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Hall of Illusions

 Filling-in Illusion

 What to do.

      Look at the figure on the left. There is a small black dot  with a dark smudge around it. Stare at the black dot without moving your eyes. The smudge will miraculously disappear!

      Try the same experiment again with the smudge on the right. This time the smudge does not disappear.

     What is going on here? Why does the smudge disappear in one instance and not the other?

What's Going On?

     The visual system  does not like steady state stimulation. There is sophisticated apparatus that allows you to view a stimulus in such a way to nullify your natural eye movements so that the image of the stimulus remains on exactly the same part of the retina as if there were no eye movements. Such apparatus is called a stabilized image system.

     You can simulate this effect with an afterimage. If you stare at a strong stimulus for about a minute and then look away you will continue to see an after image of the stimulus for some seconds and then it will disappear. But for a time you can bring it back to life by blinking. The blinking brings a temporal factor to the otherwise stabilized retinal image and it becomes visible for a while.

     Now to the disappearing disk. Most people would see the smudge in the left disappear as they stared at the black dot. Most people would not see the smudge disappear in the figure on the right. In the left figure, the darker area slowly becomes lighter as one moves away from the black dot. This gradual change from gray to white is a poor stimulus for sustaining visual perception. However, if you allow your eyes to freely move over the stimulus the perception of it will be sustained. When you fixate on the black dot and try and hold your gaze as steady as possible then the smudge should fade away and the color of the background would predominate.

     The figure on the right is exactly the same as the left except for the dark gray ring. This dark gray ring is sufficient to keep the stimulus "alive" no matter how hard you stare. When you fixate the black dot and try to hold your gaze as steady as possible, your eyes are still in constant motion. True, many of these eye movements are very tiny tremors as opposed to the large saccades or pursuit eye movements we make. Nevertheless these small tremors can keep a stimulus "alive". When the stimulus is one as in the left figure where there is a very gradual change from gray to white, the change in stimulation is so slight as to approach that encountered by the steady state condition of a stabilized image. As a result the image fades. You will undoubtedly have noticed that even when you fixated on the left field and the smudge disappeared, it would spontaneously reappear and then again fade. It reappeared because you made a large enough eye movement. The dark gray ring in the figure on the right will respond to even the very small eye tremors thus keeping the image "alive."

     Our kind thanks go to professor Peter Kaiser of York University for this demonstration and explanation.

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