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I was trying to do some stimulation for SSVEPs by strobing a solid black square and a solid white square. Weird thing was, I ended up seeing weird grid like patterns that looked like this, always of the inverted color.

  _        _
 / \      / \
 | |      | |
 \ / _--_ \ / _--_
  O |    | O |    |
 / \ ---- / \ ----
 | |      | |
 \_/ _--_ \_/ _--_
  O |    | O |    |
     ----     ----

And that sort of pattern repeats itself around the strobing area (where the convex areas are always of opposite color than the current color on the screen).

I'm inclined to think about light and dark adaptation, but it still seems weird that it'd be shaped like that in particular. So... What's going on?

EDIT: Additional info that I should've realized was necessary >_<:

  • Strobe area: ~14x10.5 cm

  • Strobe frequency: 2 frames/half-cycle @ Laptop Vsync (~60 Hz) ~ 15 Hz

  • Viewing distance: ~55-60 cm and shorter.

Screen is backlit. Not sure about brightness metrics.

The pattern isn't offset to one corner like I've drawn, it's more evenly spaced. Horizontal bars are more accentuated than vertical ones (and the dark-on-light phenomenon is more visible for me than the light-on-dark). Moving in closer increases the visibility of the effect (I guess that was to be expected), and changes the number of times the pattern repeats across the screen (but it's hard to count them). The larger the area in my visual field the more times the pattern repeats. I'd say that at about 60 cm it's just one box, but with my nose a handful of cm away from the screen (it's hard to measure by myself >_<), there are maybe three in the horizontal direction... Or at least tens of them (increasingly large as it gets toward the center in both dimensions), depending on how focused in on the screen I am.

It's really really hard to count them.

I haven't had the chance to check with other people yet, but I'll try some time (this is just something I saw while working on a personal project).

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Do other people see the same pattern as well? How big was the square and how fast was it presented? What's the spatial scale of the pattern? –  Memming Aug 11 '13 at 14:02
1  
@Memming Other people see the patterns as well, although not necessarily the same patterns I'm seeing. –  user Oct 21 '13 at 4:47
2  
It just occurred to me that cogsci.SE might have more psychophysics people. –  Memming Oct 21 '13 at 14:57
1  
I have actually experienced something similar before. What are you using to make the strobe? You can run into interesting patterns when you start strobing near the frequencies of your monitor (flashing refresh down as it's drawing). I'd also make sure it's not a programming effect. What is the difference between what you see and what other's see? –  Atl LED Oct 21 '13 at 15:33
1  
@AtlLED Oh, sorry - I can assure you that the screen is solid black and solid white (no vsync issues), and can additionally reference you to the underlying low-level code (SDL2's vsync-waiting update and GTK's GdkFrameClock for two different programs). Others have reported seeing the phenomenon, albeit with different patterns (they aren't necessarily consistent between people looking at the same screen; I have a really small sample size, though). –  user Oct 28 '13 at 22:00
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