Interactive computer systems often present information visually.  As users interact with the system, new information (data, controls, error messages) is produced and displayed.  Thus, the contents of the display change frequently.

The issue for interaction designers is: how to present important information so as to ensure that people notice it.  That turns out to be harder than you might think.

People often fail to notice many of the visual changes that occur around them.  Our brains are designed to focus our attention on the features of our environment that are most important to us – threats, opportunities, or things related to our current goals – and ignore everything else.  Basically, we are “programmed” to notice changes that we need to notice, such as a leopard walking out from behind a bush.  We are more-or-less “blind” to changes that aren’t in our focus of attention (Angier, 2008).

Cognitive psychologists’ have a name for this perceptual phenomenon: change blindness.

Several simple demonstrations of change blindness were devised by perception researchers at the University of Paris (O’Regan and Noe, 2000).  Each demonstration consists of an image that switches every few seconds to a very similar image.  In some of the demonstrations, the image-switch is instantaneous; in others, flicker is added between the two images.  How many cycles does it take for you to notice what is changing in each of these images?

Woman in kayak: http://nivea.psycho.univ-paris5.fr/ASSChtml/kayakflick.gif

Couple at dinner: http://nivea.psycho.univ-paris5.fr/ASSChtml/couple.gif

Notre Dame cathedral: http://nivea.psycho.univ-paris5.fr/ASSChtml/ND.gif

Canal bridge: http://nivea.psycho.univ-paris5.fr/ASSChtml/canalbridge.gif

Cars on street: http://nivea.psycho.univ-paris5.fr/ASSChtml/dottedline.gif

In contrast, when the focus of your attention is exactly those image features that are changing, as in the following example, you notice much more quickly what has changed:

Man with milk glass: http://nivea.psycho.univ-paris5.fr/ASSChtml/milkman.gif

People may be even more likely to miss changes that occur slowly.  Check out this video from the University of Illinois (Simons et al, 2000):

Film directors try very hard to ensure “continuity” between shots in a scene.  If an character is wearing a red shirt as a scene opens, he’d better be wearing the same red shirt in every shot in the scene; otherwise viewers would notice the change, right?  If his friend’s wine glass is almost empty in a shot in the middle of the scene, it better not inexplicably be full at the end of the scene, because the audience might notice, yes?

Well, actually, no.

The University of Illinois researchers (Simons et al, 2000) made videos to show that people often miss changes in details between movie scenes – even some not-so-subtle changes.  Here is one of those videos:

The most amazing demonstrations of how people’s focused attention causes them to miss changes are those, again devised by Simons and his colleagues, in which people interacting face-to-face with one person may fail to notice that, in the middle of the interaction, a new person replaced the original person:

A British magician named Derren Brown later tried to see how far he could stretch this phenomenon of people failing to notice when a person they are talking with is swapped for a different person:

Now, having seen these demonstrations, how sure are you that your software’s users will actually notice the error messages your software displays?  Are you still sure your website’s users will see those subtle AJAX-mediated changes in the displayed data?

If you aren’t sure, the only way to be sure is to design the display so that the changing items either are already in users’ focus of attention or somehow grab users’ focus of attention after the change.  You can do this by:

• Placing the new information where the user is looking, i.e., near where they just clicked or where the cursor is now.

• Popping the new information – e.g., an error message – up in front of the application window.

• Highlighting the new information, e.g., by displaying it in a different color than what is already on the screen, or by giving it a blinking background or border, or by wiggling it briefly. Such highlighting should not be continuous, but rather brief – a half second or so – just long enough to attract users’ attention.

Otherwise you may be designing features many users won’t notice.

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About the Author

Jeff Johnson is president and principal consultant at UI Wizards, Inc., a product usability consulting firm (www.uiwizards.). He has worked in the field of Human-Computer Interaction since 1978, as a software designer and implementer, usability tester, manager, researcher at several computer and telecommunications companies, and as a consultant. In the course of his career, he has written many articles, cowritten several books, and given numerous presentations on a variety of topics in Human-Computer Interaction.

References

1. Angier, N. (2008). Blind to change, even as it stares us in the face. New York Times. April 1, 2008. Web: www.nytimes.com/2008/04/01/science/01angi.html.
2. O’Regan, J.K. and Noe, A., “Experience is not something we feel but something we do: a principled way of explaining sensory phenomenology, with Change Blindness and other empirical consequences”, Proceedings of ASSC Conference, Brussels, June 29-July 2, 2000.
   Web: http://nivea.psycho.univ-paris5.fr/ASSChtml/ASSC.html
3. Simons, D.J., “Demos and Stimuli”, Visual Cognition Lab, University of Illinois.
   Web: http://viscog.beckman.illinois.edu/djs_lab/demos.html
4. Comp: In the 2nd reference-item (and the body-text that references it), “Noe” should have an e with two dots over it (French style), but I couldn’t figure out how to do that in Word.