Sequential contrast effects with human subjects.

Institutionalized retardates were exposed to a multiple variable-interval: extinction schedule of reinforcement in which 5-min periods of variable-interval reinforcement and 5-min periods of extinction were presented in a random order. This schedule was found to generate sequential contrast effects: response rates during variable-interval reinforcement were higher when a variable-interval period followed an extinction period than when it followed another variable-interval period. The rate of responding within a variable-interval period also was affected by the number of extinction periods preceding a variable-interval period. As the number of successive extinction periods that preceded a variable-interval period increased, the rate of responding during that variable-interval period increased. The sequential contrast effects were transient, being most evident during the early sessions and generally disappearing by the tenth session.