Local contrast in multiple schedules: the effect of stimulus discriminability.
Stimulus discriminability decides whether you see positive local contrast, negative contrast, or none at all.
01Research in Context
What this study did
Researchers worked with pigeons in a lab.
They used multiple schedules with different colors of light.
The colors were close or far apart on the spectrum.
Birds pecked for food during each color.
The team watched how hard the birds worked when colors changed.
What they found
When colors were almost the same, birds pecked more in the rich schedule.
This created positive local contrast.
When colors were very different, the extra pecking vanished.
Negative contrast showed up at any color gap.
Bigger gaps made the drop in pecking stronger.
How this fits with other research
Malone (1976) first said stimulus similarity matters.
Haemmerlie (1983) now shows the exact cut-off for positive contrast.
Parsons et al. (1981) removed stray rewards to reveal contrast.
Haemmerlie (1983) proves the form of contrast still hinges on how well the bird can tell the colors apart.
Reed (1991) later used the same idea to tweak the matching law.
Why it matters
Your client may work harder in one task if the cues look almost the same as another task with richer rewards.
Make sure the SD for the high-pay task looks very different from the SD for the lean task if you want to avoid sudden bursts or drops in responding.
Check your materials for accidental color or shape overlap.
A quick stimulus discrimination test can save you from mystery spikes or dips in data next week.
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02At a glance
03Original abstract
A three-ply multiple schedule assessed responding in a standard component as a function of the just-preceding schedule. The principal experimental condition was the difference among the wavelengths signaling the schedule components. Only the pigeons working in a narrow wavelength range showed persistent positive local contrast; that is, response rate during the standard component was higher when that component followed extinction than when it followed itself. Birds in both narrow- and medium-range groups showed persistent negative local contrast; that is, rate was lower following a relatively rich component. The dissipation of positive contrast appeared to be most clearly related to the establishment of differential responding. Negative contrast was inversely related to wavelength differences. Theories pertaining to contrast must account for the role of discrimination in both positive and negative types.
Journal of the experimental analysis of behavior, 1983 · doi:10.1901/jeab.1983.39-427