Smell etcetera (Home): Role of smell

Role of smell

Hazard warning

Smell is one of the chemical senses, the other being taste (see "Taste" page). They are so called because they sense chemicals, and smells are, of course, chemicals. With these senses we sample our environment for information. We are continuously testing the quality of the air we breathe (this will alert us to potential dangers, e.g. smoke) as well as using this sense to inform us of other relevant information, such as the presence of food or another individual. Many dangerous and toxic compounds have a sulphur atom in them (e.g. bacterial decomposition) and we are very sensitive to sulphur (think of rotten eggs or skunks smell, both contain sulphur). This sends a signal to the amgydala, we interpret it as a "bad" smell and this alerts us to possible danger. Often we don't even have to think about it, our response is sub-conscious. We are then primed to act if necessary. The chemicals detected by our sensory systems need to have certain properties. For instance, odour molecules must be small enough to be volatile (less than 300-400 relative molecular mass) so that they can vapourise, reach the nose and then dissolve in the mucus. This tells us that smell, unlike taste, can signal over long distances (an early warning device). We appear to have an innate ability to detect bad, aversive smells. One-day old babies give facial expressions that indicate rejection when given fish or rotten egg odour.
Body odour has a number of functions (see "Smell and mate selection"), one of which is to tell us when to wash. We notice a build up of our body odour, some more quickly than others (!) and we find the smell unpleasant - is this another potential hazard? Well, we need to remove dead skin and grime to prevent a build up of fungi and bacteria that could lead to disease and infection. The bacteria are one of the factors in the production of bad smell.

Memory prompt

But, is our olfactory system doing more than just giving us warnings? Yes, of course. Smell is the backdrop to our every waking moment, it is part of our consciousness and, even in sleep (see Smell and dreams) the sense of smell keeps working. It also serves a recognition function. We all have our own unique smell (some more pleasant than others! - but that's another story, see Smelly you on the "Smell - What & How" page) and can recognise and be recognised by our smell. Smell evokes memory and, to a greater extent than the other senses, evokes the mood and emotion associated with the memory. The brain regions dealing with smell (piriform cortex) and memory (hippocampus) are close together and there is a two way highway of connections between them (see interesting work by Matsunaga's lab on smell memory, the immune system and health). The amygdala is also part of this collection of brain centres, called the limbic system, and sets the the appropriate emotional tone in response to memories.

Chemical signalling

Dogs can distinguish between the smell of T-shirts worn by non-identical twins (they couldn't tell the difference between identical twins - presumably because they smell identical!). Children can distinguish between the smell of their siblings and other children of the same age. Babies recognise their own mothers' smell and mothers recognise their own babies' smell. Emotion can be communicated by smell. Dogs and horses are very sensitive to the smell of fear in humans and recently it has been demonstrated that humans can detect fear. Dr Mujica Parodi used fMRI to show that "fear sweat" (from skydivers) activates the amygdala in other people whereas gym sweat does not. Research has shown that a panel of women can discriminate between armpit swabs taken from people watching "happy" and "sad" films. Men were less good at this. Mum's learn the smell of their own babies and babies learn the smell of their own mothers. The emotions of others, for example fear, contentment, sexuality, may therefore be experienced and communicated by smell. Memory is often associated with smell. Smell and memory are intimately linked - although this phenomenon is not well understood.

We certainly underestimate the importance of smell to our well-being - ask an anosmic (someone who has lost some or all of their sense of smell). There are suggestions that smell can influence mood, memory, emotions, mate choice,

the immune system and the endocrine system (hormones). In fact the sense of smell could be said to be at the mind-body interface working in the background to modulate our response to the outside world.

Antidepressant

A proportion of those who lose their sense of smell become depressed. So you could say that smell acts as an antidepressant. This works both ways. More detail on smell and depression can be found in my page on "Smell and Mood" and I look into new, non-drug treatments for depression in "Therapeutic use of smell".

Smell and flavour

Flavour is SMELL + TASTE, but mainly smell. Without smell food tastes really dull, so does drink. When we are hungry our appetite increases and our sense of smell becomes more acute, we smell things more intensely and this eases off once we've eaten. Those that loose their sense of smell often have difficulty in controlling their weight either eating too much to compensate or not eating at all because they have lost the pleasure in food.

Smell and mate selection

I have always believed that the prime function of smell is mate selection and that this has driven (human) evolution. We like the smell of some people and don't like the smell of others. Our sense of smell is subconsciously identifying the immune system of others. It is our MHC genes (called HLA in humans), part of of our immune system, that confer our own unique body odour and we like the smell of those with very different HLA genes and do NOT like the smell of those with similar HLA genes. The attraction to the smell of those with different HLA alleles encourages outbreeding, increases disease resistance and prevents incest. It is not so much that we actively sniif out our perfect partner more that we are unlikely to end up with someone whose smell we can't stand. The more scientific details about this are given below.

HLA and mate selection - non random pairing

One of the major functions of our sense of smell is as an immune system detector. A specific set of genes determines our immune system (Human Leucocyte Antigen, or HLA, genes). We are all different (except identical twins) and this huge range of differences (called “polymorphism”) results from our innate ability to choose partners with different HLA genes to our own. This phenomenon is called “disassortative mating preference” (Penn and Potts, 1999) where unlike genotypes are attracted to one another. Evidence for this has been found in mice (Yamazaki et al., 1976; 1978; Beauchamp et al., 1988), humans (Ober et al., 1997; Jacob et al., 2002; Chaix, Cao and Donnelly, 2008), birds (Freeman-Gallant et al., 2003), reptiles (Olsson et al., 2003) and fish (Landry et al., 2001). This mating preference acts to increase disease resistance (Hughes and Nei, 1992) and prevents kin mating (incest avoidance; Weisfeld et al., J. Exp. Child Psychol. 85:279-295, 2003). Mate choice appears to be mediated by smell and this immune-dependent odour is referred to as “odourtype”. There are four hypotheses as to how our immune system can influence odourtype (reviewed by Penn, 2002): (1) HLA molecules may provide the odours themselves, (2) HLA molecules may act as carriers for volatile odorants (the carrier hypothesis), (3) metabolites of HLA-bound peptides may provide the source of volatile odours (the peptide hypothesis), and (4) HLA genes may influence the odour indirectly by shaping an individual’s personal microflora composition (the microflora hypothesis), i.e. the bacteria responsible for your smelly armpits.

This odour-based mate selection encourages disassortative pairing and there are penalties if it is subverted (it is thought that oral contraceptive use undermines this process). Couples sharing HLA alleles have a higher rate of spontaneous abortion (Ober, 1995), and significantly lower success at achieving pregnancy following artificial reproductive therapy (reviewed in Ober and van der Ven, 1997) and some studies have suggested that females reject HLA-similar sperm or zygotes (Rulicke et al., 1998). Consanguineous married couples in China have been found to have low fertility and it has been suggested that they fail to initiate pregnancy due to the increased incidence of HLA-sharing (Liu et al., 2005) a phenomenon that has been long reported in the West (Thomas et al, 1985). Ober et al. (1985) concluded that fertility is reduced in couples sharing HLA-DR alleles. HLA-dissimilar mate preference may influence the psychology of sexual attraction. As HLA sharing increased, women’s sexual responsiveness to their partners decreased, their number of extrapair sexual partners increased, and their attraction to men other than their primary partners increased, particularly during the most fertile period of their menstrual cycles (ovulation) (Garver-Apgar et al., 2006).

From the above it can be suggested that it is important to make the appropriate, genetically-compatible choice when selecting a partner. Clearly this is not an active, conscious process in humans. More likely, it exerts a negative influence, viz. couples who find the smell of their partner continuously unpleasant would be less likely to stay together. To confound this biological selection mechanism, use of the oral contraceptive interferes with womens’ ability to select HLA-compatible partners, reversing their preference, and they have been found to prefer the smell of HLA-similar men (Wedekind et al., 1995; Craig Roberts et al., 2008). Is this the end of human evolution? Actually, not a silly question and no-one has looked into it. They should.

Smell and the pill

A study by Salvatore Caruso found women taking a birth control pill were less sensitive to scents, which may negatively affect their sex drives. Researchers tested the ability of 60 women not taking the pill to detect six smells at various points in their menstrual cycles. Sense of smell was shown to be most sensitive around ovulation, a time when women are most fertile. The subjects were then put on birth control pills and their sense of smell was re-tested. After three months of pill use, the increased sensitivity to smell usually seen during ovulation was absent. This contradicts earlier work carried out by Doty and colleagues who demonstrated fluctuations in smell sensitivity during the menstrual cycle that were not affected by contraceptive pill use.
Changes in olfactory perception were seen in a study by Hummel and colleagues. They tested sensitivity to phenylethyl alcohol, androstenone and nicotine at various phases of the menstrual cycle. They found that it was only the pleasantness of androstenone that changed and sensitivity (to all three compounds) was unaffected. Naturally cycling women in the periovulatory phase were significantly more sensitive to androstenone, androsterone, and musk than women taking oral contraceptives. These findings give support to odour-specific hormonal modulation of olfaction. Renfro and Hoffman tested the sensitivity of women to 6 odours - lemon, peppermint, rose, musk, androstenone and androsterone. They found that naturally cycling women were significantly more sensitive around ovulation to androstenone, androsterone, and musk than women taking oral contraceptives.
Another study has uncovered other relationships between the sense of smell and pill use. Research by Thornhill and colleagues, showed that women taking oral contraceptives had no significant preference for the smell of males with symmetrical features vs. asymmetrical features. Symmetry in features is a measure of attractiveness and genetic fitness. Women not taking the pill preferred the smell of males with symmetrical features during their most fertile time. Pill use can also decrease sex drive in some women. However, it should be said that sex drive and choosing a mate are very complex issues that involve many physical and psychological variables.

So it seems that oral contraceptive use does affect smell and may alter sensitivity to factors that have bearing on partner choice, sex and relationships.

Smell and sleep

A study by Carskadon and Herz (Sleep 27, 402-5: 2004) looked at whether smell could be used as an alarm clock to wake you up. They used peppermint and pyridine (unpleasant fish-like odour) but neither smell acted as a satisfactory wake-up call. However, I notice that this hasn't stopped Google from funding a start-up in Paris to design and manufacture an olfactory alarm clock (Sensorwake).

Smell and dreams

A study in Germany has shown that smell can influence the quality and emotional tone of dreams. Researchers wafted the smell of roses under the noses of sleeping volunteers in the REM phase of their sleep (dreaming phase). They then woke them up and asked them to report on the content and quality of their dreams. While the subjects did not report actually smelling anything during their dreams, the emotional tone of their dream did change depending upon the stimulation. The smell of roses evoked pleasant emotions and the smell of rotting eggs had the opposite effect.

Smell and memory

Smell and memory are closely linked. Smell evokes memories. Damage to the temporal cortical region of the brain - the site of memory - does not affect the ability to detect smell, but, rather, prevents the identification of the odour. We must first remember a smell before identifying it.

What we know about smell and memory:

Aroma can be associated with an event, place, person, sensation (conditioned association) and this is stored in memory. Smell can therefore be used to change mood. The emotions surrounding the association are captured and stored by the brain.

Memory - odour memory falls off less rapidly that other sensory memory (Miles & Jenkins, 2000)

Odour memory lasts a long time - the Proust effect (see no. 4) can last a lifetime. This is not true of conditioned association smell memory (no. 1 above).

The "Proust effect" ( a type of autobiographical memory) - an odour is associated with personal experience and the same smell can recall the memory; smell is better at this memory cue effect than other senses (Chu and Downes, 2000).

Marcel Proust has lent his name to the phenomenon of memory recall in response to a specific smell (after his description of such an event in "Swan's Way") - the "Proust Effect". Whole memories, complete with all associated emotions, can be prompted by smell. This is entirely unconscious and cannot necessarily be prompted voluntarily although countless studies have shown that recall can be enhanced if learning was done in the presence of an odour and that same odour is presented at the time of recall. Useful for exam revision!