a, Schema of the social memory task described in Figure 2. Insets show the expression of Arch in the medial perforant path and medial entorhinal cortex (MEC) in a coronal brain slice (upper panel) and in sagittal brain slices (lower panel) from a mouse previously injected in the MEC with an Arch-expressing AAV. Shining yellow light on MEC inputs in dorsal CA2 during the recall phase of the task (trial 3, Arch n=7, GFP n=6) (b-e) or during the learning phase (trial 2, Arch n=8, GFP n=10) (f-i) does not impair social memory in mice expressing Arch in MEC relative to the control group expressing GFP. Scale bar: a: 1 mm. #: p<0.05, ##: p<0.01, ###: p<0.001 one-sample t-test against “0”. *: p<0.05, **: p<0.01, ***: p<0.001, ****: p<0.0001 Holm-Sidak’s post hoc test after two-way mixed-design ANOVA (in b: F=30.04 p=0.0002 for Familiarity; in d: F=22.90 p=0.0006 for Familiar L-R; in f: F=5.145 p=0.0375 for interaction Familiarity x Genotype; in h: F=14.49 p=0.0015 for Familiar L-R).