To create our perceptions, the brain makes use of two processing systems, namely the bottom-up and the top-down processing system.
- Bottom-up processing happens when we combine a number of stimuli into a single, unified perception.
- Top-down processing happens when we make sense of the sensory information that was produced by the bottom-up process.
We interpret sensory input on the basis of existing knowledge, expectations and ideas. These processing systems make up our perception. Because of our top-down processing system, it can argued that we are not in direct contact with the world because we only get to “know” our perception.
Top-down processes are useful when we look for patterns in our sensory input, but it may also lead to a bias known as a perceptual set. This bias hinders us to perceive things in other ways. We expect to see things in a certain way. Reversible figures reflect this tendency (see an example here).
It is widely believed that there are four principles that organize our perception, the so-called Gestalt principles:
- Similarity
- Proximity
- Closure
- Continuity
We make perceptual hypotheses. Perceptual hypotheses are expectations about what the sensory information represents. We may test our perceptual hypotheses, and we can do so by looking at reversible figures.
Reversible figures show how our expectations form perception, and the longer we look at these figures, we start to perceive two different perceptual phenomena in the same figure. As a result, our perceptual hypothesis has been tested.