USE OF MASK PROCEDURE TO VERIFY STIMULUS CONTROL RELATIONS IN CAPUCHIN MONKEYS

Abstract

Subtle differences between the stimulus control relations that are programmed by the experimenter and those effectively establi­shed in discriminative training are frequently found in non-human subjects or human participants with severe developmental disabi­lities. Most of the times, such differences are only identified through testing procedures. The present study evaluated the viability of using mask procedure to verify stimulus control relations in conditional discriminations (identity matching to sample) with monkeys as subjects. Two capuchin monkeys (Cebus cf. apella) participated. The procedure comprised identity matching to sample training and tests for stimulus control relations with a blank comparison (mask) replacing sometimes S+ sometimes S–. By selectively removing those stimuli in probe trails, we evaluated their participation in determining different types of controlling relations: Control by S– or Reject Control; Control by S+ or Select Control, or both, that is, Mixed Control. Results show that (1) baseline discriminations did not deteriorate during the tests and (2) choice responses in probe trials let successfully infer the on-going stimulus control relations. Subject M09 presented initially Reject Control for one of the tested relations and Mixed Control for the other relations. Subject M16 presented Mixed Control for all tested relations. The findings suggest that mask procedure is adequate to evaluate stimulus control in conditional relations with monkeys. We encourage its use in future studies on equivalence class formation with non-humans to verify if in fact the arbitrary Sample-S+ conditional relations (i.e., Select or Mixed control) were established. Keywords: mask procedure, identity matching, Cebus cf. apella.