Intense focusing on a task can make people effectively blind, even to stimuli that normally attract attention. The most dramatic demonstration was offered by Christopher Chabris and Daniel Simons in their book The Invisible Gorilla. They constructed a short film of two teams passing basketballs, one team wearing white shirts, the other wearing black. The viewers of the film are instructed to count the number of passes made by the white team, ignoring the black players. This task is difficult and completely absorbing. Halfway through the video, a woman wearing a gorilla suit appears, crosses the court, thumps her chest, and moves on. The gorilla is in view for 9 seconds. Many thousands of people have seen the video, and about half of them do not notice anything unusual. It is the counting task—and especially the instruction to ignore one of the teams—that causes the blindness. No one who watches the video without that task would miss the gorilla. Seeing and orienting are automatic functions of System 1, but they depend on the allocation of some attention to the relevant stimulus. The authors note that the most remarkable observation of their study is that people find its results very surprising. Indeed, the viewers who fail to see the gorilla are initially sure that it was not there—they cannot imagine missing such a striking event. The gorilla study illustrates two important facts about our minds: we can be blind to the obvious, and we are also blind to our blindness.
Daniel Kahneman, Thinking, Fast and Slow, New York, 2011, pp. 23-24
Since, in order to survive, we must be able to move about effectively, perception must to a certain degree achieve stable and veridical representations. It must tell us how matters stand out there. But the universe is in constant flux. We move about and other things also move. Day turns into night. Sound sources approach and recede. How can perceptual stability be achieved in the face of the ongoing flux?
We can perhaps formulate a better question by asking what aspect of the universe most needs stability. For example, is it the differences or the proportions and ratios that need to remain constant in perception? Apparently it is the proportions—the ratios. When we walk toward a house, the relative proportions of the house appear to remain constant: the triangular gable looks triangular from almost any distance. A photograph portrays the same picture whether we view it under a bright or a dim light: the ration between the light and the shaded parts of the photograph seems approximately the same even though the illumination varies. The perceived relations among the sounds of speech remain the same whether the speech is soft or highly amplified. In other words, the perceptual domain operates as though it had its own ratio requirement—not a mathematically rigid requirement, as in physics, but a practical and approximate requirement.
The usefulness of perceptual proportions and relations that remain approximately constant despite wide changes in stimulus levels is immense. Think how life as we know it would be transformed if speech could be understood at only a single level of intensity, or if objects changed their apparent proportions as they receded, or if pictures became unrecognizable when a cloud dimmed the light of the sun.
By making the perceived aspects o stimuli depend on power functions of the stimulus dimensions, nature has contrived an operating mechanism that is compatible with the need for reasonable stability among perceptual relations.
Stanley Smith Stevens, Psychophysics: Introduction to its Perceptual, Neural, and Social Prospects, New Brunswick, 1986, pp. 18-19