To determine the role of the interpositus nuclei of cerebellum in rule-based learning and optimization processes, we studied (1) successive transfers of an initially acquired response rule in a cross maze and (2) behavioral strategies in learning a simple response rule in a T maze in interpositus lesioned rats (neurotoxic or electrolytic lesions). Even though lesioned animals showed no impairment in learning the initial stimulus-response association, they had difficulties in transferring the acquired adapted response rule, and in optimizing their response strategy. These results add information on the role of interpositus nuclei in adaptation to environmental changes.Over the past decades, cerebellar circuits have been shown to play a major role in the process by which an action becomes automatic with practice in humans (Jenkins et al. 1994;Doyon et al. 1998Doyon et al. , 2002Hubert et al. 2007), and thus in the automatic execution of over-learned cognitive routines. Moreover, whereas the initial acquisition guided by reward or other feedback stimuli seems to be normal in cerebellar patients, reversal learning or transfer of an acquired response rule to a new task is impaired Ramnani 2008, 2011;Thoma et al. 2008). In animal models of cerebellar dysfunction, rule-learning deficits and impaired optimization were similarly observed. A deficit in learning the procedural component of spatial tasks was pointed out after hemicerebellectomy (for reviews, see Petrosini et al. 1998;Rondi-Reig and Burguière 2004) or in L7-PKCI transgenic mice (Burguière et al. 2005). Deep nuclei lesions were shown to play a particular role in acquisition and reversal learning (Joyal et al. 2001). More recently, lesions of the interpositus nuclei were shown to prevent the development of habit-based behavior with overtraining, without altering the initial goal-directed behavior, suggesting that deep nuclei are part of the network involved specifically in optimization processes and habit formation (Callu et al. 2007). Similarly, using the mouse mutants L7-PKCI, Burguière et al. (2010) showed that prefrontal cortex-Purkinje cell long-term depression (LTD) is not required for the initial goal-directed behavior but is required for optimization of motor responses.To determine the role of the cerebellum in rule-based learning and optimization processes, we first compared interpositus lesioned rats (IN) with Sham-operated rats in learning a simple response rule in a T maze and then transferring this rule to different maze configurations. Second,, we used Packard and McGaugh's paradigm (1996) in which animals (IN lesioned and Sham) were trained in a T maze to determine the preferential strategy by which they resolved the task after training or overtraining. Specifically, in the first experiment, bilateral electrolytic lesions of interpositus nuclei were stereologically performed, whereas in the second experiment, the IN lesioned group received bilateral microinjections of Colchicine. Details of surgery methods have been reported previously (Callu...