III.

Other Animals

It is likely that other animals dream. Mammals, including dogs, cats, and monkeys, have D-sleep periods like those of humans. During D-sleep they also show intense activity in a region of the brain called the visual cortex. In humans this brain activity corresponds to visual sensations. One study observed monkeys that were trained to push a lever whenever they saw a picture on a screen. At different times during their sleep the monkeys suddenly began to push the lever as if responding to an observed image.