Eye movements continuously jitter retinal images
When we use eye movements to scan a scene, as illustrated schematically in the left image above, the input stream of images to the visual system from the retina is continuously disrupted. The result is analogous to a continuously jittering video stream entering the visual system (movie above on the right; click on the right image). As can be seen, the image entering the visual system continuously moves even though the actual scene itself is perfectly stable. This highlights the fact that eye movements are associated with a major challenge to the visual system: the brain must employ mechanisms that ensure perceptual stability in the face of continuous image motion due to continuous eye movements.