On the sides of many aspalt roads, there is a row of bricks just before buildings. Since these are bricks, there are slits where one brick ends and another starts. As I rotate my head and try to make my eyes follow the movement, it never 100% follows the skull. Instead, my gaze is focused at some point - usually the slits - and as if my eyes where locked in that direction, even though I try to make my eyeballs follow my head, they will still be focused on that point for a short time. And after that, my eyes would suddenly jerk in the direction of my head movement, finding and settling at a new point to focus on. In short words, the eyes don't smoothly follow my head. Instead, their position of focus 'jumps'.
To explain this, I did research on small eye movements and decided that three kinds of movements would be needed to do so. Those are: a saccade, smooth pursuit, and vestibulo-ocular reflex.
Saccade
A saccade is a quick, jerking movement of the eyes. They happen when your eyes change the object of interest to focus on. Say you were looking at the 'O' key of the keyboard. When shifting your sight to, for example, 'R', your eyes will move suddenly to the left. This fast action is the saccade movement of the eyes. Saccades happen very quickly; one act of saccade lasts less than 200ms, and the eye can rotate at speeds up to 900 degrees per second.
Smooth Pursuit
Smooth pursuit is when you track a moving object and try to keep it in focus. Its speed is slower than saccades - only up to 100 degrees per second. In smooth pursuit movements, the eyes move smoothly, following the object of interest.
Vestibulo - ocular Reflex
Unlike the previous two movements, the vestibulo-ocular reflex occurs when the head rotates. When the head rotates, the semicircular canals in the vestibular system detects a flow of fluid - the endolymph. This results in a electrical messege through the nervous system and to the muscles around the eyeballs, making one side of them contract. The eyeballs are rotated in the opposite direction to the rotation of the head, allowing it to keep focus on an object.
Conclusion
After finding out these types of eye movements, I was able to deduce why my eyes would work like the way I mentioned above. Since my head, not the scene, is moving, smooth pursuit movement does not happen. Instead, due to the vestibulo-ocular reflex, my eyes keep their direction towards what they were focusing on. But since my head is keep moving, at some point, the 'lock' is released. Then the eyes would saccade to find a new point to focus on; resulting in jerky, rigid rotations of the eye.
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