Rats given magazine and bar-press training with 32% sucrose reinforcement learned faster than those given either 4% or 11% reinforcements. After all Ss were given a small, decreasing number of 11% reinforcements as the initial part of an extinction test, reliable group differences occurred, the downshifted Ss showing the greatest and the upshifted Ss the least decrement in nonreinforced performance. The results are interpreted as supporting the reality of an over-all contrast phenomenon, especially with respect to the recently implicated artifactual account emphasizing differential weight loss but as leaving open the question of whether any significant contribution is made by an overshooting effect.
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