From Chapter 8: Eyes Must Move to See
Eyes Grasp the World with Saccadic Motion
The essential rapid flickering of eyes is called saccadic motion. In 1964 Russian biophysicist Alfred Yarbus demonstrated that if this motion stops, an "empty field" is created within one to three seconds. "Empty field" means no shapes, no colors; nothing is seen.
You can observe saccadic eye movements yourself. Find someone to get close to. Watch their eyeball from the side. As your specimen looks straight ahead you will see her eye making small jerky hops. This happens even when the person thinks she is "fixing" her...