Troxler effect

How it works

Nerve cells in the visual system respond most strongly to change. When the same light falls on the same spot of the retina for a long time, the cells there fire less and less. This is called neural adaptation². Normally you never notice it, because your eyes make tiny jumps called microsaccades many times a second. These jumps keep the image moving across the retina, so nothing stays constant for long¹.

The edge of the visual field has larger receptive fields than the centre. That means a small eye movement is not enough to move a blurry peripheral object onto fresh cells. When you fixate hard, the peripheral object stays on the same cells. They adapt, and the brain fills the area with the surrounding background. The object fades out¹.

The effect is strongest for low-contrast, blurry shapes. A sharp black dot on white is hard to fade. A soft grey smudge on grey fades in seconds¹.

The Lilac Chaser illusion combines Troxler fading with afterimages. Twelve blurred pink dots blink in a circle. After a few seconds of fixation the pink dots vanish and a single green dot runs around the circle¹.

Try it

1. Draw a small dot in the middle of a page. Around it, draw a few soft, blurry grey smudges. A pencil rubbed with a finger works.
2. Hold the page about 30 centimetres away.
3. Stare at the central dot. Do not move your eyes at all.
4. After 10 to 20 seconds the smudges fade into the white paper.
5. Move your eyes. They come straight back.

The same thing works with a real scene. Fixate on one point in a dim room and the edges of your vision go flat and grey.

History and culture

The effect is named after Ignaz Paul Vital Troxler, a Swiss doctor and philosopher who described it in 1804¹. It is the basis of the strange-face illusion, where a dim mirror and a fixed stare make your own face fade and distort.