# Delboeuf illusion

Figure 1. Delboeuf illusion. The full black circles are the same size.

The Delboeuf illusion is a geometrical- optical illusion named after the Swiss mathematician and psychologist Joseph Remi Leopold Delboeuf (1831-1896) , in which two concentric circles influence each other in such a way that they approach each other in perceived size at similar diameters unequal diameters, however, the opposite occurs.

Figure 2. In these examples the red circle is always the same size. The numbers indicate the respective framing ratio.

## observation

In the Delboeuf illusion, two concentric circles seem to change their relative size: if they differ by a fraction of their diameter, then they approximate in their perceived size (assimilation). As can be seen in Figure 2, the maximum of the illusion is at a framing ratio of around 1.43. The frame ratio is the outer diameter of the black ring divided by the outer diameter of the inner or red circle. If the outer circle becomes significantly larger, then the inner circle appears to shrink again (contrast effect).

## Attempts at interpretation

• If the rings differ only slightly in diameter, then their signals in the visual system are also very similar and can influence one another. So there are at synesthetes a crosstalk (also crosstalk ) between sensory channels or their processing centers. Context effects can also influence a target in other visual perception illusions.
• If the concentric circles are very different in diameter, then the effect of size constancy can play a role: A single object in focus is not perceived in its size strictly proportional to the retinal image: Small objects are enlarged, large objects appear - relatively to this - smaller. This applies not only to a moving object, which does not become smaller as quickly as the retinal image, but also to objects of different sizes at the same distance, as long as only one is in the field of view. A small circle, viewed on its own, appears enlarged. However, if you look at a very large concentric outer circle, it does not appear larger to the same extent. Its perception scale then applies, also for the small inner circle. This is, as it were, compressed a little and then appears smaller than on its own.

## Comparable deceptions

In the Ebbinghaus illusion , too , the larger the ring of the surrounding circles, the smaller the circle appears.