Final answer:
Bottom-up processing is initiated by sensory input from the environment, leading to perception built without pre-existing cognitive representations, contrasting with top-down processing which is influenced by prior knowledge.
Step-by-step explanation:
Processing that is initiated by sensory information about the external environment is referred to as bottom-up processing. This type of processing is a system in which perceptions are built from sensory input, without any preconceived cognitive interpretations. Sensory receptors collect information from the environment, and this data is then processed up to the brain for interpretation, which allows humans to react to stimuli. In contrast, top-down processing is influenced by one's previous knowledge and expectations.