(A) Schematic diagram of the photostimulation protocol, as in Figure 1D; a total of 270 s of 20 Hz stimulation is delivered over a 30 min session. (B) Experimental protocol. Group 1 (G1, 3 SERT-Cre and 2 WT mice) received photostimulation from session 1 to 24. Group 2 (G2, 3 SERT-Cre and 2 WT mice) was exposed to the arena for 23 days, receiving photostimulation beginning only on the 24th day for six consecutive days. (C) Average speed across sessions for Group 1 (N = 3 SERT-Cre mice). Non-stimulated (pre, black) and stimulated (post, blue) intervals. Lines and shading indicate mean ± SEM here and E, G, I, J. (D) Speed in early and late sessions (as indicated in (B)) for mice in (C). Dark lines and error bars indicate (mean ± SEM). Error bars are too small to see in some cases. Gray lines indicate individual mice. *p<0.05; n.s: not significant, with paired t test. Same applies to F, H, K, L. (E, F) Same as (C, D) but for Group 2 (N = 3 SERT-Cre mice). (G) Group one stimulation effect (delta speed, difference between post- and pre-stimulation intervals) across all sessions. (H) Stimulation effect in early vs. late sessions. (I) Same as (G) but for Group 2. (J) Group one speed across sessions assessed during the pre interval only in the first block, i.e. prior to receiving any stimulation during that session (K) Speed in the first block as in (I) for early vs. late sessions (as indicated in (B)). (L) Center occupancy in early vs. late sessions for Group one and Group 2.