SUBJECTIVE CONTOURS Most contours that we can see are defined by specific boundaries, but occasionally, a contour will be apparent when in fact no contour was originally placed there. For example, in the illustration to the right, there is an apparent square overlapping the four dark circles. In actuality, only the four circles exist. Rather than appear as four unassociated circles, however, the visual system 'adds' a unifying element - the box.

Subjective contours are surprisingly robust illusions. They can occur under a variety of conditions, such as in the two examples below:

In the first of these examples, it is obvious that subjective contours are not restricted to simple, geometric figures. Instead, a strange 'wobbly' shape like the one on the left can also be produced. In addition, lines that impinge on the object do not seem to diminish its effectiveness. On the contrary, they seem to enhance the illusion of depth! In the image on the right, the opaque shape created by the subjective contour appears to be under or above the interfering lines. In other words, the visual system continues to interpret it as a realistic opaque object.
(for more information and additional images, see: Rock, Irvin (ed.). (1988). Readings from Scientific American: The Perceptual World, "Subjective Contours." New York: H. Freeman &Co. pp. 155-163.
Illusions of depth are easy to produce, particularly when two nearly identical images are combined. In the diagram below, you may be able to elicit the wallpaper effect from the pattern. In order to see it, move forward until you are practically touching the screen, and focus 'beyond' the pattern. An illusion of depth should be created, and if you move forward and backward slowly, the apparent depth should vary accordingly. (This illusion is not always the easiest to see, so don't be discouraged!)
Subjective contours can also create illusions of depth. In these cases, it appears that the subjective contour object is floating above the background. The image below provides some illusion of depth:

SUBJECTIVE CONTOUR IN MOTION This subjective contour is based on the same image as the one above. However, in this illustration, the 'box' moves between two sets of circles. In many ways, this is a much more effective illusion of depth than the stationary version.
This is a somewhat similar image. Here, the background is made up of a series of continuous lines, but a break in these lines is imposed by the visual system to create the appearance of a contour. While this is fairly effective in producing a subjective contour object, it is not as effective in terms of illusory depth as the earlier demonstration.

Modified from Ramachandran, V.S. and Cavanagh, P. (1985). "Subjective contours capture stereopsis," Nature. v.317. pp. 527-530.

SUBJECTIVE CONTOURS AND CHANGES IN APPARENT BRIGHTNESS In this image, the area just outside the inner section (apparently surrounded by the circle), appears to be grayer than the inner area. This apparent change in brightness is another interesting effect of subjective contours.