WITHIN-SESSION CHANGES IN KEY AND LEVER PRESSING FOR WATER DURING SEVERAL MULTIPLE VARIABLE-INTERVAL SCHEDULES

Rats pressed keys or levers for water reinforcers delivered by several multiple variable-interval schedules. The programmed rate of reinforc ement varied from 15 to 240 reinforcers per hour in different conditio ns. Responding usually increased and then decreased within experimenta l sessions. As for food reinforcers, the within-session changes in bot h lever and key pressing were smaller, peaked later, and were more sym metrical around the middle of the session for lower than for higher ra tes of reinforcement. When schedules provided high rates of reinforcem ent, some quantitative differences appeared in the within-session chan ges for lever and key pressing and for food and water. These results i mply that basically similar factors produce within-session changes in responding for lever and key pressing and for food and water. The natu re of the reinforcer and the choice of response can also influence the quantitative properties of within-session changes at high rates of re inforcement. Finally the results show that the application of Herrnste in's (1970) equation to rates of responding averaged over the session requires careful consideration.