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Olfactory Enrichment Supports Cognition with Aging
Keeping our brain stimulated and exposed to novelty is one of the best ways to promote neuroplasticity and maintain healthy cognition as we age . Environmental enrichment through sensory stimulation is one of the many ways to support neuroplasticity . All types of sensory stimulation may contribute to cognitive health, but olfactory stimulation has a specific advantage: the olfactory system is the only sensory system that is directly linked to the limbic system, which has crucial roles in memory, emotion, and behavior ; the other senses have only indirect connections to the limbic system via the thalamus. This privileged access to learning and memory systems may allow for stimulation of the olfactory system to help to delay memory decline via direct neural activation.
All types of sensory stimulation may contribute to cognitive health, but olfactory stimulation has a specific advantage: the olfactory system is the only sensory system that is directly linked to the limbic system, which has crucial roles in memory, emotion, and behavior.
In animals, olfactory enrichment through exposure to odorants—think of an odorant as being something with a distinctive smell—improved both memory and neurogenesis in the mouse brain [4,5]. Novelty was the critical element in this kind of stimulation, as repeated exposure to odorant mixtures did not produce these changes, while exposure to multiple odorants individually did . The reason why novelty made a difference is because it is more stimulating than familiar stimuli [6,7] and affects different aspects of cognition .
Furthermore, as people age, olfactory ability deteriorates and this loss is associated with cognitive decline and loss of both gray and white matter [8–11]. Olfactory enrichment may help to overcome these age-related changes as it has been shown to improve olfactory ability in humans with olfactory loss , to increase gray matter volumes in areas of the brain associated with learning and memory, such as the hippocampus or the entorhinal cortex [12–14], and to improve aspects of cognition in aged individuals [14–16].
The ideal setting to obtain maximal benefits from olfactory enrichment is one that allows for continued and consistent exposure to olfactory stimuli in a manner that takes into account the importance of novelty across time. Therefore, a new study tested whether minimal-effort olfactory enrichment at night in a home setting could enhance cognitive function and modify limbic pathways that support cognition in healthy older adults aged 60 to 85 years with normal cognition.
Using an odor diffuser, participants in the enriched group were exposed to an odorant for 2 h every night over the course of six months. Seven different pleasant essential oil scents (rose, orange, eucalyptus, lemon, peppermint, rosemary, and lavender) were used in the study, rotating through the seven different scents each week, with a single scent used each night. Participants were asked to turn on the diffuser when they went to bed so that the odorant was released into the air for 2 h when they went to sleep. Participants in the control group followed the same procedure but received only trace amounts of odorant. Neuropsychological tests to assess cognitive performance (verbal learning and memory, working memory, planning, and attention switching) and fMRI scans were administered at the beginning of the study and after six months.
Results showed a large 226% difference in verbal learning and memory performance between the enriched and the control group. Whereas the control group overall had poorer verbal learning performance after six months, the enriched group showed significant improvements in verbal learning, with 11 out of 12 participants showing either improved or unaltered performance (and only 1 doing worse), as compared to only 4 out of 11 in the control group (with 7 of 11 doing worse). The other neurocognitive assessments showed no significant differences.
Results showed a large 226% difference in verbal learning and memory performance between the enriched and the control group who were exposed to an odorant for 2 h every night over the course of six months.
Brain imaging studies using fMRI showed changes in response to olfactory enrichment in a major brain pathway (called left uncinate fasciculus) that plays a role in mediating episodic memory, language, and socio-emotional processing and that deteriorates with aging . In line with verbal learning performance, its activity was enhanced in the enriched group and decreased in controls.
Studies with olfactory training—an intervention involving systematic sniffing of a set of odorants—had shown improvements in semantic verbal fluency in older individuals, which declared feeling cognitively younger after completion of olfactory training . This study showed improvements in verbal learning and memory and had an important difference: it did not require any systematic actions other than turning on an odor diffuser. It was a minimum-effort intervention in which participants were passively exposed to olfactory stimulation while they slept. Furthermore, because participants were exposed to different odors each night, rather than using the same scent or combination of scents every day, novelty was introduced with each rotating odor, adding an element to the sensory stimulation that is known to support neuroplasticity .
As life expectancy increases, cognitive loss in older adults becomes a growing concern, creating the need for strategies to support cognitive function as we age. Although this was a small study, it indicates that olfactory enrichment administered at night may be sufficient to improve aspects of cognition and neural function and provide an easy to implement, low-effort, affordable approach to support brain health. Simple approaches that can be carried out at home with minimum effort are ideal because they’re easier to integrate into daily routines.
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Woo CC, et al. Overnight olfactory enrichment using an odorant diffuser improves memory and modifies the uncinate fasciculus in older adults. Front Neurosci 2023, 17:1200448. doi: 10.3389/fnins.2023.1200448
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