Inattentional blindness or perceptual blindness (rarely called inattentive blindness) occurs when an individual fails to perceive an unexpected stimulus in plain sight, purely as a result of a lack of attention rather than any vision defects or deficits. When it becomes impossible to attend to all the stimuli in a given situation, a temporary "blindness" effect can occur, as individuals fail to see unexpected but often salient objects or stimuli. The term was chosen by Arien Mack and Irvin Rock in 1992 and was used as the title of their book of the same name, published by MIT Press in 1998, in which they describe the discovery of the phenomenon and include a collection of procedures used in describing it. A famous study that demonstrated inattentional blindness asked participants whether or not they noticed a person in a gorilla costume walking through the scene of a visual task they had been given. Research on inattentional blindness suggests that the phenomenon can occur in any individual, independent of cognitive deficits. However, recent evidence shows that patients with ADHD (Attention deficit hyperactivity disorder) performed better attentionally when engaging in inattentional blindness tasks than control patients did, suggesting that some mental disorders may decrease the effects of this phenomenon. Recent studies have also looked at age differences and inattentional blindness scores, and results show that the effect increases as humans age. There is mixed evidence that consequential unexpected objects are noticed more: Some studies suggest that we can detect threatening unexpected stimuli more easily than nonthreatening ones, but other studies suggest that this is not the case. There is some evidence that objects associated with reward are noticed more. Numerous experiments and art works have demonstrated that inattentional blindness also has an effect on people's perception. Defining criteria The following criteria are required to classify an event as an inattentional blindness episode: 1) the observer must fail to notice a visual object or event, 2) the object or event must be fully visible, 3) observers must be able to readily identify the object if they are consciously perceiving it, and 4) the event must be unexpected and the failure to see the object or event must be due to the engagement of attention on other aspects of the visual scene and not due to aspects of the visual stimulus itself. Individuals who experience inattentional blindness are usually unaware of this effect, which can