Image caption Smell is closely linked to memory

The sense of smell can be improved through training, a study on rats suggests.

The study, published in Nature Neuroscience, also suggests if we do not use our sense of smell, we begin to lose it.

The New York University Langone Medical Center team says their work also raises hopes of reversing loss of smell caused by ageing or disease.

But a UK expert thought that was unlikely.

Impairment in the sense of smell is associated with Alzheimer's disease, Parkinson's disease, schizophrenia, and even normal ageing.

Exactly why smell weakens remains a mystery, but the Langone team have pinpointed a half-inch-sized area of the rat brain called the piriform (olfactory) cortex where the problems appear to occur.

Rewards

The researchers placed thirsty rats in boxes with a snout-sized hole in each of three walls and exposed them to brief blasts of odours through the middle hole.

There were three different smells: a mix of 10 chemicals from fruits, oils, and cleaning agents; the same mixture with one chemical replaced by another; and the same mixture minus one of the chemicals.

When the rodents identified one smell, they were rewarded with a sip of water by going to the hole in the left side wall, for another smell they received water by going to the right side wall.

Rats could readily distinguish between odours when a chemical had been replaced in one mixture, but when one component had simply been removed, they could not differentiate.

Sense of smell Smell is one of our oldest senses, and the one most closely linked to memory

It is probably related to simple chemo-sensation exhibited by some of the very first uni-cellular creatures on this planet

The olfactory bulb, a structure in the brain that receives nerve signals from the nose, has direct connections to other parts of the brain such as the amygdala, which controls memory, and the prefrontal cortex, which is involved in processing thoughts

The researchers anaesthetised the rats and inserted electrodes into their brains.

Within the olfactory bulb, each smell produced a different pattern of electrical activity.

But in the piriform cortex the odours that rats could tell apart produced distinct patterns of activity, while those they could not distinguish produced identical patterns.

The researchers then trained a new group of rats to discriminate between the odours the first animals could not tell apart by rewarding them over and over with sips water for choosing the appropriate hole.

In the rats' piriform cortex, activity patterns elicited by these similar odours were now different as well.

A third group of animals were trained to ignore the difference between odours the first rats could readily distinguish by giving them water at the same hole after exposure to either odour.

This effectively dulled their sense of smell: the rats could not tell one smell from the other, even for a reward.

Their loss of discrimination was reflected in the piriform cortex, which now produced similar electrical patterns in response to both odours.

'Use it or lose it'

Lead researcher Prof Donald Wilson said: "We located where in the brain loss of smell may happen.

"And we showed that training can improve the sense of smell, and also make it worse.

"Our findings suggest that while olfactory impairment may reflect real damage to the sensory system, in some cases it may be a 'use it or lose it' phenomenon."

Andrew McCombe, honorary secretary of the British Association of Otorhinolaryngologists ENT-UK, said there was good evidence that keeping mentally active was good for brain function.

However, he said training our sense of smell was only likely to be helpful in its long-term preservation.

He said: "Loss of sense of smell is fortunately not too common but it is miserable when it happens as you usually lose your sense of taste - particularly flavour - to some extent too.

"Whilst interesting research, I am not sure it's going to suddenly lead to a significant change in the way we treat loss of sense of smell which sadly is usually permanent and complete when it happens."