Sex, handedness and side of nose modulate human odor perception

[Involvement of the trigeminal nerve system in the sense of smell]

A systematic review of literature on the issue of involvement in the sense of smell, as well as the interaction between the trigeminal and olfactory nerves, was carried out. The article discusses the features of the chemical perception systems, as well as the treatment of olfactory disorders using transcranial electrical stimulation of the trigeminal nerve.

Judgements of attractiveness of the opposite sex and nostril differences in self-rated mood: The effects of androstenol

Androstenol has been reported to influence judgements of attractiveness and to affect participants' mood. In the present study, participants were asked to sniff androstenol or a control odour (pure ethanol) unilaterally with the left or right nostril. Subsequently, they rated the attractiveness of photographs of the opposite sex and their own feelings on four mood scales. Participants rated the photographs as significantly more attractive after sniffing androstenol compared with the control odour. This did not depend upon androstenol being perceived as pleasant. Androstenol made male participants feel more lively, and both male and female participants more sexy, when sniffed through the right compared with the left nostril. Participants rated themselves as more irritable and aggressive when exposed to androstenol through the left nostril. The findings are discussed in relation to the effects of arousal on attraction and in the context of current theories of hemispheric differences in emotion.

Sniffing out significant "Pee values": genome wide association study of asparagus anosmia

OBJECTIVE

 To determine the inherited factors associated with the ability to smell asparagus metabolites in urine.

DESIGN

 Genome wide association study.

SETTING

 Nurses' Health Study and Health Professionals Follow-up Study cohorts.

PARTICIPANTS

 6909 men and women of European-American descent with available genetic data from genome wide association studies.

MAIN OUTCOME MEASURE

 Participants were characterized as asparagus smellers if they strongly agreed with the prompt "after eating asparagus, you notice a strong characteristic odor in your urine," and anosmic if otherwise. We calculated per-allele estimates of asparagus anosmia for about nine million single nucleotide polymorphisms using logistic regression. P values <5×10^-8 were considered as genome wide significant.

RESULTS

 58.0% of men (n=1449/2500) and 61.5% of women (n=2712/4409) had anosmia. 871 single nucleotide polymorphisms reached genome wide significance for asparagus anosmia, all in a region on chromosome 1 (1q44: 248139851-248595299) containing multiple genes in the olfactory receptor 2 (OR2) family. Conditional analyses revealed three independent markers associated with asparagus anosmia: rs13373863, rs71538191, and rs6689553.

CONCLUSION

 A large proportion of people have asparagus anosmia. Genetic variation near multiple olfactory receptor genes is associated with the ability of an individual to smell the metabolites of asparagus in urine. Future replication studies are necessary before considering targeted therapies to help anosmic people discover what they are missing.

Left: Right Differences in Psychophysical and Electrodermal Measures of Olfactory Thresholds and Their Relation to Electrodermal Indices of Hemispheric Asymmetries

The study of lateralization processes in olfaction in human subjects has given rise to many contradictory findings. Indeed, sensorial cerebral asymmetry in olfaction depends on several factors (nature of task, quality of stimulus, characteristics of subjects, etc.) and could be also related to differences between the nostrils. In this field, few studies have assessed simultaneously the left–right nostril differences and the hemispheric asymmetry. The present work dealt with this question in the same population with the same odorants, procedures, and stimulations. Seven different concentrations of four specific odorants (two pleasant and two unpleasant) were used by single nostril stimulation with 30 dextral subjects (20 women and 10 men). Threshold detection in unilateral stimulation was investigated using electrodermal response to confirm the first psychophysic measure. Moreover, bilateral recordings of electrodermal activity (EDA) with unilateral stimulation were used as a measure of functional hemispheric asymmetry. Analysis showed no differences between the two nostrils for the threshold detection regardless of the method used (psychophysic or EDA response). However, most subjects presented a constant direction of electrodermal asymmetry whichever nostril was stimulated and whichever odorant stimulus used. The constant bilateral differences in EDA recordings are discussed in terms of asymmetrical activation of the hemispheres.

Analysis of the human osseous nasal shape--population differences and sexual dimorphism

OBJECTIVES

In this study, the shape of the outer osseous nose in a German and a Chinese sample is analyzed using a dense set of semi-landmarks. Shape differences related to population and sex as well as directional and fluctuating asymmetry were statistically evaluated and also visualized.

MATERIALS AND METHODS

Shape differences in the bony nose were investigated between a large sample of CT scans of German (140 ♀, 127 ♂) and Chinese (135 ♀, 132 ♂) crania. We used semi-automatic methods to represent the shape of this region as a dense point-cloud, consisting of 370 three-dimensional bilateral coordinates. Both the symmetric and asymmetric modes of shape variation were addressed.

RESULTS

Strong differences in nasal shape were found between the two populations, while sex was found to play a minor role in explaining the observed shape variation. The expression of sexual dimorphism was similar in both populations. Differences attributed to population affinity and to sexual dimorphism were both found to affect the shape of the ossa nasalia and the projection of the spina nasalis. The correlation with population/sex was weak for directional asymmetry, but strong for fluctuating asymmetry. The nasal region is more asymmetric in Germans than in Chinese, with males displaying more asymmetry than females in both populations.

DISCUSSION

While the bony nose is well suited for predicting population affinity, regarding the populations under investigation, its value for sexing unknown individuals is rather moderate. The similar expression of sexual dimorphism in those otherwise very dissimilar populations indicates common factors responsible for these differences.

The perception of odor is not a surrogate marker for chemical exposure: a review of factors influencing human odor perception

CONTEXT

Clinical toxicologists perform risk assessments and clinical evaluations for patients with potential exposure to airborne toxicants in which the patient's self-reported perception of odor may be the only indicator that an exposure may have taken place.

OBJECTIVE

To review the factors that may affect the human ability to perceive chemical odors and relate those odors to specific chemical exposures.

METHODS

The medical literature, from 1950 through 2012, was searched using the OVID database and the PUBMED database. The searches returned 238 articles, of which 113 involved human studies and were published in the English language. Of these 113 articles, 40 articles discussed odor issues and thus were chosen as specifically relevant to the topic. Bibliographies of all articles were also searched for other relevant references and this found six additional articles, making a total of 46. FACTORS THAT MAY AFFECT OLFACTION AND THE ABILITY TO PERCEIVE ODOR: Genetic/population: Ethnic background is associated with widely differing odor detection abilities and thresholds. A significant genetic influence for the ability to smell and perceive odor has been reported. Gender: Women are superior to men in their ability to identify odors. Age. Increasing age is correlated with higher odor detection thresholds. Medical conditions: A variety of medical conditions have been associated with deficits in olfaction, including diseases of the nose and sinuses, multiple sclerosis, and schizophrenia. Alcoholism and smoking: Abuse of alcohol results in impaired olfactory sense, and smoking tobacco products alters odor detection threshold in a dose-related manner. Occupational and environmental factors: Repeated inhalation of any chemical results in olfactory fatigue over relatively short time frames that leads to a decreased ability to accurately detect and identify an odor. Recent exposure to relatively high concentrations of a chemical has been shown to affect sensitivity to that particular odorant, altering subsequent detection thresholds by up to three orders of magnitude. Applicability of proposed odor thresholds: Humans are only able to identify three to four components of complex olfactory mixtures and the odorants present in the mixture affect which individual components are detected. Odorants present in suprathreshold concentrations in a mixture may effectively mask the presence of odorants present in perithreshold concentrations. Self-rating of olfactory function may not correlate with actual olfactory ability. It is even more difficult to accurately determine intensity of an odor in a quantifiable way. For example, under conditions of constant stimulation with hydrogen sulfide, perceptual intensity was reported to decrease exponentially with time of stimulation. Concomitant visual stimulation also affects odor intensity. Some chemicals, such as hydrogen sulfide, may induce reactions in humans related solely to their odor, even when they are present in concentrations substantially lower than those levels usually associated with the development of adverse clinical effects. There is a wealth of literature suggesting that the intensity of perceived odor, the degree of irritation, and the reported health effects of exposure to an odorant chemical are affected by psychological state and bias. Multiple theories have been proposed to explain the cognitive basis for perceived illness in association with the perception of odor. The concept of odor has been reported to be intrinsically and cognitively associated with illness rather than with health. Assigning negative bias to an odor prior to an exposure results in the reporting of significantly more health-related symptoms following exposure. This suggests that those symptoms are not mediated by the odor directly, but rather by an individual's cognitive associations between odor and health.

CONCLUSIONS

Attempts to verify exposure intensity based on the report of a perceived odor is unreliable and has no useful application in legitimate exposure assessment paradigms. Detection of an odor does not imply a medically significant exposure to a toxicant and, due to subject bias and the difficulty of detecting individual odorants in mixtures, may not constitute an exposure to the purported substance.

Olfactory lateralization: odor intensity but not the hedonic estimation is lateralized

An earlier study in humans comparing the olfactory sensitivity of both nostrils revealed a small but significant advantage of the right nostril for detection and for olfactory quality discrimination. However lateralization was not evaluated for the perception of odor intensity and hedonic evaluation (pleasantness/unpleasantness). Thus we investigated lateralization of olfactory intensity and hedonic evaluation in right-handed healthy volunteers (n=186) from the HeDoS-F database (Hedonic Database of Smell-Franconia). For olfactory evaluation the Sniffin' Stick Test was employed with the parameters detection, discrimination, identification and extended by analogue hedonic and intensity rating scales. Over all odors subjects rated the perceived intensity significantly higher following stimulation of the right compared to the left nostril. The analysis of the single odors of the Sniffin' Stick Test consistently confirmed higher intensity ratings for the right compared to the left nostril reaching a statistically significant difference for 10 out of 16 odors. In contrast we found no significant differences between the nostrils for the hedonic estimates over all odors. Differences in odor detection, discrimination and identification did not reach a statistically significant level, but for all these parameters the scores of the right nostril were slightly higher compared to the left nostril. For odor identification, however, a statistical tendency was observed. Based on our results we concluded that olfactory intensity estimates represent the most sensitive parameter of olfactory lateralization.

Sex differences in the citrus lemon essential oil-induced increase of hippocampal acetylcholine release in rats exposed to a persistent painful stimulation

The microdialysis technique was used to study the ability of essential oil from citrus lemon to modulate hippocampal acetylcholine (ACh) release in male and female rats. Animals were allowed to inhale this odor while experiencing a persistent nociceptive input (50 microl formalin, 5%) or under control conditions (sham-injection). In males, exposure to the essential oil did not change the time course and magnitude of the ACh increase induced by pain. In females, the pain-induced increase of ACh was delayed and increased by exposure to lemon essential oil. The present results indicate that lemon essential oil affects the ACh release differently in male and female rats during a painful condition.

Lateralization of brain activation to imagination and smell of odors using functional magnetic resonance imaging (fMRI): left hemispheric localization of pleasant and right hemispheric localization of unpleasant odors

PURPOSE

Our goal was to use functional MRI (fMRI) of brain to reveal activation in each cerebral hemisphere in response to imagination and smell of odors.

METHOD

FMRI brain scans were obtained in 24 normal subjects using multislice fast low angle shot (FLASH) MRI in response to imagination of banana and peppermint odors and in response to smell of corresponding odors of amyl acetate and menthone, respectively, and of pyridine. Three coronal sections selected from anterior to posterior brain regions were used. Similar studies were obtained in two patients with hyposmia using FLASH MRI and in one patient with hyposmia using echo planar imaging (EPI) both before and after theophylline treatment that returned smell function to or toward normal in each patient and in two patients with birhinal phantosmia (persistent foul odor) and global phantogeusia (persistent foul taste) with FLASH and EPI fMRI before and after treatment with neuroleptic drugs that inhibited their phantosmia and phantogeusia. Activation images were derived using correlation analysis. Ratios of hemispheric areas of brain activation to total hemispheric brain areas were calculated for FLASH fMRI, and numerical counts of pixel clusters in each hemisphere were made for EPI studies. Total pixel cluster counts in localized regions of each hemispheric section were also obtained.

RESULTS

In normal subjects, activation generally occurred in left (L) > right (R) brain hemisphere in response to banana and peppermint odor imagination and to smell of corresponding odors of amyl acetate and menthone. Whereas there were no overall hemispheric differences for pyridine odor, activation in men was R > L hemisphere. Although absolute activation in both L and R hemispheres in response to banana odor imagination and amyl acetate smell was men > women, the ratio of L to R activation was women > men. In hyposmic patients studied by FLASH fMRI, activation to banana odor imagination and amyl acetate smell was L > R hemisphere both before and after theophylline treatment. In the hyposmic patient studied with EPI before theophylline treatment, activation to banana and peppermint odor imagination and to amyl acetate, menthone, and pyridine smell was R > L hemisphere; after theophylline treatment restored normal smell function, activation shifted completely with banana and peppermint odor imagination and amyl acetate and menthone smell to L > R hemisphere, consistent with responses in normal subjects. However, this shift also occurred for pyridine smell, which is opposite to responses in normal control subjects. In patients with phantosmia and phantogeusia, activation to phantosmia and phantogeusia before treatment was R > L hemisphere; after treatment inhibited phantosmia and phantogeusia, activation shifted with a slight L > R hemispheric lateralization. Localization of all lateralized responses indicated that anterior frontal and temporal cortices were brain regions most involved with imagination and smell of odors and with phantosmia and phantogeusia presence.

CONCLUSION

Imagination and smell of odors perceived as pleasant generally activated the dominant or L > R brain hemisphere. Smell of odors perceived as unpleasant and unpleasant phantosmia and phantogeusia generally activated the contralateral or R > L brain hemisphere. With remission of phantosmia and phantogeusia, hemispheric activation was not only inhibited, but also there was a slight shift to L > R hemispheric predominance. Predominant L > R hemispheric differences in brain activation in normal subjects occurred in the order amyl acetate > menthone > pyridine, consistent with the hypothesis that pleasant odors are more appreciated in L hemisphere and unpleasant odors more in R hemisphere. Anterior frontal and temporal cortex regions previously found activated by imagination and smell of odors and phantosmia and phantogeusia perception accounted for most hemispheric differences.

Unilateral olfactory perception and magical ideation

We assessed olfactory detection thresholds and discrimination abilities in 40 healthy right-handers (20 women and 20 men). All subjects were also required to complete the Magical Ideation (MI) scale, a well-validated 30-item schizotypy inventory. Over both nostrils, we found elevated thresholds for subjects with high MI scores (at or above the median score of 9.0) compared with those with low scores. In men but not women, specifically left-nostril acuity was inversely correlated to MI raw scores. MI was unrelated to olfactory discrimination performance. These results suggest an association, at least in healthy men, between even moderate signs of schizotypy and deficits in odor detection. The selective impairment of left-nostril performance adds to the growing evidence for left temporal lobe functional abnormalities in people high on MI. This laterality effect is known from previous studies in patients with schizophrenia. However, as a rule, in psychiatric patients olfactory identification rather than simple detection performance was found to be impaired, indicating that the integration of odor information is affected at different levels of processing in schizotypy compared with schizophrenia. Work with completely normal subjects may reasonably complement clinical studies of olfactory perception. Among its advantages are the good subject compliance and the absence of medication effects.

Complexity of olfactory lateralization processes revealed by functional imaging: a review

Currently available techniques used in neurosciences and particularly cerebral imaging are contributing to a better understanding of human perception and the treatment of sensorial information. In this field, the chemical senses (taste and smell) have received little attention when compared with the auditory, visual and tactile senses. Nevertheless, recent research has been trying to overcome this. Brain regions involved in the processing of olfactory information have been investigated in several studies including a large number dealing with the question of lateralization. However, functional asymmetry in olfaction has still not been resolved and the resulting data have not been homogeneous. In this field, the contribution of cerebral imaging studies is very important insofar as it shows that the processes of functional lateralization in olfaction depend on many factors (nature of stimulus, nature of task, characteristics of subjects ...) that future research will have to consider.

Olfaction and hemispheric asymmetry: unilateral stimulation and bilateral electrodermal recordings

The study of hemispheric asymmetry in olfaction in human subjects has given rise to many publications, but the findings have often been contradictory. This study used bilateral electrodermal activity recordings with unilateral stimulation as a measure of functional hemispheric asymmetry. A specific odorant (lavender) was used by monorhinic (single nostril) stimulation on 30 dextral subjects (20 females and 10 males). Intraindividually, the results showed no difference between the two nostrils, but all subjects exhibited a constant direction of electrodermal asymmetry: 20 subjects systematically showed a greater response amplitude for the right hand and 10 subjects systematically showed a greater response amplitude for the left hand, whatever hemisphere stimulated.

Behavioral lateralization during spontaneous smelling tasks

The smelling behavior of 52 right-handed subjects was videotaped during tasks involving identification and recognition of different odors. Analysis showed that men more often used the right nostril than the left whatever the odor. There was no significant difference for the women. These results support a more marked cerebral asymmetry in men than in women and a main involvement of the right cerebral hemisphere in the olfactory processes at least by right-handed men.

Gender effects on odor-stimulated functional magnetic resonance imaging

On standardized tests of odor identification and odor detection, women tend to score better than men at nearly all age groups. We sought to determine if these findings would translate to differences between the sexes in the volume of activated brain when odors are presented to subjects as the stimulants for functional magnetic resonance imaging (FMRI) experiments. The activation maps of eight right-handed women (mean age 25.3 years old, range 20-44, S.D. 8.3 years) were compared with those of 8 right-handed men (mean age 30.5, range 18-37, S.D. 6.5 years) given the same olfactory nerve stimuli in an FMRI experiment at 1.5 T. Olfactory stimuli were delivered to the patients in a passive fashion using a Burghart OM4-B olfactometer with a nose piece inserted into the patients' nostrils. We used agents (eugenol, phenyl ethyl alcohol, or phenyl ethyl alcohol alternating with hydrogen sulfide) that were selective for olfactory nerve stimulation in the nose. The odorants were delivered to both nostrils for 1 s every 4 s during a 30 s 'on-period'. During the 30 s 'off-period', the patient received room air at the same flow rate. The women's group-averaged activation maps showed up to eight times more activated voxels than men for specific regions of the brain (frontal and perisylvian regions). The left and right inferior frontal regions showed a statistically significant increase in activation in women at p<0.01. In general, more women showed activation than men. The results suggest that (1) FMRI activation maps in subject groups can demonstrate correlates to psychophysical tests of olfaction, and (2) one must control for gender when performing odor-stimulated FMRI experiments.

Odor recognition memory is better under bilateral than unilateral test conditions

Two odor memory tests were administered unilaterally (left and right) and bilaterally to the same set of 24 men and 24 women on two test occasions. These tests were (i) a "multiple-target" test (MTT) in which three target stimuli were selected, after 10-, 30-, and 60-sec retention intervals, from stimulus sets containing both target and distracter stimuli, and (ii) a 9-item "single-target" three-choice test (STT), in which single stimuli were selected from stimuli sets containing two distracters. Overall, odor memory scores were higher under bilateral than unilateral testing, and higher on the second than on the first test session. Unilateral testing resulted in a monotonic delay-related forgetting function. No differences were observed in the odor memory test scores of the left and right sides of the nose, and no significant correlations were present between these scores and scores on a battery non-olfactory memory tests. On the STT, the female, but not the male, subjects demonstrated better performance on the second, than on the first, test session. The results of this study imply that odor memory is facilitated centrally by bilateral activation, and that a memory system may exist for odors which is distinct from other memory systems.

Olfactory lateralization in the chick

Chicks using their right nostril (and so with direct olfactory input to the right hemisphere), and presented simultaneously with two objects identical in visual appearance with the rearing object, and differing only in odour, chose that which smelled like the rearing object. Chicks using the left nostril chose equally readily but at random. Earlier work, using similar tests, has shown special interest of the right hemisphere in change in visual properties of familiar stimuli, suggesting that analysis of a wide range of properties of a familiar stimulus may be an important function of the right hemisphere in the chick, with consequent detection of novelty.

Hand preference and age in the United States

A survey of 1,177,507 U.S. men and women between the ages of 10 and 86 included questions regarding hand preference for writing and throwing. Three effects were observed. Individuals with at least some left motoric bias comprised a smaller percent of the population with advancing age. This finding provides large-scale confirmation of a previously described phenomenon. Among sinistrals, concordance for writing and throwing was 2.2 times as prevalent as left-writing with right-throwing, and 4.1 times as prevalent as right-writing with left-throwing. These sinistral subpopulations displayed distinct and stable prevalence prior to age 50 and changing patterns of prevalence subsequent to age 50. The results confirm a decrease with age in the prevalence of sinistrality, but indicate that age-specific rates of mixed- and left-handedness are distinct. The implications for hypotheses regarding age-related change in the prevalence of sinistrality are discussed.

A twin study of odor identification and olfactory sensitivity

Interindividual variation in odor identification and olfactory sensitivity has been explained primarily with reference to age, sex and/or experiential factors. A twin study of olfaction can, therefore, substantially contribute to current research in this area. Thirty-nine monozygotic and twenty dizygotic twin pairs have completed the University of Pennsylvania Smell Identification Test (UPSIT), an olfactory preference questionnaire, and two odor detection threshold tests (phenyl ethyl alcohol and butanol). A genetic influence on odor identification, as assessed by the UPSIT, has been demonstrated. Future plans and directions for this research program are discussed.

Reciprocity versus rhythmicity in spontaneous alternations of nasal airflow

Active anterior rhinomanometry was used to observe nasal airflow in five men and four women (ages 18-30). Measurements were obtained for each nasal passage every 5 min throughout an uninterrupted 8-hr session. Facial skin temperature from the left and right side of the face was recorded simultaneously from thermocouples. Observations were made during the months of May and June; subjects were allowed to maintain their routine diurnally active schedules prior to observation. Airflow in the two passages showed a significant negative correlation (i.e. was reciprocal) in 44% of subjects (N = 9). Autocorrelation and spectral analysis of the airflow data found evidence of periodicity in 39% of individual nostrils and 56% of subjects. Mean estimated period was 4.5 +/- 1.0 hr (range 3.5-6.0 hr). Only 22% of subjects showed statistical evidence of periodicity in both nostrils (i.e. a "nasal cycle"). Left- and right-side facial skin temperatures changed in parallel rather than reciprocally, but showed evidence of periodicity in 50% of hemifacial time series (56% of subjects), with an estimated period of 3.8 +/- 1.0 hr (range 2.3-5.0).