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Contrast effects as a function of shifts in delay of water reward.
The effect of delay of reward upon acquisition of an instrumental response was examined using water deprived rats in a straight runway. Following stable performance in the . acquisition (preshift) phase, subjects receiving 0, £ or 16 sec access to a water drinking tube were subdivided and either maintained on preshift delay, or shifted to the other preshift delay intervals. The postshift performance was examined for evidence of positive and negative contrast effects. It was found that delay of water reward produced significant differences in acquisition such that the shorter the delay interval the faster the running speed. This effect was also found in the second phase, in which the same relationship held between length of delay and speed, regardless of earlier acquisition experience. Although a graphical, albeit statistically nonsignificant, PCE was observed in all sections of the runway, no NCE was found. The results were discussed in terms of Hull's general behavior theory, Amsel's frustration theory, Capaldi's sequential theory, Helson's adaptation-level theory and Logan's micromolar theory.