Inattentional blindness is the failure to notice a fully-visible, but unexpected object because attention was engaged on another task, event, or object.

One of the most famous tests on the subject is the invisible Gorilla test. Users are asked to watch a video, which has two teams passing balls between each other and counts the number of passes by one team. During the video, a person in a gorilla suit walks through the middle of the crowd. The surprising output of this experiment is that around 50% of the people don’t mention they have seen a gorilla.

The concept has huge implications on UX design. An assumption that something on the screen is necessarily noticed by the user isn’t valid anymore and this opens the door to various interpretations.

Sources:

Inattentional blindness, Daniel J.Simons : http://www.scholarpedia.org/article/Inattentional_blindness

Müller-Birn, C., Lecture „05-2 HCI Human Memory“