Humans and many animals do not look at a scene in fixed steadiness; instead, the eyes move around, locating interesting parts of the scene and building up a mental, three-dimensional 'map' corresponding to the scene (as opposed to the graphical map of avians, that often relies upon detection of angular movement on the retina). When scanning immediate surroundings or reading, human eyes make jerky saccadic movements and stop several times, moving very quickly between each stop.