Every moment, the visual system processes enormous amounts of detail.
Edges, colors, shadows, movement, distances, and shapes all contribute to our understanding of the environment.
Yet we rarely experience those details individually.
Instead, we perceive complete objects and meaningful scenes.
When looking at a photograph of two people, for example, we usually recognize the people before consciously analyzing the position of each finger, elbow, or shoulder.
This ability is extraordinarily useful.
Without it, recognizing familiar environments would require an exhausting amount of deliberate attention.
However, the same efficiency that makes human vision so effective can occasionally lead to mistakes.
The brain relies on expectations.
It assumes that a visible hand belongs to a nearby arm, that a particular arm connects to a corresponding shoulder, and that the bodies of two people occupy separate physical positions even when parts of them overlap.
Most of the time, these assumptions are accurate.
But a photograph can remove the depth information that normally helps us distinguish one body from another.
