Second-order stimuli do not always increase overall response rates in second-order schedules of reinforcement in the rat.
BACKGROUND
Second-order schedules of reinforcement have been used extensively to model reward-seeking and drug-seeking behaviour. Second-order stimuli within second-order schedules have been shown to enhance response rates during operant responding for natural reinforcers and drug reinforcers. This has led some to view second-order schedules of drug reinforcement as a model maintained of drug-seeking in addicts by drug-associated stimuli. However, the functional role of the second-order stimulus within second-order schedules is complex.


OBJECTIVE
We investigated the role of second-order stimuli within a second-order schedule of reinforcement [FI 4 min (FR10: S)] maintained by sweetened water reinforcement.


METHODS
Eight rats were trained to press a bar on a second-order schedule of reinforcement and tested in the presence and absence of the second-order stimulus.


RESULTS
In contrast to most previous work, overall bar-pressing rates were significantly increased when the second-order stimulus was omitted (second-order stimulus omission: 0.17 Hz (+/-0.04, 95% CI); second-order stimulus present: 0.13 Hz (+/-0.04, 95% CI)). However, second-order stimuli also changed the pattern of responding whereby rats would make a bout of bar presses prior to the presentation of the second-order stimulus and then pause briefly after the second-order stimulus. In the absence of second-order stimuli, responding was uniformly high. Control measures, such as the ability of the second-order stimulus to evoke checking for the primary reinforcers, indicated that the second-order stimulus was associated with the primary reinforcer.


CONCLUSIONS
These results demonstrated that although second-order stimuli maintained responding and caused the rat to check for primary reinforcement, overall response rates were increased when the second-order stimuli were omitted. This has implications for interpreting the results of studies where overall response rates within second-order schedules have been the only measure used to assess the effects of potential anti-addiction drugs. Future studies could be improved by performing a second-order stimulus omission test analysing both the overall response rates and the temporal organization of responding with respect to the second-order stimulus.
Wilson
David I G
DI
School of Psychology, St Mary's College, University of St Andrews, South Street, St Andrews, Fife KY16 9JP, UK. digw@st-and.ac.uk
Bowman
E M
EM
D000704:Analysis of Variance; D000818:Animals; D001070:Appetitive Behavior / Q000187:drug effects; D003216:Conditioning, Operant / Q000187:drug effects / Q000502:physiology*; D005108:Extinction, Psychological; D008297:Male; D051381:Rats; D012055:Reinforcement Schedule; D012054:Reinforcement, Psychology; D013549:Sweetening Agents / Q000494:pharmacology; D013997:Time Factors
D013549:Sweetening Agents