David G. Laing

The human sense of smell

1 Anatomy, Physiology and Chemistry.- 1. The Ultrastructure of the Human Olfactory Mucosa.- I Introduction.- II The biopsy technique.- III Ultrastructure of the human nasal mucosa.- IV Comparative anatomy of microvillar cells from different chemosensory neuroepithelia.- V The histopathology of olfactory dysfunction.- VI Summary.- References.- 2. Anatomy of the Human Olfactory Bulb and Central Olfactory Pathways.- I Introduction.- II Central olfactory components.- III Anterior olfactory nucleus.- IV Olfactory tubercle.- V Primary olfactory cortex.- VI Connections of the primary olfactory cortex.- VII Beyond the primary olfactory cortex.- VIII Neocortical projections.- IX Subcortical projections.- X Functional Organization of the olfactory system.- XI Summary.- References.- 3. Physiology of Olfactory Reception and Transduction: General Principles.- I Introduction.- II Cellular Organization of the olfactory mucosa.- III Sensory transduction.- IV Action potential initiation and transmission.- V Summary and conclusions: emerging first principles.- References.- 4. Molecular Structure and Smell.- I Introduction.- II Structure - odor relationships.- III Structure - odor relationships for musky odorants.- IV Structure - odor relationships for sandalwood odorants.- V Structure - odor relationships for bell pepper odorants.- VI Conclusion.- References.- 2 Measurement of Olfactory Responses.- 5. Psychophysical Measurement of Odor Perception in Humans.- I Introduction.- II Odor quality Classification.- III Measurement of absolute olfactory sensitivity.- IV Measurement of suprathreshold olfactory function.- V Sensory attribute scaling.- VI Subject factors which influence olfactory test measures...- VII Overview.- References.- 6. Human Electro-Olfactograms and Brain Responses to Olfactory Stimulation.- I Introduction.- II Stimulation.- III The human electro-olfactogram.- IV The human olfactory evoked potential.- I Summary.- References.- 3 Development and Senescence.- 7. Olfactory Function in Neonates.- I Introduction.- II Olfaction in the neonate.- III Summary and conclusions.- References.- 8. Olfactory Preferences in Children and Adults.- I Introduction.- II The origins of olfactory preferences.- III Developmental changes in olfactory preferences: Older children and adults.- IV Discussion.- V Conclusions.- References.- 9. Influences of Aging on Human Olfactory Function.- I Introduction.- II Age-related changes in olfactory perception.- III Physiological basis for age-related changes in odor perception.- IV Summary and questions for the future.- References.- 4 Basic Characteristics of Human Olfaction.- 10. Olfactory Adaptation.- I Introduction.- II Effects of adaptation on the olfactory threshold.- III Effects of adaptation on perceived intensity.- IV Effects of adaptation on reaction times.- V Physiological measurements of adaptation.- VI Discussion.- References.- 11. Memory for Odors.- I Introduction.- II Odor-evoked memory.- III Odorimagery.- IV Different forms of memory.- V Episodic odor memory.- VI Odor identification.- VII Conclusions.- Reference.- 12. Characteristics of the Human Sense of Smell when Processing Odor Mixtures.- I Introduction.- II Phenomena observed with odor mixtures.- III What do we smell in mixtures?.- IV Role of cognitive factors in mixture perception.- V Predicting the odor intensity of mixtures.- VI Mechanisms of mixture perception.- VII Summary.- References.- 13. Comparison of Odor Perception in Humans and Animals.- I Introduction.- II Olfactory sensitivity.- III Suprathreshold intensity.- IV Odor quality perception.- V Summary and conclusions.- References.- 5 Clinical and Health Aspects of Olfaction.- 14. Olfactory Dysfunction.- I Introduction.- II Evaluation of smell complaints.- III Psychophysical measurement in a clinical setting.- IV Frequent causes of olfactory dysfunction.- V Conclusions.- References.- 15. Epidemiology and its Application to Olfactory Dysfunction.- I Introduction.- II Epidemiologie principles.- III Application of epidemiologic principles to studies of olfactory dysfunction.- IV Risk factors for olfactory dysfunction.- V Research issues and direction.- VI Summary and conclusions.- References.- 16. Influence of Drugs on Smell Function.- I Introduction.- II The physiological basis of drug actions on olfaction.- III Norepinephrine in the olfactory bulb.- IV Summary.- References.- 17. Effects of Odors on Mood and Behavior: Aromatherapy and Related Effects.- I Introduction.- II Belief in the effects of odors on mood.- III Historical meaning of aromatherapy.- IV Potential mechanisms for an influence of smell on mood.- V Measurement of mood changes to odors.- VI Unresolved issues and future directions.- References.

The analysis of odor mixtures by humans : evidence for a configurational process

Humans have a limited capacity to analyze odor mixtures with three to four being the maximum [Physiol Behav 46 (1989) 809.]. This study investigates the large loss of information about odor identity that occurs in mixtures and aims to determine the information on which identification and failure to identify is based. In Experiment 1, 14 subjects used a selective attention procedure to identify odorants in stimuli consisting of one to four components. As expected, substantial difficulties were encountered in identifying more than two odorants, and chance level scores were obtained for the group for each of the odorants in the quaternary mixture. In Experiment 2, 21 subjects used a profiling procedure consisting of 146 descriptors to describe the odor qualities perceived in the same stimuli used in Experiment 1. The results indicated that for some odorants, loss of a major characteristic quality occurred even in binary mixtures, but that many of the features of some odorants remained in the quaternary mixture. Comparison of the data from the two experiments indicated that identification of most of the prominent qualities of an odorant was not necessarily sufficient for identification of the odorant in a mixture. In contrast, the loss of some prominent features did not always result in non-identification. A configurational hypothesis of olfaction, analogous to that for facial and object recognition, is proposed to account for the data and the processes underlying odor identification in mixtures.

Odour mixture suppression: evidence for a peripheral mechanism in human and rat

Rarely do we encounter a single odorant in our environment. Perception of odours, therefore, usually depends on the reception and neural processing of many components. However, little is known about how and where odour mixtures are processed. Evidence is presented here that suppression of one odour by another, a common result of mixing odours, is primarily a peripheral event. Having demonstrated with human subjects that perception of one or both odorants in two-component mixtures is dependent on the polarity and perceived intensity of the odorants, the same mixtures were presented to rats that had been injected with a metabolic marker, [3H]2-deoxyglucose (2-DG). By measuring the metabolic activity in the glomeruli of the rat olfactory bulb, where the axons of the receptor cells terminate, it was found that in a mixture where humans had perceived only one odour, there is a dramatic reduction in metabolic activity of glomeruli specific to the suppressed odour. In mixtures where both odorants were perceived, metabolic activity characteristic of both components was observed. These findings indicate that similar mechanisms underlie the perception of odour mixtures in the two species. Since metabolic activity revealed by 2-DG in glomeruli occurs predominantly in presynaptic receptor axons, the reduced activity seen after stimulation with odour mixtures indicates that a mechanism for mixture suppression begins at the receptor cells. Therefore, the ability of one odorant to suppress another in a mixture is probably determined by their relative chemical polarities, which effects access to and competition for membrane receptor sites in the olfactory epithelium.

The influence of odor type on the discrimination and identification of odorants in multicomponent odor mixtures.

Using a limited set of odorants, previous studies have indicated that the ability of humans to discriminate and identify the components of olfactory mixtures is limited to approximately four. However, the ability to generalize these results may have been limited by specific neural or cognitive interactions among the particular odorants used. In the present experiment, 41 subjects examined the influence of odor type (different individual odorants), from two very different odor sets, on the perception of the components of complex mixtures. One set contained odors that were selected by an expert panel to blend well in mixtures (good blenders), whereas the other contained odors that blended poorly in mixtures (poor blenders). The stimuli were common, dissimilar odorants of equivalent, moderate intensity, each of which was a single chemical. A computer-controlled air dilution olfactometer delivered a single odorant or a mixture containing up to eight odorants. Although the poor blenders were more easily discriminated, this superiority was displayed within a narrow range, and the ability of subjects to identify mixture components with either odor set was limited to approximately four. The results indicate that, whereas odor type can alter which odorants will be perceived in a mixture, the limited capacity to discriminate mixture components is independent of the type of odorants. These findings are discussed in terms of their implications for olfactory coding.

A Limit in the Processing of Components in Odour Mixtures

We investigated the hypothesis that physiological limitations restrict the ability of humans to identify components in an odour mixture. Subjects were trained to identify the test odours, and were required to detect a single highly familiar odorant in stimuli consisting of one, four, eight, twelve, and sixteen odorants by using a selective-attention procedure. The stimuli were delivered by a computer-controlled sixteen-channel air-dilution olfactometer which provided samples of each of the sixteen odorants to be of equal perceived intensity for each subject. Identification fell to chance level when sixteen odorants were present. It is proposed that the profound loss of information was primarily due to inhibition of olfactory receptor cells by the odorants through competitive mechanisms, and the subsequent loss of odour identity through changes in the spatial code that may be used to identify odorants.

The influence of chemical complexity on the perception of multicomponent odor mixtures

The present study investigates the hypothesis that complexobject odors (odors that emanate from flowers, foods, sewage, etc.) that consist of dozens of odorants are processed and encoded as discrete entities, as if each was a single chemical odor. To test this hypothesis, the capacity of trained subjects to discriminate and identify the components of stimuli consisting of one to eight object odors was determined. The results indicated that subjects could only identify up to four object odors in a mixture, which is similar to earlier findings with mixtures that contained only single chemical odors. The limited capacity was also reflected in the number of odors selected, regardless of whether the choices were correct or incorrect, in confidence ratings, and in decision times. The identification of a limited number of object odors in every mixture that was presented suggests that both associative (synthetic) and dissociative (analytic) processes are involved in the perceptual analysis of odor mixtures.

A comparison of the ability of 8-9-year-old children and adults to detect taste stimuli

Conflicting data exist in the literature regarding the maturity of the human sense of taste during childhood and if gender influences gustatory development. To investigate these 2 questions, taste detection thresholds for the 4 common tastants sucrose, sodium chloride, citric acid, and caffeine were established for 61 young adults and 68 children aged 8-9 years old, using a paired-comparison forced-choice procedure. No significant differences were found between the mean thresholds of women and men, or between those of female children and adults. In contrast, male children had significantly higher thresholds for all 4 tastants than adult females, for all tastants except caffeine than adult men, and for sucrose and sodium chloride than female children. It is concluded that the taste sensitivity of 8-9-year-old males, although well developed, has not fully matured, and that taste sensitivity is not affected by gender in young adults.

Rapid quantitative assessment of fungiform papillae density in the human tongue

Fungiform papillae density, which can be used in a variety of circumstances as an indicator of taste function [L.M. Bartoshuk, V.B. Duffy, I.J. Miller, PTC/PROP tasting: anatomy, psychophysics and sex effects, Physiol. Behav. 56 (1994) 1165-1171; I.J. Miller, F.E. Reedy, Variation in human taste bud density and taste intensity perception, Physiol. Behav. 47 (1990) 1213-1219; J.R. Zuniga, N. Chen, C.L. Phillips, Chemosensory and somatosensory regeneration after lingual nerve repair in humans, J. Oral Maxillofac. Surg. 55 (1997) 2-13], was measured on the dorsal surface of the anterior tongue of living humans using a digital camera and a videomicroscope. Both procedures provided similar results, with the camera providing a more rapid, portable and flexible imaging procedure. Subsequently, the camera was successfully used to identify small regions of the anterior tongue which provide reliable measures of fungiform papillae density that correlate highly with the total number of fungiform papillae on the anterior tongue.

Selective attention and the perceptual analysis of odor mixtures

Two psychophysical methods were used to investigate the capacity of humans to identify the constituents of odor mixtures consisting of up to six components. With one method subjects were required to identify all the components present in each stimulus; with the other, a selective attention procedure was used where subjects had to identify only one component at each trial. Little difference was found between the levels of identification obtained with both methods, reinforcing the finding that humans have great difficulty in identifying more than three components in an odor mixture and indicating that it is unlikely that olfactory adaptation influenced the identification process.

The limited capacity of humans to identify the components of taste mixtures and taste-odour mixtures

The capacity of humans to identify the components of taste mixtures and odour – taste mixtures was investigated in two experiments. Subjects were trained to identify the components presented alone and to use a ‘yes/no’ procedure to identify them in mixtures. All stimuli were presented with a retronasal (by mouth) technique. A maximum of three tastants were identified in both types of mixtures, only one tastant was identified in five-component taste mixtures, and no component was identified in four-component odour – taste mixtures. Importantly, in no instance was the olfactory stimulus identified in any mixture with tastes, including binary mixtures. Loss of identity of the odorant in binary and ternary mixtures may have been due to suppression as a consequence of temporal processing, or to the absence of an association between the odorant and tastants that had established an identifiable percept. In contrast, poor identification of the components of the quaternary odour – taste mixture and quinternary taste mixture is attributed to the limited capacity of working memory. Overall, the poorer ability to identify components in odour–taste mixtures than in taste mixtures indicates that interactions occurred between the two senses, challenging the proposal that odours and tastes are processed independently when present in complex chemosensory stimuli.