Discrimination of highly similar stimuli as members of different equivalence classes

Learning to discriminate between physically similar stimuli as members of different classes can be relevant in certain situations. This study investigated effective methods of displaying two pairs of quasi-identical stimuli, as samples and/or comparisons, during the training of baseline conditional discriminations. The goal was to enable participants to form three 3-member equivalence classes and discriminate similar stimuli as members of distinct equivalence classes. Eighteen adults underwent arbitrary relations (AB/AC) training. A multiple-probe design assessed maintenance and emergence of stimulus relations. Participants were randomly assigned to one of six training conditions across three experiments. Conditions 1, 2, and 5 presented quasi-identical stimuli successively as samples during training. Condition 3 presented quasi-identical stimuli successively as comparisons, whereas Condition 4 presented quasi-identical stimuli simultaneously as comparisons. Condition 6 presented each pair of quasi-identical stimuli simultaneously as a sample and a comparison. Condition 4 uniquely resulted in successful equivalence class formation for all participants. Conditions 3 and 6 failed to form equivalence classes, whereas Conditions 1, 2, and 5 did not yield baseline learning. These findings highlight the relevance of presenting quasi-identical stimuli as simultaneous comparisons (Condition 4) to foster equivalence class formation. Understanding optimal training conditions has implications for discussions on the acquisition of simple discriminations required in training simultaneous conditional discriminations.