Investigates the effect of delay of reinforcement upon human discrimination learning with particular emphasis on the form of the gradient within the first few seconds of delay. In previous studies subjects are usually required to make an instrumental response to a stimulus, this is followed by the delay interval, and finally, the reinforcement occurs. The present procedure did not require any instrumental response during the experimental treatment. Instead, a pair of stimuli was presented for a brief period, a delay interval occured, and then a light appeared over the stimulus that the subject was instructed to remember. In the present situation the subject is required to associate a stimulus with the reinforcement light. Classical conditioning literature indicates that an interstimulus interval of about .50 sec. produces optimal conditioning. Even small changes in either direction from this interval produce performance decrements. Thus, evidence from both animal discrimination learning and classical conditioning support the importance of the first second or two of the interval between the occurrence of two stimuli that are to become associated. In this series of experiments, particular attention was given to delay intervals between 0 and 2.0 sec. with other delays ranging up to 5.0 sec. (Author/RC)