In this classic optical illusion, called simultaneous brightness contrast, the two disk appears to have a different luminance while being actually the same as the surrounding frame. This is because the background gradient introduces a varying contrast when the disks move along it. This illusion is thought to originate from a low level in the visual system (retina or LGN) before the information from both eyes get combined in the visual cortex.

How do the properties of the disks affect the perception of this illusion? This widget allows you to play with some important parameters that affect the strength of the illusion, for example contrast, disk size or separation.

To learn more about this classical visual illusion, check this recent "Popular Mechanisms" article:

  Science Finally Explains This Classic Optical Illusion

References:

  Sinha et al (2020) Mechanisms underlying simultaneous brightness contrast: Early and innate . Vision Research 173:41–49

  Chevreul (1839) De la loi du contraste simultané des couleurs et de l'assortiment des objets colorés. Paris: Pitois-Levrault

Here is the math behind this stimulus:

  x1 = (side/2)*cos(2*pi*time/4)
  y1 = separation/2
  x2 = -(side/2)*cos(2*pi*time/4)
  y2 = -separation/2
  r1 = sqrt(pow(x-x1,2)+pow(y-y1,2))
  r2 = sqrt(pow(x-x2,2)+pow(y-y2,2))
  disks = r1<radius | r2<radius
  frame = abs(x)<side/2 & abs(y)<side/2
  z = frame*(1-disks)*x/(side/2)

The whole stimulus is generated in real-time using a GLSL shader that runs right inside your WebGL-compatible browser. The plain Math behind the stimulus was converted to this optimized GLSL shader using the new Psykinematix Pro Edition. Translation to Matlab and Python code is also possible !

This whole widget was also fully generated using Psykinematix Pro Edition. The parameters that control the stimulus properties through the sliders are the same as the ones you would define as dependent or independent variables when using the stimulus in an actual psychophysical experiment run in Psykinematix. The widget creation is otherwise fully customizable with your own logo, copyright, links, etc.

To learn more about the widget creation, click on the above "Made With" button !