Assessing unit-price related remifentanil choice in rhesus monkeys.

Researchers let rhesus monkeys choose between two levers. Each lever delivered the opioid remifentanil through an IV line.

The catch: one lever gave a bigger dose but required more lever presses. The team kept raising the 'price' (responses per milligram) to see when the monkeys would switch to the cheaper dose.

Even when the big dose cost twice or three times as many presses, the monkeys still picked it most of the time.

Only at sky-high prices did preference flip. Standard unit-price curves predict a smooth decline; these curves stayed flat until the cost was extreme.

Delmendo et al. (2009) ran a similar price test with kids and candy. Children quickly traded down when the price rose. The monkey data look like a contradiction, but the difference is reinforcer type: candy is mild; remifentanil is an ultra-fast, potent drug.

Catania et al. (1974) saw neat dose-based matching with cocaine. Remifentanil breaks that pattern. The authors argue the drug's ultra-short half-life makes each large hit extra reinforcing, so the usual price rule collapses.

Killeen (1978) showed monkeys defend daily calories when food and water costs change. The remifentanil study finds a parallel: the animals defend large-dose intake until the response cost is almost prohibitive.

If you use economic logic to set reinforcement schedules, remember that magnitude can override price for highly potent reinforcers. In clinical practice, raising effort may not cut consumption until the requirement is extreme. Pair high effort with alternative, lower-magnitude reinforcers to loosen the grip of the preferred one.