I get _strong_ twisting from http://www.math.brown.edu/~rkenyon/gallery/squarespiral.gif (vs squarespiral2), and ascribed it to differential fatigue of my motion receptors. (Generalized moving bar illusion.) On 2017-06-01 09:21, Fred Lunnon wrote:

At second attempt, I did detect a slight twisting effect.

I imagine this is the same mechanism that causes the room apparently to rotate in the reverse direction after one suddenly ceases spinning about a vertical axis (how do ballerinas cope, I wonder?); or the floor to roll and pitch after one disembarks from a small boat.

For many hrs after a local (1989, Richter 7) quake, the ground felt gelatinous. I assumed it was objectively measurable seismicity. Perhaps not. --rwg

I doubt whether retinal cell configuration has any part to play in such effects, which are manifestations of a general mental process, occurring at many levels, which seeks to filter out constant background from our perception, leaving us conscious only of any unpredictable variation.

I once encountered a spectacular example of this phenomenon, involving hearing rather than sight. Standing beside the Victorian wall clock in

the

hall of my home late one quiet night, I failed to hear its customary loud tick, and concluded that it must have broken down. Visual inspection however revealed that the pendulum continued to swing; and as I stared at it in some perplexity, over several seconds its tick gradually faded in to become once more clearly perceptible.

Similarly as Sarah Silverman recently pointed out, it is impossible to register one's own body odour, whatever olfactory violence one may inadvertantly be perpetrating upon one's neighbours. The distinctive feature of sight in this context is that relative motion possesses sign, making its (sudden) absence more directly contrasted with presence.

Fred Lunnon

On 6/1/17, Bill Gosper <billgosper@gmail.com> wrote:

...

What's supposed to happen after 20 sec?

You're supposed to quickly redirect your stare to top left of your screen.

(That's my sixties cred shot to pieces.) WFL

Or just your retinal cone cells. If you do not see disturbing, startling twisting,

consider an ophthalmological consult. --Bill

On 5/31/17, Bill Gosper <billgosper@gmail.com> wrote:

On 2017-05-29 10:55, Stuart Anderson wrote:

To make the tiles really dance ...

Danny Caligari

;

https://www.google.com.au/amp/s/lamington.wordpress.com/2013/01/13/kenyons-s...

...

Bad URL there. Use

http://www.math.brown.edu/~rkenyon/gallery/squarespiral2.gif

While you're there, stare at

http://www.math.brown.edu/~rkenyon/gallery/squarespiral.gif

for ~20 secs, then the top corner of your screen. --rwg