A mnemonic theory of odor perception

Valid olfactory impairment tests can help identify mild cognitive impairment: an updated meta-analysis

Background

Olfactory testing is emerging as a potentially effective screening method for identifying mild cognitive impairment in the elderly population.

Objective

Olfactory impairment is comorbid with mild cognitive impairment (MCI) in older adults but is not well-documented in subdomains of either olfactory or subtypes of cognitive impairments in older adults. This meta-analysis was aimed at synthesizing the differentiated relationships with updated studies.

Methods

A systematic search was conducted in seven databases from their availability to April 2023. A total of 38 publications were included, including 3,828 MCI patients and 8,160 healthy older adults. Two investigators independently performed the literature review, quality assessment, and data extraction. The meta-analyses were conducted with Stata to estimate the average effects and causes of the heterogeneity.

Results

Compared to normal adults, MCI patients had severe impairments in olfactory function and severe deficits in specific domains of odor identification and discrimination. Olfactory impairment was more severe in patients with amnestic mild cognitive impairment than in patients with non-amnestic MCI. Diverse test instruments of olfactory function caused large heterogeneity in effect sizes.

Conclusion

Valid olfactory tests can be complementary tools for accurate screening of MCI in older adults.

Is Novelty Detection Important in Long-Term Odor Memory?

Memory for odors is believed to be longer-lasting than memory for visual stimuli, as is evidenced by flat forgetting curves. However, performance on memory tasks is typically weaker in olfaction than vision. Studies of odor memory that use forced-choice methods confound responses that are a result of a trace memory and responses that can be obtained through process of elimination. Moreover, odor memory is typically measured with common stimuli, which are more familiar and responses may be confounded by verbal memory, and measure memory in intentional learning conditions, which are ecologically questionable. Here we demonstrate the value of using tests of memory in which hit rate and correct rejection rate are evaluated separately (i.e., not using forced-choice methods) and uncommon stimuli are used. This study compared memory for common and uncommon odors and pictures that were learned either intentionally (Exp. 1) or incidentally (Exp. 2) and tested with either a forced-choice or a one-stimulus-at-a-time (“monadic”) recognition task after delays of 15 min, 48 h or 1 week. As expected, memory declined with delay in most conditions, but depended upon the particular measure of memory and was better for pictures than odors and for common than uncommon stimuli. For common odors, hit rates decreased with delay but correct rejection rates remained constant with delay. For common pictures, we found the opposite result, constant hit rates and decreased correct rejection rates. Our results support the ‘misfit theory of conscious olfactory perception’, which highlights the importance of the detection of novelty in olfactory memory and suggests that olfactory memory should be studied using more ecologically valid methods.

Olfactory Perception in Relation to the Physicochemical Odor Space

A growing body of research aims at solving what is often referred to as the stimulus-percept problem in olfactory perception. Although computational efforts have made it possible to predict perceptual impressions from the physicochemical space of odors, studies with large psychophysical datasets from non-experts remain scarce. Following previous approaches, we developed a physicochemical odor space using 4094 molecular descriptors of 1389 odor molecules. For 20 of these odors, we examined associations with perceived pleasantness, intensity, odor quality and detection threshold, obtained from a dataset of 2000 naïve participants. Our results show significant differences in perceptual ratings, and we were able to replicate previous findings on the association between perceptual ratings and the first dimensions of the physicochemical odor space. However, the present analyses also revealed striking interindividual variations in perceived pleasantness and intensity. Additionally, interactions between pleasantness, intensity, and olfactory and trigeminal qualitative dimensions were found. To conclude, our results support previous findings on the relation between structure and perception on the group level in our sample of non-expert raters. In the challenging task to relate olfactory stimulus and percept, the physicochemical odor space can serve as a reliable and helpful tool to structure the high-dimensional space of olfactory stimuli. Nevertheless, human olfactory perception in the individual is not an analytic process of molecule detection alone, but is part of a holistic integration of multisensory inputs, context and experience.

French Vividness of Olfactory Imagery Questionnaire: A Potential Tool for Diagnosing Olfactory Loss by Assessing Olfactory Imagery?

Several studies have shown a significant relationship between smelling and olfactory imagery abilities. The primary aim of the present study was to validate a French version of the Vividness of Olfactory Imagery Questionnaire (VOIQ). The secondary aim was to investigate its capability to differentiate individuals with smell loss from healthy individuals. After having elaborated a French translation of the VOIQ (fVOIQ), we evaluated olfactory imagery abilities of 387 French participants who anonymously self-completed the fVOIQ: 121 pathologic individuals (hyposmic and anosmic), 244 normosmic individuals (healthy non-expert), and 22 fragrance experts. Significant split-half reliability as expressed by Spearman correlation coefficients for the global sample, as well as for each group separately, indicated the excellent internal consistency of the fVOIQ. Moreover, results revealed a significant effect of the smelling ability group on fVOIQ score, suggesting that daily olfactory stimulation is fundamental to maintaining the ability to create a vivid image and that severe loss of smell may result in progressive impairment of olfactory imagery. Our fVOIQ and the original English version seemingly have similarly high benefit in differentiating experts and normosmic individuals based on their olfactory imagery ability. Moreover, the fVOIQ seems capable of differentiating individuals with loss of smell from healthy individuals. These findings demonstrate the reliability and validity of the fVOIQ, and its capability to differentiate individuals’ smelling ability according to their olfactory imagery ability.

Configural memory of a blending aromatic mixture reflected in activation of the left orbital part of the inferior frontal gyrus

Blending aromatic mixtures components naturally fuse to form a unique odor - a configuration- qualitatively different from each component's odor. Repeated exposure to the components either in the mixture or separately, favors respectively, configural and elemental processings. The neural bases of such processes are still unknown. We examined the brain correlates of the experienced-induced configural processing of a well-known model of binary blending odor mixture, the aromatic pineapple blending (AB, ethyl maltol + ethyl isobutyrate). Before fMRI recording, half of the participants were repeatedly exposed to the mixture (AB, group Gmix), with the other half exposed to its separate components (A and B; Gcomp). During the fMRI recording, all participants were stimulated with the mixture (AB) and the components (A and B). Finally, participants rated the number of odors perceived for each stimulus. Gmix perceived the AB mixture as less complex than did Gcomp. While Gcomp perceived the mixture as more complex than its components, Gmix did not. These results show the presence of experience-induced configural or elemental processing of the AB mixture in each group. Contrasting the brain activity of Gcomp and Gmix, when stimulated with AB, revealed higher activation in the left orbital part of the inferior frontal gyrus. This result sheds light on this area's function, commonly found activated in olfactory studies, and closely connected with the lateral orbitofrontal cortex. We discuss the role of this area as a mediator of configural percepts between temporal and orbitofrontal areas involved in configural memory processes.

The importance of smell and taste in everyday life: Dysfunction in COVID-19 patients

Human-to-human transmission of coronavirus (SARS-CoV-2) - COVID-19 (corona virus disease 2019) - is characterized by a pandemic exponential rate and the patients with mild to moderate infection have odor and taste problems that represent a new atypical disease. A new viral syndrome of acute anosmia or "new loss of taste or smell" without rhinitis and nasal obstruction or rhinorrhea has been placed on the list of symptoms that may occur 2 to 14 days after exposure to the COVID-19 virus. Two months after declaring the COVID-19 pandemic in May 2020, the World Health Organization (WHO) has recognized changes in the perception of smell and taste as symptoms of this disease. The described cardinal symptoms are more common in the population of young patients and able-bodied people which facilitates the spread of disease. Significantly higher prevalence of patients with COVID-19 who have lost their taste and smell is treated at home (rare hospitalization), lung damage is rare, as well as oxygen therapy with mild lymphopenia. Different scenarios of SARS-CoV-2 viral infection can be assumed: it is probable that the virus does not enter directly into olfactory sensory neurons (they do not have ACE2 and TMPRSS2 receptors), but it is localized to vascular pericytes and causes inflammatory processes and vasculopathies. On the other hand, direct infection of non-neuronal cells which contain said receptors is possible. Those are specific cell types in the olfactory epithelium such as sustentacular, horizontal basal cells, as well as Bowman's glands, which leads to massive degeneration and loss of olfactory neurons. The sense of taste is a complex sensation that is the result of the interaction of smell, taste, temperature and texture of food. The virus damages cranial nerves, epithelial receptors and blood vessels leading to taste damage (ageusia or dysgeusia). A multidisciplinary approach with epidemiological, clinical and basic research is needed to elucidate the mechanism of sensorineural odor and taste loss caused by coronavirus.

The Smell of Synthetic Biology: Engineering Strategies for Aroma Compound Production in Yeast

Yeast—especially Saccharomyces cerevisiae—have long been a preferred workhorse for the production of numerous recombinant proteins and other metabolites. S. cerevisiae is a noteworthy aroma compound producer and has also been exploited to produce foreign bioflavour compounds. In the past few years, important strides have been made in unlocking the key elements in the biochemical pathways involved in the production of many aroma compounds. The expression of these biochemical pathways in yeast often involves the manipulation of the host strain to direct the flux towards certain precursors needed for the production of the given aroma compound. This review highlights recent advances in the bioengineering of yeast—including S. cerevisiae—to produce aroma compounds and bioflavours. To capitalise on recent advances in synthetic yeast genomics, this review presents yeast as a significant producer of bioflavours in a fresh context and proposes new directions for combining engineering and biology principles to improve the yield of targeted aroma compounds.

On the Communicative Function of Body Odors

Humans use multiple senses to navigate the social world, and the sense of smell is arguably the most underestimated one. An intriguing aspect of the sense of smell is its social communicative function. Research has shown that human odors convey information about a range of states (e.g., emotions, sickness) and traits (e.g., individuality, gender). Yet, what underlies the communicability of these states and traits via smell? We fill this explanatory gap with a framework that highlights the dynamic and flexible aspects of human olfactory communication. In particular, we explain how chemical profiles, associative learning (i.e., the systematic co-occurrence of chemical profiles with state- or trait-related information), and top-down contextual influences could interact to shape human odor perception. Our model not only helps to integrate past research on human olfactory communication but it also opens new avenues for future research on this fascinating, yet to date poorly understood, field.

Perceived haze, stress, and negative emotions: An ecological momentary assessment study of the affective responses to haze

The aim of this study was to examine the mediating role of stress in the association between people's perceived haze and negative emotions in daily life. Using ecological momentary assessment, 95 college students reported their perceived haze, stress, and negative emotions twice a day over the course of 2 weeks. The results showed a positive relationship between perceived haze and negative emotions. More importantly, this association was significantly mediated by levels of stress. Findings suggested that people who perceived more severe haze may report higher stress levels, which in turn may lead to increases in negative emotions.

A quantitative meta-analysis of olfactory dysfunction in mild cognitive impairment

BACKGROUND

The connection between Alzheimer's disease (AD) and olfactory deficits is well documented and further, alterations in olfactory functioning may signal declines in functions associated with dementia. The aim of the present comprehensive meta-analysis was to investigate the nature of olfactory deficits in mild cognitive impairment (MCI).

METHODS

Articles were identified through computerised literature search from inception to 30 June 2016 using PubMed, MEDLINE and PsychInfo databases. In order to control for differences in sample size during effect size computation, studies were weighted according to their inverse variance estimates.

RESULTS

31 articles (62 effects) were identified, which included 1993 MCI patients and 2861 healthy older adults (HOA). Included studies contrasted odour identification, discrimination, detection threshold and/or memory between cases and controls. Moderate to large and heterogeneous effects were seen for olfactory deficits in MCI relative to HOA (d=-0.76, 95% CI -0.87<δ<-0.64). Moderator analysis revealed that tests of odour identification yielded larger effect sizes than those of odour detection threshold or memory. In addition, a potential interaction between age and sex was observed, with male patients carrying a larger burden of olfactory deficit and older female patients performing better on olfactory tests.

CONCLUSIONS AND RELEVANCE

Olfactory deficits are present and robust in MCI. Odour identification is most impaired in MCI, which parallels the most prominent sensory deficit seen in AD. As such, a simple-to-administer test of odour identification warrants inclusion in the screening of individuals at risk for developing AD.

Phenomenological Differences between Familiar and Unfamiliar Odours

Unfamiliar odours are harder to discriminate than familiar odours. We explored the phenomenal basis of this difference. In experiments 1a and 1b, participants profiled odour quality for two sets of familiar and unfamiliar odours. In both cases unfamiliar odours were redolent of more odour qualities than familiar stimuli. In experiment 2, participants received (i) a set of familiar and unfamiliar odours and learnt their names, and (ii) a further set of familiar and unfamiliar odours to which they were exposed. Participants then profiled these stimuli as well as a further unexposed set of familiar and unfamiliar odours. Exposure, but not naming, led to a significantly smaller difference between the familiar and unfamiliar stimuli, in terms of their redolence to other odours, when compared to unexposed control stimuli. Unfamiliar exposed odours were also judged as less redolent than unexposed unfamiliar odours. These observations are consistent with a mnemonic basis for odour-quality perception.

Cognitive Compensations for Blindness in Children: An Investigation Using Odour Naming

Historically, blindness has been associated with compensation for the loss of vision by the other senses. However, research to date has focused on perceptual compensations, largely ignoring possible cognitive compensations. We explored the notion that cognitive skills of blind children may facilitate performance in apparently perceptual tasks, by investigating the cognitive factors related to naming a familiar odour. Eighty-three children participated in olfactory and cognitive tasks (thirty-two early-blind, five late-blind, fourteen low-vision, and thirty-two sighted). In the olfactory tasks, the early-blind children performed significantly better than the sighted children on the odour-naming task but not on the odour-sensitivity task. From the cognitive tasks, scores on a nonvisualisable word-pairs task and a sound – word-pairs task were significantly higher for early-blind children and were highly correlated with odour-naming score. The early-blind children outperformed the sighted controls on a task of directed attention. The groups did not differ on memory for a story or for visualisable word pairs. The results suggest that blind children enjoy an advantage in tasks that assess nonvisual memory for paired associates and directed attention, and that superiority on these tasks facilitates performance in the odour-naming task. Other data suggest that sighted children rely on visualisation as a strategy to aid their performance on the cognitive tasks, and are disadvantaged when these strategies cannot be utilised.

Odour Perception: An Object-Recognition Approach

Object recognition is a crucial component of both visual and auditory perception. It is also critical for olfaction. Most odours are composed of 10s or 100s of volatile components, yet they are perceived as unitary perceptual events against a continually shifting olfactory background (ie figure—ground segregation). We argue here that this occurs by rapid central adaptation to background odours combined with a pattern-matching system to recognise discrete sets of spatial and temporal olfactory features—an odour object. We present supporting neuropsychological, learning, and developmental evidence and then describe the neural circuitry which underpins this. The vagaries of an object-recognition approach are then discussed, with emphasis on the putative importance of memory, multimodal representations, and top—down processing.

Intramodal Olfactory Priming of Positive and Negative Odors in Humans Using Respiration-Triggered Olfactory Stimulation (RETROS)

Priming describes the principle of modified stimulus perception that occurs due to a previously presented stimulus. Although we have begun to understand the mechanisms of crossmodal priming, the concept of intramodal olfactory priming remains relatively unexplored. Therefore, we applied positive and negative odors using respiration-triggered olfactory stimulation (RETROS), enabling us to record the skin conductance response (SCR) and breathing data without a crossmodal cueing error and measure reaction times (RTs) for olfactory tasks. RT, SCR, and breathing data revealed that negative odors were perceived significantly more arousing than positive ones. In a second experiment, 2 odors were applied during consecutive respirations. Here, we observed intramodal olfactory priming effects: A negative odor preceded by a positive odor was rated as more pleasant than when the same odor was preceded by a negative odor. Additionally, a longer identification RT was found for the second compared with the first odor. We interpret this as increased "perceptual load" due to incomplete first odor processing while the second odor was presented. Furthermore, intramodal priming can be considered a possible reason for the increase of identification RT. The use of RETROS led to these novel insights into olfactory processing beyond crossmodal interaction by providing a noncued unimodal olfactory test, and therefore, RETROS can be used in the experimental design of future olfactory studies.

What's that smell? An ecological approach to understanding preferences for familiar odors

How do odor preferences arise? Following Palmer and Schloss's (2010, PNAS, 107, 8877-8882) ecological valence theory of color preferences, we propose that preference for an odor is determined by preferences for all objects and/or entities associated with that odor. The present results showed that preferences for familiar odors were strongly predicted by average preferences for all things associated with the odors (eg people liked the apple odor which was associated with mostly positive things, such as apples, soap, and candy, but disliked the fish odor, which was associated with mostly negative things, such as dead fish, trash, and vomit). The odor WAVEs (weighted affective valence estimates) performed significantly better than one based on preference for only the namesake object (eg predicting preference for the apple odor based on preference for apples). These results suggest that preferences for familiar odors are based on a summary statistic, coding the valence of previous odor-related experiences. We discuss how this account of odor preferences is consistent with the idea that odor preferences exist to guide organisms to approach beneficial objects and situations and avoid harmful ones.

The muted sense: neurocognitive limitations of olfactory language

Most people find it profoundly difficult to name familiar smells. This difficulty persists even when perceptual odor processing and visual object naming are unimpaired, implying deficient sensory-specific interactions with the language system. Here we synthesize recent behavioral and neuroimaging data to develop a biologically informed framework for olfactory lexical processing in the human brain. Our central premise is that the difficulty in naming common objects through olfactory (compared with visual) stimulation is the end result of cumulative effects occurring at three successive stages of the olfactory language pathway: object perception, lexical-semantic integration, and verbalization. Understanding the neurocognitive mechanisms by which the language network interacts with olfaction can yield unique insights into the elusive nature of olfactory naming.

Tactile roughness perception in the presence of olfactory and trigeminal stimulants

Previous research has shown that odorants consistently evoke associations with textures and their tactile properties like smoothness and roughness. Also, it has been observed that olfaction can modulate tactile perception. We therefore hypothesized that tactile roughness perception may be biased towards the somatosensory connotation of an ambient odorant. We performed two experiments to test this hypothesis. In the first experiment, we investigated the influence of ambient chemosensory stimuli with different roughness connotations on tactile roughness perception. In addition to a pleasant odor with a connotation of softness (PEA), we also included a trigeminal stimulant with a rough, sharp or prickly connotation (Ethanol). We expected that—compared to a No-odorant control condition—tactile texture perception would be biased towards smoothness in the presence of PEA and towards roughness in the presence of Ethanol. However, our results show no significant interaction between chemosensory stimulation and perceived tactile surface roughness. It could be argued that ambient odors may be less effective in stimulating crossmodal associations, since they are by definition extraneous to the tactile stimuli. In an attempt to optimize the conditions for sensory integration, we therefore performed a second experiment in which the olfactory and tactile stimuli were presented in synchrony and in close spatial proximity. In addition, we included pleasant (Lemon) and unpleasant (Indole) odorants that are known to have the ability to affect tactile perception. We expected that tactile stimuli would be perceived as less rough when simultaneously presented with Lemon or PEA (both associated with softness) than when presented with Ethanol or Indole (odors that can be associated with roughness). Again, we found no significant main effect of chemosensory condition on perceived tactile roughness. We discuss the limitations of this study and we present suggestions for future research.

Learning to smell danger: acquired associative representation of threat in the olfactory cortex

Neuroscience research over the past few decades has reached a strong consensus that the amygdala plays a key role in emotion processing. However, many questions remain unanswered, especially concerning emotion perception. Based on mnemonic theories of olfactory perception and in light of the highly associative nature of olfactory cortical processing, here I propose a sensory cortical model of olfactory threat perception (i.e., sensory-cortex-based threat perception): the olfactory cortex stores threat codes as acquired associative representations (AARs) formed via aversive life experiences, thereby enabling encoding of threat cues during sensory processing. Rodent and human research in olfactory aversive conditioning was reviewed, indicating learning-induced plasticity in the amygdala and the olfactory piriform cortex. In addition, as aversive learning becomes consolidated in the amygdala, the associative olfactory (piriform) cortex may undergo (long-term) plastic changes, resulting in modified neural response patterns that underpin threat AARs. This proposal thus brings forward a sensory cortical pathway to threat processing (in addition to amygdala-based processes), potentially accounting for an alternative mechanism underlying the pathophysiology of anxiety and depression.

Identification of odorant-receptor interactions by global mapping of the human odorome

The human olfactory system recognizes a broad spectrum of odorants using approximately 400 different olfactory receptors (hORs). Although significant improvements of heterologous expression systems used to study interactions between ORs and odorant molecules have been made, screening the olfactory repertoire of hORs remains a tremendous challenge. We therefore developed a chemical systems level approach based on protein-protein association network to investigate novel hOR-odorant relationships. Using this new approach, we proposed and validated new bioactivities for odorant molecules and OR2W1, OR51E1 and OR5P3. As it remains largely unknown how human perception of odorants influence or prevent diseases, we also developed an odorant-protein matrix to explore global relationships between chemicals, biological targets and disease susceptibilities. We successfully experimentally demonstrated interactions between odorants and the cannabinoid receptor 1 (CB1) and the peroxisome proliferator-activated receptor gamma (PPARγ). Overall, these results illustrate the potential of integrative systems chemical biology to explore the impact of odorant molecules on human health, i.e. human odorome.

[Olfactory influences on the perceived effects of lip balm]

This study examined the influence of olfactory impressions on the perceived effects of lip balm. Twenty female college students used two types of lip balm: one with natural essences (lemon or vanilla) and one without. Then they rated their perceived experience of the balms. The results showed that the participants felt that the balm with lemon essence was significantly smoother than the ones with vanilla or without natural essences. Furthermore, the participants felt that the balm with vanilla essence was significantly stickier than the ones with lemon or without natural essences. Also, the participants felt that the balm with lemon essence had marginally better moisturizing properties than the one with vanilla. These results suggest that olfactory cognition influences the perception of tactile sensations.

Smelly primes - when olfactory primes do or do not work

In applied olfactory cognition the effects that olfactory stimulation can have on (human) behavior are investigated. To enable an efficient application of olfactory stimuli a model of how they may lead to a change in behavior is proposed. To this end we use the concept of olfactory priming. Olfactory priming may prompt a special view on priming as the olfactory sense has some unique properties which make odors special types of primes. Examples of such properties are the ability of odors to influence our behavior outside of awareness, to lead to strong affective evaluations, to evoke specific memories, and to associate easily and quickly to other environmental stimuli. Opportunities and limitations for using odors as primes are related to these properties, and alternative explanations for reported findings are offered. Implications for olfactory semantic, construal, behavior and goal priming are given based on a brief overview of the priming literature from social psychology and from olfactory perception science. We end by formulating recommendations and ideas for a future research agenda and applications for olfactory priming.

A "Misfit" Theory of Spontaneous Conscious Odor Perception (MITSCOP): reflections on the role and function of odor memory in everyday life

Our senses have developed as an answer to the world we live in (Gibson, 1966) and so have the forms of memory that accompany them. All senses serve different purposes and do so in different ways. In vision, where orientation and object recognition are important, memory is strongly linked to identification. In olfaction, the guardian of vital functions such as breathing and food ingestion, perhaps the most important (and least noticed and researched) role of odor memory is to help us not to notice the well-known odors or flavors in our everyday surroundings, but to react immediately to the unexpected ones. At the same time it provides us with a feeling of safety when our expectancies are met. All this happens without any smelling intention or conscious knowledge of our expectations. Identification by odor naming is not involved in this and people are notoriously bad at it. Odors are usually best identified via the episodic memory of the situation in which they once occurred. Spontaneous conscious odor perception normally only occurs in situations where attention is demanded, either because the inhaled air or the food smell is particularly good or particularly bad and people search for its source or because people want to actively enjoy the healthiness and pleasantness of their surroundings or food. Odor memory is concerned with novelty detection rather than with recollection of odors. In this paper, these points are illustrated with experimental results and their consequences for doing ecologically valid odor memory research are drawn. Furthermore, suggestions for ecologically valid research on everyday odor memory and some illustrative examples are given.

Time to smell: a cascade model of human olfactory perception based on response-time (RT) measurement

The timing of olfactory behavioral decisions may provide an important source of information about how the human olfactory-perceptual system is organized. This review integrates results from olfactory response-time (RT) measurements from a perspective of mental chronometry. Based on these findings, a new cascade model of human olfaction is presented. Results show that main perceptual decisions are executed with high accuracy within about 1~s of sniff onset. The cascade model proposes the existence of distinct processing stages within this brief time-window. According to the cascade model, different perceptual features become accessible to the perceiver at different time-points, and the output of earlier processing stages provides the input for later processing stages. The olfactory cascade starts with detecting the odor, which is followed by establishing an odor object. The odor object, in turn, triggers systems for determining odor valence and edibility. Evidence for the cascade model comes from studies showing that RTs for odor valence and edibility assessment are predicted by the shorter RTs needed to establish the odor object. Challenges for future research include innovative task designs for olfactory RT experiments and the integration of the behavioral processing sequence into the underlying cortical processes using complementary RT measures and neuroimaging methods.

Quality-space theory in olfaction

Quality-space theory (QST) explains the nature of the mental qualities distinctive of perceptual states by appeal to their role in perceiving. QST is typically described in terms of the mental qualities that pertain to color. Here we apply QST to the olfactory modalities. Olfaction is in various respects more complex than vision, and so provides a useful test case for QST. To determine whether QST can deal with the challenges olfaction presents, we show how a quality space (QS) could be constructed relying on olfactory perceptible properties and the olfactory mental qualities then defined by appeal to that QS of olfactory perceptible properties. We also consider how to delimit the olfactory QS from other modalities. We further apply QST to the role that experience plays in refining our olfactory discriminative abilities and the occurrence of olfactory mental qualities in non-conscious olfactory states. QST is shown to be fully applicable to and useful for understanding the complex domain of olfaction.

Olfactory perception, cognition, and dysfunction in humans

The main functions of olfaction relate to finding food, avoiding predators and disease, and social communication. Its role in detecting food has resulted in a unique dual mode sensory system. Environmental odorants are 'smelled' via the external nostrils, while volatile chemicals in food-detected by the same receptors-arrive via the nasopharynx, contributing to flavor. This arrangement allows the brain to link the consequences of eating with a food's odor, and then later to use this information in the search for food. Recognizing an odorant-a food, mate, or predator-requires the detection of complex chemical blends against a noisy chemical background. The brain solves this problem in two ways. First, by rapid adaptation to background odorants so that new odorants stand out. Second, by pattern matching the neural representation of an odorant to prior olfactory experiences. This account is consistent with olfactory sensory physiology, anatomy, and psychology. Odor perception, and its products, may be subject to further processing-olfactory cognition. While olfactory cognition has features in common with visual or auditory cognition, several aspects are unique, and even those that are common may be instantiated in different ways. These differences can be productively used to evaluate the generality of models of cognition and consciousness. Finally, the olfactory system can breakdown, and this may be predictive of the onset of neurodegenerative conditions such as Alzheimer's, as well as having prognostic value in other disorders such as schizophrenia. WIREs Cogn Sci 2013, 4:273-284. doi: 10.1002/wcs.1224 For further resources related to this article, please visit the WIREs website.

What is a fresh scent in perfumery? Perceptual freshness is correlated with substantivity

Perfumes are manufactured by mixing odorous materials with different volatilities. The parameter that measures the lasting property of a material when applied on the skin is called substantivity or tenacity. It is well known by perfumers that citrus and green notes are perceived as fresh and they tend to evaporate quickly, while odors most dissimilar to ‘fresh’ (e.g., oriental, powdery, erogenic and animalic scents) are tenacious. However, studies aimed at quantifying the relationship between fresh odor quality and substantivity have not received much attention. In this work, perceptual olfactory ratings on a fresh scale, estimated in a previous study, were compared with substantivity parameters and antierogenic ratings from the literature. It was found that the correlation between fresh odor character and odorant substantivity is quite strong (r = −0.85). ‘Fresh’ is sometimes interpreted in perfumery as ‘cool’ and the opposite of ‘warm’. This association suggests that odor freshness might be somehow related to temperature. Assuming that odor perception space was shaped throughout evolution in temperate climates, results reported here are consistent with the hypothesis that ‘fresh’ evokes scents typically encountered in the cool season, while ‘warm’ would be evoked by odors found in nature during summer. This hypothesis is rather simplistic but it may provide a new insight to better understand the perceptual space of scents.

A preliminary investigation of olfactory function in olfactory and auditory-verbal hallucinators with schizophrenia, and normal controls

INTRODUCTION

It is well established that people with schizophrenia have impaired olfactory perception. However, another olfactory abnormality that occurs in schizophrenia--olfactory hallucinations (OHs)--has received almost no attention.

METHODS

This preliminary study compared a small sample of olfactory (OH; n=14) and auditory-verbal (AVH; n=11) hallucinators with schizophrenia, with matched healthy controls (NC; n=21), on tests of odour detection threshold, identification, and hedonics, and bespoke tests relating to possible causes of OHs.

RESULTS

Both OH and AVH participants were equally impaired on tests of odour identification, but neither had any impairment in detection. However, although the AVH group had hedonic impairments, these were not evident in the OH and NC groups. Examination of the possible causes of OHs revealed abnormalities in olfactory habituation. In addition, we observed a far greater rate of past episodes of brief unconsciousness in the OH group.

CONCLUSIONS

The presence of habituation deficits and past episodes of brief unconsciousness, and absence of olfactory affective impairment, have not been identified before as correlates of OHs, suggesting these factors may be worthy of further investigation.

A time-based account of the perception of odor objects and valences

Is human odor perception guided by memory or emotion? Object-centered accounts predict that recognition of unique odor qualities precedes valence decoding. Valence-centered accounts predict the opposite: that stimulus-driven valence responses precede and guide identification. In a speeded response time study, participants smelled paired odors, presented sequentially, and indicated whether the second odor in each pair belonged to the same category as the first (object evaluation task) or whether the second odor was more pleasant than the first (valence evaluation task). Object evaluation was faster and more accurate than valence evaluation. In a complementary experiment, participants performed an identification task, in which they indicated whether an odor matched the previously presented word label. Responses were quicker for odors preceded by semantically matching, rather than nonmatching, word labels, but results showed no evidence of interference from valence on nonmatching trials. These results are in accordance with object-centered accounts of odor perception.

The role of perceived pollution and health risk perception in annoyance and health symptoms: a population-based study of odorous air pollution

OBJECTIVES

Health effects associated with air pollution at exposure levels below toxicity may not be directly related to level of exposure, but rather mediated by perception of the air pollution and by top-down processing (e.g., beliefs that the exposure is hazardous). The aim of the study was to test a model that describes interrelations between odorous air pollution at non-toxic exposure levels, perceived pollution, health risk perception, annoyance and health symptoms.

METHODS

A population-based questionnaire study was conducted in a Swedish community of residents living near a biofuel facility that emitted odorous substances. Individuals aged 18-75 years were selected at random for participation (n = 1,118); 722 (65 %) agreed to participate. Path analyses were performed to test the validity of the model.

RESULTS

The data support a model proposing that exposure level does not directly influence annoyance and symptoms, and that these relations instead are mediated by perceived pollution and health risk perception.

CONCLUSIONS

Perceived pollution and health risk perception play important roles in understanding and predicting environmentally induced annoyance and health symptoms in odorous environments at non-toxic levels of exposure.

Implicit and explicit olfactory memory in people with and without Down syndrome

This study examined differences in implicit and explicit memory performance between people with Down syndrome (DS), their siblings, children matched on mental age, and university undergraduates, using olfactory stimuli. The DS and mental-age matched participants were also compared on two tasks of executive function. The data revealed implicit memory for olfactory stimuli. Further, people with DS performed similarly to each control group on the implicit memory task, but performed significantly poorer than all control groups on the explicit memory task. Impairment to executive functioning was identified as a possible cause of this deficit in explicit memory as people with DS performed more poorly than the mental-age matched controls on both tasks of executive function.

Attention and olfactory consciousness

Understanding the relation between attention and consciousness is an important part of our understanding of consciousness. Attention, unlike consciousness, can be systematically manipulated in psychophysical experiments and a law-like relation between attention and consciousness is waiting to be discovered. Most attempts to discover the nature of this relation are focused on a special type of attention: spatial visual attention. In this review I want to introduce another type of attention to the discussion: attention to the olfactory modality. I will first clarify the position of attention to smells in a general taxonomy of attention. I will then review the mechanisms and neuroanatomy of attention and consciousness in the olfactory system before using the newly introduced system to provide evidence that attention is necessary for consciousness.

Labeling, Identification, and Recognition of Wine-Relevant Odorants in Expert Sommeliers, Intermediates, and Untrained Wine Drinkers

In this study we examined the development of wine expertise. We asked four groups—untrained wine drinkers, second- and third-level trainee sommeliers, and professional sommeliers—to engage in a range of olfactory tasks to assess perceptual and semantic aspects of expertise. These tasks included identification, recognition, and description of a range of domain-specific and common odour stimuli, including wines. Trainee sommeliers were significantly poorer at identification of wine-relevant odours than untrained wine drinkers and professional sommeliers. Trainee and professional sommeliers were, however, significantly better than untrained wine drinkers in a delayed matching-to-sample wine-recognition task, but not in the case of other odorous stimuli. The wine-description task demonstrated a degree of skill, in terms of specificity and quantity of wine-relevant descriptors, as a function of expertise. These results, of one of the first studies of examining wine expertise by a cross-sectional developmental approach, indicate that perceptual aspects of expertise are probably rapidly acquired, being present even in the second-and third-level trainees, while semantic expertise is slower to develop, and may incur time for the identification of wine-specific odorants during training.

Carbon chain length and the stimulus problem in olfaction

Understanding how odour quality perception is encoded in its molecular properties arguably poses one of the most significant problems in olfaction. Determining the odour structure-quality relationships of structurally similar odorants could provide a key tool to this problem. We tentatively explored whether a mixture of two molecules, differing only in carbon chain length (C), would yield the same percept as a single odorant with an intermediate carbon chain length, akin to colour vision, or be perceived as a different quality. Ability to discriminate between pairs of iso-intense solutions of n-butanol (4C), n-propanol (3C), n-pentanol (5C), and an intermediate 50/50 molecular weight mixture of n-propanol and n-pentanol (3C/5C) was assessed in 20 healthy young adults. We found that participants were able to discriminate 4C from the 50/50 molecular weight mixture of n-propanol and n-pentanol (3C/5C), and also from the other alcohols. In conclusion, we successfully replicated previous data demonstrating that participants are able to discriminate between structurally similar alcohols, and, more importantly, the present study shows that an odour mixture of two molecules differing only in carbon chain length is clearly distinguishable from a single odorant with an intermediate carbon chain length. These findings suggest that although carbon chain length matters to odour quality, carbon chain length is not a physical continuum within homologous series of substances that corresponds to a single qualitative dimension akin to the wavelength-hue relation for monochromatic light.