PMC

Electrical stimulation of ACC during observational fear learning. (A) Diagram of the apparatus and treatment used for observational fear conditioning. (B) Representative photograph indicating the location of the electrode used for electrical stimulation in the right ACC. (C and D) Electrical stimulation of the right ACC (n = 18) increased both the observational freezing behavior (F_1,29 = 25.122, P < 0.001; C) and the 24-h memory recall (F_1,29 = 7.385, P = 0.011; D) compared with the nonstimulated controls (n = 16). Electrical stimulation into the left ACC (n = 21) had no effect on observational fear learning (F_1,33 = 1.008, P = 0.323; C) and 24-h contextual memory recall (F_1,33 = 0.041, P = 0.841; D) compared with nonstimulated controls. (E and F) Total freezing time showed an enhanced effect on right electrical stimulation into the right ACC in observational fear learning (E) and 24-h contextual memory recall (F) compared with nonstimulated controls or mice with stimulation into the left ACC. Trains were delivered every 1 s for 4 min during training in observational fear conditioning. Error bars represent SEM. *P < 0.05, **P < 0.01.

Lateralization of ACC in observational fear learning. (A) Diagram of the apparatus and treatment used for observational fear conditioning. (B) Representative photograph indicating the location of the implanted cannula in the right ACC. (C) Mice with lidocaine injections into the right ACC (n = 6) before training failed to acquire fear compared with those receiving saline injections (n = 9). (D) Contextual memory 24 h after the training in C. There were significant differences in the observational training (F_1,13 = 13.877, P = 0.003) and the 24-h contextual memory (F_1,13 = 9.748, P = 0.008). (E and F) Administration of lidocaine into the left ACC before training had no influence on the acquisition of observational fear (F_1,21 = 0.0000000489, P = 1, E) and 24-h contextual memory recall after training (F_1,21 = 0.00194, P = 0.965; F) (lidocaine, n = 11; saline, n = 12). Error bars represent SEM. *P < 0.05, **P < 0.01.

Control of observational fear is not lateralized at thalamus. (A and B) Mice with ibotenic acid injection into right thalamic nuclei (n = 6) 1 wk before training exhibited impaired acquisition compared with those receiving saline injections into right thalamus (n = 11) in observational fear (F_1,15 = 12.307, P = 0.003, A) and 24-h contextual memory test (F_1,15 = 11.295, P = 0.004, B). (C and D) Unilateral injection of ibotenic acid into left thalamic nuclei (n = 5) also caused impaired observational fear learning during training (F_1,14 = 27.979, P < 0.001; C) and 24-h contextual memory (F_1,14 = 16.245, P = 0.001; D) compared with those receiving saline injection into left thalamus (n = 11). There was no difference between the left and right thalamic lesioned groups in the freezing level in the learning (A and C) (F_1,9 = 0.000995, P = 0.976) and the 24-h memory recall test (B and D) (F_1,9 = 0.488, P = 0.502). Error bars represent SEM. *P < 0.05, **P < 0.01.

Lesion of the thalamic regions in reciprocal connection with ACC. Tracer injection of the anterograde Phal (green; A–D) or retrograde CTB (red; E–H) tracers into the ACC (A and E, respectively) revealed strong bidirectional projections with the AM (B, C, F, and G) and MD/IL (C, D, G, and H) thalamic nuclei. Note bilateral corticothalamic (stars in B and C) but unilateral thalamocortical projection. Insets: White frames, enlarged in F, show that only the ispilateral AM (Right) contains CTB-positive cell bodies. The contralateral side possesses only axonal staining (Left) (for details, see and Fig. S2). (I, a) Neurochemical lesion in the thalamus (arrowheads) visualized with NeuN immunostaining (magenta). (I, b) Combination of anterograde tracing with Phal from ACC (green) and lesion in the same animal demonstrates that the thalamic territory in connection with ACC was targeted. (J) Schematic drawings of three left (blue) and three right (red) thalamic lesions at a single AP level. A grid from the Paxinos mouse brain atlas was used to identify the location (). Phal-labeled corticothalamic fibers (white) in the background show that the lesions targeted the appropriate nuclei. Fig. S3 shows the total extent of the lesions. mt, mammillothalamic tract; Re, reuniens thalamic nucleus; VM, ventromedial thalamic nucleus. (Scale bars, 500 μm.)