Pre-attentive processing is the subconscious accumulation of information from the environment. All available information is pre-attentively processed. Then, the brain filters and processes what is important. Information that has the highest salience (a stimulus that stands out the most) or relevance to what a person is thinking about is selected for further and more complete analysis by conscious i.e attentive processing.
We switch between conscious viewing and pre attentive vision every time we shift our gaze. Pre attentive vision may only last a few milliseconds if we are engaged in a visual task. Or, it can last up to five seconds if we are not engaged in a visual task. Visual tasks include reading this sentence, viewing a webpage and advertisement, looking for an exit sign in a public space, a company logo on a piece of direct mail, or an employee at a retail store. But, for our vision system, these tasks translate into looking for colors, shapes, etc.