This optical illusion deceives you.

How come we sometimes perceive things that aren't? We love stories of malevolent ghosts and spirits, therefore we presume there's another world. One optical illusion may be explained by Troxler's Fading or Effect, a strange psychological phenomenon.

In 1804, Swiss physician and philosopher Ignaz Paul Vital Troxler (1780-1866) discovered this phenomenon. This perception trick describes what happens when you focus on one visual spot. No need to be long—10 seconds will do. Images and colors may fade from your peripheral vision.

See the black cross in the center of the image to fade the spots in this “lilac chaser” illusion in a few seconds. Unless you see a moving blue-green point, the cross and grey background will remain. After looking away, you may notice several green specks.

Research suggests the effect is connected to how the visual system adapts neurons that perceive stimuli. Our mind will replace unchanging stimuli with background information (or color).

How Does It Work ?

 Saccades—involuntary eye movements even while the gaze is settled—are associated to “sensory fading” or “filling-in”. The Illusions Index states that the “local neural adaptation of the rods,

 cones and ganglion cells in the retina” causes a motionless image or scene to fade in a few seconds if we dwell on a point.

The impact is stronger with low-contrast or fuzzy stimulus images.

Studies demonstrated the impact occurs in the eyes and brain, but there's no clear explanation for this odd visual experience.

Weekend box office: Ghostbusters: Frozen Empire opens with $45 million