The development of functional response units: The role of demarcating stimuli

An experiment with rats examined the roles of demarcating stimuli and diffe rential reinforcement probability oil the development of Functional respons e units. It examined the development of units in a probabilistic, free-oper ant situation in which the presence of demarcating stimuli was manipulated. In all conditions, behavior became organized into two-response sequences f ramed by changes in local reinforcement probability. A tone demarcating the beginning and end of contingent response sequences facilitated the develop ment of functional response units, as in chunking, but the same units devel oped slowly in the absence of the tone. Complex functional response units d eveloped even though reinforcement contingencies remained constant. These f indings demonstrate that models of operant learning must include a mechanis m for changing the response unit as a function of reinforcement history. Ma rkov models may seem to be a natural technique for modeling response sequen ces because of their ability to predict individual responses as a function of reinforcement history; however, no class of Markov chain can incorporate changing response units in their predictions.