An investigation of peak shift and behavioral contrast for autoshaped and operant behavior.
Peak shift can travel across body parts and reinforcers, so watch for gradient bumps wherever you run tight discrimination training.
01Research in Context
What this study did
Varley et al. (1980) asked whether peak shift shows up only with key pecks or also with other moves. They used pigeons that stepped on a treadle for food or to turn off loud noise. After the birds learned to tell two colors apart, the team ran generalization tests to see where the birds now responded most.
The same birds also had key-peck sessions with grain. This let the authors compare peak shift across two body parts and two kinds of reinforcement.
What they found
Treadle pressing showed a clear peak shift. The birds stepped fastest to a color that was even farther from the bad color than the one they were trained on. This happened whether the reward was food or escape from noise.
Key pecks did not shift. The birds pecked most at the exact color that had always paid off. Same birds, same lab, but different response, different result.
How this fits with other research
Reynolds (1968) already showed that suppressing one stimulus pushes the whole gradient away. C et al. prove this push works for foot responses and for negative reinforcement, not just key pecks with grain.
Reynolds (1966) warned that long training can wipe out peak shift. C et al. used standard-length training, so their shift stayed. The two papers do not clash; they map when the effect appears and when it fades.
Fujita (1983) later got positive contrast with the same treadle response. Together the studies tell us the pigeon's foot is sensitive to both contrast and peak shift, giving you more options than the usual key peck.
Why it matters
If you run discrimination programs, do not assume peak shift is a key-peck quirk. Expect learners to respond strongest to stimuli that are "extra safe" after tough discriminations, whether the response is a hand push, switch flip, or step. When you see odd generalization blips, check if the topography or reinforcer type could be letting, or blocking, the shift.
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02At a glance
03Original abstract
Instrumental treadle press and nonreinforced key peck responses were monitored during discrimination training and generalization testing in pigeons on positive and negative reinforcement schedules. In Experiment 1, six pigeons pressed a treadle for food on a multiple variable-interval extinction schedule. In Experiment 2, three pigeons pressed a treadle to avoid shock on a multiple free-operant avoidance extinction schedule. Different color keylights signaled S+ and S- components. Some positive behavioral contrast occurred during discrimination training, but the effect was small. Pecking occurred to the S+ keylight in Experiment 1 but not in Experiment 2. On stimulus generalization tests, all subjects displayed a positive peak shift when pressing the treadle for food or to avoid shock. However, peak shift was not found for nonreinforced "autopecks" on the stimulus key, although an area shift was observed in Experiment 1. This is the first demonstration of peak shift for pigeons pressing treadles and the only reliable demonstration of peak shift when negative reinforcement maintained responding. These results, in combination with previous demonstrations of peak shift for rats pressing levers and pigeons pecking keys, indicate that peak shift is a general by-product of operant discrimination learning, since it occurs across a variety of the organisms, responses, and reinforcers.
Journal of the experimental analysis of behavior, 1980 · doi:10.1901/jeab.1980.33-101