Time cost of alternation reduced demand for food in a closed economy.

A principal purpose of the present study was to characterize rats' demand for food by increasing the required delay between the alternation of leverpresses. While living in cages that were exposed to a 12:12 light:dark cycle, rats were required to alternate leverpresses for their entire daily food ration. After a press on one lever resulted in the delivery of a food pellet, a red cuelight was illuminated for a fixed duration that equaled the imposed delay between successive leverpresses. After the imposed delay elapsed and the cuelight was turned off, a press on the alternate lever resulted in the delivery of another food pellet. Increases in the imposed delay from 0.01 s to 16 s reduced mean percent correct (alternation) and mean food intake, although both were reduced more rapidly and to a greater extent in the light than in the dark photoperiod. This interaction resulted from a reduction of meal size in the light and a compensatory increase in meal size in the dark as the imposed delay increased. Rats also defended their daily intake in the dark against increasingly larger imposed delays by spending increasingly larger amounts of time alternating and by increasing total (correct + incorrect) presses. These results suggest that photoperiod is a determinant of the motivational value of food as a reinforcer for instrumental behavior in a closed economy.