Responding changes systematically within sessions during conditioning procedures.

McSweeney et al. (1993) watched rats press a lever for food during one long session. The schedule stayed the same the whole time — no extra cues, no changes.

They counted how fast the rats pressed every few minutes to see if the rate moved even when nothing else changed.

Response rate did not stay flat. It climbed early, then slid down for the rest of the hour.

The same pattern showed up again when they used water instead of food.

Lowe et al. (1995) ran the same setup with water and saw the same climb-then-fall curve. This direct replication tells us the drift is real, not tied to one reinforcer.

Lewon et al. (2019) went further. They gave mice a bite of food right before a water session. Water responding jumped even higher early on. Their work extends the 1993 finding by showing that a quick change in hunger can nudge the within-session curve up or down.

Schneider et al. (1967) looks like a clash at first — shock paired with food made rats press faster, not slower. But the shock was brief and signaled more food, so the early boost fits the same rule: early minutes can spike when something raises the value of the reinforcer.

If you run a 30-minute discrete-trial program, do not assume the client’s motivation is steady. Probe early, then probe again at minute 20. You may see faster correct responses at the start and a slow fade after 15 minutes. Short breaks or a quick preference check mid-session can reset the curve and save you from labeling the later dips as “non-compliance.”