A long-term study of nearly 3,000 older adults (57-85) has found that those who couldn’t identify at least four out of five common odors were more than twice as likely as those with a normal sense of smell to develop dementia within five years.
Of the participants, some 14% could name just three out of five, 5% could identify only two scents, 2% just one, and 1% couldn’t identify a single smell.
In the past few months, several studies have come out showing the value of three different tests of people's sense of smell for improving the accuracy of MCI and Alzheimer's diagnosis, or pointing to increased risk. The studies also add to growing evidence that a decline in sense of smell is an early marker for mild cognitive impairment and Alzheimer’s. Indeed, it appears that this sensory loss is a very early symptom, preceding even the shrinking of the entorhinal cortex (the first brain region to show signs of atrophy).
A couple of studies reported at the recent Annual Conference of the British Psychological Society are intriguing.
In the first study, 180 healthy adults completed questionnaires relating to their mood before being given either a drink of peppermint tea, chamomile tea or hot water. Twenty minutes later, their memory and cognition were tested, followed by another mood questionnaire.
Confirming the common experience of the strength with which certain smells can evoke emotions or memories, an imaging study has found that, when people were presented with a visual object together with one, and later with a second, set of pleasant and unpleasant odors and sounds, then presented with the same objects a week later, there was unique activation in particular brain regions in the case of their first olfactory (but not auditory) associations. This unique signature existed in the hippocampus regardless of how strong the memory was — that is, it was specific to olfactory associations. Regardless of whether they were smelled or heard, people remembered early associations more clearly when they were unpleasant.
Yeshurun, Y., Lapid, H., Dudai, Y., & Sobel, N. (2009). The Privileged Brain Representation of First Olfactory Associations. Current Biology, 19, 1869-1874. Retrieved from http://www.cell.com/current-biology/abstract/S0960-9822(09)01857-0
http://www.physorg.com/news176649240.html
Older adults who have difficulty identifying common odors may have a greater risk of developing mild cognitive impairment, increasingly recognized as a precursor to Alzheimer’s disease. A study of nearly 600 older adults (average age 79.9) found that 30.1% developed mild cognitive impairment over the five-year period of the study. Risk of developing mild cognitive impairment was greater for those who scored worse on an odor identification test given at the start of the study. For example, those who scored below average (eight) were 50% more likely to develop MCI than those who scored above average (11). This association did not change when stroke, smoking habits or other factors that might influence smell or cognitive ability were considered. Impaired odor identification was also associated with lower cognitive scores at the beginning of the study and with a more rapid decline in episodic memory (memory of past experiences), semantic memory (memory of words and symbols) and perceptual speed. The odor test involved identifying 12 familiar odors given four possible alternatives to choose from.
Wilson, R. S., Schneider, J. A., Arnold, S. E., Tang, Y., Boyle, P. A., & Bennett, D. A. (2007). Olfactory Identification and Incidence of Mild Cognitive Impairment in Older Age. Arch Gen Psychiatry, 64(7), 802-808. Retrieved from http://archpsyc.ama-assn.org/cgi/content/abstract/64/7/802
http://www.eurekalert.org/pub_releases/2007-07/jaaj-dio062807.php
A study in which students played a computer version of a common memory game in which you turn over pairs of cards to find each one's match found that those who played in a rose-scented room and were later exposed to the same scent during slow-wave sleep, remembered the locations of the cards significantly better than people who didn't have that experience (97% vs 86%). Those exposed to the odor during REM sleep, however, saw no memory boost. Imaging revealed the hippocampus was activated when the odor was presented during slow-wave sleep. Having the smell available throughout sleep wouldn’t help, however, because we adapt to smells very quickly. Being exposed to the smell when being tested didn’t help either. Nor did experiencing the odor during slow-wave sleep help when the memory task involved a different type of memory — learning a finger-tapping sequence — probably because procedural memory doesn’t depend on the hippocampus.
Rasch, B., Buchel, C., Gais, S., & Born, J. (2007). Odor Cues During Slow-Wave Sleep Prompt Declarative Memory Consolidation. Science, 315(5817), 1426-1429. Retrieved from http://www.sciencemag.org/cgi/content/abstract/315/5817/1426
http://www.physorg.com/news92647884.html
http://www.nature.com/news/2007/070305/full/070305-10.html
A study involving 75 female students found that those who were exposed to chemicals from fear-induced sweat performed more accurately on word-association tasks than did women exposed to chemicals from other types of sweat or no sweat at all. When processing meaningfully related word pairs, the participants exposed to the fear chemicals were significantly more accurate than those in either the neutral sweat or the control (no-sweat) condition. When processing word pairs that were ambiguous in threat content, such as one neutral word paired with a threatening word or a pair of neutral words, subjects in the fear condition were significantly slower in responding than those in the neutral sweat condition.
Chen, D., Katdare, A. & Lucas, N. 2006. Chemosignals of Fear Enhance Cognitive Performance in Humans. Chemical Senses, Advance Access published on March 9, 2006
http://www.eurekalert.org/pub_releases/2006-03/ru-sof033106.php
We all know the power of smell in triggering the recall of memories. New research has found the specific area of the brain involved in this process - a section of the hippocampus called CA3. The hippocampus has long been known to play a crucial part in forming new memories. It appears that the CA3 region of the hippocampus is crucial for recalling memories from partial representations of the original stimulus.
Wilson, M. A., Tonegawa, S., Nakazawa, K., Quirk, M. C., Chitwood, R. A., Watanabe, M., … Johnston, D. (2002). Requirement for Hippocampal CA3 NMDA Receptors in Associative Memory Recall. Science, 297(5579), 211-218. Retrieved from http://www.sciencemag.org/cgi/content/abstract/297/5579/211
http://www.eurekalert.org/pub_releases/2002-05/bcom-tr052902.php
http://news.bbc.co.uk/hi/english/health/newsid_2017000/2017321.stm
A small study involving 20 people has found that those who were exposed to 1,8-cineole, one of the main chemical components of rosemary essential oil, performed better on mental arithmetic tasks. Moreover, there was a dose-dependent relationship — higher blood concentrations of the chemical were associated with greater speed and accuracy.
The olfactory bulb is in the oldest part of our brain. It connects directly to the amygdala (our ‘emotion center’) and our prefrontal cortex, giving smells a more direct pathway to memory than our other senses. But the olfactory bulb is only part of the system processing smells. It projects to several other regions, all of which are together called the primary olfactory cortex, and of which the most prominent member is the piriform cortex.
190-million-year-old fossil skulls of Morganucodon and Hadrocodium, two of the earliest known mammal species, has revealed that even at this early stage of mammalian evolution, mammals had larger brains than would be expected for their body size. High-resolution CT scans of the skulls have now shown that this increase in brain size can be attributed to an increase in those regions dealing with smell and touch (mammals have a uniquely well developed ability to sense touch through their fur).
Previous research suggesting loss of smell function may serve as an early marker of Alzheimer's disease has now been supported by a finding that in genetically engineered mice, loss of smell function is associated with amyloid-beta accumulation in the brain, and that amyloid pathology occurs first in the olfactory region. It was striking how sensitive olfactory performance was to even the smallest amount of amyloid presence in the brain as early as three months of age (equivalent to a young adult).
Herz, R.S. (1997). The effects of cue distinctiveness on odor-based context dependent memory. Memory and Cognition, 25, 375-380.
The effect of smell on learning and memory was investigated in an experiment that used three different ambient odors (osmanthus, peppermint, and pine).
Osmanthus was used to see whether there was a difference in performance depending on whether the smell was novel or familiar. Peppermint and pine were used to see whether the appropriateness or inappropriateness of the smell made a difference to memory.
In the experiment, subjects were individually shown into a room in which the odor was present. Their attention was called to the smell, and to ensure their attention to the smell, they were given a questionnaire to fill out about the room environment. They were left alone in the room for ten minutes to promote encoding of contextual cues.
The experimenter then read out a list of 20 common nouns, pausing after each one for the subject to describe an event that the word reminded them of. Memory for the words was tested 48 hours later.
It was found that word recall was best when the novel odor (osmanthus) was present during learning and again at testing. Among the familiar odors, recall was better if the smell was contextually inappropriate (peppermint). The improvement in recall only occurs when the odor is present at both encoding (learning) and retrieval (testing). Clearly, smell is a good contextual cue.