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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?

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  • $\begingroup$ Related: What degree of control do we have on eye movements? $\endgroup$ – Arnon Weinberg Oct 28 at 2:14
  • $\begingroup$ @ArnonWeinberg indeed related, I was thinking about posting this on medical sciences SE, but I have to wait a bit... I’m looking more for the reason why saccades usually happen when nothing is there to watch move smoothly. $\endgroup$ – Cotton Headed Ninnymuggins Oct 28 at 2:18
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    $\begingroup$ Cross-posting is discouraged across the SE network. FYI: it is off-topic on Health (and indeed closed over there) as it is a Neuroscientific question. $\endgroup$ – AliceD Oct 28 at 8:17
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    $\begingroup$ "Why" questions can be difficult if not impossible to answer. I feel like the Q&A Arnon linked to does a pretty good job of explaining that there are two systems for changing eye gaze: a saccadic one and a smooth pursuit one. So at one level the answer is "because smooth pursuit only works to track a moving object and you aren't tracking a moving object so you use the other one." If you want to know "why the system was built this way" we have at Biology.SE biology.stackexchange.com/questions/35532/… $\endgroup$ – Bryan Krause Oct 28 at 16:09
  • $\begingroup$ The reason it is so difficult to smoothly move your gaze without having a moving object to focus on is that people have a very strong tendency to “foveate.” Your fovea is the part of your retina that has the best visual acuity for details. When you look at an object, you direct your fovea to that object. When you try to cast your gaze smoothly across the room, your eye makes frequent, short stops when your fovea “locks onto” an object before moving on. $\endgroup$ – tombk Oct 31 at 21:38
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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).

Reference
- Purves et al. (eds). Neuroscience, 2nd ed. Sunderland (MA): Sinauer Associates (2001). Types of Eye Movements and Their Functions

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    $\begingroup$ Not sure where I saw this but some people can close their eyes and move their finger back and forth in front of their eyes while smoothly tracing where they would see their finger $\endgroup$ – Cotton Headed Ninnymuggins Oct 29 at 21:08

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