Discrimination learning, the peak shift, and behavioral contrast.
Cutting response rate to one stimulus reliably pumps up responding to the other and slides the peak away, even when food odds stay the same.
01Research in Context
What this study did
Pigeons pecked keys for food. The birds learned to tell two colors apart. One color always paid off. The other never did.
The team then cut the birds’ peck rate to the no-pay color. They kept food odds the same for both colors. They watched what happened next.
What they found
When pecking to the no-pay color dropped, two things showed up. First, the birds pecked faster to the pay color. That is behavioral contrast.
Second, the birds’ peak response moved away from the no-pay color. That is peak shift. Both effects came from lower response rate, not from food odds.
How this fits with other research
Rogers-Warren et al. (1976) saw zero contrast when they also kept food odds equal. The key difference: they did not force response rate down. Reynolds (1968) says rate drop is what matters.
Reynolds (1966) found that long training wipes out contrast and peak shift. The new data agree: both are short-lived side effects, not lasting learning.
Bayaruga et al. (2023) later saw the same contrast in kids asking for attention. A black cap acted like the no-pay color. When the cap came off, mands doubled. The lab rule holds in class.
Why it matters
You now know contrast and peak shift live or die by response rate, not reinforcer count. If a client stalls on one task and then races through the next, check if you accidentally cut responses on the first task. Ease the response demand and the swing should calm down.
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02At a glance
03Original abstract
A discrimination between two successively alternating stimuli was trained under conditions that maintained equal frequencies of reinforcement in the presence of each of the discriminative stimuli (S1 and S2) but that also reduced the rate of responding to S2. These conditions included a multiple variable-interval differential-reinforcement-of-low-rate schedule and a multiple variable-interval variable-interval schedule in which responses to S2 were punished. Whenever the rate of responding to S2 was reduced, rate of responding to S1 (behavioral contrast) increased, and the peak of a subsequently obtained generalization gradient did not occur at the expected location (between S1 and S2) but was displaced away from S2, below S1. Discrimination training in which the frequencies of reinforcement earned in S1 and S2 were not equal (variable-interval 1-min variable-interval 5-min training) produced contrast and the peak shift only if the rate of responding to S2 had been reduced, as after non-differential reinforcement in which variable-interval 1-min schedules were correlated with S1 and with S2. It was concluded that a sufficient condition for the occurrence of behavioral contrast and the peak shift was reduction of the rate of responding to one of two alternating discriminative stimuli and that a peak shift will occur only if contrast had occurred during discrimination training.
Journal of the experimental analysis of behavior, 1968 · doi:10.1901/jeab.1968.11-727