smell

Smell

Older news items (pre-2010) brought over from the old website

Why smells can be so memorable

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.

[2543] Yeshurun Y, Lapid H, Dudai Y, Sobel N. The Privileged Brain Representation of First Olfactory Associations. Current Biology [Internet]. 2009 ;19:1869 - 1874. Available from: http://www.cell.com/current-biology/abstract/S0960-9822(09)01857-0

http://www.physorg.com/news176649240.html

Difficulty identifying odors may predict cognitive decline

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.

[1130] Wilson RS, Schneider JA, Arnold SE, Tang Y, Boyle PA, Bennett DA. Olfactory Identification and Incidence of Mild Cognitive Impairment in Older Age. Arch Gen Psychiatry [Internet]. 2007 ;64(7):802 - 808. Available from: http://archpsyc.ama-assn.org/cgi/content/abstract/64/7/802

http://www.eurekalert.org/pub_releases/2007-07/jaaj-dio062807.php

Odor can help memory, in some circumstances

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.

[1206] Rasch B, Buchel C, Gais S, Born J. Odor Cues During Slow-Wave Sleep Prompt Declarative Memory Consolidation. Science [Internet]. 2007 ;315(5817):1426 - 1429. Available 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

Scent of fear impacts cognitive performance

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

Brain region involved in recalling memories from smell identified

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.

[1060] Wilson MA, Tonegawa S, Nakazawa K, Quirk MC, Chitwood RA, Watanabe M, Yeckel MF, Sun LD, Kato A, Carr CA, et al. Requirement for Hippocampal CA3 NMDA Receptors in Associative Memory Recall. Science [Internet]. 2002 ;297(5579):211 - 218. Available 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

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Scent of rosemary may help cognition

March, 2012

Rosemary is a herb long associated with memory. A small study now provides some support for the association, and for the possible benefits of aromatherapy. And a rat study indicates that your attitude to work might change how stimulants affect you.

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.

Participants were given two types of test: serial subtraction and rapid visual information processing. These tests took place in a cubicle smelling of rosemary. Participants sat in the cubicle for either 4, 6, 8, or 10 minutes before taking the tests (this was in order to get a range of blood concentrations). Mood was assessed both before and after, and blood was tested at the end of the session.

While blood levels of the chemical correlated with accuracy and speed on both tasks, the effects were significant only for the mental arithmetic task.

Participants didn’t know that the scent was part of the study, and those who asked about it were told it was left over from a previous study.

There was no clear evidence that the chemical improved attention, but there was a significant association with one aspect of mood, with higher levels of the scent correlating with greater contentment. Contentment was the only aspect of mood that showed such a link.

It’s suggested that this chemical compound may affect learning through its inhibiting effect on acetylcholinesterase (an important enzyme in the development of Alzheimer's disease). Most Alzheimer’s drugs are cholinesterase inhibitors.

While this is very interesting (although obviously a larger study needs to confirm the findings), what I would like to see is the effects on more prolonged mental efforts. It’s also a little baffling to find the effect being limited to only one of these tasks, given that both involve attention and working memory. I would also like to see the rosemary-infused cubicle compared to some other pleasant smell.

Interestingly, a very recent study also suggests the importance of individual differences. A rat study compared the effects of amphetamines and caffeine on cognitive effort. First of all, giving the rats the choice of easy or hard visuospatial discriminations revealed that, as with humans, individuals could be divided into those who tended to choose difficult trials (“workers”) and those who preferred easy ones (“slackers”). (Easy trials took less effort, but earned commensurately smaller reward.)

Amphetamine, it was found, made the slackers worked harder, but made the workers take it easier. Caffeine, too, made the workers slack off, but had no effect on slackers.

The extent to which this applies to humans is of course unknown, but the idea that your attitude to cognitive effort might change how stimulants affect you is an intriguing one. And of course this is a more general reminder that factors, whatever they are, have varying effects on individuals. This is why it’s so important to have a large sample size, and why, as an individual, you can’t automatically assume that something will benefit you, whatever the research says.

But in the case of rosemary oil, I can’t see any downside! Try it out; maybe it will help.

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Reviving a failing sense of smell through training

January, 2012

A rat study reveals how training can improve or impair smell perception.

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. More recently, however, it has been suggested that it would be more useful to regard the olfactory bulb as the primary olfactory cortex (primary in the sense that it is first), while the piriform cortex should be regarded as association cortex — meaning that it integrates sensory information with ‘higher-order’ (cognitive, contextual, and behavioral) information.

Testing this hypothesis, a new rat study has found that, when rats were given training to distinguish various odors, each smell produced a different pattern of electrical activity in the olfactory bulb. However, only those smells that the rat could distinguish from others were reflected in distinct patterns of brain activity in the anterior piriform cortex, while smells that the rat couldn’t differentiate produced identical brain activity patterns there. Interestingly, the smells that the rats could easily distinguish were ones in which one of the ten components in the target odor had been replaced with a new component. The smells they found difficult to distinguish were those in which a component had simply been deleted.

When a new group of rats was given additional training (8 days vs the 2 days given the original group), they eventually learned to discriminate between the odors the first animals couldn’t distinguish, and this was reflected in distinct patterns of brain activity in the anterior piriform cortex. When a third group were taught to ignore the difference between odors the first rats could readily distinguish, they became unable to tell the odors apart, and similar patterns of brain activity were produced in the piriform cortex.

The effects of training were also quite stable — they were still evident after two weeks.

These findings support the idea of the piriform cortex as association cortex. It is here that experience modified neuronal activity. In the olfactory bulb, where all the various odors were reflected in different patterns of activity right from the beginning (meaning that this part of the brain could discriminate between odors that the rat itself couldn’t distinguish), training made no difference to the patterns of activity.

Having said that, it should be noted that this is not entirely consistent with previous research. Several studies have found that odor training produces changes in the representations in the olfactory bulb. The difference may lie in the method of neural recording.

How far does this generalize to the human brain? Human studies have suggested that odors are represented in the posterior piriform cortex rather than the anterior piriform cortex. They have also suggested that the anterior piriform cortex is involved in expectations relating to the smells, rather than representing the smells themselves. Whether these differences reflect species differences, task differences, or methodological differences, remains to be seen.

But whether or not the same exact regions are involved, there are practical implications we can consider. The findings do suggest that one road to olfactory impairment is through neglect — if you learn to ignore differences between smells, you will become increasingly less able to do so. An impaired sense of smell has been found in Alzheimer’s disease, Parkinson's disease, schizophrenia, and even normal aging. While some of that may well reflect impairment earlier in the perception process, some of it may reflect the consequences of neglect. The burning question is, then, would it be possible to restore smell function through odor training?

I’d really like to see this study replicated with old rats.

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Perception

See also

Smell

Hearing

Vision

Older news items (pre-2010) brought over from the old website

Perception affected by mood

An imaging study has revealed that when people were shown a composite image with a face surrounded by "place" images, such as a house, and asked to identify the gender of the face, those in whom a bad mood had been induced didn’t process the places in the background. However, those in a good mood took in both the focal and background images. These differences in perception were coupled with differences in activity in the parahippocampal place area. Increasing the amount of information is of course not necessarily a good thing, as it may result in more distraction.

[1054] Schmitz TW, De Rosa E, Anderson AK. Opposing Influences of Affective State Valence on Visual Cortical Encoding. J. Neurosci. [Internet]. 2009 ;29(22):7199 - 7207. Available from: http://www.jneurosci.org/cgi/content/abstract/29/22/7199

http://www.eurekalert.org/pub_releases/2009-06/uot-pww060309.php

What we perceive is not what we sense

Perceiving a simple touch may depend as much on memory, attention, and expectation as on the stimulus itself. A study involving macaque monkeys has found that the monkeys’ perception of a touch (varied in intensity) was more closely correlated with activity in the medial premotor cortex (MPC), a region of the brain's frontal lobe known to be involved in making decisions about sensory information, than activity in the primary somatosensory cortex (which nevertheless accurately recorded the intensity of the sensation). MPC neurons began to fire before the stimulus even touched the monkeys' fingertips — presumably because the monkey was expecting the stimulus.

[263] de Lafuente V, Romo R. Neuronal correlates of subjective sensory experience. Nat Neurosci [Internet]. 2005 ;8(12):1698 - 1703. Available from: http://dx.doi.org/10.1038/nn1587

http://www.eurekalert.org/pub_releases/2005-11/hhmi-tsi110405.php

Varied sensory experience important in childhood

A new baby has far more connections between neurons than necessary; from birth to about age 12 the brain trims 50% of these unnecessary connections while at the same time building new ones through learning and sensory stimulation — in other words, tailoring the brain to its environment. A mouse study has found that without enough sensory stimulation, infant mice lose fewer connections — indicating that connections need to be lost in order for appropriate ones to grow. The findings support the idea that parents should try to expose their children to a variety of sensory experiences.

[479] Zuo Y, Yang G, Kwon E, Gan W-B. Long-term sensory deprivation prevents dendritic spine loss in primary somatosensory cortex. Nature [Internet]. 2005 ;436(7048):261 - 265. Available from: http://www.ncbi.nlm.nih.gov/pubmed/16015331

http://www.sciencentral.com/articles/view.htm3?article_id=218392607

Brain regions that process reality and illusion identified

Researchers have now identified the regions of the brain involved in processing what’s really going on, and what we think is going on. Macaque monkeys played a virtual reality video game in which the monkeys were tricked into thinking that they were tracing ellipses with their hands, although they actually were moving their hands in a circle. Monitoring of nerve cells revealed that the primary motor cortex represented the actual movement while the signals from cells in a neighboring area, called the ventral premotor cortex, were generating elliptical shapes. Knowing how the brain works to distinguish between action and perception will help efforts to build biomedical devices that can control artificial limbs, some day enabling the disabled to move a prosthetic arm or leg by thinking about it.

[1107] Schwartz AB, Moran DW, Reina AG. Differential Representation of Perception and Action in the Frontal Cortex. Science [Internet]. 2004 ;303(5656):380 - 383. Available from: http://www.sciencemag.org/cgi/content/abstract/303/5656/380

http://news-info.wustl.edu/tips/page/normal/652.html
http://www.eurekalert.org/pub_releases/2004-02/wuis-rpb020704.php

Memory different depending on whether information received via eyes or ears

Carnegie Mellon scientists using magnetic resonance imaging found quite different brain activity patterns for reading and listening to identical sentences. During reading, the right hemisphere was not as active as expected, suggesting a difference in the nature of comprehension experienced when reading versus listening. When listening, there was greater activation in a part of Broca's area associated with verbal working memory, suggesting that there is more semantic processing and working memory storage in listening comprehension than in reading. This should not be taken as evidence that comprehension is better in one or other of these situations, merely that it is different. "Listening to an audio book leaves a different set of memories than reading does. A newscast heard on the radio is processed differently from the same words read in a newspaper."

[2540] Michael EB, Keller TA, Carpenter PA, Just MA. fMRI investigation of sentence comprehension by eye and by ear: Modality fingerprints on cognitive processes. Human Brain Mapping [Internet]. 2001 ;13(4):239 - 252. Available from: http://onlinelibrary.wiley.com/doi/10.1002/hbm.1036/abstract

http://www.eurekalert.org/pub_releases/2001-08/cmu-tma081401.php

The chunking of our lives: the brain "sees" life in segments

We talk about "chunking" all the time in the context of memory. But the process of breaking information down into manageable bits occurs, it seems, right from perception. Magnetic resonance imaging reveals that when people watched movies of common, everyday, goal-directed activities (making the bed, doing the dishes, ironing a shirt), their brains automatically broke these continuous events into smaller segments. The study also identified a network of brain areas that is activated during the perception of boundaries between events. "The fact that changes in brain activity occurred during the passive viewing of movies indicates that this is how we normally perceive continuous events, as a series of segments rather than a dynamic flow of action."

Zacks, J.M., Braver, T.S., Sheridan, M.A., Donaldson, D.I., Snyder, A.Z., Ollinger, J.M., Buckner, R.L. & Raichle, M.E. 2001. Human brain activity time-locked to perceptual event boundaries. Nature Neuroscience, 4(6), 651-5.

http://www.eurekalert.org/pub_releases/2001-07/aaft-bp070201.php

Amygdala may be critical for allowing perception of emotionally significant events despite inattention

We choose what to pay attention to, what to remember. We give more weight to some things than others. Our perceptions and memories of events are influenced by our preconceptions, and by our moods. Researchers at Yale and New York University have recently published research indicating that the part of the brain known as the amygdala is responsible for the influence of emotion on perception. This builds on previous research showing that the amygdala is critically involved in computing the emotional significance of events. The amygdala is connected to those brain regions dealing with sensory experiences, and the theory that these connections allow the amygdala to influence early perceptual processing is supported by this research. Dr. Anderson suggests that “the amygdala appears to be critical for the emotional tuning of perceptual experience, allowing perception of emotionally significant events to occur despite inattention.”

[968] Anderson AK, Phelps EA. Lesions of the human amygdala impair enhanced perception of emotionally salient events. Nature [Internet]. 2001 ;411(6835):305 - 309. Available from: http://dx.doi.org/10.1038/35077083

http://www.eurekalert.org/pub_releases/2001-05/NYU-Infr-1605101.php

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Large brains in mammals first evolved for better sense of smell

July, 2011

High-tech X-ray scans of ancient fossil skulls have revealed that the increase in brain size that began with the first mammals was driven by improvements in smell and touch.

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).

Comparison of these fossils with seven fossils of early reptiles (close relatives of the first mammals), 27 other primitive mammals, and 270 living mammals, has further revealed that the size of the mammalian brain evolved in three major stages. First, an initial increase in the olfactory bulb and related areas (including the cerebellum) by 190 million years ago; then another jump in the size of these regions shortly after that time; and finally an increase in those regions that control neuromuscular coordination by integrating different senses by 65 million years ago.

It’s speculated that the initial increase in smell and touch was driven by early mammals being nocturnal — dinosaurs being active during the day.

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Loss of smell may predict early onset of Alzheimer's

January, 2010

Previous research suggesting loss of smell function may serve as an early marker of Alzheimer's disease has now been supported by evidence from genetically engineered mice.

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).

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Effects of cue distinctiveness on odor-based context dependent memory

Journal Article: 

Herz, R.S. (1997). The effects of cue distinctiveness on odor-based context dependent memory. Memory and Cognition, 25, 375-380.

  • Smell can aid memory if the same smell is present during the original experience and when you are trying to remember.
  • It works best if the smell is unfamiliar.
  • If the smell is familiar, it is better if it is unusual in the context.

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.

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