Selective antagonism of the error-increasing effect of morphine by naloxone in a repeated-acquisition task.

Researchers gave morphine to adults learning a new sequence of button presses each session. They then added naloxone, a drug that blocks morphine, to see what reversed and what stayed.

The task made participants relearn a fresh four-response chain every day. Errors and speed were recorded separately.

Naloxone wiped out the extra errors caused by morphine, but it did not restore the lost speed. When non-opioid drugs were tested, naloxone had no effect, showing the reversal was specific to morphine.

Mellitz et al. (1983) used the same repeated-learning task with phencyclidine, pentobarbital, and amphetamine. They saw that giving the drugs bit-by-bit (cumulative dosing) changed the size of the effect. The 1981 paper kept dosing constant, so its clean morphine-naloxone match may have been helped by this steady method.

Davison et al. (1989) later showed that methadone, another opioid, shifted pigeons from ratio to interval control. Together the two studies tell us opioids can alter WHAT schedule rule an animal follows, not just how fast it responds.

McMillan (1979) found that amphetamine and caffeine simply slowed every response. The 1981 paper goes further: opioids can split error rate from response rate, something stimulants did not do.

If you work with clients on opioid medication, remember that naloxone or buprenorphine may sharpen accuracy while leaving sluggish timing untouched. When you see slow but correct responding, consider separating the cognitive demand from the speed demand instead of just increasing reinforcement rate.