The Neuropsychology of Smell and Taste

G. Neil Martin

• Publisher: Psychology Press
• Year: 2013
• ISBN:
  9781135090173
• Edition: 1
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

Smell and taste are our most misunderstood senses. Given a choice between losing our sense of smell and taste, or our senses of sight and hearing, most people nominate the former, rather than the latter. Yet our sense of smell and taste has the power to stir up memories, alter our mood and even influence our behaviour.

In The Neuropsychology of Smell and Taste, Neil Martin provides a comprehensive, critical analysis of the role of the brain in gustation and olfaction. In his accessible and characteristic style he shows why our sense of smell and taste do not simply perform basic and intermittent functions, but lie at the very centre of our perception of the world around us. Through an exploration of the physiology, anatomy and neuropsychology of the senses; the neurophysiological causes of smell and taste disorders, and their function in physical and mental illness, Neil Martin provides an accessible and up-to-date overview of the processes of gustation and olfaction.

The Neuropsychology of Smell and Taste provides a state-of-the-art overview of current research in olfactory and gustatory perception. With sections describing the effect of odour and taste on our behaviour, and evaluating the contribution current neuroimaging technology has made to our understanding of the senses, the book will be of interest to researchers and students of neuropsychology and neuroscience, and anybody with an interest in olfaction and gustation.

1 Smell and Taste

An introduction to the psychology of chemosensation

1.1 Unique features of smell and taste

Smell and taste are chemosenses, that is, they are sensory systems that respond to chemical stimulation and whose chemical stimuli bind to receptors to create a sensation. Both are two of the most neglected and unusual in the sensory panoply in that each exhibits features that uniquely and dramatically separate it from the dominant senses of vision and audition, and even somatosensation. For example:

• Olfaction is the only sense with receptors directly exposed to the environment;
• Humans have an ability to detect hundreds, if not thousands, of different odours but only five or so tastes. However, the same humans are notoriously poor at identifying such odours;
• There is no agreed classification system for odour; there is more agreement for taste;
• Unlike vision, hearing and touch there is no olfactory dimension that relates stimuli to sensation; it has no predictable frequencies nor limited spectra (Mackay-Sim and Royet, 2006);
• Also unlike vision and audition, the olfactory system requires a third of the genome; vision requires three genes; audition requires a structure that develops from genes coding for other aspects of development (Mackay-Sim and Royet, 2006);
• The olfactory cortex has three layers, unlike most other sensory cortices;
• Taste and smell receptors regenerate every sixty days – thus, our current chemoreceptors did not exist two months ago;
• Smell is probably the most manipulable and confusable of the senses: people can be convinced that an odourless substance is odorous or that they are smelling something they are not;
• Taste is invariably confused with smell although smell provides the greatest contribution to food flavour;
• Olfactory dysfunction may be a better marker of risk of degenerative disease (e.g. Alzheimer’s Disease (AD)) than more conventional neurophysiological or clinical measures.

Formally, smell is known as olfaction and taste as gustation and the chemicals that stimulate each sense are called odorants or tastants. In the case of gustation, sensation is produced by the interaction between the tastant on the tongue and the depolarization that occurs in the taste bud field it stimulates. In the case of odour, the molecules are inhaled via the external nares (nostrils) with the air that carries them, and are processed, via transduction, by the olfactory apparatus at the top of the nose and beyond (Chapter 3 describes this pathway and process in detail). An odorant is an odour compound which means that it is volatile (and, therefore, evaporates quickly) and hydrophilic and lipophilic (it can dissolve in oil and water). However, the sense of smell also delivers olfactory information from another source of respiration, other than external: from inside the mouth.

1.2 Orthonasal and retronasal breathing

Typical olfactory perception involves two types of breathing – orthonasal and retronasal. With orthonasal breathing, odour molecules enter the anterior or external nares, travel through the nasal cavity and are transported to the olfactory apparatus at the top of the nose and onward to the cortex. Retronasal breathing occurs in the oral/buccal cavity where odorants stimulate the posterior or interior nares of the pharynx (the receptors here are called nasopharyngeal receptors and are supplied by two cranial nerves (CNs), neither of which are olfactory), and travel to the olfactory apparatus and the olfactory receptors at the top of the nose (Burdach and Doty, 1987). It is this process that creates food flavour.

A fruit juice inserted into the mouth and rolled on the tongue while the nose is pinched shut, will be almost impossible to identify, although the identification of the juice’s taste (that it is sweet or sour) will be relatively unaffected. However, if the nostrils are released, identification of the juice will be almost immediate because the internal nasopharyngeal receptors have been stimulated by the odour molecules released by the tastant and these molecules have stimulated the epithelium retronasally, via the back of the mouth. The failure of retronasal perception is the reason why when individuals suffer colds and the ‘flu, they claim to be unable to taste food. What they actually mean is that they cannot smell the food – they can easily determine whether the food tastes salty, sweet and so on if pressed. What they are unable to do is identify the flavour (and, therefore, the food). The reason for this inability is the impairment in retronasal perception of odour. In this sense, therefore, the sense of smell is both an exteroreceptor and an interoreceptor. Unless we are ingesting, the exteroreceptive function is the most common and important, ‘giving warning of enemies and other noxious things and guiding the animal to mates, food and other desirables’ (Herrick, 1933). There are also psychophysical differences between orthonasal and retronasal breathing. Thresholds for odours are lower in the former and odours may be perceived more intensely (Voirol and Daget, 1986), especially when coupled with tastants administered to the tongue (Gillan, 1983), and identified more accurately (Pierce and Halpern, 1996).

Odour is the primary determinant of food flavour – providing a crucial interoceptive function – but it has important interactions with taste (and other sensations) to create the flavour ensemble (discussed in detail in Chapter 6). Retronasal breathing increases the judged intensity of taste whereas orthonasal breathing has little effect on these judgements. As Brillat-Savarin wrote in his Physiology of Taste,

I am not only convinced that without the co-operation of smell there can be no complete degustation, but I am also tempted to believe that smell and taste are in fact a single sense, whose laboratory is the mouth and whose chimney is the nose… When smell is intercepted, taste is paralysed.

Moncrieff (1967) had observed that if the eyes and nose were closed, pureed apple and onion would be perceived as being the same olfactorily. That is, they would be unidentified. However, they would be discriminated on the basis of nonolfactory cues: the apple would taste more sour and the onion sweeter or sharper. Other pairings that would be impossible to identify if olfaction was impaired include red wine and unsweetened black coffee, raw apple and potato, tomato and orange, diluted raspberry and sweetened milk, sweetened milk and sweetened milk with vanilla, and peach and apricot. These might be discriminated from each other reasonably well – red wine is lighter and less bitter than coffee, raw apple is sweeter/sourer than potato – but not correctly identified, thus demonstrating the importance of olfaction to the identification of food flavour.

In view of the above, it is not difficult to empathize with the refrain and lament of Hollingworth and Poffenberger (1917) in their The Sense of Taste: ‘Why should it be the rule that, since the taste and smell qualities are to be confused, smell should so commonly sacrifice its claim, so that odours are called tastes rather than vice versa?’ One answer to this conundrum, they suggest, is the illusion of predominance created by the cutaneous stimulation in taste. A curious phenomenon, illustrated by the von Skramlik test, is that odours can be perceived through the mouth. If a person pinches their nose and inhales and exhales through the mouth, a sensation will emerge (Mozell et al., 1969). That all food-related identification appears to involve the tongue, however, a physical structure that senses food and its various characteristics from temperature and texture to its actual taste, may account for the dispropportionate importance placed on it when we perceive flavour.

Smell is the first chemosensory custodian of survival: we can sniff food for signs of spoiling or decomposition. Off-milk, meat and vegetables, if their appearance provides no indication or harm or rotting, can be detected in this way and are prevented from ingestion. If its scent and appearance indicate that food is ingestible, taste is the final custodian and its most important role is to prevent gastrointestinal risk. Thus bitter substances are normally perceived as less pleasant than are sweet or salty (and sour) tastants – most poisons are bitter-tasting, suggesting an evolutionary role for the development of receptors allowing us to detect bitterness. Preference for tastes with a strong bitter component tends to develop during young to late adulthood – thus a liking for coffee, green vegetables, whisky, gin and other bitter-tasting foods is virtually absent in children (not that you would normally feed children whisky and gin). Unfamiliar food also elicits withdrawal behaviour or curiosity short of eating (a gustatory neophobia). Bitter substances or spoiled, contaminated food elicit a disgust reaction, evidenced by changes in physiognomy, feelings of revulsion and behavioural withdrawal (Rozin and Fallon, 1987). At the core of food/taste disgust are feelings of danger, distaste and knowledge of the nature or origin of the food (Rozin and Fallon, 1987). Analysis of the features of food that people find disgusting suggests that there are two dimensions: textural properties (unpleasant) and reminders of ‘livingness' or ‘animalness’ (Martins and Pliner, 2006). Predictors of disgust include culture, history of exposure and the ability to detect disgust-eliciting taste. There is also evidence that this disgust can extend to moral disgust. Recent work in social psychology suggests that people in clean-scented environments engage more in charity work, express reciprocal trust, enagage with unknown people more and show more interest in voluntary work (Lilenquist et al., 2010). Eskine et al. (2011) found that people who were given a teaspoon of a bitter substance to taste expressed greater moral disgust when judging controversial topics such as incest or the acceptability of eating a dead dog. The more conservative respondents responded even more robustly after tasting the substance. Of course, this effect could be due to intensity, rather than bitterness, and a useful experiment would alter the intensities of this and other tastes.

1.3 Smell and taste: basic features and assumptions

Phylogenetically, smell and taste are two of the oldest senses, if not the oldest. The subcortical structures we now identify as the limbic system – structures involved in our most basic behaviours such as hunger, thirst, sexual drive, homeostasis, aggression – were originally described as the rhinencephalon or ‘smell–brain’, a term given to these regions and structures by Paul Broca. They are evolutionary contemporaries of thermosensation and nociception (pain).

Smell and taste are also both near senses, or ‘short distance sensory modalities’, to use Sherrington’s (1906) term. That is, the stimuli that result in transduction of sensory signals are close to the site of transduction (the nose and mouth). This contrasts with audition and vision, which are far senses, or ‘long distance sensory modalities’ (you can identify a building or a sound from ten miles or 10m away).

But despite this similarity, olfaction is the more flexible sense. Taste requires molecules to make contact with receptors on the tongue, located inside the mouth – which is normally shut – and this involves the conscious act of introducing stimuli into a cavity and making direct contact with the stimulus producing the sensation. This contrasts starkly with its partner which is tireless and open – the process of respiration means that the olfactory system is continuously working, inhaling air which carries molecules that comprise odorants. This is a feature it also shares with audition. Taste, on the other hand, is similar to vision and more like somatosensation – in that while receptors are theoretically available to be stimulated, there needs to be an agentic action in order to create stimulation in taste and touch (and the eyelid needs to be opened for vision – although a closed lid, of course, does not prevent visual stimulation). The sense of smell is automatic but the degree of response depends on the degree of automaticity. The most obvious illustration of this is that sniffing may be more effective than natural breathing in allowing olfactory perception.

There is also some debate over whether smell and taste are analytic or synthetic senses. The prototypical synthetic sense is vision. Yellow combined with red will not result in a bit of yellow and a bit of red but orange: seeing orange means we are none the wiser as to whether it is orange or a combination of two wavelengths. Audition, conversely, is considered to be the prototypical analytic sense; it does not blend sounds and we can distinguish the auditory components that contribute to sound combinations (such as musical instruments in a song). Olfaction is probably, on balance, a synthetic sense because we can discriminate, at best, three or four odours in an odour mixture – no more (Laing and Glemarec, 1992) – and odours can be blended like colour frequencies. Taste is considered an analytic sense because the qualities of taste do not combine to form new tastes.

A sniff is shorter and more vigorous than a breath and it assists olfactory perception. LeMagnen (1945/1946) found that participants’ sensitivity to eucalyptus improved when the number of inhalations increased from twelve to ninety-six per minute. Similarly, Rehn (1978) found a threefold increase in sensitivity to the odour of pyridine sensitivity when inhalations increased from 4.5 to sixty per minute (the odour of pyridine is distinctive of the top notes of Oxo or coffee). However, there are negative findings – Teghtsoonian et al. (1978) found no improvement in intensity perception when sniffs increased from 15.2 to 32.4 per minute. Laing (1983) observed that the average human inhales at a rate of thirty breaths per minute, with a volume of 200cm3 and a duration of 0.4 seconds per inhalation but also reported that varying the number of sniffs, the size of sniffs and the interval between sniffs does not improve odour perception. What Laing suggests is that sniffing confirms a person’s perception of the odour, rather than determines it.

To test the efficiency of sniffing and scent tracking in humans, Porter et al. (2007) asked thirty-two participants to follow a 10m trail of chocolate essential oil in open grass while participants were kneeling and blindfolded. Two-thirds were able to do this effectively. With increased training – three times a day, three days a week for two weeks – the amount of deviation from the scent trail reduced and speed along the trail increased. Sniffing also increased over three days and this sniffing increased with increasing speed along the trail. In terms of sniffing velocity, the right naris was found to have a velocity of 0.45msecs^–1; in the right, this was 0.30msecs^–1. The right nostril also had an advantage in terms of spatial reach – the maximum spatial distance at which an odour could be detected. Its reach was 1.5–2cm to the right; the left was 1–1.5cm to the left. In a final experiment, Porter et al. compared monorhinal with birhinal tracking – sniffing with one nostril resulted in decreased accuracy (30 per cent compared with 66 per cent) and increased slowness (20 per cent slower). This asymmetrical airflow and its psychological consequences is described in Chapter 3.

1.4 Classification of smell and taste

There is more agreement on the classification of tastes than odours. In 1755, Polycorpe Poncelet proposed the existence of seven disti...

Table of contents

• Cover
• Title
• Copyright
• Dedication
• Contents
• List of illustrations
• Series preface
• Preface
• List of abbreviations
• 1 Smell and taste: An introduction to the psychology of chemosensation
• 2 Individual differences in smell and taste: Age, sex, personality and culture
• 3 Smell and taste: Anatomy, development, neuroanatomy and neurophysiology
• 4 Psychophysiological and neuroimaging studies of smell and taste
• 5 Disorders of smell and taste, and diseases associated with chemosensory impairment
• 6 The neuropsychology of flavour: Multisensory interaction at the behavioural and neural level
• References
• Index