In a classic study on depth perception, Gibson and Walk placed infants of various ages on a fabric-covered runway that ran across the center of a clever device called a visual cliff. In the study, .

In the study, they put ten babies on a surface that looks unsafe and risky. On the other side were their mothers who were calling babies to crawl to them.  Eight out of ten infants did not continue crawling. Psychologists can analyse infants` reaction (emotional response) to the visual cliff even before they start crawling. For instance, Campos and his work group in the 1970s came to the conclusion that infants (one-month-old) do not have the change in heart rate when they are put on the cliff with their faces down. A A month older infants had decreased heart rate when put in the same position with the face down which interprets as them being interested in what was happening. And finally, when they did the experiment with children who are nine-month-old, their heart rates quickened on the cliff, which was a sign of fear.

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