Within-session changes in key and lever pressing for water during several multiple variable-interval schedules.
Water can replace food in VI procedures, but watch for faster within-session drops at high reinforcement rates.
01Research in Context
What this study did
Lowe et al. (1995) watched rats press a lever and peck a key for tiny sips of water. The team used multiple VI schedules in each session. They tracked how fast the animals responded minute-by-minute.
Water was the only pay-off. No food, no shock, just plain H₂O.
What they found
Response rates rose early, then drifted down as the session went on. The same hill-shaped curve seen with food pellets showed up again.
At the richest VI value the drop was steeper for water than for food. Reinforcer type matters when pay is dense.
How this fits with other research
McSweeney et al. (1993) first drew the within-session curve for any reinforcer. The 1995 paper adds water to that map and shows the shape holds.
Lewon et al. (2019) later showed that pre-session food can boost later water pressing. Their work widens the lens from minute-by-minute drift to day-by-day motivation.
Hymowitz (1976) also used water, but looked at shock-induced suppression, not VI rates. Same fluid, different question.
Why it matters
If you run sessions longer than ten minutes, expect early highs followed by a slide. The slide is steeper when the schedule is rich and the reinforcer is water. Track minute-by-minute data instead of averaging the whole session. When you switch from food to water, thin the schedule or add brief breaks to keep response levels steady.
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Join Free →Graph responses in one-minute bins; if the line dives after minute 5, add a 30-s break or thin the schedule.
02At a glance
03Original abstract
Rats pressed keys or levers for water reinforcers delivered by several multiple variable-interval schedules. The programmed rate of reinforcement varied from 15 to 240 reinforcers per hour in different conditions. Responding usually increased and then decreased within experimental sessions. As for food reinforcers, the within-session changes in both lever and key pressing were smaller, peaked later, and were more symmetrical around the middle of the session for lower than for higher rates of reinforcement. When schedules provided high rates of reinforcement, some quantitative differences appeared in the within-session changes for lever and key pressing and for food and water. These results imply that basically similar factors produce within-session changes in responding for lever and key pressing and for food and water. The nature of the reinforcer and the choice of response can also influence the quantitative properties of within-session changes at high rates of reinforcement. Finally, the results show that the application of Herrnstein's (1970) equation to rates of responding averaged over the session requires careful consideration.
Journal of the experimental analysis of behavior, 1995 · doi:10.1901/jeab.1995.64-75