Monday, May 19, 2008

Grouping by contrast

Here is an illusion from a brief article that I wrote with Kai Hamburger (“Grouping by contrast—figure-ground segregation is not necessarily fundamental,” Perception, 2007).


The five disks in the top row of the display all change gradually from black to gray to white and then abruptly back to black; in the bottom row, the five disks all change gradually from white to gray to black and then abruptly back to white.

Click on the “add/remove gradient” button to place gradient rectangles behind the disks. The switch in background from solid to gradient creates a dramatic change in the way the disks are perceived. The disks themselves have not changed, but now motion appears to sweep from right to left across each row. The perceived motion is similar – in some respects – to the perceived motion in the window shade illusion, since it follows the location of minimum contrast between the disks and the background.

There are many features about this shift in perception that we could talk about, but I would like to draw attention to what I think the display says about how we organize visual information. Notice that when the solid background is present, you tend to group the disks into two horizontal rows of five, but when the gradient backgrounds are present, you tend to track the motion and group the disks into five vertical columns of two disks each (so far, all viewers have automatically grouped the disks in these ways).

This shift in perceptual grouping is really unusual. When you group the disks into columns, you pair a white disk in one row with a black disk in the other row even though each of these disks has the same luminance level as the four other disks in its row. A horizontal grouping would seem like a reasonable way to organize the disks because – just like in the contrast asynchrony – the disks in each row become light and dark at the same time. However, it seems that the visual system prefers to group vertically because the disks in each column have similar contrast levels relative to their respective backgrounds.

In the article that I link to above, Kai Hamburger and I suggest that contrast-based grouping poses a bit of a puzzle. The Gestalt approach to visual perception proposes that the visual system organizes the world into simple perceptual units in accordance with well-known Gestalt laws (for example, similarity, symmetry, proximity, closure, common fate). Central to the Gestalt approach is the idea that the visual system organizes the world in terms of “Figure” and “Ground” (for an example, see Rubin’s face/vase illusion). Contrast information, however, cannot really be considered part of the Figure perceptual unit or the Ground perceptual unit because the contrast information cuts across the figure/ground border (the contrast information represents the luminance levels of the disks relative to the luminance levels of the background).

The illusion in this post illustrates a condition in which the visual system privileges contrast information over object similarity in order to organize the visual scene. At some level, this should not be surprising; a visual scene can be described in terms of a variety of stimulus dimensions (spatial scale, luminance, contrast, temporal changes, chromaticity, etc.). The visual system contains parallel neural channels, each of which responds to only a small range in a few of these dimensions. Presumably, the neural processes that organize the visual scene must do so by selecting a sub-population of the neural channels. However, since we generally think about the world in terms of objects that remain relatively stable regardless of context, it can be surprising to see the effect that contrast information can have on how we organize the visual scene.

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I was on the road last week, attending the Vision Sciences Society conference (sorry for the gap in the blog). In my last post, I mentioned the then upcoming, now past, best illusion of the year contest. Incredibly interesting new illusions were presented, and the event was a great success. My lab had two entries in the top ten, but (alas) our entries did not place 1st, 2nd, or 3rd this year. You can see the winning entries at this link. I also encourage you to visit the website of the major sponsor for the event: the Mind Science Foundation. They have many resources regarding consciousness that may be of interest to readers of this blog.

Next week: an illusion of spatial scale.

1 comment:

Albert Soler said...

After reading about this blog's winning entry on the 'The Break of a Curve Ball', (Which was absolutely fascinating!), for neuralcorrelate.com's Best Visual Illusion of the year (2009), I decided to check out the older posts and came across "Grouping By Contrast".

I wanted to point out that if I completely cover either row with thick cardboard, the illusion remains. In fact, if I cover up all circles except one, the illusion still remains! Without the gradient, a single circle appears to pulsate from the outer edge inwards concentrically towards the center. With the gradient, it looks somewhat like a model of the waxing/waning Moon -- appearing to move right to left.

 
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