Shanahan et al. first identified the brain activity patterns that are associated with different visual stimuli, including tools and animals (upper left); the 2x2 grid illustrates a certain pattern of brain activity. Afterwards (lower left), participants had to perform learning tasks while a specific smell (e.g., lemon) was presented for each visual category (e.g., tools). During slow-wave sleep, participants were exposed to the specific smell again. In a brain region called the ventromedial prefrontal cortex, the smell of lemon induced brain patterns similar to those observed for tools during the learning phase in some participants (upper middle). In other participants, the smell of lemon did not induce this pattern in this brain region (lower middle). The participants with a higher replay of brain patterns during sleep had a better memory performance after sleep (upper right) than those without (lower right).