Abstract
To test Hull''s Theorem 14, 6 groups of rats were trained in a 2 choice discrimination box under various combinations of discriminanda. With 3 discriminanda, 2 negative stimuli and 1 positive stimulus, the rate of discrimination learning was significantly slower than with only 2 discriminanda, 1 positive and 1 negative. The ordinal relationships of the rates of learning under various other combinations were compatible with the summation gradient hypothesis.

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