Perception is often thought of as being synonymous with the term sensation, but this is an almost total misunderstanding of the concept. William James (a pioneering American psychologist and natural philosopher) may have put it best when he said, “Whilst part of what we perceive comes through our senses from the object before us, another part (and it may be the larger part) always comes out of our own mind.” “Sensation” refers to the immediate response of our sensory receptors (eyes, ears, nose, mouth, fingers, etc.) to such basic stimuli as light, color, and sound. “Perception”, on the other hand refers to the process by which sensations are selected, organized, and interpreted by the individual.

People only process a small amount of information available to them, and an even smaller amount is actually attended to and thus given meaning.  The process of perception involves long-term memory and experience and is therefore subject to assumptions and biases and involves considerable hypothesizing and guessing with regard to what is and what is not important. “Attention” refers to the extent to which processing activity is paid to a particular stimulus.  Attention is allocated based on the expectations and past experience of the individual. Witnesses often suffer from sensory overload (i.e., exposure to far more information than they are willing or able to process.) Inattentional blindness may result when the focusing of attention on certain tasks or scene elements prevents noticing other stimuli (this is a common cause of car/motorcycle collisions).