“…A number of studies have demonstrated, in varied situations, that increased vigor of response follows interruption of, delay of, or failure to reward. For example, Marzocco (14), Rohrer (16), and Sheffield (18) present data in which the effect of failure or delay of reward is presumed to manifest itself immediately. On the other hand, Geier and Tolman (8), who obtained measures of activity during delay, showed an increasing effect (on activity) over trials, while Logan (13) showed a gradual decline of response strength over trials as the result of the introduction of an increased delay.…”