Contrast effects in multiple fixed-interval reinforcement schedules.
Expect quick, short-lived rate changes when learners move from rich to lean fixed-interval schedules.
01Research in Context
What this study did
Innis (1978) watched pigeons peck on two back-to-back fixed-interval schedules.
The first schedule lasted 3 min, the second 1 min.
Birds switched between these two timers many times a session.
What they found
Right after the switch, birds slowed down in the 3-min part.
They sped up in the 1-min part.
These blips faded fast—most change sat in the first minute of each part.
How this fits with other research
Fantino (1969) saw the same quick blips years earlier.
Green et al. (1975) found a late surge before a switch to no food.
That looks like a clash, but the difference is schedule type: L used variable-time, K used fixed-time.
McLean et al. (1981) later showed that early-minute effects shrink as the part drags on, backing K’s short-window claim.
Why it matters
If you run mixed DRL or FI sessions, expect a brief dip or jump right after each schedule change.
Measure the first minute separately; that is where contrast lives.
Plan prompts or extra cues for that window so the dip does not look like skill loss.
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02At a glance
03Original abstract
Pigeons were exposed to a multiple fixed-interval one-minute fixed-interval three-minute schedule of reinforcement following training on either a multiple fixed-interval one-minute fixed-interval one-minute schedule or a multiple fixed-interval three-minute fixed-interval three-minute schedule. For all birds, large negative local contrast effects developed during the first of four three-minute intervals in a component; response rate was depressed and postreinforcement pause lengthened in this interval. Positive local contrast effects were evident during the first of 12 one-minute intervals in a component for five of six birds; at asymptote, the pause was very short and response rate slightly elevated during this interval. Overall positive contrast was generally transient and varied considerably across subjects, while overall negative contrast effects, if they occurred, appeared only after a large number of sessions.
Journal of the experimental analysis of behavior, 1978 · doi:10.1901/jeab.1978.29-233