Sorry for the longish question. To clarify in advance, I'm not medically concerned, just scientifically curious.

After a long day spent reading various pieces of text on a computer, I noticed that I was perceiving some sort of "after-images" whenever I looked at relatively plain white surfaces (e.g. paper, painted wall, porcelain sink, fridge door... later I noticed the same images with my eyes closed). The "after-images" very strongly resembled faint grey text, apparently consisting of letterlike symbols arranged into wordlike patterns in parallel lines and even paragraph- or column-like structures, with a range of different "font" sizes and orientations, but all close to horizontal.

Whenever I tried to focus on some particular part of the after-images, a horizontally-scrolling line or lines of "text" (in a relatively small "font") would appear in a roughly rectangular area in the centre of my field of vision. The scrolling was always from right to left, as though I was reading the text from left to right. This effect made it overall difficult to try to "read" what any of the afterimages "said", and they were at any rate continually changing, but I did occasionally get the barely-conscious, perhaps-imagined fleeting impression or sensation of particular words or part-words, often outside the position my eyes were actually "focused on".

Closing one eye and then the other led the images to become slightly less distinct, especially in more peripheral parts of my field of vision, but with a sense of fast scrolling upwards of the "text", as though I were quickly scrolling down through it on a computer display. On reopening both eyes, I found that I continued to notice this upward-scrolling effect, and with some effort of attention could induce or reduce its prominence.

Whilst writing these observations down, I was aware of the implausibility of literal visual after-images providing an explanation for this phenomenon, due to the dynamic content of the images. It occurred to me that in some ways they seemed to more closely resemble a visual form of the "Tetris effect" commonly associated with long periods of repetitive activitity. But I've never before associated the Tetris effect with "lexical" images, especially such visually clear ones. Has anything like this been seen or studied before? What mechanism might explain it?

I would guess this is something akin to the after effect of looking at a very bright light which in effect bleaches that portion of the retina. It is a not uncommon effect when staring at one image for a very long time. Usually most people don't see it since they don't look at a white background soon after. I would also expect that being tired may also exacerbate it. Either way this should disappear after 15-30 minutes and certainly should be gone by the next morning.

If you are in any way concerned about your vision you should consult an optometrist in the first instance.