Jeff Johnson, UI Wizards, Inc.

Interactive computer systems typically 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 that people notice it. That turns out to be more of a challenge than you might think.

It seems that people commonly fail to notice many of the visual changes that occur in their environment. Our brains are designed to focus on the features of our environment that are most important to us - those most relevant to what we are trying to do right now - and to ignore everything else. That makes us more or less 'blind' to events that aren't in our focus of attention.

Cognitive psychologists call this perceptual phenomenon change blindness.

Several simple demonstrations of change blindness were devised by perception researchers at the University of Paris (O'Regan and Noë, 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, while in others flicker is added between the two images. How many cycles does it take for you to notice what has changed in each of these images?

• Woman in kayak
• Couple at dinner
• Notre Dame cathedral
• Canal bridge
• Cars on street

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

• Man with milk glass

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

• Farm scene
• Medal award ceremony
• Family at home
• Café scene

Film directors try very hard to ensure 'continuity' among shots in a scene. If a 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 commonly miss changes in details between movie scenes - even some not-so-subtle changes. See what you notice in these demonstrations (note: Flash required):

• Subtle changes
• Non-subtle changes

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 a second person may fail to notice that, in the middle of the interaction, a third person replaced the second one:

• Original experiment
• Follow-up experiment

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're not sure, the only way to be sure is to design the display so that the changing items are either already in users' focus of attention or somehow grab the 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.

References

O'Regan, J.K. and Noë, 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

Simons, D.J., 'Demos and Stimuli,' Visual Cognition Lab, University of Illinois, 2009.
Web: http://viscog.beckman.illinois.edu/djs_lab/demos.html