Visual filling-in and fading

Fix your gaze on the red dot at the centre of this figure. After a while, the blue circle gradually “dissolves” and is replaced by the white colour of the background. This phenomenon of visual fading or filling-in reveals another aspect of our perception.

Visual filling-in is one of the mechanisms that completes the parts of an object that remain hidden and, for example, prevents us from being aware of our physiological blind spot—at the position where the optic disc is located—or of other scotomas.

The fading of the ring in the first figure is known as the Troxler effect and is the basis of very popular internet illusions such as the “lilac chaser” (we cannot reproduce it here as it is a moving figure, but see, for example: https://es.wikipedia.org/wiki/Perseguidor_del_lila).

There are static versions such as the following, created by Professor Kitaoka:

Troxler fading (or Troxler bleaching) is only one example of visual filling-in. If we fix our gaze (preferably covering one eye) on the central point in the disc above, the entire grey area surrounding it will also gradually “disappear”; alternatively, it may be the central spot itself that does so (in the green disc).

This phenomenon has been explained at a basic physiological level as the result of adaptation or “fatigue” of retinal neurones in response to a constant and unmoving stimulus, which therefore tends to fade. However, this does not explain why it is replaced by the colour of the background. Other theories invoke mechanisms operating in the cerebral cortex, whether at a low level (isomorphic filling-in) or at a higher level (symbolic filling-in), a question that remains open to debate.

This phenomenon has been explained at a basic physiological level as the result of adaptation or “fatigue” of retinal neurones in response to a constant and unmoving stimulus, which therefore tends to fade. However, this does not explain why it is replaced by the colour of the background. Other theories invoke mechanisms operating in the cerebral cortex, whether at a low level (isomorphic filling-in) or at a higher level (symbolic filling-in), a question that remains open to debate.

Professor Rafael I. Barraquer, medical director of the Barraquer Ophthalmology Centre