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I’ve wondered about this for quite a long time, but never thought to ask it anywhere:
I’m assuming most people know what it’s like to watch a bird flying by, you can steadily concentrate as the bird flies by, and it seems your eyes move at a constant smooth rate.
As opposed to watching a bird or any object or thing, I can’t seem to use my eyes to sweep across the scene if nothing is moving.
Why can’t I gaze around at a constant rate when I have nothing to watch?
Short answer
Smooth-pursuit eye movements require something to pursuit. Intentionally attempting to make such an eye movement in the absence of an appropriate moving visual target will
result in a saccade instead.
Background
Your question concerns two types of basic eye movements, namely saccades and smooth-pursuit eye movements.
Saccades are rapid movements of the eyes to change the point of fixation. They occur for instance during reading and while gazing across a room. Saccades can be made voluntarily or involuntary (Purves, 2001).
Smooth pursuit movements are slower eye movements that keep a moving stimulus focused on the retina. One can voluntarily decide whether to follow a moving stimulus or not. However, making a smooth-pursuit movement in the absence of a moving stimulus is next to impossible for most people. Instead, when attempting to do this, a saccade is made instead. Performing a smooth-pursuit movement in the absence of a moving target seems to be possible, but requires a lot of training (Purves, 2001).
