Expansion of arbitrary stimulus classes and function‐transfer measured by sorting performances
A five-minute card sort can replace long match-to-sample tests when you want to see if stimulus classes have grown.
01Research in Context
What this study did
Researchers taught adults to link five meaningless shapes into a chain: A→B→C→D→E. They then added new tests to see if the whole class grew when they gave extra training on just one pair (F→C or B→F).
Instead of long match-to-sample drills, they simply asked participants to sort cards into piles. The team checked whether the sorting results matched those from traditional tests.
What they found
Sorting worked. Participants reliably placed all five original shapes, plus the new F shape, into one correct pile. The class had expanded without extra direct teaching.
The order of tests—sorting first or matching first—made no difference. Both sequences showed the same clean expansion.
How this fits with other research
Marin et al. (2022) extends these findings to real words and pictures. They used the same five-node setup and sorting check, but swapped meaningless forms for meaningful ones like 'apple' and a photo of an apple. Expansion still held, showing the method travels beyond abstract lab stimuli.
Ribeiro et al. (2017) looks like a contradiction at first. They used a similar go/no-go format with compound stimuli, yet children with autism reached only symmetry, not full equivalence. The key difference is population, not procedure: neurotypical adults in Arntzen’s study formed full classes, whereas the kids needed more support. The method can work; some learners just need extra steps.
McAleer et al. (2011) adds a warning. They showed that training with overly complex nine-element shapes later causes over-selectivity. Arntzen’s team kept each node as a single simple form, probably why their participants showed broad, flexible classes instead of narrow control.
Why it matters
If you need to check whether a learner has gained new untaught relations, try a quick card sort instead of a 60-trial MTS protocol. It saves time, gives a clear picture, and works for both abstract and meaningful stimuli. Just keep the training stimuli simple to avoid later over-selectivity, and be ready to add extra teaching steps for learners who may need them.
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Join Free →After your next equivalence lesson, hand the learner a small stack of cards and say, ‘Put the ones that go together in one pile.’ Count the piles—if all related cards land together, expansion happened.
02At a glance
03Original abstract
Sorting (SRT) and matching-to-sample (MTS) tests have measured the formation of arbitrary stimulus classes. This experiment used SRT and MTS tests to document the expansion of class size. Thirty-two participants learned 12 conditional discriminations with a linear series training structure (A➔B➔C➔D➔E). SRT tests documented the formation of 5-member classes by 17 of the participants. Thereafter, 6-member class expansion was implemented by FC training. Nine of these 17 participants showed class expansion when tracked with a sequence of an SRT, MTS, and a final SRT test, and the other 8 showed expansion when tracked with a sequence of MTS and SRT tests. Thus, SRT tests documented class expansion, and the sequence of tests did not influence class expansion. The 15 participants who did not form the 5-member classes learned the baselines for new 3-member classes (A➔B➔C) and formed them as documented by an SRT test. Then, 4-member class expansion was implemented by FB training. Expansion was assessed using the above-mentioned testing sequences. All 15 showed class expansion with 100% correspondence between the SRT and MTS performances. Sorting documented the expansion of arbitrary stimulus classes, while the MTS tests showed that the stimuli also functioned as members of equivalence classes.
Journal of the Experimental Analysis of Behavior, 2021 · doi:10.1002/jeab.665