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3/13/04

Invisible Gorillas and Inattentional Blindness

Daniel Simons, head of Visual Cognition Lab at University of Illinois, video-taped two teams, one with white shirts, the other with black. They move around one another in a small room, tossing a basketball.

For his experiment, he recruited students as subjects to watch the video. The subjects' task was to count the number of passes by members of the white team, which is not easy, given the continual, inter-weaving movement of players. They must remain intent on the movement of the basketball.


After 35 seconds a gorilla walks into the room, thumps her chest, and in 9 seconds leaves the room.

After the subjects have watched the clip, the experimenter asks them if they saw the gorilla.
Out of over 10,000 subjects, half have not seen the gorilla. They even accused the experimenter of showing two different clips, but became believers when he replayed the only clip. Simons calls their lapse inattentional blindness.

Their experience reveals false assumptions we make about perception.

The normal, common sense belief is that an internal observer sits behind the eyes dutifully recording all sensory input. This does not happen. We have no internal observer. Instead, different parts of the brain process inputs and in the processing they may totally omit data for the sake of attention continuity.

My brother-in-law once turned left directly in front of an oncoming pickup. He said he looked right through the truck and never saw it, because he had already assumed traffic to be clear. I used to ride motorcycles and found that drivers did not always see me. Even though I had my headlight on, cars turned left in front of me even though I was almost at the intersection. I decided to ride only in open country. I once rode a bicycle twenty five miles daily and was peddling thirty five miles an hour down a hill when a car passed and immediately turned right in front of me. I narrowly missed adding splatters of red to its side.

In terms of inattentional blindness, the gorilla experiment can be used to undermine the belief that a self is watching each event and anything unanticipated will be noticed.

Saccades indicate no little man or woman inside, no humunculus in control. With saccading, in order to process visual information in sharp images some input is omitted. Otherwise, blurred images would be recorded on the brain, and although these are actual data, they are not useful information. Saccading is not done by any observer, but by neurons. Visually, a saccade is a fast eye movement in which a scene isn't regarded steadily. Instead the retinal line of sight jerks around so to notice important details and build a mental map. This allows the eye to sense parts of the scene with greater resolution so to make efficient use of brain and neurons. Without saccading, an optic nerve larger than the eyeball would be required to perceive a scene in high resolution, as well as a brain several times larger than the current one.

On a separate, but concluding note, if understanding of visual deceptions became part of common sense, law courts would place less reliance on certain types of eye-witness testimony.

Click here for Simons'web site.

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