Impaired latent inhibition in Nogo-A−/− mice. A, Expression of freezing behavior toward the tone CS across the three conditioning trials immediately after pre-exposure in the conditioned freezing paradigm. Freezing is expressed as the percentage time freezing per trial. Freezing in the pre-exposed (PE) subjects was lower than in the non-pre-exposed (NPE) subjects, constituting the LI effect. B, Freezing to the context 24 h after conditioning. This is expressed as the overall mean percentage time freezing. There was no effect of genotype or pre-exposure. C, Freezing to the tone CS 48 h after conditioning. Freezing is expressed as the mean percentage time freezing before tone onset on the left and as the mean percentage time freezing across the tone period on the right. In the 3 min before the onset of the tone, there was no difference between any of the groups for the levels of freezing. Reduced freezing levels to the tone in the PE group compared with the NPE group in Nogo-A+/+ animals indicates the presence of LI (***p < 0.001 based on a priori contrast). In contrast, Nogo-A−/− mice failed to show significant LI. This difference between the genotypes was supported by the presence of a significant two-way interaction between pre-exposure and genotype (p < 0.05). Moreover, there was a genotype effect in the PE condition (*p < 0.05), whereas both NPE groups were comparable. All values are mean ± SEM. KO, Nogo-A−/− (n = 9 PE, 9 NPE); WT, Nogo-A+/+ (n = 15 PE, 15 NPE).