participants who are reinforced with feedback contingent on selecting the correct comparison stimulus that matches the sample during training sessions (i.e., selecting B 1 when presented with A 1 ).Tests for the emergence of untrained relations occur in the absence of reinforcement after these initial relations are taught. If the participant responds with a high level of accuracy on testing trials for emergent relations, it is inferred that the stimuli have joined in an equivalence class and now function interchangeably.While the direct reinforcement used in MTS accounts for the acquisition of the explicitly taught relations, it does not account for the emergent relations that appear in testing phases [9,10]. This finding limits the extent to which operant conditioning alone can explain equivalence class formation because the emergent relations themselves occur without any explicit reinforcement or training. In an attempt to examine the necessary and sufficient conditions for the emergence of untrained relations, Leader and Barnes-Holmes [11] demonstrated a procedure that resembles respondent rather than operant conditioning procedures called respondent-type training (ReT) that can reliably facilitate emergent relations between stimuli without any response requirement on the part of the participant. In ReT, the participant only needs to observe the stimuli as they occur in front of them. The stimuli are "paired" through contiguous temporal arrangement with one another. Shorter delays occur within pairs of