The effect of multiple S delta periods on responding on a fixed-interval schedule.

The team used a fixed-interval schedule with pigeons. Birds pecked a key for food every few minutes.

Between food deliveries they added short S-delta periods. These were brief blackout cues that told the bird "no food now."

They wanted to know if these pauses would break the smooth scallop pattern we usually see on FI schedules.

The birds still showed the same curved response pattern. They paused after food, then slowly sped up until the next delivery.

Small local changes happened, but the overall scallop stayed intact. The pattern did not need a smooth response chain to survive.

Gibbon (1967) later showed the scallop can live on brief conditioned reinforcers alone. Together the two papers prove the pattern is not tied to steady food every cycle.

Nevin (1969) re-described the scallop as an abrupt two-state jump: long pause, then quick switch to fast pecking. DEWS (1962) showed the jump still happens even when you poke holes in the middle with S-delta cues.

Kodera et al. (1976) stuffed extra food into the pause and saw almost no change. Both studies agree: once FI timing control is set, outside events have little say.

If you run FI schedules in the lab or use them to build flexible pausing in learners, know the pattern is tough. Brief interruptions, extra cues, or even missing food do not erase the curve. You can add brief S-delta or conditioned-reinforcer flashes without fear of wrecking timing. This frees you to layer instruction or give sensory breaks while the FI rhythm holds steady.