Covert verbal mediation in arbitrary matching to sample.

Covert verbal mediation was examined in an arbitrary matching-to-sample (MTS) preparation with a high-verbal group (college students) and a low-verbal group (adults with intellectual disabilities). Arbitrary relations were established between nonsense words, visual symbols, objects, and hand signs. Task difficulty was balanced for the groups based on errors during acquisition. All participants experienced a hand sign condition, and three MTS conditions each with a unique configuration of the comparison array: fixed location, random location, and all symbols the same. The same symbol condition was designed to impede a participant's ability to label individual symbols. The results showed that disrupting labeling adversely affected MTS performance for high-verbal participants, but not for low-verbal participants. The data suggest that high-verbal participants depended on mediating verbal behavior and joint control to assist them in finding the correct comparison stimulus. Low-verbal participants could not benefit from verbal mediating variables and likely relied on unmediated contingencies, or some form of nonverbal mediation. For the high-verbal group, 19 different putative emergent relations were identified as occurring at various stages of acquisition between the sample stimulus and the selection response. These emergent relations likely provided supplementary sources of stimulus control that participated in evoking MTS selection behavior.