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Mean percent choice (and standard deviation) of the larger-dose alternative as a function of the FR programmed on that alternative. The response requirement associated with the smaller-dose alternative was one-third that of the larger-dose alternative. Circles represent comparisons employing 0.3 and 0.1 µg/kg/inj. Squares represent comparisons employing 0.9 and 0.3 µg/kg/inj.
