The Effect of Ratio and Interval Training on Pavlovian- Instrumental Transfer in Mice Brian J. Wiltgen1,2*, Courtney Sinclair2, Chadrick Lane2, Frank Barrows2, Martı´n Molina2, Chloe Chabanon-Hicks2 1 Center for Neuroscience, Department of Psychology, University of California Davis, Davis, California, United States of America, 2 Department of Psychology, University of Virginia, Charlottesville, Virginia, United States of America Abstract Conditional stimuli (CS) that are paired with reward can be used to motivate instrumental responses. This process is called Pavlovian-instrumental transfer (PIT). A recent study in rats suggested that habitual responses are particularly sensitive to the motivational effects of reward cues. The current experiments examined this idea using ratio and interval training in mice. Two groups of animals were trained to lever press for food pellets that were delivered on random ratio or random interval schedules. Devaluation tests revealed that interval training led to habitual responding while ratio training produced goal-directed actions. The presentation of CSs paired with reward led to positive transfer in both groups, however, the size of this effect was much larger in mice that were trained on interval schedules. This result suggests that habitual responses are more sensitive to the motivational influence of reward cues than goal-directed actions. The implications for neurobiological models of motivation and drug seeking behaviors are discussed. Citation: Wiltgen BJ, Sinclair C, Lane C, Barrows F, Molina M, et al. (2012) The Effect of Ratio and Interval Training on Pavlovian-Instrumental Transfer in Mice. PLoS ONE 7(10): e48227. doi:10.1371/journal.pone.0048227 Editor: Xiaoxi Zhuang, University of Chicago, United States of America Received June 27, 2012; Accepted September 21, 2012; Published October 29, 2012 Copyright: 2012 Wiltgen et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This research was supported by a grant from the National Institute on Drug Abuse (R03DA029165-01) to B.J.W. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: bjwiltgen@ucdavis.edu Introduction Humans and animals learn about the consequences of their actions via instrumental conditioning [1]. This process increases the probability of adaptive behavior through reward learning and reduces inappropriate responding via punishment [2]. The ability to associate our actions with the outcomes they produce (i.e. goal- directed learning) allows us to select responses that are appropriate for particular situations and motivational states. Although many of our behaviors are initially goal-directed, they become habitual when continuously rewarded [3,4]. Unlike actions, habits are not controlled by their consequences and tend to be automatically and reflexively elicited by stimuli in the environment. The transition from action to habit is normally adaptive, as it allows reliably reinforced behaviors to become efficient and automatic [5]. However, this process can also be maladaptive in situations like those leading to drug addiction [6–9]. Drug seeking is initially goal-directed and maintained by the rewarding effects produced by drugs of abuse. However, after repeated experiences behavior becomes automatic and independent of its consequences. This fact explains many features of drug addiction that make it difficult to overcome. For example, drug-seeking behavior persists even when drugs are no longer rewarding to the addict and instead produce many unwanted, aversive consequences [8,10]. Another feature of addiction is that abstinence is very difficult to maintain. Two motivational factors are thought to contribute to this: exposure to drug-related cues and time-dependent increases in craving and desire that accompany withdrawal (i.e. incubation) [6,10]. These motivational processes can be modeled in animals, which has led to a detailed characterization of the anatomical circuits and the cellular and molecular mechanisms that underlie the motivation of goal-directed actions [6–8,10–12]. However, much less is known about the effects of reward cues on habitual responding [13]. This is a critical gap in our knowledge, as addictive behavior in humans is largely characterized by automatic, stimulus controlled, habit-like responding [6,10]. Therefore, the goal of the current study was to determine if habitual behaviors are more strongly motivated by reward cues than goal-directed actions. Actions and habits can easily be studied in rodents. Habits are defined as instrumental behaviors that are insensitive to changes in reward value. For example, devaluing a reward by pairing it with illness will reduce instrumental actions but not habits that produce the same reward [14–16]. Habits often develop with time and can be fostered with specific training procedures. For example, ratio schedules, where reward delivery is contingent on the number of responses made, promote the development of action-outcome associations. In contrast, interval schedules, where reward delivery is contingent on responding after a specific amount of time has passed, promote the development of habits [17–19]. In the current experiments, mice were trained to lever press for food reward that was delivered after a certain number of responses had been made (i.e. ratio schedule) or after a certain amount of time had elapsed since the previous reward (i.e. interval schedule). Each of these procedures is described in detail below. After confirming that these schedules led to actions and habits, respectively, we then examined the impact of reward cues on responding. PLOS ONE | www.plosone.org 1 October 2012 | Volume 7 | Issue 10 | e48227