Bottom-up processing is analysis that begins with the sense receptors in the body and works up to the brain's integration of sensory information. This form of processing suggests that information is received in small units, and built into larger units that carry meaning. This is the classic example of perception being brought on through sensation. For example, if you are reading a book, you experience (sense) the letters one at a time, and then organize the letters into words, the words into phrases or clauses, then sentences. This means that one level of perception can only be accessed after one of the previous levels has been "unlocked". A sort of hierarchy.

This idea of bottom-up processing working independently is unlikely due to top-down processing, (in which we process from the top of the perception hierarchy and let it flow down to our senses), which occurs simultaneously with bottom-up processing in most cases. Such as the fact that when we read, we are often able to guess the next word in the sentence; in a movie, we are often able to guess the next line, or part of the plot before we see it.

The theory of direct perception, by James J. Gibson, states that all one needs for perception to occur is a visual system and an environment in which to operate. All the cues necessary for perception to take place, according to Gibson's theory, are already present in the environment, and it is not necessary to invoke top-down processing.