Nerves from a part of the rat brain known as the basal forebrain are thought to secrete the chemical acetylcholine into the brain's main vision center, also called the primary visual cortex, reinforcing connections that were recently active.

MSN: Goggle-wearing rats show link between learning, rewards

When the scientists injected the rats with a neurotoxin that blocks those acetylcholine-releasing nerves, however, the rats' brains continued to remember the licking time for each signal, suggesting the acetylcholine and basal forebrain weren't needed to maintain those associations.

MSN: Goggle-wearing rats show link between learning, rewards