Accidental illusion

NOTE: You can see a webapp I put together to investigate this in more detail here.

While creating two-dimensional sinusoidal gratings and investigating their local properties, I accidentally created an illusion today, which I find pretty strong (though the strength of it does depend on its size and your distance to it).

I created the grating shown below:

and was dropping down some circular windows on it to see if I could recreate the same image by making a bunch of small sinusoids and combining them. What I ended up seeing was:

To me, it looks like there's an illusory grating that's perpendicular to the actual one, but if you focus on the location of those supposed stripes, putting them in the center of your gaze, they disappear. This is pretty strong to me, even overwhelming the original grating unless I focus directly on it. It changes with size / distance though, so that if I make it really small or stand far away, it looks more cross-hatched, a combination of the two patterns, than either one alone.

I'm not entirely sure why this is happening (and, discussing it with the lab, neither is anyone else). It seems similar to the following classic illusion, where the Xs in either side of the image look like their yellow / blue, respectively, but are actually the same color.

A similar illusion is shown below, where the two squares are the same grayscale level, but appear to be different.

In all of these, the illusion relies on the fact that our perception of brightness and color of an object is influenced by the contrast with the stuff around that object and not just the object itself. This relates to a recurring theme in vision and perception more generally: we are not just passively perceiving a perfect copy of the world, our sensory systems perform computation on our input, which highlights some pieces of information and throws away other pieces. This happens so fast and automatically that we often don't realize it's happening until we see illusions like the above, which make it obvious.