Ex-vivo cell recordings demonstrate successful optical IL stimulation.

(A) Wild-type Long-Evans rats (n = 2 females and 2 males) were bilaterally infused in the IL with a ChR2 virus. (B) The top representative raw trace is a cell-attached recording from a transfected IL neuron with LED train stimulations. In the same neuron under whole-cell current clamp, LED pulses excited the IL neuron as evidenced by enhanced action potentials (AP) from resting state baseline (bottom raw trace). (C) AP frequency was enhanced during LED illumination compared to their baseline state (n = 10 cells). (D) Micrographs showing a ChR2 transfected IL neurons that was labelled with biocytin.

Prior experience with fear extinction enables IL stimulation to facilitate subsequent fear extinction.

(A) Schematic representation of the behavioral design used in Experiment 1. The LEDs were activated in the Ext-ON and ReExt-ON groups during the second fear extinction session. Ext-OFF: n = 3 females and 7 males; Ext-ON: n = 4 females and 5 males; ReExt-OFF: n = 5 females and 6 males; ReExt-ON: n = 2 females and 6 males. (B) Mean percent freezing during all experimental stages. Data are shown as mean ± SEM. Test data include individual data points for female (filled circle) and male (open circle) rats. Asterisks denote significant effect (** p < 0.01).

Prior experience with backward fear conditioning enables IL stimulation to facilitate subsequent fear extinction.

(A) Schematic representation of the behavioral design used in Experiment 2. The LEDs were activated in all groups during the fear extinction session. No-EYFP: n = 5 females and 5 males; No-ChR2: n = 3 females and 4 males; Back-EYFP: n = 5 females and 5 males; Back-ChR2: n = 4 females and 5 males. (B) Mean percent freezing during all experimental stages. Baseline freezing (before first tone presentation) at test is provided due to significant differences. Data are shown as mean ± SEM. Test data include individual data points for female (filled circle) and male (open circle) rats. Asterisks denote significant effect (** p < 0.01; *** p < 0.001).

Prior experience with backward fear conditioning enables IL stimulation to facilitate subsequent fear extinction.

(A) Schematic representation of the behavioral design used in Experiment 3. The LEDs were activated in all groups during the fear extinction session. Diff-EYFP: n = 4 females and 3 males; Diff-ChR2: n = 2 females and 4 males; Same-EYFP: n = 4 females and 3 males; Same-ChR2: n = 2 females and 4 males. (B) Mean percent freezing during all experimental stages. Baseline freezing (before first tone presentation) at test is provided to compare with Experiment 3. (C) Schematic representation of the behavioral design used in Experiment 4. No: n = 12 females and 4 males; Back: n = 8 females and 8 males. Data are shown as mean ± SEM. (D) Mean percent freezing during all experimental stages. Data are shown as mean ± SEM. Test data include individual data points for female (filled circle) and male (open circle) rats. Asterisks denote significant effect (* p < 0.05; *** p < 0.001; n.s., nonsignificant).

Inhibitory memories encoded within the IL lack motivational information.

(A) Schematic representation of the behavioral design used in Experiment 5. The LEDs were activated in all groups during the fear extinction session. Diff-EYFP: n = 4 females and 5 males; Diff-ChR2: n = 4 females and 5 males; Same-EYFP: n = 6 females and 3 males; Same-ChR2: n = 6 females and 2 males. (B) Number of magazine entries during backward appetitive conditioning. (C) Mean percent freezing during remaining experimental stages. Data are shown as mean ± SEM. Test data include individual data points for female (filled circle) and male (open circle) rats. Asterisks denote significant effect (* p < 0.05).

Stimulus familiarity does not enable IL stimulation to facilitate fear extinction.

(A) Schematic representation of the behavioral design used in Experiment 6. The LEDs were activated in all groups during the fear extinction session. For-EYFP: n = 3 females and 4 males; For-ChR2: n = 4 females and 4 males; Diff-EYFP: n = 2 females and 4 males; Diff-ChR2: n = 2 females and 4 males. (B) Number of magazine entries during forward and backward appetitive conditioning. (C) Mean percent freezing during remaining experimental stages. Data are shown as mean ± SEM. Test data include individual data points for female (filled circle) and male (open circle) rats. Asterisks denote significant effect (** p < 0.01).

Prior experience with appetitive extinction enables IL stimulation to facilitate subsequent backward fear conditioning.

(A) Schematic representation of the behavioral design used in Experiment 7. The LEDs were activated in all groups during backward fear conditioning. EYFP: n = 5 females and 5 males; ChR2: n = 5 females and 4 males. (B) Number of magazine entries during forward appetitive conditioning and appetitive extinction. (C). Mean percent freezing during remaining experimental stages. Data are shown as mean ± SEM. Test data include individual data points for female (filled circle) and male (open circle) rats. Asterisks denote significant effect (* p < 0.05).