A selective attention test is used to demonstrate that people are often unaware of certain visual stimuli when they are focusing on other aspects of what they see. This type of test is used to show how little people observe in the world around them and how they can miss unusual events when focusing on a task. There are many different types of tests that can be devised, though the most famous one was developed by Dr. Daniel Simons, a professor of psychology.
In Dr. Simons's selective attention test, a test subject watches a video of six people passing two basketballs around. Three of the people are dressed in black and the other three are dressed in white. The test subjects are asked to count the number of times the people dressed in white pass the ball and to disregard the actions of the people dressed in black. Visually, the video contains a great deal of information because all six people move around continuously, and the test subject must keep track of the movement of one of the basketballs in the video.
Near the middle of this selective attention test, a person dressed in a gorilla suit walks through the scene, pausing in the center to beat its chest before exiting the video on the other side. The test subjects are then asked how many times the basketball was passed between the players in white. Once they answer, they are asked if they noticed the gorilla. Nearly half of the people who participated in the study did not notice the presence of the gorilla.
This selective attention test demonstrates that people may not notice things that enter their field of vision when preoccupied with other tasks, such as counting. This is because the brain has a limited amount of working memory available and devotes it to the task at hand rather than the observation of the environment. This effect is called inattentional blindness.
Psychologists can make use of a selective attention test to learn more about the way that the brain processes attention and memory. The use of the "invisible gorilla" selective attention test demonstrated inattentional blindness, while further tests developed by Dr. Simons also demonstrated that subjects who expected to see one thing, such as a gorilla, often do not notice other changes made to the environment. These tests also show that people are often mistaken about how much they notice and remember about what they have seen.