![]() 4).Īcross all six days, the number of active cells stayed constant, but after conditioning ~45% more responded 1, 10 to the CS + ( Fig. CS-evoked Ca 2+ transients closely resembled those expected from past electrical recordings 12 ( Extended Data Fig. ![]() baseline rank-sum test) were sparse, interspersed and largely distinct ( Fig. Across a six-day protocol, cells responding to the CS + or CS – ( P ≤ 0.01 evoked signals vs. Mice with and without implanted microendoscopes had comparable CS +-evoked fear expression, visible as conditioned freezing 15, 17 ( Extended Data Fig. As a control, we repeatedly presented another tone (CS –) without the US ( Fig. To study associative memory 1, 12, 14 – 16, we repeatedly paired an auditory cue (CS + 25 × 200-ms-tone-pulses per presentation) with a foot-shock US. This intermingling may help the BLA to link temporally associated signals of different types via local circuit interactions 10 – 14. The cells that responded to these stimuli were sparse and interspersed across BLA 10, 11. We first examined neural responses to tones and electric shocks in awake mice ( Extended Data Fig. Traces in b and d were down-sampled to 200 ms time bins. Colored cells responded to CS + or CS – tones. ( d) Activity traces of cells responsive to CS + or CS – presentations before conditioning. Values are respectively averaged over 5 and 4 stimulus presentations, before and after conditioning. percentages of time 12 mice froze during CS + and CS – presentations. ( c) Upper, Conditioning protocol, with numbers of stimuli. ( b) Traces of spontaneous Ca 2+ activity from 15 BLA neurons. ( a) A miniature microscope and implanted microendoscope allowed large-scale neural Ca 2+ imaging. This differs from past electrophysiological studies of BLA, which lacked access to ensemble activity patterns and had limited recording durations 1, and studies of immediate early gene (IEG) activation 9, 10, which poorly reports declines, temporal patterns and gradations of electrical activity.Ĭa 2+ imaging of BLA neural activity across a six-day fear conditioning protocol. To track BLA neural ensemble activity in behaving mice, we combined time-lapse microendoscopy, a head-mounted microscope 6, 7 and expression of the GCaMP6m 8 Ca 2+-indicator in excitatory neurons ( Fig. Neural ensembles might allow more robust storage, but how cell ensembles encode associative memories and whether this fits the Hebbian model remain unknown. However, the dynamics of individual fear cells seem too stochastic to support reliable memory storage 1. This prompted a Hebbian model in which ‘fear cells’ with co-active inputs conveying the paired CS–US presentations potentiate their responses to subsequent CS presentations 1, 3, 5. Past work found BLA neurons with potentiated responses to a CS, such as an auditory tone, after associative conditioning with an aversive US 1 – 3. These findings support a supervised learning model in which activation of the US-representation guides the transformation of the CS-representation.Īssociative fear conditioning induces a long-term memory that requires BLA 1 – 3 but not hippocampal 4 activity. Throughout, the strength of the ensemble-encoded CS-US association predicted each mouse’s level of behavioral conditioning. During extinction training with repetitive CS presentations, the CS-representation became more distinctive without reverting to its original form. This bi-directional plasticity mainly occurred after conditioning and reshaped the CS ensemble neural representation to gain similarity to the US-representation. Fear conditioning induced both up- and down-regulation of individual cells’ CS-evoked responses. Using a miniature fluorescence microscope, we tracked BLA ensemble neural Ca 2+ dynamics during fear learning and extinction over six days in behaving mice. Here we studied how cell ensembles in the basal and lateral amygdala (BLA) encode associations between conditioned and unconditioned stimuli (CS, US). The brain’s ability to associate different stimuli is vital to long-term memory, but how neural ensembles encode associative memories is unknown.
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