Memory and sleep are intricately linked in the learning process. Studies on rats have shown that spatial information such as the path through a maze is stored in the hippocampus region of the brain. This has also been shown in humans, and the ability to memorize information has been strongly linked to sleep for some time. However, there are competing models on what the hippocampus and slow wave sleep are doing to reinforce memories. On one side, researchers believe that the hippocampus repeatedly relives the learning experience, and that slow wave sleep serves to keep the learning process covert. The other main contender, called downscaling, involves the slow wave sleep suppressing other synaptic activity so that the new connections are reinforced. Although these ideas are similar in that the activities of the hippocampus and slow wave sleep are nearly identical, they have rather different implications in their role. The problem is that the studies that are used to determine the role of the hippocampus and sleep are passive observational studies of how the brain responds to learning tasks, which makes it very difficult to sort out cause and effect.

A new study by Rasch and colleagues in Science shows that improved recall could be obtained by deliberately manipulating hippocampal activity during different periods of sleep. Increased hippocampal activation was achieved through controlled exposure to odors, which is the only external stimulus that does not disturb the sleeper. It is also known that odors are very good at causing a contextual recall - odors directly activate the hippocampus. Volunteers were given a spatial learning task, involving memorizing the locations of certain cards on a grid. During the task some of the participants were exposed to a spray containing a rose scent, while the control subjects were exposed to a scentless spray. During sleep, some of the participants were exposed to the rose scent again during slow wave and others during rapid eye movement periods of sleep. Care was taken to vary the amount of scent in the air to avoid desensitizing the nasal odor receptors.

The results showed that participants who were exposed to the odor during learning and during slow wave sleep had better recall than the controls, who either had no odor exposure at all, or exposure during learning and rapid eye movement sleep. Not only that, the researchers also tested participants on non-spatial tasks - tapping out a sequence on a keyboard - which isn't though to involve the hippocampus at all. They found that participants subjected to odoriferous activation of the hippocampus did not perform any better than controls.

After the learning task tests were completed, the researchers confirmed that odors were activating the hippocampus by re-exposing subjects to the odor during slow wave sleep while in an fMRI machine. Although these studies cannot rule out downscaling, they are certainly far more supportive of the repetitive learning hypothesis. For those of you who have spatial tasks to learn, you now have a new tool