Selective visual attention allows a person to take in a great deal of visual data from the environment but attend to only a small portion of it. There is a great deal of information coming into a person's brain through the eyes, so the brain processes only a small part of it, choosing to ignore the rest, an effect known as inattentional blindness. Though the brain continues to receive data from the entire field of vision at once, most of this data is ignored most of the time. Instead, selective visual attention decreases the focus of the field of vision to a smaller section of the total field so that a person can focus on the details that are important at the moment.
There are two types of visual processing that happen within the human brain. The first is called pre-attentive processing, and occurs across the entirety of a person's field of vision simultaneously. This type of processing allows a person to notice changes in the environment. For instance, an object that was previously still that has suddenly started to move or an object of a different color amongst objects of the same color will draw a person's selective vision. Evolutionarily, this method of pre-processing data allowed humans to survive in a hostile environment.
Once an object or objects have been selected out of the field of pre-attentive processing data, a person has focused attention on it. This object becomes the focus of the eyes and is given a large portion of the working memory. The pre-attentive field remains active, however, and can alert a person to another change in the environment if one appears. Attention may be focused through the use of either the top-down or the bottom-up approach. In the top-down approach, deviations from the current environment trigger the selective vision, but in the bottom-up approach, attention is focused based on prior experiences and long-term memory expectations.
When people devote their focus to one thing in their field of vision, it is possible to become blind to other objects or events that the person can see. This happens because the brain determines that these other things are not as important as whatever the person is currently focusing on. The range of focus of selective visual attention can be as small as one degree of the total field of vision. Scientists are not sure how large a range of focus a person can have.