Fig. 1

a–h Hippocampal theta activity increases during training in the novel object recognition task. Male Wistar rats implanted with electrode arrays in the CA1 region of the dorsal hippocampus (n = 6) were habituated to an open-field arena (HAB; 20-min/d/4d) and 24-h later trained in the novel object-recognition task (TR). Long-term ORM was evaluated 24-h thereafter (TT). a (Top) Graphic representation of the experimental design. (Bottom) Behavioral data showing exploration events distribution and hippocampal LFP recordings during TR for a representative rat. b (Top) Mean exploration time (% and s), mean number of exploration events, and representative trajectory during TR. (Bottom) Mean exploration time and number of exploration events per minute during TR. c (Top) Exploration events and theta activity during TR for a representative rat. (Bottom) Representative power spectral density plots during TR. d (Top) Spectrograms highlighting theta activity during exploration and inter-exploration (IE) events. (Middle) Mean locomotion speed during exploration events and IE. (Bottom) Representative power spectral density plots for exploration events and IE. e (Top) Mean theta power for exploration events and IE computed for interquartile intervals. (Bottom) Representative plot showing cumulative theta power during TR. f (Left) Mean power spectral densities for exploration and IE during TR. (Top right) Mean theta power and (Bottom right) mean theta peak frequency during TR. g (Top) Raw data, filtered theta and theta envelope for a representative exploration event. (Bottom) Spatial distribution of theta during TR for a representative rat. h (Top) Discrimination index during TR and TT. (Bottom) Mean exploration time (%) during TT. i–n Optogenetic inactivation of the medial septum causes amnesia. Two groups of rats expressing Arch-T in MS (Light_OFF, n = 22 and Light_ON A, n = 19) were trained in the novel object recognition task exactly as in A, except that Light_ON A animals received yellow light (565-nm) stimulation during object A exploration events whereas Light_OFF animals were not stimulated. Long-term ORM was evaluated 24-h later. i Graphic representation of the experimental design. j Trajectory during TR for representative Light_OFF and Light_ON A rats. k Mean distance travelled during TR and TT. l Mean exploration time (s) and number of exploration events during TR and TT. m Discrimination index during TR and TT. n Mean exploration time (%) during TR and TT. Individual values in Additional file 1: Tables S1, S2