You are looking at two curtains. At the center of the curtain on the left is a circular region that looks like a shadow; at the center of the curtain on the right is a circular region that looks like a spotlight. As would be expected from our understanding of the real world, the region in the shadow looks darker than the region in the spotlight.
But this is so only when the disks are vertically oriented. Actually, the two disks are physically the same.
Move the slider to rotate the disks. When the disks are turned, they no longer “line up” with the background, and as a result, they no longer appear to be a shadow and spotlight. In this view, the disks appear to be filled with the same gradient pattern (and actually are).
The disks can be moved. You can get a similar effect by clicking on and dragging a disk to place it next to the other, on the same background.
Brief Comments: One of my favorite places on the web is The Situationist, a blog that explores how the “situation” (or context) affects interpretation. The site has numerous examples of how objects, people, and events in one context are interpreted differently from the same objects, people, and events placed in another context.
The visual display above presents an example of the effects of the visual “situation.” In one situation (vertical orientation for the disks), the viewer interprets the disks with reference to the background context (i.e., the two curtains). One disk looks like a shadow on the curtain, and the other looks like a spotlight. The disks are therefore interpreted as a dark spot and a lighter spot on the curtains. In another situation (horizontal orientation), the viewer is able to separate the disks from the context of the curtains and therefore will identify the disks as having the same shading.
From my perspective, the most surprising aspect of this display is that the spots look the same when oriented horizontally. Why should it be surprising that the disks look the same? Consider the relation of this visual display to an old and well known situational effect: “simultaneous contrast.” In a simultaneous contrast display, a gray patch appears brighter when placed against a dark background than when placed against a bright background. In the visual display above, the left curtain is brighter than the right curtain, and so it makes sense that the left disk appears darker than the right disk when the disks are oriented vertically. But when the disks are oriented horizontally, it is as if one situational effect – simultaneous contrast – disappears in the presence of a different situational effect (orientation).
Why does the effect of orientation apparently supersede the effect of simultaneous contrast when we interpret the appearance of the disks? The illusion above is a response to research conducted by Bart Anderson and Jonathan Winawer (here is a link to their 2005 Nature article, and to Bart Anderson’s demonstration webpage). Please read their article to learn the details of their interpretations.
My interpretation differs from theirs. I will write more about interpretations of simultaneous contrast and orientation in future posts.
Visual scientists have argued for more than a century about the causes of simultaneous contrast. If you are curious about this topic, here is a link to a recent book by Alan Gilchrist that gives a history of the research into lightness illusions.