Both eyes usually move together because they work in concert to provide a single, unified visual experience. This is known as binocular vision, and it allows for depth perception and a more comprehensive view of the environment. The reason both eyes move in sync is due to the coordinated actions of several eye muscles and the neural pathways that control them.
Here’s a simplified explanation of how it works:

1. Optic Nerve: Each eye has its own optic nerve that sends visual information to the brain. However, the brain combines these two streams of information to create a single, three-dimensional perception of the world.
2. Eye Muscles: The movement of the eyes is controlled by six external muscles—four recti muscles (two for each eye) that control horizontal and vertical movement, and two oblique muscles (one for each eye) that control tilt movements. These muscles work in pairs, with one muscle from each eye contracting to move the eye in a particular direction.
3. Neural Pathways: The coordination of eye movements is managed by several neural pathways, including the oculomotor pathway, which originates in the brainstem and controls most of the eye muscles. The frontal eye fields in the brain also play a role in coordinating eye movements.
4. Convergence: When looking at a close object, the eyes must work together to focus on it. This coordination of focusing is called convergence, and it is controlled by the brain to ensure that both eyes are looking at the same point.
5. Stereopsis: Binocular vision allows for stereopsis, the ability to perceive depth and distance. This is possible because each eye has a slightly different view of the world, and the brain compares these two views to create a sense of depth. For this to work effectively, both eyes must be aligned and move together.
6. Equatorial forces: When the eyes move, the lens is pressed against the eye’s equator. If the eyes did not move together, the lens would not be able to focus properly, leading to blurred vision.

In summary, the synchronization of eye movements is essential for accurate and coherent vision. It allows us to perceive the world in three dimensions and to have a stable and focused visual field. While it’s possible for the eyes to move independently of each other under certain circumstances (such as when looking in different directions or when one eye is occluded), the default mode is for both eyes to work together seamlessly.