Olfaction Research Paper

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When you sniff an odor you probably believe that your experience is a direct result of what you are smelling—that is, the stimulus dictates what you smell. Many olfactory scientists held this commonsense view until fairly recently, but it is looking increasingly wrong. Rather, what one smells appears to be defined by what the stimulus reminds your olfactory system of. If you smell caramel, for example, it smells sweet, yet sweetness is a quality associated with a completely different sensory system—taste. Therefore this experience of a “tasty smell” must be a consequence of memory. This point is made most strikingly when olfactory memory itself is damaged, leaving patients able to smell but unable to experience an odor’s unique quality. That is, roses and gasoline, for example, or indeed any odor, all smell alike—a sort of olfactory “grey.” In this research-paper we examine how the olfactory system works and the crucial role that memory plays in this process, as well as the methods special to olfactory research and the applications that arise from the study of olfaction.

Olfactory capabilities differ. Some animals excel at smelling. Women nearly always outperform men on olfactory tasks. Humans may also possess, along with other animals, two olfactory systems (main and accessory), each with differing capabilities. There are also striking differences in how the main olfactory system processes information at a neural level and in its cognitive processes, when contrasted with the visual and auditory systems. The future direction of olfactory research is exciting; it should help our understanding of the more complex visual and auditory systems that may have evolved from it. It will make a unique contribution to the study of human consciousness. Finally, it has growing practical relevance to the study of human health and disease.

Olfaction is not what you think. Although most of us know that we need eyes to see and ears to hear, most of us do not know that we need a nose to “taste,” yet olfaction is central to our enjoyment of eating and drinking. Indeed, most of us probably regard olfaction as nothing more than an occasional sniff of a flower or the heady whiff of garbage or somebody else’s unwashed body. But olfaction pervades our life in many ways, betraying whether we are likely to develop Alzheimer’s disease or schizophrenia, and indeed, if some are to be believed, influencing whether others find us attractive, as well as allowing us to recover forgotten memories from childhood. However, as interesting as these things are, they are not the reasons that most psychologists choose to study smell. Rather, it is the beguiling mystery of how it works, a conundrum that is still not resolved and at the moment forms the central focus for many psychologists working in this field. This research-paper starts by directly addressing this issue and, as you will see, the picture that emerges is indeed bizarre—what you smell is a memory.

Theory

Function

In both humans and animals, the basic function of the olfactory system is to detect and recognize biologically relevant chemicals (i.e., food, mates, kin, predators). This is no easy matter. The environment is full of chemicals that can be detected by the olfactory system. In addition, nearly all odors—which we perceive as being discrete entities—are in fact combinations of 10s or 100s of different volatile (i.e., smellable) chemicals. For example, chemical analysis of coffee reveals that it has 655 different detectable volatiles; bread, 296; grapefruit, 206; carrot, 95; and even rice has over 100. Yet, in each of these cases, and more generally with most if not all odors, what we perceive is a discrete entity or odor object—the smell of coffee, baking bread, or whatever. Theories of olfaction have to account for how we might accomplish this. Before we turn to examine the theories that have been proposed, it is important to consider what we know about the anatomy and sensory physiology of the olfactory system. This physiology constrains any psychological theory, offers its own insights into how the system might work, and presents us with some further problems that any psychological theory must account for. We will concentrate on humans for the moment and defer discussion of the similarities and differences between human and animal olfaction until later (see Comparisons section).

Anatomy and Sensory Physiology

Two major advances have shaped contemporary thinking about olfaction. The first is genetics, derived in no small part from seminal work conducted by Linda Buck and Richard Axel, who won the Nobel Prize for physiology for their study of olfactory receptors. When we detect an odor we do so in two discrete ways. The first is by sniffing, termed orthonasal olfaction. The act of sniffing is important, as it directs attention to the stimulus and engages brain areas involved in smell, even if there is no odor present. Damage to brain areas involved in coordinating sniffing can impair an otherwise functional sense of smell, and this is believed to underpin the olfactory deficits observed in patients with Parkinson’s disease. The second way we detect odors is not generally associated with smelling and is termed retronasal olfaction. When we eat and drink, volatile chemicals are released, and these diffuse up the back of the throat (via the nasopharynx) to the same set of olfactory receptors that are stimulated when we actively sniff something. Most people are not aware of the major role of olfaction in eating and drinking. For example, people who have damaged their olfactory system often report that they “cannot taste”—yet testing reveals an intact sense of taste (sweet, sour, salty, bitter via receptors on the tongue) but a damaged sense of smell. You can readily appreciate this distinction between taste and smell by pinching your nose while eating—all one is left with, in the main, is taste.

The olfactory receptors are located on a sheet of tissue called the olfactory epithelium. There is one sheet for the left nostril and one for the right, located just below eye level within the upper part of the nasal cavity. Volatile chemicals diffuse into a thin layer of mucus that covers the olfactory epithelium. This mucus layer is important; if it is abnormally thick (as when you have a head cold), you cannot smell properly. When it is functioning correctly, it probably assists the binding of chemicals to the olfactory receptors. Most important, it allows the body to remove the stimulus (i.e., chemicals) once they have been detected—a problem not faced by the visual or auditory systems. The mucus layer does this by containing proteins that break down or bind to the chemicals and thus lock them into the mucus, which then gradually flows over the olfactory epithelium, down the throat, and into the stomach.

The key player is the olfactory receptor neuron. On its receptor end, which extends into the mucus layer, it expresses G-protein receptors, and it is these that bind with chemicals and lead to depolarization and an action potential. Buck and Axel’s work revealed that in mice, there are about 1,000 different sorts of G-protein receptor (just contrast this with the visual system for a moment, with its 4 types of receptor—3 cones and 1 rod). Humans have somewhere between 300 and 500 different sorts of G-protein receptors. These can be arranged into families and subfamilies, based upon their similarity. Similar types of G-protein, not surprisingly, bind to similar types of chemical, but the specific feature of the chemical that they detect (i.e., chemical bond, particular chemical type [aldehyde, alcohol, etc.], chemical shape, etc.) is not well understood. Importantly, each type of G-protein has quite a broad spectrum of sensitivity—it will bind with many types of chemical. This makes it very unlikely that there are specific relations between one receptor and one percept (i.e., there are no “coffee-detecting” receptors).

Each olfactory receptor neuron only expresses one type of G-protein receptor. The arrangement of G-protein receptor types across the olfactory epithelium is not random— certain families of G-protein receptors tend to be located on certain areas of the epithelium—but the functional significance of this is not well understood. Information from each type of G-protein receptor (recall that in humans this is somewhere in the range of 300 to 500 types) converges onto clusters of neurons called glomeruli, which are in the olfactory bulb. The bulb itself is located underneath the brain and sits a centimeter or so above the olfactory epithelium—one for the left side and one for the right. There are approximately the same numbers of glomeruli as there are G-protein receptor types, and each type of G-protein receptor tends to converge onto just one or two glomeruli. This is significant, as it is highly likely that the information that is passed to the brain (as we will see in a moment) is in the form of a pattern of activity across all of the glomeruli. This pattern has both a spatial (some glomeruli are active, by degree, and some are not) and a temporal form (pattern of activity changing over time). This spatial and temporal form is probably unique for every different combination of chemicals that the olfactory system encounters (i.e., coffee will have its own unique spatial and temporal pattern of glomerular activity, as will carrots, white wine, or urine).

The glomerular output then passes largely intact down the lateral olfactory tract into the primary olfactory cortex. This is not true cortex, but paleocortex, with fewer cell layers than found in the neocortex. Nonetheless, it is here that many neuroscientists suspect that three key processes occur. First, if the glomerular output pattern is new, it is learned. Second, where the glomerular output pattern is old, it is recognized via a pattern-matching system. This may then give rise to our conscious experience of smell. Third, the pattern-matching process allows the brain to recognize a combination of chemicals, when other chemicals are simultaneously present in the environment, and to recognize combinations of chemicals when some of their components are missing (a degraded stimulus). This pattern-matching process is further assisted by another feature of the primary olfactory cortex, its rapid ability to adapt to chemicals that are present in the environment, thus allowing anything new to be quickly detected. The major downside of the pattern-matching process is that it appears to prevent analysis of the component parts of an odor, making smell a very synthetic, rather than an analytic, sense.

At this point, what we know becomes far sketchier, but a second major advance, namely neuroimaging, has allowed us to identify the areas in the brain that are also involved in odor processing. Whereas our discussion so far has focused on recognizing particular odor objects, there is far more to it. Most odors we smell are perceived in conjunction with other sensory systems (i.e., we see the potato chips we are about to eat, we hear the crunch in our mouth and taste their saltiness). We also respond emotionally to many odors, sometimes very strongly so, especially if they are fecal, rotting, or body odors (the emotion of disgust). Finally, we have some capacity to imagine smells, to name them, and to expect them to have certain properties (e.g., people perceive white wine that has been colored red to have a smell like red wine!). We will look now at the areas of the brain that neuroimaging has revealed and the functions they might have.

The primary olfactory cortex has connections to the amygdala. Neuroimaging suggests that the amygdala is involved in our experience of a strong emotional response to certain odors. The primary olfactory cortex also projects to the hypothalamus, and this almost certainly relates to the role of olfaction in hunger and satiety. Most important, the primary olfactory cortex projects to the orbitofrontal cortex. It does so in two ways—first, directly, and second, indirectly, via the thalamus. The direct projection of olfactory information to the neocortex sets olfaction apart from all other sensory systems, all of which enter the neo-cortex solely via the thalamus. The orbitofrontal cortex is involved in a variety of olfactory-related functions, including multimodal processing, judgments of edibility (is this a food-related odor?), and in higher cognitive functions such as naming and familiarity judgments.

Psychology

Since the first experimental psychologists studied olfaction in the late 1800s and early 1900s, a predominant focus has been on understanding the relation between the stimulus, the receptor that detects it, and the experience that it purportedly produces. Solving this so-called “stimulus problem” has been a major preoccupation of olfactory scientists. Such an emphasis is not surprising, as historically the same approach was adopted in audition (e.g., relation between sound frequency and pitch perception) and vision (e.g., relation between electromagnetic frequency and color perception). However, although research in audition and vision “moved on,” with an increasing ecological focus in both on the perception of objects (e.g., the sound of “thunder” or the sight of a “tree”), olfactory research remained firmly anchored to a stimulus-problem approach.

There were benefits, but not ones researchers probably wanted. First, early investigators believed, reasonably enough, that there was only a limited set of receptors. It then followed that there should be only a limited set of primary odor qualities, with each primary associated with the activation of each specific receptor type (e.g., smoky, spicy, floral, etc.). That is, one should be able to identify a limited set of odor qualities (primaries), combinations of which would account for all possible olfactory experiences. No such set of odor primaries has ever been identified, and not for want of trying. Second, again, if there were a limited set of receptors, then some unlucky souls should be born without one or more receptor types (akin to color blindness) and this should allow one to determine the relation among stimulus, receptor, and percept. Again, researchers could not identify such consistent relations; those that they did identify differed significantly in form to their conceptual parallels in color blindness. Finally, although there are clearly relations between the percepts produced by chemically similar stimuli (i.e., aldehydes tend to smell fruity), it has never been possible to accurately predict what a novel chemical will smell like. For these reasons, although empirical research in the psychology of olfaction has made enormous strides over the last 100 years, theoretically there was little movement.

Things started to change in the 1980s with work on the neurophysiology of olfaction and with the genetic studies described above. The neurophysiology suggested that primary olfactory cortex functioned as a pattern-matching system, whereas the genetic data suggested what information this system might utilize. The implication of these developments slowly became apparent, especially when researchers took results from psychological work into account. A key study was conducted by Michael Rabin

and William Cain (1984), who found that participants were poorer at discriminating unfamiliar odors and that this ability could be improved by passive exposure to such odors. This result, confirmed in a number of other laboratories, is very puzzling from a “stimulus-problem” perspective. The stimulus remains the same, but with increasing familiarity the person gets better at discriminating it from other unfamiliar odors. Moreover, Rabin and Cain then showed that the ability of people to discriminate the components of an odor mixture is also dependent upon familiarity. For example, if participants sniff an unfamiliar odor (A) and then they are presented with a mixture that contains this unfamiliar odor (A) mixed with another unfamiliar odor (B), they are very poor at saying whether A is present in the AB mixture. Performance improves markedly when one of the two odors is familiar and is best when both are familiar. Again, a stimulus-problem approach to olfaction is hard pressed to account for such findings, as the stimulus never changes.

The failure of the stimulus-problem approach to account for experimental findings has become more and more apparent. One such finding is that people can appear to smell things that are not in fact there. If participants are exposed to an unfamiliar odor mixture (e.g., smoky-cherry), when they later smell the cherry odor alone, they report that it smells slightly smoky. They also report, when smelling the smoky odor alone, that it smells slightly cherry-like. A more objective approach, using discrimination, reveals much the same result. If participants have experienced an odor mixture (e.g., smoky-cherry), they are much poorer at discriminating the two components (e.g., smoky odor vs. cherry odor) than they are at discriminating other odors that they have smelled an equal number of times, but never in a mixture. Findings of this type suggest a decoupling of what we perceive from what we are actually smelling and imply that what we are actually perceiving is a product of memory, not of the stimulus itself (i.e., in this case, a memory of smoky-cherry).

Findings such as these have led to a more contemporary theoretical view. When the olfactory system encounters chemical stimuli, it generates a pattern of neural activity that is then matched against previous patterns of activity that are stored in odor memory (this may reside in the primary olfactory cortex or it may be distributed around several structures, including the orbitofrontal cortex and parts of the limbic system). When there is a match between the incoming pattern and one stored in memory we perceive a particular odor, be it cheese or coffee. When there is no match, we experience a rather vague percept, one that is not readily discriminated from other such unfamiliar smells. In addition, when the pattern is unfamiliar it is also learned, such that when it is encountered again the odor starts to take on a more definite perceptual form—it becomes an odor object.

This type of mnemonic (memory-based) theory can readily account for many of the findings described above. For example, if one encodes an odor mixture (e.g., smokycherry), then later smelling the smoky odor alone should partially match the stored pattern (memory) of “smoky-cherry,” resulting in perception of a cherry-like note in the smell. More important, it has no problem in accounting for why chemicals that share similar structural features (e.g., aldehydes) can sometimes smell alike, as structurally similar chemicals will produce similar patterns of neural activity and thus will activate similar odor memories. From this mnemonic perspective, the relation between the structural features of the chemical (the stimulus) and what we perceive is seen as correlational, but crucially not causal. The cause of a particular olfactory experience is the patterns that are activated in odor memory.

The strength of any particular theory is its ability to generate predictions and, as we alluded to above, to account for existing findings. We can derive two predictions from the mnemonic (memory) theory. First, as one encounters more odors, one should become progressively better at discriminating different smells. Put more bluntly, children should be poorer at odor discrimination than adults. Researchers have tested this claim extensively, using familiar, moderately familiar, and unfamiliar odors (as judged by adults). For all three odor types, adults and older children (11-year-olds) are consistently better at discrimination than six-year-old children. This difference could result from other causes. For example, adults and older children are better at naming odors and this also improves discriminative ability. However, using an articulatory suppression task (saying “the, the, the….”), which makes it hard to engage in any form of verbal processing such as naming, still reveals significantly poorer performance in 6-year-olds. Finally, it could be that younger children simply cannot perform the discrimination task. However, this possibility was also eliminated by having the children perform a complex visual discrimination task. Performance here did not differ from that of adults. Thus, younger children do indeed appear to be poorer at discriminating odors.

A second prediction is that any damage to odor memory should produce a profound deficit in olfactory perception, akin perhaps to visual agnosia, whereby a person can see (sensation) but cannot perceive visual objects (perception). There is a wealth of neuropsychological evidence in olfaction to support this type of deficit. The most striking example comes from the most well-known neuropsychological case, HM. HM had a bilateral temporal lobe resection, which included most of his primary olfactory cortex. The consequences for HM were profound, especially in the dense anterograde amnesia that followed his surgery (i.e., he could not remember anything new postsurgery). His olfactory perception was also profoundly affected. He could detect the presence or absence of odors as well as a normal person. He could also readily discriminate between a strong and a weak smell. However, he was totally unable to discriminate between different odors when they were equated for intensity (strength)—that is a rose, feces, and a fish supper all smelled alike. The conclusion that has been drawn from HM and similar cases is that even when the stimulus can be detected, if it cannot be recognized via the memory-based pattern-matching system, one cannot perceive odor quality—all odors come to smell alike.

Multisensory Processing

So far we have considered olfaction in a vacuum, yet, as noted above, when we smell we normally experience percepts in other sensory modalities at the same time. Olfaction is of special interest in this regard because of its involvement in “flavor.” When we eat and drink a variety of independent sensory systems are activated, with smell, taste, and touch being the principal players. However, what we experience is a synthetic experience, especially between taste and smell. Paul Rozin reported that of 10 languages he surveyed, none had terms that clearly distinguished the olfactory component of eating and drinking, suggesting that at a linguistic level people are unaware of the contribution made by smell to flavor. Second, and more recently, several experiments have found that participants are very poor at discriminating the components of taste-smell mixtures in the mouth.

It is possible to account for these types of findings by recourse to much the same type of mnemonic model that we just discussed. For example, participants experience a particular combination of taste and smell, and these are then learned. Two consequences flow from this. First, the taste and the smell should now be harder to tell apart, in the mouth, than they were before. Researchers have not tested this prediction directly. Second, taste and smell might acquire each other’s characteristics. Researchers have explored this prediction extensively. However, things are not as straightforward as one might first imagine. If, for example, you ask participants to “taste” sucrose (sweet) to which an odor has been added (say, lychee) and then later ask them to smell lychee odor alone, they generally report that the lychee odor now smells sweet, and, indeed, sweeter than it did prior to pairing with sucrose. However, if you now experience sucrose alone it does not result in an experience of lychee odor. This is a quite remarkable state of affairs, namely that an odor (a discrete sensory modality) can induce perception of a taste (another discrete sensory modality)—synesthesia. More recent evidence suggests that animals too may experience something substantially similar.

Critics have argued that the perception of sweetness produced by an odor is metaphoric—we speak of “sweet” people but we do not mean that they taste sweet. The research evidence does not support this interpretation. Odor sweetness is akin to that produced by sucrose on the tongue, and this result is suggested by the phenomenon of sweetness enhancement. Here a tasteless but sweet-smelling odor is added to a sucrose solution and this results in participants rating this mixture as sweeter than the sucrose alone. The degree to which this enhancement occurs is best predicted by how sweet the odor is judged to smell, suggesting that smelled “sweetness” and tasted sweetness are perceptually similar.

The experience of odor sweetness is almost certainly mnemonic. Nothing about the odor stimulus changes as a consequence of experiencing an odor and sucrose together. The similarity to the results described earlier lends yet more weight to the notion that what we experience when we smell an odor is not primarily driven by what is actually there, but rather by what it reminds us of—redolence.

Methods

Experimental approaches to studying olfaction in both humans and animals utilize many methods common to other areas of sensory psychology—threshold, discrimination, identification, and so on. However, olfaction offers certain specific challenges along with some rather specialized methods; these are examined in the following.

The Stimulus

The selection of odors for an experiment requires careful attention. As should now be apparent, individual experience with an odor can affect its discriminability. We also know that individual experience with an odor tends to produce (in the main) more positive hedonic responses toward it and that such odors are also judged to smell stronger. A more general issue concerns the use of single pure chemicals or more ecologically valid stimuli (e.g., coffee) in experiments. Although single pure chemicals offer ease of replication, they do not reflect the type of stimulus that the olfactory system generally has to contend with. Certain researchers, such as Robyn Hudson (1999), have argued that progress in olfactory research has been slowed by an unwillingness to utilize complex ecologically valid odors.

When selecting odors, care should be taken to specify the experimental requirements and the population under test. For learning experiments, odors probably need to be unfamiliar to most participants, although for identification, the odors likely need to be familiar. With older participants, who are known to have poorer odor naming and acuity and reduced perception of odor intensity, more intense stimuli might be needed to compensate for such deficits. In younger participants, especially children, care needs to be taken with toxicity and with the presence of potential allergens, especially products containing nuts.

Adaptation

A special problem faced by olfactory researchers is in providing sufficient time between stimuli to prevent adaptation from occurring. Unfortunately, there are no well-standardized guidelines upon which to base a decision, although E. P. Kosters’s doctoral thesis on odor adaptation probably has the most complete set of data about adaptation; it is a valuable source of information. Whereas a visual object remains visible throughout the viewing period, repeated presentation of an odorant will lead to progressive loss of the ability to perceive it with possible cross-adaptation to other odors, too. This varies on both an individual basis and on an odor-by-odor basis, making it hard to prescribe general guidelines. Nonetheless, at least 15 to 20 seconds should intervene between each stimulus, with longer intervals (60 seconds) if the same stimulus is used repeatedly.

Specialized Methods

Two specialized methods warrant mention. The first is the most widely used neuropsychological test of olfactory ability, the Smell Identification Test developed by Richard Doty (SIT, previously known as the UPSIT; 1997). This test is composed of 40 microencapsulated odors. For each odor, the participant scratches the microencapsulated dot and releases the smell, which the person then sniffs. The person then has to identify the odor by selecting one of four response options provided. The test is scored out of 40 and norms are available by age and gender. This test has been used extensively to explore olfactory deficits in neuropsychological and psychiatric populations, in industrial settings, as a screening test, and with normal participants in a variety of experimental contexts. The test has excellent reliability and validity and provides a snapshot of a person’s olfactory capabilities. Its major limitation is that it does not indicate the precise nature of any deficit.

For certain types of olfactory research in both animals and people, precise metered delivery of an odorant is required. This typically involves a specialized piece of equipment—an olfactometer. These are usually built to order, and vary in the speed and accuracy with which they can deliver an odorant to the nose and in their ability to do so without any auditory cue (i.e., clicking of valves). The number of channels is also an important consideration, in terms of the number of odors the researcher can readily administer within an experimental session. Delivery may also need to be to breathing rate, so that the odor is delivered at the moment of inspiration. Several research groups around the world build olfactometers, and published details are available in the References about how to construct one.

Applications

The psychological study of olfaction has applications to many diverse areas; I examine a few important examples in this section. The place to start, perhaps, is perceptual expertise, as this relates back in many ways to issues raised early about learning and memory. Within the olfactory domain perceptual expertise manifests in many forms: wine, beer, dairy, and cheese tasters; flavorists (who create food flavors); perfumists (who create perfumes and many perfume-like additives for laundry powder, wood polish, etc.); and expert sensory evaluation panels. All of these individuals usually have a fairly extensive training that typically involves learning the name for many of the chemical compounds that they may encounter within their domain of expertise.

There have been several investigations of how perceptual experts differ from novices. One apparently surprising finding is that such individuals are not as good as one might at first believe. Claims that wine tasters may identify the soil type, hillside, vintage, and so forth are exaggerated. Similarly, the often verbose descriptions of a wine’s qualities are unreliable, as research has shown that when experts are asked to provide such written descriptions and then later match them to the same set of wines, although they are above chance and significantly better than naive controls, the actual size of the effect is small. Moreover, when assessed purely in their discriminative capacity (i.e., to tell one wine from another), experts fare no better than amateur wine drinkers. Their expertise only becomes apparent when they are asked to provide a verbal description. It appears that experts may move back and forth between a perceptual experience (the wine’s flavor) and a verbal description more readily than amateur drinkers (i.e., experts are less prone to verbal overshadowing).

One way to eliminate the need for experts, and perhaps sniffer dogs as well, is to design electronic noses that may detect tainted food, drugs, explosives, contraband food, or whatever. Although there has been some success in this regard, nothing can yet match the acute sensitivity and accuracy of a dog’s nose. Moreover, dogs can be trained to identify several different targets, whereas most electronic noses are limited to detecting a single chemical and may have trouble with odors in which there are considerable variations in chemical composition. In fact, it is likely that major advances in electronic noses will come from software that mimics the type of associative memory with which we have been endowed.

Some smells may genuinely indicate danger (e.g., smoke, natural or poison gas). In the United States alone, 3 million people live near an oil refinery, with exposure to high levels of aromatic hydrocarbons, and over 33 percent of Americans regard air pollution as a major problem. Certain environmental odors can be offensive and produce significant loss of enjoyment for those forced to live with them, even though the level of the pollutant may not be sufficient to directly cause ill health. These problems may occur from traffic fumes, paper mills, chemical plants, sewage farms, and animal husbandry (abattoirs, feed lots, etc.). However, these types of odors may be perceived as noxious and injurious to health and produce somatic symptoms, consistent with the person’s beliefs about the odor. Tobacco smoke is a case in point. Whereas 30 years ago few complained about having to share public space with smokers and their smoke, changing perceptions of what secondhand smoke means have led to a “moralization” of tobacco smoke. This allows people to express outrage at smokers for polluting the local environment, even when limited exposure can produce no detectable harm. More generally, the precise measurement of noxious odors is problematic, as they rarely correlate with subjective impressions of the problem. Nonetheless, there is a developing research agenda that seeks to understand how people’s beliefs affect their perception of odors and how exposure to noxious smells influences health and behavior.

Tests of olfactory function may provide useful warning signs of impending disease. In people who have a heightened risk of developing schizophrenia, poor odor naming is the single best predictor of whether they will later develop this disease. Similarly, the earliest indicator of the presence of Alzheimer’s disease is a notable reduction in odor-naming ability (and discriminative capacity), which may occur many years before more overt cognitive deterioration is evident. The use of odor naming as a prognostic tool is still in its earliest stages, but it is likely to play an important role in the future, especially as tests such as the SIT (see Specialized Methods section) appear very sensitive to abnormalities in frontal-lobe function (e.g., in ADHD, OCD).

Finally, the study of olfaction in animals has a number of practical applications. The identification of various insect pheromones has led to the development of insect traps that utilize pheromones to attract pest species. Similarly, with mosquitoes, there have been significant advances in understanding what makes some individuals more prone to be bitten than others, and it is likely that new forms of chemical control will eventuate from this research. In addition to new weapons against insect pests, advances in rodent control based upon their innate fear response to odors that are secreted by cats and foxes are also under development. These odors can be readily collected from cats and foxes, but researchers have yet to identify the key ingredients.

Comparisons

Four forms of comparison, which have all attracted considerable research attention, appear to be particularly valuable:

Microsmatic animals (sense of smell is of lesser importance; e.g., humans and primates) versus macrosmatic animals (sense of smell is very important; e.g., rodents and dogs)
The main olfactory system (on which this research-paper is mainly focused) versus the accessory olfactory system (a system that functions to detect specific chemicals, often related to reproduction)
Men versus women
Olfaction versus other sensory modalities in humans

Microsmatic Versus Macrosmatic Mammals

Animals differ in their sensory specialization—well-developed eyes are of little use to night-flying bats or rodents. For olfaction, there has been a traditional divide between microsmatic and macrosmatic animals. Although this divide has some utility, it probably reflects our ignorance about the olfactory capabilities of many species that have never been examined in any depth (e.g., monotremes, whales, dolphins). Nonetheless, some mammals clearly have a greater reliance on their sense of smell, notably dogs, rodents, and certain carnivores. In these animals, especially dogs and rodents, which have received the most attention, the size of the olfactory mucosa is proportionally larger and contains more olfactory receptor neurons than that in species with a lesser reliance upon smell such as humans. Whereas we have around 10cm2 of olfactory epithelium, dogs such as German shepherds have up to 169cm2, boxers have up to 120cm2, and even Pekinese have around 30cm2. Similarly, the number of olfactory receptor neurons is also greater in dogs—humans have around 10 million, fox terriers have 150 million, and sheepdogs have 220 million. Functionally, this allows animals such as dogs to detect odors at concentrations well below what we can detect. Consequently they are sensitive to trace levels of drugs or explosives, or even underground gas leaks. However, it is not clear what other benefits this sensitivity may confer. As we described, much olfactory processing relies upon higher cortical functions (recognition), and although rodents and dogs may have proportionally more cortex devoted to olfaction than humans, the degree to which they are better at recognition may not be so large. Clearly there are some advantages. Dogs can, for example, readily discriminate the odors of different people (which we find hard) and even discriminate identical twins (with difficulty), whereas rats appear able to analyze odor mixtures in a way we cannot. Their main advantage, however, is sheer sensitivity.

Stimulus-Driven Versus Mnemonic Olfactory Systems

So far we have concentrated on general olfactory abilities and thus on the main olfactory system. However, nearly all animals, both vertebrates and invertebrates, have both a main and an accessory olfactory system. Whereas the main olfactory system is plastic and flexible—that is, it can recognize many odors and acquire new ones—the accessory olfactory system is characterized by its general inflexibility and specificity. That is, it appears to be hardwired to detect certain specific chemicals or mixtures of chemicals. Not surprisingly, these turn out to be chemicals that often have important biological functions, and many of these fall under the rubric of “pheromones”—chemicals secreted by one member of a species that have an instinctual effect on the behavior or physiology of another member of that species.

Whereas the existence of an accessory olfactory system and pheromones in insects, fish, rodents, and some other mammalian species as well (e.g., rodents) is solidly established, the existence of both an accessory olfactory system in humans and human pheromones is highly controversial. There clearly is evidence that humans have cells located in the nasal septum that appear similar to those found in other mammalian accessory olfactory systems (the vomeronasal organ), but whether these cells can actually detect chemicals and influence behavior has not been established. As for the effect of pheromones on human behavior, this is a field awash with claims, some of which are supported and many of which rely on weak evidence. Menstrual synchrony driven by chemical cues is probably the most replicated finding, albeit not without its critics. Here, women who live in close proximity tend to progressively synchronize their menstrual cycles. However, whether or not certain chemicals such as 5-alpha-andrsotenone act as a chemical attractant (secreted by men and attracting women) is a highly controversial claim and is currently unresolved, although such chemicals are widely marketed.

Men Versus Women

On virtually every sort of olfactory task—sensitivity, recognition, and identification—women tend to outperform men. This finding is well established and robust, although its cause is not well understood. One possibility is that higher circulating levels of estrogen in women may enhance olfactory function. This might explain why olfactory abilities are better preserved into old age in women than in men, as well as the variations in olfactory acuity that occur in women during the menstrual cycle. It does not explain the observation, however, that prepubescent females may exceed prepubescent males on olfactory tasks, although the data here are somewhat limited and not all studies report such differences.

A second possibility is that women may simply attend to olfactory stimuli more than men—they certainly dream of smells more frequently—as they may have a greater interest in things olfactory as a consequence of gender stereotyping (i.e., cooking, perfume, etc.). Recent attempts to test this reveal that women still outperform men even when unfamiliar odors are used (i.e., those with which they could have had no prior experience) and where level of motivation does not differ either. Moreover, National Geographic conducted a large smell survey of over 1 million people during the early 1990s, across many different nations and cultures (i.e., with presumably different types of gender stereotyping), and yet this too found consistent female advantage in performance. Finally, some have suggested that women’s better performance on olfactory tasks is an evolutionary adaptation reflecting their putative role in gathering food supplies of plant origin (whereas men hunted). It has been argued that this would place a selective pressure on women for keener olfactory capabilities, so as to distinguish edible plants and avoid toxic ones. Such evolutionary explanations may account for superior female abilities, but they do not explain their biological basis.

Olfaction Versus the Other Senses

At both the neuroanatomical and the psychological level, olfaction differs qualitatively from the primary senses of vision and audition. As we noted earlier, olfaction has a unique neuroanatomy in being the only sense to have both a direct route (to the orbitofrontal cortex) and an indirect route (via the mediodorsal nucleus of the thalamus) to the neocortex. The functional significance of this distinction is a matter of debate, but some have suggested that it might account for the universal form of synesthesia (i.e., “tasty smells”) that olfaction exhibits.

At the psychological level, olfaction appears to differ in several respects. The first concerns the presence of discrete short-term odor memory capacity, as distinct from a long-term odor memory capacity. Such a distinction is usually made on four grounds—capacity differences, coding differences, neuropsychological dissociations, and the serial position effect. To date, no consistent evidence from any of these domains has been identified, and although some contend that olfaction does have a discrete short-term store, this is contentious.

A second difference is that olfaction appears to be primarily a synthetic sense with little available capacity to discriminate the component parts of the stimulus. In contrast, although we may appreciate that a telephone is one visual object or that “Penny Lane” is a Beatles song, we can identify many individual components within these stimuli, a feat that we cannot readily accomplish in olfaction even after specialist training.

A third difference is the more constrained capacity for odor imagery. Some suggest here a qualitative difference, claiming that olfaction has no imagery capacity whatsoever, and there is certainly evidence to back this claim. Even where evidence of imagery is obtained it tends to be more circumscribed in its capacity than in vision or audition, but the evidence of olfactory hallucinations strongly suggests that under certain circumstances people clearly do perceive an odor when one is not in fact there.

A fourth difference, and the one that has been most extensively explored, is the olfactory-verbal “gap.” Most people can readily name visual objects, yet most people have enormous difficulty naming common odors when their visual cues are absent (e.g., sniffing coffee from an opaque bottle). The origins of this effect are poorly understood, even though the phenomenon itself is robust. Tyler Lorig has suggested that olfaction and language may compete for the same neural resource, making it difficult to name an odor. Others suggest that it is the neuroanatomical remoteness of language areas from primary olfactory processing areas and the sparsity of connections between them that result in impoverished naming. More prosaically, it may in part be a consequence of practice, as teaching participants to name odors does produce a rapid improvement in performance, and outside of the laboratory most of us do not have to rely upon the blind naming of odors.

Finally, it has long been claimed that odors may serve as powerful retrieval cues for distant autobiographical memories—the Proustian phenomenon (Proust describes how the smell of a madeleine biscuit dipped in tea led to the evocation of forgotten childhood memories). Certainly, odors do appear able to act as retrieval cues for older memories in adult participants, more so than comparable nonolfactory cues, and odors clearly invoke memories that are more emotional as well. But as with so many things olfactory, what we do not know far exceeds what we do.

Future Directions

For nearly all of the areas discussed in this survey of olfaction, the paucity of data is striking. Olfaction is heavily underresearched, a likely consequence of our reliance as a species on vision and audition. However, the study of olfaction is important for reasons that may not be readily apparent. First, it offers a glimpse into how the visual and auditory object recognition systems may have developed. This is because a developed chemical sense predates, in evolutionary terms, complex visual and auditory systems. Second, the olfactory system is an ideal place to study consciousness. In relative terms, the system is far less complex than the visual and auditory systems and so may provide a better place to tackle the hard question of consciousness (i.e., why smell feels different from sight or hearing and, more generally, how we experience such feelings at all). To give a more specific example, the apparently universal synesthesia that is demonstrated in the olfactory system (“tasty smells”) offers an excellent tool with which to explore the nature of consciousness. Since the time of William James, scientists have pondered whether specific areas of the neocortex were associated with specific sensory qualities (e.g., visual or auditory, etc.). Synesthesia is an excellent way of studying this problem—that is, if a person can no longer taste sweet things, can he or she still experience sweet smells? Do these similar percepts rely upon the same area of the brain? Third, whereas olfaction may be less important to humans, it is certainly very important to many other species—and determining how the system functions may lead to important advances in the control of agricultural pests, insect vectors of disease (e.g., mosquitoes, and pest animals such as rodents.

Perhaps the most important area of research activity in olfactory psychology at the moment is in determining whether (and how) odor object recognition may function. As of the writing of this research-paper, there is a growing consensus, both empirically and theoretically, that this is a more useful way of thinking about olfaction instead of concentrating on the relation between the stimulus (odor) and the response it produces. There are as yet almost no research papers that actually set out to test predictions derived from odor object recognition theory, indeed the theory itself is still poorly developed. It is these sorts of issues that will likely capture the attention of a new generation of olfactory psychologists in the 21st century.

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