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Let's think for a moment about what is required for perception the assignment of meaning to incoming stimuli to occur, First, some aspect of the environment‑some stimulus has to be picked up by the person (e.g., has to be seen). Next, that stimulus has to be transformed and held, somehow. A body of knowledge has to be available and brought to bear on the stimulus (e.g., cat, cut, and cot are the three‑letter words beginning with c and ending with t). Finally, sonic decision has to be made‑a meaning must be assigned (It's an "a").
The very common phenomenon of identifying the letter "a" seems far more complex when we consider what may happen during the process of perception. One important observation is the fact that perception takes time‑identifying the "M" figure (or any other stimulus, for that matter) is not instantaneous. Recall that the stimulus must be picked up and transformed, memory must be called up, the stimulus must be compared to what is in memory, and a decision must be made. The fact that perception requires time leads to a problem of sorts. Because environments may change rapidly (as when watching a film or driving a car), a stimulus could stop being available before a meaning was assigned. (Imagine seeing the word: DOOR projected by a slide projector for, say, one tenth of a second.) Unless there is some way in which we can "hold" that stimulus for a while, our perceptual processes would have to stop in midstream. The experience of watching a movie, for example, would be terribly frustrating if stimulus after stimulus disappeared before we could interpret their meaning. Our experience, however, tells us that such breakdowns in our perceptual processes occur infrequently. This is because our cognitive systems are equipped to register sensory information.