Fig. 1. Experimental paradigm and optogenetic perturbation.

(A) Framework of MEC and LEC to DG communication conveying information about environmental global cues for spatial learning and object identity, respectively. We hypothesize that different gamma oscillations route these messages to specific hippocampal targets. (B) Photograph of the cheeseboard maze. (C) Histological section showing expression of Dlx-ChR2-mCherry in interneurons (orange) in MEC (sagittal slice) or LEC (coronal slice) and location of optic fibers (dashed lines). Blue is DAPI staining. (D) Schematic of the surgical implants. Three optic fibers in MEC or LEC bilaterally and silicon probe recordings in dorsal hippocampus. (E) Schema with timeline of the experiment. F) Entorhinal local perturbation and recording experiment. (F) (i) Average firing response across all entorhinal inhibitory cells showing a significant increase in their mean firing rates during 30 s of 53-Hz stimulation (1.6 ± 0.3 z-scored firing rate compared with baseline, n = 16 units from 2 rats, P < 0.001 sign-rank test). Each line is the average response of a single unit across 20 stimulation trials. Dashed lines indicate stimulation onset and offset. (ii) Average autocorrelogram for all inhibitory cells during stimulation show strong ~53-Hz modulation. (iii) Distribution of firing probability of all inhibitory units as a function of the phase of light stimulation (dashed blue line). (G) Same as (F) but for excitatory neurons (n = 86 units from 2 rats). On average, excitatory cells decreased their firing rates during stimulation (− 0.27 ± 0.06, P < 0.001, sign-rank test), although a minority of them increased their firing. Note that suppression of pyramidal cells is sixfold smaller than the magnitude of firing rate increase of interneurons.