Lesions of rat infralimbic cortex result in disrupted retardation but normal summation test performance following training on a Pavlovian conditioned inhibition procedure

The prefrontal cortex has a well-established role in the inhibition of inappropriate responding. Converging evidence implicates the infralimbic (IL) region of the rat medial prefrontal cortex (MPFC) in the maintenance of extinction [Quirk et al. (2000) J. Neurosci., 20, 6225-6231; Rhodes & Killcross (2004) Learn. Mem., 11, 611-616], a process generally considered to be mediated by the formation of inhibitory associations. An appetitive Pavlovian conditioned inhibition paradigm was used in order to investigate further the role of the IL MPFC in the control over behaviour by inhibitory associations. In two separate experiments, the effect of IL lesions on summation and retardation tests of conditioned inhibition was assessed. IL lesions did not affect summation test performance, indicating that lesioned animals were able to acquire inhibitory associations between a stimulus and reward and could express them normally when placed in competition with an excitatory cue. However, the retardation of excitatory conditioning that is normally seen when a stimulus with inhibitory properties is subsequently paired with reward was abolished in IL-lesioned animals. This suggests a selective role for the IL MPFC in the competition for behavioural control between the inhibitory and excitatory associations of a single stimulus.