“…It is unclear whether, under the current procedures, contingent houselight illumination functioned as a signal helping to differentiate functional units of behaviors, as a conditioned reinforcer established through some unidentified operation, or as an unconditioned reinforcer. As previously cited, numerous studies have shown that the onset of illumination can function to reinforce responding (e.g., Girdner, 1953;Kish, 1955;Kling, Hurwitz, & Dalhagen, 1956;Marx, Henderson, & Roberts, 1955;Reed, Collinson, & Nokes, 1995;Segal, 1959;Stewart, 1960).…”