|Martin Rolfs et al, from Nature|
These saccades happen largely unconsciously, about three times each second. Before each saccade, our visual system anticipates where it's going to jump next. A small part of our attention is distributed to possible future targets.
The act of anticipation helps stabilize the image when it appears after the jump. During the moment of anticipation, the brain is beginning to form "theories" of what it's going to see.
Artists creating optical illusions enjoy experimenting with this dynamic of our perception, playing our fovea (the detailed center of visual attention) against our peripheral retina.
How does this affect us as picture makers? It's important to know that the viewer is interpreting your image not only with their fovea , but also with their peripheral attention, which responds to broader cues. That's why it's so important to step back, squint, look in the mirror, and try to regard your composition from a distance, so that it will read well peripherally, too.
Science Friday video about Martin Rolfs' work