Adjunctive behavior is not entirely dead, it just changed its name! Gabriela E. López-Tolsa and Ricardo Pellón Universidad Nacional de Educación a Distancia


Adjunctive behavior is not entirely dead, it just changed its name!
Gabriela E. López-Tolsa and Ricardo Pellón
Universidad Nacional de Educación a Distancia

A recent blog entry wondering what ever happened to adjunctive behavior was posted on the ABAI history blog recently. In that entry, Kupfer and Allen (2023) point out towards the relevance of adjunctive behavior as an understudied phenomenon that could be related to several human problematic behaviors.

Answering their question as whether adjunctive behavior is dead or just on remission: it is alive, and well, just had a change of identity! Adjunctive behavior was initially described in 1961 by Falk, who reported that food-deprived rats would drink excessively when exposed to an intermittent food schedule (Falk, 1961). This is what we now know as schedule-induced drinking (SID; formerly, psychogenic polydipsia, adjunctive polydipsia, schedule-induced polydipsia), the most relevant example of adjunctive (schedule-induced) behaviors (Pellón et al., 2020). In recent years, the term schedule-induced behavior –mentioned only in the last paragraph of Kupfer and Allen’s blog entry– has been favored over the term adjunctive behavior, as it is our opinion that it better describes its origin and maintenance mechanisms (López-Tolsa, 2019; López-Tolsa & Pellón, 2021), given that it is not currently considered as adjunct to any other primary behavior, but principal behavior whose explanation is the basis of voluntary behavior in general (Segal, 1972).

After the initial understanding of schedule-induced behaviors as a different type of behavior that was neither respondent nor operant, but a result of sensitization (Wetherington, 1982), or that was correlated with moments in which probability of reinforcement was low (after Staddon, 1977), a great amount of research carried out in the last 35 years points towards schedule-induced behavior being a positively reinforced operant (Killeen & Pellón, 2013). The development of SID is a reliable phenomenon that occurs under a variety of food-reinforcement schedules, including fixed and variable time and interval schedules, and ratio schedules. Essentially, if there is a bottle of water available in the experimental chamber and an appropriate inter-reinforcement interval duration (typically, between 10 and 120 s), food-deprived rats receiving food intermittently will develop SID. Moreover, SID is sensible to variables to which more traditional operant behavior is sensitive, such as lick-contingent food delays (Lamas & Pellón, 1997; Pellón & Blackman, 1987; Pellón & Pérez-Padilla, 2013; Pérez-Padilla & Pellón, 2007), delays to reinforcement initiated by licks (Pellón & Pérez-Padilla, 2013), level of contingency between lick occurrence and reinforcement delivery (Álvarez et al., 2016), and manipulation of different dimensions of the reinforcers (Castilla & Pellón, 2013).

Furthermore, schedule-induced behavior is the center of a relatively recent debate regarding the mechanisms responsible for the organization and maintenance of behavior. There seems to be an agreement on the idea that reinforcers affect not only the immediately preceding or target behaviors, but all behaviors that occur during the inter-reinforcement intervals (Catania, 1971; Catania et al., 1988; Killeen, 1994, 2023; Killeen & Pellón, 2013; López-Tolsa & Pellón, 2021). Schedule-induced and target behaviors –a separation that is likely made by the experimenter, not by the organism behaving– compete with each other (Pellón & Killeen, 2015), and are organized onto relatively invariant patterns of behaviors that, if the conditions are not changed, are likely to be repeated (López-Tolsa & Pellón, 2021; Ruiz et al., 2016). On the other hand, whether the process responsible for the selection of behaviors is the molecular reinforcement or the molar induction is still quite open to debate (Baum, 2018; Killeen, 2023). In any case, it is the authors’ opinion, that the study of schedule-induced behaviors and their distribution and relationship among them, may aid in the resolution of such debate.

On a more translational note, schedule induction has been considered as the main mechanism to support the development of running in the animal model of anorexia nervosa named activity-based anorexia (ABA). In the ABA procedure, rats are given access to food for one hour, and access to a running wheel for the rest of the day. Under these conditions, rats will develop an excessive amount of running in the wheel that translates into a rapid weight lose that, if the experiment is not stopped, could lead to the death of the rats. Changes in the distribution of running throughout the day point towards running being strengthened due to the intermittency of the delivery of food (a fixed time 24 h food schedule) (de Paz et al., 2020). Moreover, schedule-induced behaviors have also been used as animal models for other excessive behaviors, such as compulsive disorder (Moreno & Flores, 2012) and have been related to other relevant basic phenomena such as timing (López-Tolsa & Pellón, 2021; Ruiz et al., 2016) and impulsivity (Ramos et al., 2019; Sjøberg et al., 2021).
We agree with Kupfer and Allen (2023) that schedule-induction should be further studied, and that it might be responsible for a greater percentage of human responses than we could have initially predicted. Understanding how behaviors might be acquired through their proximity –not contiguity– with reinforcers that were arranged to strength other specific target behaviors, seems like a relevant endeavor.


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