Influences on cocaine tolerance assessed under a multiple conjunctive schedule of reinforcement.

Ho and colleagues gave pigeons daily cocaine before sessions. Birds pecked a key under a conjunctive FI-FR schedule. They had to wait through a fixed interval, then finish a fixed ratio to earn food.

The team varied the FI value across birds. They wanted to see if longer waits changed how fast tolerance to cocaine appeared.

Cocaine first slowed pecking, but after many days the birds returned to baseline speed. This tolerance showed up no matter how long the FI was set.

In short, the ratio part of the schedule drove the effect; the interval length made no difference.

Garcia (1974) first used the same conjunctive schedule with pigeons. That paper looked at single doses of pentobarbital and d-amphetamine, not chronic cocaine. It laid the groundwork Ho later built on.

Byrd (1980) seems to disagree. In chimpanzees, cocaine raised FI key-pressing but left FR pressing unchanged. Ho now shows that after daily exposure, the ratio part still matters for tolerance, not for acute rate changes. Species and question differ, so both findings can stand.

Goldman et al. (1979) also saw cocaine tolerance in pigeons, but under a learning task. Together, these studies say tolerance happens across very different schedules; the common link is repeated drug exposure, not the task details.

If you run repeated sessions with stimulant-like challenges (think caffeine, sugar, or screen time), expect behavior to rebound even if the wait time or delay changes. The critical lever is the response requirement, not the timing rule. When you design interventions, focus on the work demand that stays constant across conditions.