Neural ensemble dynamics underlying a long-term associative memory

The brain’s ability to associate different stimuli is vital for long-term memory, but how neural ensembles encode associative memories is unknown. Here we studied how cell ensembles in the basal and lateral amygdala encode associations between conditioned and unconditioned stimuli (CS and US, respectively). Using a miniature fluorescence microscope, we tracked the Ca 2+ dynamics of ensembles of amygdalar neurons during fear learning and extinction over 6 days in behaving mice. Fear conditioning induced both up- and down-regulation of individual cells’ CS-evoked responses. This bi-directional plasticity mainly occurred after conditioning, and reshaped the neural ensemble representation of the CS to become more similar to the US representation. During extinction training with repetitive CS presentations, the CS representation became more distinctive without reverting to its original form. Throughout the experiments, the strength of the ensemble-encoded CS–US association predicted the level of behavioural conditioning in each mouse. These findings support a supervised learning model in which activation of the US representation guides the transformation of the CS representation. Use of a head-mounted miniature microscope in awake, behaving mice reveals that neural ensembles in the basal and lateral amygdala encode associations between conditioned and unconditioned stimuli in a way that matches models of supervised learning. Of mice and memory decoding Most of the work exploring the substrates underlying associative memory formation has involved molecular and cellular aspects, but less is known about how neural ensembles encode these associations between stimuli. Here, Mark Schnitzer and colleagues utilize chronic microendoscopy imaging in behaving mice to observe cell ensembles within the amygdala that represent conditioned and unconditioned stimuli. As associations are formed and strengthened, the representation of the conditioned stimulus increasingly matches that of the unconditioned stimulus, with the opposite occurring during extinction of the association. These findings support a supervised learning model that could be tested in neural areas beyond the amygdala.