Is it a bird? Is it a plane? Is it your mind playing games?
Our brain works primarily by focusing our attention on stimuli that interests us. It does this via a filtering mechanism. This allows us to tune out conversations or look for specific colors in a crowded room. Other processes also operate similarly by filtering out information our brain deprioritizes without our awareness. In cases like this, our priority is primarily focused on survival instincts like movement and brightness. “If something is moving, it’s often fairly important to your survival.”
The eye is constantly making small movements and taking in our surroundings, but our visual system has to blur any background jitter, otherwise, we wouldn't experience the world as we do. It is easy for us to perceive the movement of small objects, but when made bigger it becomes more difficult to detect.
Scientists have found an explanation as to why this occurs. The brain prioritizes the detection of movement important for survival, which tends to be smaller. The brain suppresses the movement of backgrounds, which makes it more difficult to perceive the movement of large objects, so it learns to block it out- like a mouse running through a field who doesn't notice the motion of the grass and trees.
In a training exercise conducted on a group of adults, it was determined that older participants had more difficulty spotting small moving objects against a moving background. It was believed that with practice, the older participants gradually became better at spotting this type of motion. The scientists soon realized that even though there seemed to be improvement, the participant's ability to detect small moving objects was still impaired. The improvement occurred due to the fact that the subjects were less distracted, but had gotten worse at detecting larger movements. Our sensitivity is lower because our brain uses a strategy to make objects being perceived stand out more while blurring out the rest.