Temporal discrimination and a free-operant psychophysical procedure.

de Villiers (1980) let pigeons peck freely while lights stayed on for different lengths of time.

The birds chose one key after short lights and another after long lights.

No trials, no breaks—just continuous responding—so the author could watch timing without stopping the session.

The pigeons sorted the durations almost perfectly.

When the team changed how much food each choice earned, the birds only shifted bias—they did not lose the ability to tell time.

The free-operant method gave the same clean data older trial methods gave, but with less downtime.

Reynolds (1966) had already shown pigeons can time their own pauses. de Villiers (1980) adds that free-operant setups do the job without forced trials.

Rutter et al. (1987) later used the same lab and birds to prove reinforcer ratio only nudges bias, not sensitivity—backing the 1980 split.

Pinheiro de Carvalho et al. (2012) then asked if birds use absolute or relative rules in timing. They found absolute control, sharpening the picture de Villiers (1980) started.

Cippola et al. (2014) moved the logic to college students, showing exclusion responding works with durations too—taking the animal model into human work.

If you need a client to keep working while you measure timing—say, staying on task for 10 s before a break—use a free-operant layout. Let the learner respond the whole time, then watch choice shifts to see whether errors are about timing skill or just preference. You get cleaner data and fewer stop-start battles.