Kanizsa has shown, with demonstrations such as the one presented here, that it is possible for us to perceive contours which, in fact, do not exist. Such contours are called illusory contours or subjective contours.
1.) Start by clicking on the 'Start Demo' button below. A new frame will pop up.
(Note: if you are using MS Internet Explorer or a PC version of Netscape, you
may want to resize your new frame. Specifically, adjust the frame until the
pac-men figures are approximately an equal distance apart in the horizontal
and vertical directions. Also, you will need to click your mouse somewhere inside
the new frame to get things rolling for either of these 2 platforms.)
2.) Look at the image. You should see 4 pac-men. By pressing the number '1' on your keyboard, gradually increase the diameter of the pac-men. As the pac-men grow closer together, you should, alternatively, be able to perceive a black square in front of and partially hiding 4 white disks. But notice that contour outlining the square is not physically present in the space between any 2 pac-men. Hence, such contour, when perceived, is illusory and subjective.
3.) How close do the pac-men have to be to one another for you to perceive the (illusory) square?
Play with this some more to find out:
Pressing '1' on your keyboard increases the diameter of the pac-men.
Pressing '0' on your keyboard decreases the diameter of the pac-men.
Pressing spacebar restarts the demo from its initial state.