Here's a test.
1. Make sure the image below fills your computer screen.
2. Look at the image at a distance of 8-12 inches or 20-30 centimeters from the screen.
3. Stare at the black dot in the middle of the white star for 30 seconds. Keep your focus on the black dot or the test will not work.
4. After 30 seconds, shift your focus to the black dot in the middle of the white rectangle. Once again, you must focus, you absolutely must hold your focus on the black dot in the middle of the white square after the 30 seconds pass, or this will not work.
What did you see?
Did you see a grey star on the white square.? If you didn't, reread the instructions, and take the test again. Make sure you are close enough to the image.
This time the issue is color contrast. The difference between white and black creates excessive activity in the eye's ability to adjust to dark and light - and this fatigues the eye. The same thing happens when you try to read white papers on a black or dark desk.