You can demonstrate the existence of such cells in your own brain. If you stare continuously at the red dot on the right of (c), you will notice that after a few seconds the illusory rectangle fades even though you still see the bricks and Pacmen. The cells signaling the illusory edges are "fatigued" by the steady fixation, which hyperactivates them and depletes them of their chemical neurotransmitters. If you move your eyes, they reappear, because a new set of cells is recruited. Apparently these illusory contour cells are more easily fatigued than those signaling the real edges of the bricks and Pacmen.
Finally, take a look at (g), devised by Kanisza. It initially appears to be an opaque horizontal rectangle with holes through which you can see another (horizontal) rectangle. But with some effort, you can "imagine" it as a semitranslucent veil-like smaller rectangle sitting on top of the black holes (or disks) on the larger one, and suddenly you see the illusory contours "completing" gaps across the holes. Thus, the complex rules of image segmentation incorporating the physical laws of transparency can be "applied" to the scene. Cells in the earliest stages of visual processing may signal illusory edges, but top-down modulation based on visual attention can reject or accept the contours depending on overall consistency with the scene.
This article was originally published with the title The Reality of Illusory Contours.