The human brain has access to a massive amount of visual sensory data. Researchers have conducted some interesting studies to figure out how our brain decides what small window of that data we direct our attention to. The researchers determined that when we notice an object appearing, we pay attention to it for an extended period of time (1/3 of a second). But, when we notice an object disappearing we immediately direct our attention elsewhere.

[The appearing object effect] was first observed by Michael Posner — if an object appears in one part of our field of view, it temporarily delays our ability to detect another object appearing near it. The effect begins about a third of a second after the first object appears and lasts about a second. If the second object appears sooner than that, we actually notice it quicker. Subsequent research revealed that the effect became progressively smaller at greater and greater distances from the spot where the first object appeared — surprisingly, we’re quicker to spot other objects appearing farther away from the original object.

See Cognitive Daily for the full article, including the research on disappearing objects.