Rationale: Observing responses bring sensory receptors into contact with environmental stimuli. In the observing-response procedure, periods in which an operant response (e.g. pressing a lever) is reinforced by drug deliveries alternate with periods in which this response is never reinforced (i.e. extinction). These alternating periods of drug availability versus extinction are not signaled. Observing responses (i.e. presses on a second lever) produce brief stimuli signaling whether drug is available or not for responses on the first lever. Little is known about how parameters of the drug reinforcer affect drug-stimulus observing.
Objectives: The effects of changes in the unit price (responses/reinforcer magnitude) of self-administered ethanol on rats' observing were examined. Also, the effects of an observing-response-produced ethanol stimulus on ethanol consumption were examined by comparing consumption during signaled and unsignaled periods of ethanol availability.
Methods: Rats self-administered oral ethanol in the observing-response procedure. The unit price of ethanol in the observing-response procedure was increased by increasing the response requirement for ethanol across conditions.
Results: Observing and response rates on the ethanol lever increased and then decreased with increases in the unit price of ethanol. However, ethanol-lever responding and ethanol consumption during periods when ethanol was available were less sensitive to increases in price when the observing-response-produced ethanol stimulus was present.
Conclusions: Observing varies as an orderly function of unit price of a drug reinforcer, and drug stimuli produced by observing responses can make drug consumption less sensitive to increases in price. This procedure may provide an animal model of both attending to drug stimuli and the resultant effects of these stimuli on drug taking.