Ratio and Interval Training Figure 2. Reward cues motivate habitual responses more than goal-directed actions. A) Mice underwent Pavlovian conditioning for 8 days where conditional stimuli (CSs) were paired with the same food rewards used in instrumental training. Both ratio and interval groups showed an increase in conditional responding (magazine entries) across training days. The data are expressed as an elevation ratio = (CS entries/ITI entries). B) During the transfer test, mice trained on ratio schedules did not show an increase in lever pressing when the CSs were presented. In contrast, mice trained on interval schedules showed a significant increase in lever pressing during the CS presentations relative to the ITI period. This increase was not selective as it was observed on the lever leading to the same outcome as the CS and the lever leading to a different outcome (i.e. general transfer). C) Mice underwent additional Pavlovian training for 8 days. D) A second transfer test was conducted and this time mice trained on ratio schedules showed a significant increase in lever pressing when the CSs were presented. This increase was not selective and observed on both levers. Mice trained on interval schedules showed increased responding during the CS presentations that was also non-selective. The amount of transfer was significantly larger in the interval group compared to the ratio group. Error bars represent 6 SEM. *p,0.05. doi:10.1371/journal.pone.0048227.g002 devaluing one of the food rewards prior to a choice extinction test. In the ratio group, this procedure led to a selective reduction in responding on the lever associated with the devalued outcome. In contrast, mice trained on interval schedules responded similarly on both levers indicating that their behavior was not mediated by the knowledge of specific response-outcome associations [4]. Whether a response is encoded as an action or a habit is determined, in part, by the correlation between the behavior and the outcome it produces [3,4]. In a ratio schedule there is a strong correlation between responding and reward. The more the animal responds the more food it gets. The less it responds, the less food it gets. On an interval schedule, responding is also required to produce food. However, there is a weak correlation between the amount of responding and the amount of reward. An animal that responds at a very high rate gets the same amount of food as an animal that responds at a low rate. As a result, the experienced correlation between response and reward on an interval schedule is very different from that experienced on a ratio schedule. Previous work has shown that lower correlations promote habit formation [4]. To determine the motivational impact of reward cues on instrumental responding we conducted a Pavlovian-instrumental transfer test in these groups. We found that reward cues exerted a stronger influence on habits than goal-directed actions. This finding is consistent with a previous study that used over-training to produce habitual responding [13]. Together, these results suggest that habits are particularly susceptible to sensory cues that are associated with reward. Habits may have been more responsive to reward cues because our procedures produced a general form of Pavlovian-instrumen- tal transfer. The presentation of conditional stimuli increased responding on the lever associated with the same reward and on the lever associated with a different reward. This result likely occurred because of the similarity between the rewards used in our experiments (two food pellets). We previously observed selective transfer in mice when food pellets and a sucrose solution were used [21]. General transfer is mediated by an emotional response that is common to both outcomes while selective transfer occurs when the CS activates specific sensory features of the reward [13,22]. Habits PLOS ONE | www.plosone.org 4 October 2012 | Volume 7 | Issue 10 | e48227