, cells tuned to different directions produce roughly equal responses. So exposure to a particular direction of motion alters the balance in favor of cells tuned to the opposite direction, leading to the illusion.
posted by Specklet at 2:43 PM on June 9, 2012

Or, check out this explanation.
posted by Specklet at 2:49 PM on June 9, 2012

OK, that's nicely laymanlier, thanks.

Only: why would we be wired for balance in our cell-tuning (and thus suffer this kind of temporary upset of the balance)? What's advantageous (in normal circumstances) about such a set-up?
posted by progosk at 2:49 PM on June 9, 2012

Unless I'm missing something, all these links are talking about illusory motion in the opposite direction, not the contraction you report, progosk.

Can you elaborate?
posted by jamjam at 2:53 PM on June 9, 2012

Erm... yes, I noticed they only mention general upward correction after downward movement.

My spontaneous recollection was of there being an overall shrinking effect, so yes with upward movement, at least at the bottom, but not evenly distributed throughout the entire height of my field of vision. If I think more about it - apart from likely distorting the exact memory of what I saw with some conceptual overlays - I can't be entirely sure the top of the afterimage was effectively going down, or was perhaps just stationary, so with the point of contraction not actually central to the image, but more coinciding with the stationary top of the waterfall.

I'd have to make the trip back to the waterfall to check more exactly...
posted by progosk at 3:08 PM on June 9, 2012

I may see what you're getting at, but if I do it's not the same as what they're calling the waterfall effect, though it is a form of compensation.

Suppose you are watching a log come over the lip of the waterfall and go down.

Since you know its a log and isn't changing its dimensions, you see it as the same size all the way down-- but that may not be what your eye is reporting to your brain.

The log is actually really speeding up as it falls, and this may cause its effective image on your retina to lengthen, because you essentially register the upstream end at one moment and the downstream end at a subsequent moment as happening at the same time due to the log's acceleration, and this causes your retina to report to your brain that the log has gotten longer, but your brain knows better and applies a compensation to the retina's report which gets larger the farther down the waterfall you are looking.

Then when you look away at something else, and that thing would be a certain distance down the waterfall if you were looking at the fall, your brain still applies its compensation program, and you see the object as vertically contracted an appropriate amount for that distance down the fall.

That would be an amazingly sophisticated compensation, if that's what's happening.

To account for the fact that the vertical contraction was apparently maximal at the middle of the fall, well... my hands are a little tired from all the waving they just did, and I need to rest up for a bit.

Wait! Maybe toward the middle of such a long fall the water has reached terminal velocity, and since speed is now constant, the brain turns off the program.
posted by jamjam at 3:55 PM on June 9, 2012 [1 favorite]