Impaired sense of smell often signals cognitive decline, but "odor formation" may help.

As we age, we often have odor problems (called olfactory dysfunctions). Older people might not be able to identify an odor or differentiate one smell from another. In some cases, they may not be able to detect an odor.

Difficulties in identifying odors are common in people with neurodegenerative diseases, including Alzheimer's disease.

In the absence of a known medical cause, an alteration of the sense of smell can be a predictor of cognitive decline. It is estimated that older people who have difficulty identifying common smells are twice as likely to develop dementia in five years than those who do not lose a lot of odors.

Olfactory dysfunction is often present before the onset of other cognitive symptoms, although this loss may remain undetected.

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In addition to being a possible early indicator of Alzheimer's disease, olfactory problems can pose safety risks, such as the inability to smell gas, smoke or rotten food.

Odor capacity is also closely related to our ability to taste, so deficiencies can lead to loss of appetite and thus nutritional deficiencies. In turn, olfactory deficits can affect the quality of life and increase the risk of depression.

But it is becoming increasingly clear that olfactory or olfactory training can improve one's ability to feel. These results may offer some hope to older people with olfactory difficulties and a deterioration in their quality of life.

How is our sense of smell related to our brain?

The process of smell activates the complex olfactory network in the brain. When we smell a rose, for example, receptors in the nose detect the many molecules that make up the smell of the rose.

This information is then sent to the many areas of the brain (including the olfactory bulb and olfactory cortex, hippocampus, thalamus, and orbitofrontal cortex) that help us process the information pertaining to this odor.

To name the rose, we access our stored knowledge of its odorous molecule model, based on past experience. Identifying the smell as belonging to a rose is therefore considered a cognitive task.

What is the odor training?

Odor training has been studied in various animals, from flies to primates. Animals exposed to multiple odors develop an increased number of brain cells and connections between them. This process has been shown to improve the learning and memory of smells.

In humans, olfactory training has generally consisted in feeling a range of robust odors representing the main categories of smells: flowery (like rose), fruity (lemon), aromatic (eucalyptus) or resinous (nail of cloves). Participants may be asked to focus on particular odors, to try to detect certain odors or to note the intensity of odors.

Generally, the training is repeated daily for several months. Periods longer than three months are suggested for the elderly.

This training has been shown to improve people's ability to identify and distinguish odors. To a lesser extent, it can help detect odors in people with various forms of odor loss, including those with a brain-related disability such as a head injury or disease from Parkinson.

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It is important to note that a recent study on olfactory training in the elderly has revealed that it not only improves the odor recognition performance, but that it is also badociated with a improvement of other cognitive abilities.

For example, those who have undergone odor training have improved their verbal fluency (improved ability to name category-related words), compared to control participants who have completed the sudoku exercises.

How does odor training work?

Neuroplasticity, the ability of our brain to continually change based on experience, may be the key to the functioning of odor formation.

Neuroplasticity involves the creation of new connections and / or the strengthening of existing connections between neurons (brain cells), which can lead to changes in thinking skills or behavior. We can see evidence of neuroplasticity when we practice a technique such as playing an instrument or learning a new language.

The olfactory network is considered particularly neuroplastic. Neuroplasticity can therefore be the basis of the positive results of odor training, both in terms of improving olfactory capacity and ability to increase capacity for other cognitive tasks.

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Could odor training be the new brain training?

Brain training aimed at maintaining or improving cognitive function has been the subject of numerous studies in elderly people with dementia or who are at risk of suffering from it.

Established cognitive training approaches generally involve participants in using learning strategies with visual or auditory stimuli. To date, no formal cognitive training has been attempted with the aid of odors.

However, by using the considerable neuroplasticity of the olfactory network and evidence-based cognitive training techniques, olfactory and cognitive deficits can be targeted, particularly in elderly people at risk of dementia. It seems possible that we can train our brain through the nose.