Stimulus generalization: the ordering and spacing of test stimuli.
Pack your generalization probes too tightly and the gradient disappears, the peak shift vanishes, and you may miss true stimulus control.
01Research in Context
What this study did
Stevenson (1966) worked with pigeons to see how test-stimulus spacing changes generalization curves.
Birds first learned to peck a colored key when it showed one brightness.
Later the researcher showed many brightnesses, either close together or far apart, and recorded how much the birds pecked each one.
What they found
When the test lights were packed close, the generalization curve flattened.
Wide spacing kept a sharp peak and also made the classic “peak shift” appear.
In short, crowding the probes blurs stimulus control.
How this fits with other research
Schroeder et al. (1969) extended the same lab setup by adding a 24-hour gap between training and testing.
They also saw flatter curves after the delay, but a quick warm-up brought the steep slope back.
Raslear (1975) conceptually replicated the spacing idea with rats hearing different sound volumes.
Closer probe sounds pulled the animals’ “middle” judgment toward one end, showing the spacing rule works across species and senses.
MIGLER (1964) warned that averaging birds together can hide these orderly patterns, so single-case plots are essential.
Why it matters
When you probe for generalization in clinic or classroom, keep your test items far enough apart to see a clear gradient.
If you must use many close stimuli, expect a flatter curve and do not mistake it for poor discrimination.
Add a brief warm-up if the learner last touched the task yesterday; Schroeder et al. (1969) showed that tiny refresher keeps the gradient sharp.
Plot each learner’s data separately, following MIGLER (1964), before you average.
Want CEUs on This Topic?
The ABA Clubhouse has 60+ free CEUs — live every Wednesday. Ethics, supervision & clinical topics.
Join Free →Space your probe stimuli at least two clear steps apart and run a quick warm-up trial if the learner last practiced yesterday.
02At a glance
03Original abstract
Twenty-four pigeons learned a successive discrimination between 500 mmu (S+) and 574 mmu (S-). When tested in extinction, some birds received stimuli around S+, with no S- presentations. These birds showed a positive peak shift, with maximum responding not at 550 mmu, but displaced to 538 mmu and 544 mmu. Other birds were tested with stimuli around S-, with no S+ presentations. These birds showed a negative shift, with least responding not at 574 mmu, but at 586 mmu. Though the first group was tested around S+ and the second around S-, total responding between groups did not differ. When retested on the other half of the continuum, however, birds that had gone from the S+ half to the S- half responded fewer times than those that had gone from the S- half to the S+ half. In a second experiment, reducing stimulus spacing from 6 mmu to 2 mmu produced flatter gradients and decreased the amount of positive shift. In a third experiment, birds were tested across the whole continuum with stimuli presented in serial order. A sequence from 538 mmu to 586 mmu produced no responding after the first part of the session; a sequence from 586 mmu to 538 mmu produced responding throughout the session.
Journal of the experimental analysis of behavior, 1966 · doi:10.1901/jeab.1966.9-457