People fail to notice when a person they're talking to is swapped for someone else during a brief interruption. Proving we perceive far less of reality than we think.
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The door study
In 1998, psychologists Daniel Simons and Daniel Levin conducted a now-famous experiment on a university campus. An actor would stop a pedestrian to ask for directions. Partway through the conversation, two people carrying a large wooden door would walk between the actor and the pedestrian, completely blocking their view of each other for about one second. During this interruption, the original actor was replaced by a different person. The new actor was of a similar build but wore different clothes and had a different voice. Astonishingly, only about 50% of the pedestrians noticed the swap.
This failure to see a major change in the immediate environment is called change blindness. It is not a failure of eyesight, but a lapse of attention. The human brain receives an immense amount of visual information and can only process a fraction of it. To cope, it creates a general sense of a stable world, but only focuses attention on a few specific items at a time. This location, the University of British Columbia, is a research center into how this selective attention works. The UBC Visual Cognition Lab, led by Professor Ronald Rensink, investigates the mechanisms of our visual experience and its limitations.
The flicker paradigm
Much of the foundational work on change blindness at UBC uses an experimental method called the "flicker paradigm," developed by Ronald Rensink. In these experiments, a person views an image that alternates with a slightly modified version of the same image. Between each alternation, a brief blank screen is inserted. This blank screen—the "flicker"—is critical. It prevents the brain's automatic motion-detection systems from immediately spotting the location of the change.
Without the motion cue, the observer has to search the scene item by item, a surprisingly difficult and slow process. People can look directly at a major change, such as a building disappearing or an engine appearing on an airplane's wing, and still not "see" it for many seconds. This demonstrates that our internal representation of the world is not a detailed, photograph-like snapshot. Instead, our brain operates on a "just-in-time" basis, grabbing details as needed. This is an efficient system, but it makes us susceptible to missing large changes that happen outside our narrow focus of attention. Rensink's work shows that attention is necessary to perceive change; without it, we are functionally blind to even obvious differences.
💡Fun Facts
A related phenomenon is "inattentional blindness," famously shown in an experiment where about half of observers, tasked with counting basketball passes, failed to see a person in a gorilla suit walk through the scene and thump their chest.
Expertise does not prevent change blindness. In one study, 83% of radiologists failed to spot a picture of a gorilla, 48 times the size of a typical cancer nodule, that was inserted into a set of lung scans they were examining.
The phenomenon also applies to our own choices. In "choice blindness" studies, people who choose a picture of a person they find more attractive often fail to notice when the experimenter uses sleight-of-hand to present them with the face they just rejected.
In the original "door study," pedestrians younger than the experimenters (who were college students) were more likely to notice the swap, while older pedestrians were less likely. This suggests we pay more detailed attention to people we perceive as part of our own social "in-group."
