Jun 18 2010

Change Blindness

It should come as no surprise that we don’t notice everything that we see. We all experience this on a regular basis – there is a great deal of visual information in our field of view but we only pay attention to a small fraction of it. Yet at the same time something within our vision can capture our attention if it flashes, moves, or otherwise changes dramatically enough. Interestingly, despite our common and frequent experience with the limitations of our own visual attention, people tend to have overconfidence in their ability to notice details and are often surprised when an important detail goes unnoticed.

Here are some fun examples of a what neuroscientists call change blindness. First from Richard Wiseman – the color changing card trick. And here is a great one from Derren Brown. Failure to notice one person being swapped for another may even have implications for eye-witness testimony.

Change blindness is the failure to notice a visual change in our field of view. It is closely related to but distinct from inattentional blindness, which is a failure to notice an element of a scene at all (not specifically a change).

Scientists have been exploring the nature of change blindness. As an aside, I am always amazed at the depth of detail I can find in any narrow scientific question. It seems that there is almost always a small group of researchers who have delved to an incredible depth on even the smallest question. And that is what I found with the change blindness literature. But I think I have wrapped my head around the major themes of this research – spurred by a recent study using computer technology to aid in more accurate testing for change blindness.

Top Down vs Bottom Up

Also as with many scientific questions, there are competing views that both likely contain an element of truth. With change blindness the two competing views are described as the top-down hypothesis and the bottom- up hypothesis, referring to what it is about the change of a scene that grabs our attention. Top down theories involve our understanding of the context of a scene. In the new study they give the example of searching for a computer in a picture of an office. Our attention goes to the desk because we understand that a computer is most likely to be found there.

Bottom up theories focus on the visual salience of image components such as contrast, movement, and lighting – the more basic elements of our visual processing.

It seems that both of these process are at work. Depending on our goal, we use our top-down knowledge of what should be in a scene to search for something of interest. But even in the midst of such a task our attention can be drawn to a high contrast object or something that begins to flash in our visual field. This makes sense from a function point of view – we need to balance out ability to focus our visual attention but still notice important changes in our visual field. But this is also a trade off, like many functions in biology. The more we focus on details, the less we may notice change, and the more we attend to the overall scene, the less we can attend to details and context.

We very much have limited visual and cognitive processing resources, and we allocate them as needed – but we can’t do everything. You have probably noticed this in everyday life. Sometimes you may be very focused on something and may be completely inattentive to events in your environment – what we might call distracted. At other times you may be “on alert” and attentive to everything happening around you.

In any case – researchers are dickering over the relative contribution of bottom up vs top down factors in change blindness. The recent study makes two contributions to this ongoing debate. First, they argue that prior research looking at the contribution of top down factors in change blindness were contaminated by unintentional bottom up changes as well. For example, when one object was manually removed from an image or replaced with another object (a top-down context change), the salience of the picture (contrast, etc.) may have also been inadvertently altered. They therefore propose a computer algorithm to makes changes to a scene without changing the overall visual salience of the scene, and therefore better separating these two variables.

Computer Study

What they did was show subjects a scene followed by a gap and then followed again by the same scene with one subtle change. This is a flicker test, and prior research has shown that it is more difficult to notice changes with the gap in between than when one scene is immediately replaced with the changed scene – the flicker disrupts our ability to notice the change (here in an example). In this video accompanying the BBC article on the study they show some of the pictures used in the study.

With their improved method the authors found that people are more sensitive to removing or replacing an object in a scene than they are to color changes (which explains the color changing card trick).


The details of this research are interesting and may lead to practical applications, such as designing road signs that better grab drivers’ attention. It will likely also be used by marketing companies to design ads and commercials that will demand our attention – the attention of consumers is a commodity that companies will pay for.

For most people, however, the big picture is important – we have a limited capacity to attend to details or changes in those details. Yet, we are overconfident in our abilities. This extends to memory as well as sensory attention. This overconfidence causes much mischief, such as placing excessive faith in eyewitness testimony. It also contributes to the anecdotal evidence that comprises the bulk of the arguments made for many paranormal beliefs. When people see a strange object in the sky, for example, and think it is an alien spacecraft, or at least a genuine mystery, they are generally being overconfident in their ability to have noticed important details that might have provided a more prosaic explanation.

Understanding change blindness is therefore important to the humility inherent in a scientific and skeptical point of view. It is also very helpful in getting husbands out of hot water when they fail to notice a change in their wife’s hair color or style. Well, maybe not.

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