This is because when your brain is faced with a lot of stimulation it will tune out the non-relevant parts.
It’s called the Troxler effect and the mind fills in the area where the circle used to be with the white that surrounds it.
Troxler Effect: “Troxler’s fading or Troxler’s effect is a phenomenon of visual perception. When one fixates a particular point, after about 20 seconds or so, a stimulus away from the fixation point, in peripheral vision, will fade away and disappear. The effect is enhanced if the stimulus is small, is of low contrast or equiluminant, or is blurred. The effect is enhanced the further the stimulus is away from the fixation point.
Troxler’s fading was discovered by Ignaz Paul Vital Troxler in 1804.
Troxler’s fading has been attributed to adaptation of neurons in the visual system vital for perceiving a stimulus. It is part of the general principle in sensory systems that an unvarying stimulus soon disappears from our awareness. For example, if a small piece of paper is dropped on the inside of one’s forearm, it is felt for a few seconds, then the sensation is no longer present; this is because the tactile neurons have adapted. But if one jiggles one’s arm up and down, giving varying stimulation, one continues to feel the paper until it falls off one’s arm.
A similar fading can be seen of a fixated stimulus when its retinal image is made stationary on the retina, a stabilized retinal image. Stabilization can be done in at least three ways. First one can mount a tiny projector on a contact lens. The projector shines an image into the eye. As the eye moves, the contact lens moves with it, so the image is always projected onto the same part of the retina. Second, one can monitor eye movements and move the stimulus to cancel the eye movements. Third, and this is the technique most people will know, one can induce an afterimage, usually by an intense, brief flash, such as when one is photographed using a photographic flash. This causes an image to be bleached onto the retina by the strong adaptation of the rods and cones. In all these cases, the stimulus fades away after a short time and disappears.
Troxler’s fading can occur without any extraordinary stabilization of the retinal image in peripheral vision because the neurons in the visual system beyond the rods and cones have large receptive fields. This means that the small, involuntary eye movements made when fixating something fail to move the stimulus onto a new cell’s receptive field, giving unvarying stimulation.”