Stimulus-specific contrast effects during operant discrimination learning.
Response rates spike next to extinction stimuli and dip next to reinforced stimuli, especially when the discrimination is tough.
01Research in Context
What this study did
Researchers watched pigeons peck keys during discrimination training. They lined up color stimuli like a rainbow and picked two as S-plus and S-minus.
Birds got grain for pecking near the reinforced color and nothing near the extinction color. The team then checked how hard the birds pecked at colors in between.
What they found
Pecking sped up next to the extinction color and slowed next to the reinforced color. This contrast effect was stronger when the two key colors looked more alike.
The harder the discrimination, the bigger the swing in response rate.
How this fits with other research
Reynolds (1966) saw the opposite: after long training, contrast faded away. The two studies seem to clash, but S kept birds training for many more sessions. Short runs show strong contrast; long runs wash it out.
Richards (1974) ran a near-copy of the setup and also found a brief peak shift toward the extinction edge. The 1975 paper adds that the size of that shift grows with discrimination trouble.
Locurto et al. (1980) followed up and showed these local bumps actually bend the whole generalization curve. Early contrast spikes are not noise; they sculpt what the bird learns next.
Why it matters
When you teach a new discrimination, expect hot spots and cold spots next to your S-plus and S-minus. If the stimuli look almost the same, the swing can be large. Track these side-effects early; they can guide later generalization. If the swing is unwanted, keep training—data say it calms down with more sessions.
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Join Free →Graph responses to the two stimuli beside your S-plus and S-minus; adjust teaching blocks if you see big unwanted swings.
02At a glance
03Original abstract
In two experiments, pigeons' responding was equally reinforced in the presence of four line-orientation stimuli. Responding was then reinforced when only two of the four orientation stimuli were present; the remaining two orientations appeared during extinction. Response rates were often highest in the stimulus adjacent to the orientations presented during extinction and often lowest in that orientation adjacent to the orientations presented with reinforcement. These effects were stronger and more persistent when the stimuli were separated by a smaller angle, rendering the discrimination more difficult. These and other data suggest that discrimination training may not be accurately explained in terms of the simple effects of reinforcement and nonreinforcement associated with isolated stimuli, nor by accounts that depend upon stimulus generalization. Recent accounts of contrast that depend upon "emotionality" produced by nonreinforced responding or upon reinforcement-elicited responses are also difficult to apply to these data.
Journal of the experimental analysis of behavior, 1975 · doi:10.1901/jeab.1975.24-281