Local contrast and Pavlovian induction.
Stimulus similarity tunes local contrast—expect short-lived spikes or dips when new cues resemble the old ones, but only if procedural confounds are gone.
01Research in Context
What this study did
Malone (1976) looked at how stimulus similarity changes local contrast.
The team used multiple schedules with pigeons. They switched the color or line angle between rich and lean components.
They watched whether pecking spiked or dropped right after each switch.
What they found
Both positive and negative contrast showed up, but only when the two stimuli looked somewhat alike.
The jumps were brief and came back each time new S+ or S- cues appeared.
This hints that Pavlovian induction, not just different pay rates, drives the brief bursts.
How this fits with other research
Dews (1978) soon backed the Pavlovian idea: signal-key pecks rose when the key merely signaled upcoming rich pay, even though the schedule stayed the same.
Parsons et al. (1981) later pushed back. They got contrast only after they removed accidental reinforcement and separated cue control from rate shifts. Their data say the effect is fragile, not automatic.
Haemmerlie (1983) widened the lens. With wavelength cues, narrow color gaps gave positive contrast, while bigger gaps gave stronger negative contrast. Together the three papers show stimulus control is powerful, but procedural details decide whether you see the effect.
Why it matters
Your client may show brief response bursts or dips when tasks change. Check how different the new cue looks or sounds. If it is only a small change, watch for a quick spike; if it is very different, watch for a drop. Before you call it contrast, rule out accidental reinforcement or timing quirks. These checks keep your data clean and your interventions sharp.
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Join Free →When you swap task cues, record the first 30 s of responding—if you see a jump or drop, note how alike the old and new cues are before deciding it is true contrast.
02At a glance
03Original abstract
Two experiments examined the effects of number and similarity of stimuli on local contrast. In the first experiment, local contrast effects differed in magnitude as a function of the similarity among stimuli; greater positive local contrast appeared when stimuli were less similar, though this effect sometimes reversed for very dissimilar stimuli. In the second experiment, both positive and negative local contrast appeared transiently during the course of training a discrimination including two quite dissimilar stimuli. When two new stimuli were added, both effects reappeared in several cases. The effects remained when the discrimination was rendered more difficult by substituting a new stimulus very similar to one of the original pair. These and other data suggest that local contrast depends on the same factors that produce Pavlovian induction; in the absence of an alternative account, Pavlov's interpretation may be useful in suggesting further research that will help identify the mechanisms involved in both classical and operant discrimination learning.
Journal of the experimental analysis of behavior, 1976 · doi:10.1901/jeab.1976.26-425