Brightness induction

from Michael’s   Visual Phenomena & Optical Illusions


What to see

[A somewhat weak effect, mainly interesting to vision insiders.]
The above demo presents two aspects: (1) the classic simultaneous brightness induction, and (2) one part of an experiment from 2020 (Sinha et al.), demonstrating that its mechanism is local, probably retinal.

What to do

  1. After opening this page (or after Reset) you see a luminance gradient (light left, dark right) and two “probe” squares. Now fixate the red cross and ask yourself: do the two probes appear equally bright? Probably not, although they have the same luminance. You can try to adjust with the lowest slider until brightness is about equal. Now uncheck Outer and notice that on a homogenous background they are no longer equal.
    This is classic simultaneous brightness induction.

  2. Now press Reset and switch on the Inner rectangle, switch off the probes. Fixate the red cross and ask yourself: Is the inner rectangle homogenous along its length? It should be, roughly.
    When you are satisfied with the homogeneity, switch on probes again. Do they appear equally bright? To me the right one, again, is a little brighter, if less so then in (1).


I am using two terms here, brightness and luminance. Brightness means how bright something appears. Luminance is the technical term for “how many photons come out”. While usually brightness increases with luminance, the former depends on context in space and time, as shown here.

  1. Simultaneous brightness induction is thought to be caused by the contrast between the surround (Outer) and the probe. This contrast leads to inhibition, exaggerating any luminance difference, Enhancing small contrasts is thought to be an evolutionary advantage and has been found throughout the animal kingdom.

  2. This experiment suggests that the inhibitory mechanisms at work are not the cognitive level: The Inner bar is homogenous in brightness throughout. However, it is set to actually darker on the right side to counteract effect (1). Yet this “invisible” luminance gradient still affects the brightness of the probes. A likely conclusion is a retinal locus for the underlying processing. This comes with little surprise, but there are also experiments (using dichoptic presentation) suggesting that it is beyond the retina. Well, that’s typical in biology: If there are contradicting explanations, they may all be right – mixed, or along the various stages of the visual pathway.


Sinha P, Crucilla S, Gandhi T, Rose D, Singh A, Ganesh S, Mathur U, Bex P (2020) Mechanisms underlying simultaneous brightness contrast: Early and innate. Vision Research 173:41–49

They also report this fascinating additional observation:

“…conventional simultaneous brightness contrast … [in] … congenitally blind children whom we were able to treat surgically [n=9, at age 8–14]. The results demonstrate an immediate susceptibility to the simultaneous brightness illusion after sight onset [testing within 48 h after surgery].”