LIGHT ONSET AND TERMINATION AS REINFORCERS FOR RATS LIVING UNDER NORMAL LIGHT CONDITIONS

Abstract

The initial observations of the reinforcing effect of response-contingent light onset (Marx, Henderson, & Roberts, 1955; Kish, 1955) have been repeatedly confirmed by other workers. There has not been comparable agreement, however, concerning the effect of light cessation following response. Hurwitz (1956) and Barnes and Kish (1957) failed to find a reinforcing effect, while Roberts, Marx, and Collier (1958) reported a significant increase in response when light cessation was made contingent upon bar pressing. The light cessation contingency provides a test of the stimulus change interpretation of the reward effect of light, and thus is of considerable theoretical interest. If stimulus change is the rewarding aspect of light onset, light offset as a stimulus change should also be reinforcing. The present experiment provides a further test of the effect of cessation of dim light following bar pressing. METHOD Sabjects.-Ss were 28 female hooded rats, 100 to 200 days old, bred in the psychology department rat colony. They lived in the colony living quarters under a 12-hr., 12-hr. light-dark cycle from birth and were kept in these quarters between experimental sessions. Appurdtgs und procedare.-Ss were tested individually in a 1-ft. by 1-ft. by 1-ft. unpainted plywood test chamber with a milk glass ceiling and a 1%-in. diameter brass half-disc lever (requiring 30-gr. pressure) protruding from one wall. A light source in a hood over the test chamber produced an illumination level of .5 ml. when the circuit was closed; when it was open the chamber was dark. Lever depression activated an electric counter recording response frequency; during test sessions the lever also controlled the light circuit. If the prevailing light condition was dark, S could turn the light on and keep it on as long as he held the bar down; if S was tested in the light, he could turn the light off and keep it off by holding the bar down. Release of the bar restored the prevailing light condition. An outline of the experiment is given in Table 1 which shows the light conditions present in the test chamber during the eight daily 15-min. experimental sessions. Both termination of light and light onset were used, and pretest light conditions were balanced for the two different contingencies; half

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