Your brain cells are sometimes like hyperactive children that get bored quickly. For instance, without a constant change of scenery, neurons in your visual system will literally tune out incoming information in search of fresh stimulation.
Experiment 1 Fix your gaze on the black "eye" in the center of Figure A for 20 seconds, while trying not to blink. After six or seven seconds, the dotted circle will begin to fade in and out of your awareness.
Called Troxler fading, for Ignaz Paul Vital Troxler, a Swiss scientist who described the phenomenon in 1804, the effect is believed to be caused by a rapid decrease in the eye's response to images that are stabilized on the retina. Notice that if you scan your eyes back and forth across the black eye, causing slight shifts in the images on your retinas, the dotted circle will not vanish.
You might wonder why the fatigue that makes the dotted circle disappear when your gaze is stabilized doesn't also cause the eye in the center of the figure to fade. The most likely explanation is that even when your eyes are locked on a fixed point, the stability of retinal images is affected by involuntary micro-movements in your eye muscles. Relatively large voluntary eye movements prevent the fading of the dotted circle at the periphery of your gaze, where visual details must be large to be noticed. In contrast, tiny, involuntary eye movements block fatigue in the center of your gaze, where very small objects are visible. Observe that when you stare directly at one part of the dotted circle, positioning it in the center of your vision, where very small details can be resolved, that this segment of the circle does not fade.
Experiment 2 The previous test supplied indirect evidence that micro eye movements exist, but direct evidence is hard to come by because the sensors that must detect such movements—your eyes—are the same sensors that are constantly being repositioned by the micro-movements. It's like trying to catch your eyes moving in the mirror. Under very special circumstances, however, micro-movements can be coaxed out into the open.
Stare at the red dot in Figure B for 15 seconds, then quickly shift your gaze to the blue dot. The negative afterimage of the black-and-white grid will jump around relative to the blue dot. Because the negative afterimage is perfectly stabilized on your retina, any motion of this afterimage relative to the live image of the blue dot must be caused by small movements of your eyes.
These micro-movements may simply be, in part, an unavoidable consequence of the background noise inherent in the human neuromuscular control system. But without these seemingly involuntary movements, nothing you lay your eyes upon would ever be clear.
Surf to a Web site dedicated exclusively to the work and life of Ignaz Troxler: www.troxlerforum.ch.