Take a look at these pictures.

Each picture depicts four shapes -- irregular vertical columns spanning the height of the picture. It's easy to tell which letter is on a column and which is not, right? If our readers are typical, over 90 percent would agree that a is on a column and b is not. But why? The space defined by the irregular vertical lines is equal in both cases. The only difference between the two figures is which direction the "pointy" curves face and which direction the convex, "smooth" curves face. Yet nearly everyone agrees that areas defined by the convex curves (like those surrounding a above) are shapes, and other areas are background.

This principle, of convex curves denoting "shapes" and not "background," has been known for decades. It's one of dozens of Gestalt rules for determining what parts of the things we see go together to form shapes and what constitutes the background -- what's the figure and what's the ground. These rules can also depend on what we're interested in. Consider the view out my office window:

I might be looking at this scene because I want to turn on my lamp, in which case I'd be primarily interested in separating the shape of the lamp from my window and the trees outside. But I might want to know whether it's raining, so I'd be more interested in what's outside the window. Or I might be thinking about buying new blinds for the window, in which case I'd be looking at the slats of my blinds and how much light they do or don't allow in from outside.

It's a complicated picture, and our visual system easily breaks it into its components based on rules that have been sorted out by visual psychologists over the past century. But what is less certain is exactly how the visual system applies these rules. Do we have to be consciously thinking about what part of the picture we'd like to see? Or are some of these rules automatically applied, without us even noticing?

Ruth Kimchi and Mar