(A) Schema illustrating unilateral vs bilateral optogenetic stimulation. Each mouse was implanted with bilateral optic fibers, and stimulation was delivered either to one side (‘unilateral #1’), the contralateral side (‘unilateral #2’) or to both sides (‘bilateral’). (B) Percent trials evoking freezing only (white) or freezing followed by an activity burst (black) in unilateral vs bilaterally stimulated ChR2 mice. **p < 0.01; Two-Way ANOVA, Bonferroni correction. n = 2 animals for each condition, each animal was stimulated either through one or the other of the two optic fibers (‘Unilateral #1, Unilateral #2’), or through both (‘Bilateral’). (C) Threshold stimulation intensity required to generate freezing alone or freezing followed by an activity burst, during the photostimulation period. n = 9 per condition. (D) Percentage of trials evoking freezing alone, or freezing followed by an activity burst, at respective stimulation intensities. **p < 0.0001; Two-Way ANOVA, Bonferroni correction. (E) Representative images of Fos+ (red) and SF1+ (green) neurons from animals exhibiting optogenetically induced freezing alone (upper), or freezing followed by an activity burst. Last column is higher magnification view of boxed area in adjacent ‘Overlay’ column. Arrow indicates cells double labeled for Fos+ (red) and SF1+ (green) (F) Percentage of SF1+ neurons that are Fos+ in eYFP (white) or ChR2 (black) mice following photostimulation trials eliciting freezing alone (‘Freezing’) or freezing followed by an activity burst (‘Activity Burst’). n = 3–5 mice per condition. (G) Percentage of trials evoking freezing alone, or freezing followed by an activity burst, in response to different photostimulation frequencies. n = 5 animals per condition. (H) Onset latency for activity burst as a function of increasing light intensity (x-axis) and frequency (colored bars). Arrow indicates condition that elicited activity burst without prior freezing. (I) Representative velocity trace displaying light-induced activity burst without preceding freezing. Arrow indicates expanded trace from boxed region. n = 2 animals for each condition. Values are displayed as mean ± SEM. ****p < 0.0001; ***p < 0.001; **p < 0.01; *p < 0.05.