When we remember an important event, we almost automatically recall the place associated with the memory. But how do our brains make and record this link between the event and the place? Researchers of the University of Amsterdam (UvA) have discovered that two areas of the brain replay the information during deep sleep in rapid succession and ten times as fast as when we are awake. This confirms an element of a theory on how memories are formed which proposes that the hippocampus acts as the conductor of the orchestra that is our memory. The results of the research were published on Tuesday in the scientific journal PLoS Biology.
This is the first time research has been conducted into how one brain structure (the hippocampus) transfers information on spatial navigation to another structure (ventral striatum), which codes information on rewards, such as food. UvA doctoral candidate Carien Lansink of the Swammerdam Institute for Life Sciences and a number of the institute's employees fitted four rats with electrodes that allowed detailed measurement of the electrical activity of groups of neurons in the two brain structures. The rat first repeatedly ran around a track on which it could find tasty treats. Afterwards, the rat fell into a sleep during which dreams and deep sleep alternated with each other.
Replay during sleep
The electronic patterns measured during while the rat was active bore a strong resemblance to the patterns recorded during the deep sleep, something not at all observed while the rat was dreaming. This ‘replay' during deep sleep ran ten times as fast as when the rat was active. For example, if the rat while awake needed three seconds to get from position A to position B where the reward could be found, the corresponding electronic pattern in both areas of the brain took only 0.3 seconds during sleep. During sleep, therefore, spatial information is linked extremely rapidly to reward information, far faster than occurs during the actual waking experience.
This study shows for the first time that the hippocampus is the first brain structure to become active when stored information is reactivated (replayed). This confirms an element of a theory proposed 38 years ago about memory formation. The theory proposes that, when awake, our brain channels sensory information to the hippocampus, which stores it in compact form. During sleep - when the brain is free from external stimuli - the hippocampus is the first part of the brain to become active again. Starting with the reactivation of spatial information, this process stimulates other areas of the brain to do the same. This role of ‘conductor' corresponds with the finding that hippocampus cells during the deep sleep start replaying memories while the reward information only resurfaces afterwards in the ventral striatum.
C.S. Lansink, P. Goltstein, J. Lankelma, R.J.N.M.A. Joosten, B.L. McNaughton, C.M.A. Pennartz, 2009. 'Hippocampus leads ventral striatum in cross-structural replay of contextual and reward information'. PLoS Biology, 18 August 2009.