Vertical motion clarity appears to be vastly better than horizontal motion clarity

[VMA]

I have no idea why this may be the case or whether it affects everyone.

How to try this for yourself:

Go to the UFO test page and either rotate the screen you are viewing on or the page contents with an add-on of your choice. Also try looking at the background, looking cross-eyed at the background, and looking at the FPS.

What I noticed:

As you start to rotate the screen, the UFO gets progressively clearer until it becomes the most clear when purely vertical. When rotating again past purely vertical it starts to become blurrier again until it is fully horizontal where it appears the blurriest, unless I am looking at the FPS.

What I initially thought was happening:

Perhaps somehow the way a progressive scan LCD refreshes influences the clarity unevenly in different directions, whether this is the case or not I am unsure but the same effect can be observed both by rotating the display relative to the content and the content itself which suggests this is not the case.

What I think is happening now:

It appears to be more natural to focus in the same way as you would when looking at a static element, without looking at a static element, when the scroll is vertical.

Questions:

[*]Why do you think this is happening?
[*]Does anyone else also experience it?