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Very often, the human eye and brain can recognize familiar shapes even if all but a few significant points are left out. It is this creative power of the human eye and brain that may someday allow profoundly deaf people to talk in sign language over a special "telephone". Researchers have found that thirteen spots of light on each hand and one spot on the nose--to show head motion and provide a reference point for hand position--are sufficient for the performing of American Sign Language. The bright spots are produced by attaching pieces of a special tape to the nose, fingertips, and wrists and adjusting a closed-circuit television system so that only the bright spots appear on the screen of the receiver. After a few minutes of familiarization, pairs of deaf people are able to communicate freely over television. Deaf people and other sign readers can, of course, understand sign language on commercial television. But widespread link-ups of television systems for personal communication are economically impolite because a standard picture requires about one thousand times more information than a telephone circuit can carry. The researchers think if likely through the simplified picture of twenty-seven moving dots used in their experiments can be reduced to the capacity of a telephone line.

What causes the bright spots appearing on the screen of the receiver（）