Local contrast in behavior allocation during multiple-schedule components.
Local contrast shows up early in a schedule part, and a small time tweak fixes the matching law.
01Research in Context
What this study did
Reed (1991) watched pigeons work on a two-part schedule. One part always paid the same. The other part switched between rich and lean pay.
The team tracked how the birds split their pecks minute by minute. They wanted to see if pay changes next door altered behavior right away.
What they found
In the first quarter of the steady-pay part, the birds shifted their pecks. This quick shift is called local contrast.
The old matching law missed this early bump. A small time tweak in the equation fit the data better.
How this fits with other research
Dews (1978) saw the same early bump and said it came from Pavlovian links between key color and payoff. Reed (1991) keeps that idea but shows the matching law can still work if you let time in.
Parsons et al. (1981) found contrast only after they removed accidental pay for wrong pecks. Reed (1991) used those clean controls, so the new math builds on their setup.
White (1995) showed that stimulus control fades the longer a part lasts. Reed (1991) fits this decay: contrast is strongest early, then fades as control weakens.
Why it matters
If you run multiple schedules with clients, expect quick spikes or drops right after a switch. These early blips are real, not noise. Track the first minute separately and you will see local contrast that the plain matching law hides. Add a short time window to your data sheet and you will make better predictions.
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02At a glance
03Original abstract
Allocation of responses between two keys was studied during two alternating multiple-schedule components. Responses were recorded in successive quarters of each component. Variable-interval reinforcer schedules on the two keys were constant throughout the experiment for one (constant) component and were varied over conditions on one key for the other, producing changes in reinforcer ratios for the varied component. Behavior allocation for the first quarter of the constant component was inversely related to varied-component reinforcer ratios, a form of local contrast, but this relationship was not observed later in the component. During the first quarter of the varied component, slopes of matching lines were high and decreased later in the component. It is argued that this form of local contrast cannot be explained in terms of reallocation of extraneous reinforcers between components, and that the matching law for concurrent operants does not capture some sources of control over behavior allocation. A simple extension of the matching law is offered that adequately describes behavior changes during both components. A version of this formulation can predict contrast effects in absolute response rates.
Journal of the experimental analysis of behavior, 1991 · doi:10.1901/jeab.1991.56-81