An experimental technique for determining whether an animal or young child can remember the solution to a problem after a lapse of time. The method is used in studying thinking processes and learning ability.Adult human beings can learn any number of tasks that involve delays of short or even long duration. They can respond to the command “Forward . . . March!” where the reaction is only slightly delayed, and they can find a letter which they put away a year before. But how early in life are we capable of such delayed reactions? And do any animals possess this ability? Psychologists have felt that the answers to these questions might throw light on thinking and learning ability, since some kind of mental process, such as language,memory traces, or other symbolic activity, seems to be needed to bridge the gap when reactions are delayed. This idea has led to a number of revealing experiments.In a typical setup, a young child is shown how to obtain an object which he has seen the experimenter hide. He is then seated in front of three small boxes, and watches while the experimenter places a toy in one of the boxes and closes the lid. The child is then turned around, or his eyes are covered for a period, and after that he is asked to find the toy. Hunter (1917) found that thirteen-month-old children performed correctly 80 per cent of the time with delays up to fifteen seconds. This ability increased with age and mastery of language, and many five-year- olds were consistently accurate after intervals of a month or more. It was found that these children frequently verbalized during the first observation, “It’s under the middle box,” and when they were tested after the delay, they would say something like “Oh, yes, I saw it under the middle box.” Hunter’s experiments indicated that delayed reaction is possible without the use of language, but that language greatly expands this ability.The first experiments with animals seemed to indicate that chimpanzees and dogs could only make correct choices if they remained oriented toward a cup under which food had been placed. Modifications in the apparatus later proved that delayed responses could be made by some animals without this orienting posture. The period of delay did not approach that of adult human beings, but in some cases it amounted to many minutes or even days. Further experiments demonstrated that monkeys were capable of a particularly advanced form of delayed reaction. Their favorite food, a banana, was placed under a cup, and after a delay they were allowed to retrieve it. Then a piece of lettuce (not a favorite food) was placed over the banana and this time they refused to take the reward, and some even had a typical temper tantrum. However, they remained in the vicinity as if they were waiting for the banana (Tinklepaugh, 1928). This experiment, together with the previous studies, showed that many animals are capable of using some form of “symbolic activity”—that is, thinking—even though they do not have language to aid them.Although these experiments have shown that some animals use thinking processes similar to those of human beings, they have also helped to distinguish between men and animals. It was found that chimpanzees cannot use color as a cue even though they have good color vision. Children, on the other hand, used position alone until they were a year of age, but color became the dominant cue when they grew older (Miller, 1934). Moreover, the time interval was strictly limited for all animals, while older children and adults were able to make correct responses even after a period of months or even years. Language is believed to be the prime factor in accounting for this difference.Delayed-reaction experiments have been used to good advantage in brain research. It had long been suspected that the forwardmost part of the brain, the prefrontal area, is extremely important in higher learning and especially in thinking that is directed to the future. This theory has received support from the fact that when the prefrontal lobes of monkeys were removed, they usually failed in the delayed-reaction task if the interval was more than a few seconds. They also seemed unable to concentrate on the task, although they became more docile and calm.These findings have an interesting sequel. When they were mentioned during a professional convention, a neurosurgeon, J. W. Watts, conceived the idea that deeply disturbed human beings might become calmer and less concerned about their problems if the prefrontal area was eliminated. As a result, this operation was performed on many mental patients who seemed to be otherwise incurable. Although it has effectively alleviated emotional disturbance in many cases, it has usually reduced the patient’s ability to think and plan,and for this reason is now used only as a last resort.