Spaced training enhances memory and prefrontal ensemble stability in mice

It is commonly acknowledged that memory is substantially improved when learning is distributed over time, an effect called the “spacing effect”. So far it has not been studied how spaced learning affects the neuronal ensembles presumably underlying memory. In the present study, we investigate whether trial spacing increases the stability or size of neuronal ensembles. Mice were trained in the “everyday memory” task, an appetitive, naturalistic, delayed matching-to-place task. Spacing trials by 60 min produced more robust memories than training with shorter or longer intervals. c-Fos labeling and chemogenetic inactivation established the involvement of the dorsomedial prefrontal cortex (dmPFC) in successful memory storage. In vivo calcium imaging of excitatory dmPFC neurons revealed that longer trial spacing increased the similarity of the population activity pattern on subsequent encoding trials and upon retrieval. Conversely, trial spacing did not affect the size of the total neuronal ensemble or the size of subpopulations dedicated to specific task-related behaviors and events. Thus, spaced learning promotes reactivation of prefrontal neuronal ensembles processing episodic-like memories. •Spacing trials enhances memory, yet impairs learning in the “everyday memory” task•The dorsomedial prefrontal cortex is activated during and is necessary for this task•Ensemble activity patterns are more similar between subsequent spaced trials•Trial spacing does not affect ensemble size Glas et al. show that increasing the time interval between trials affects learning and memory in mice. The memory-enhancing effect of trial spacing is accompanied by increased stability of the ensemble response pattern in the dorsomedial prefrontal cortex, whereas the ensemble size is unaffected.