The first study by Jan Born's group, shows that odours specifically seem to enhance consolidation of declarative (more factual) memories during a sleep phase called slow-wave sleep (named for I guess the EEG frequency). Elegant work from Brice McNaughton's lab in particular had shown that rats seem to replay recent experiences during sleep. They were looking at these 'place cells' in the hippocampus (an area of the brain thought to be the seat of memory formation). These cells are specific to particular locations, so that each time a rat takes a different route through its maze, a different sequence of place cells fires. Subsequent studies found that sequences of place-cell firing that occur as a rat explores a new environment are replayed the next time the rat sleeps, as if the rat were retracing its steps during sleep.
The same may well apply to humans, recently a Belgian team used PET to image brain activity in men learning to navigate through a scene from the game Duke Nukem. Sure enough, the same regions of the hippocampus that were active while task was being learnt were active during slow wave sleep.
What this study did, was to build on all this collerative data, and try and try and boost memory consolidation, by boosting brain activity during sleep. The researchers had the subjects play a video version of the card game Memory wherein they had to learn and regurgitate the positions of card pairs showing the same image in a group of 30 cards. Each matched pair appeared for a few seconds with all the other cards facing down. Some subjects were concomitantly treated to a puff of rose scent, the reasoning being that the scent would be associated with the task in Pavlovian fahion. After going through all the pairs, the researchers tested the subjects' recall by turning one of the 30 cards face up and asking them to find its match.
Once the subjects entered slow-wave sleep, the researchers gave some of them a puff of rose-scented air. The result? To quote this review:
functional magnetic resonance imaging (fMRI) scans in sleeping subjects revealed that the odor activated the hippocampus in those who had experienced it previously, even though the EEG showed no disruptions in the subjects' slumber. Although they didn't remember smelling roses in their sleep, the subjects who got the fragrant prompt remembered the matched pairs better the next day, getting 97% correct compared to 86% for subjects who'd received no odor while sleeping. Subjects who got the rose odor either while awake or while in REM sleep, on the other hand, showed no memory boost; nor did presenting the odor during slow-wave sleep help subjects who hadn't been exposed to rose during the training session...There's also a free to access NYT article about it here
Born's findings fit with a popular view of how the brain files memories away for long-term storage, a process neuroscientists call memory consolidation. According to this hypothesis, memories are first encoded by the hippocampus and later--perhaps in a matter of hours or days--transferred for long-term storage to the cerebral cortex, or neocortex.
The second smell related paper studied aging in an organism close to my heart, the fruit fly Drosophila. Across a number of systems from worms to flies to rodents it has pretty been convincingly demonstrated that dietary restriction extends lifespan.
What Scott Pletcher's group found was that simply exposing flies to the odour of yeast (fly food) while on a restricted diet resulted in shortened lifespan compared to dieting flies not exposed to this odour. Just the smell of food seems to be enough to offset some of the gains of dieting.They then proceeded to look at a mutant fly that lacks a co-receptor protein that results in flies with a drastically reduced ability to perceive odours, and sure enough this mutant fly strain lived much longer than normal flies, whether dieting or otherwise. It's quite a remarkable finding I think, for it suggests some sort of neural control over the aging process.
Indeed in worms work from Cynthia Kenyon et al, suggestS that mutating different smell and taste receptors has different effects on aging, some extend lifespan, some shorten it. Surprising effects I think of sensory stimulation.
Unfortunately, both the papers were in Science Magazine, which while being a top rated journal and all is subscription only..
Anyways, while I suppose I can give myself brownie points for choice of topic, can't say the same for how the research has panned out :-)