Your brain cells are sometimes like hyperactive children that get bored quickly. For instance, without a constant change of scenery, neurons in your visual system will literally tune out incoming information in search of fresh stimulation.
Fix your gaze on the black "eye" in the center of Figure A for 20 seconds, while trying not to blink. After six or seven seconds, the dotted circle will begin to fade in and out of your awareness.
Called Troxler fading, for Ignaz Paul Vital Troxler, a Swiss scientist who described the phenomenon in 1804, the effect is believed to be caused by a rapid decrease in the eye's response to images that are stabilized on the retina. Notice that if you scan your eyes back and forth across the black eye, causing slight shifts in the images on your retinas, the dotted circle will not vanish.
You might wonder why the fatigue ...
