In a sense the visual system also 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 after image. 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 upper left disappear as they stared at the black dot. Most people would not see the smudge disappear in the upper right.
In the upper left, the darker area slowly becomes lighter as one moves away from the black dot. This gradual change from black 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 the smudge should fade away and the color of the background would predominate.
The upper right figure is exactly the same as the upper 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 upper left 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 the upper 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 lower left will respond to even the very small eye tremors thus keeping the image "alive."
Disappearing smudge
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