Adult subjects were recruited from the New York City metropolitan area and tested between March 2005 and May 2006. All figures except Figure
are based on 391 subjects (210 women, 181 men). The median age of the subject was 34.6 years with a range from 19 to 75 years. For the analysis, subjects were divided into two groups, comprising the 196 older subject (“>35”) and the 195 younger subjects (“<35”). Of these 391 subjects, 91 were born outside the United States. 178 subjects self-identified as Caucasian, 97 as African-American, 31 as Asian, and 4 as Native American. There were no Pacific Islanders in this study. 70 subjects selected “Other” for self-reported race. Of these 70 subjects, 51 self-identified as Hispanics. Overall, 305 subjects self-identified as Non-Hispanics, 77 as Hispanics, 28 as underweight, 202 as of just the right weight, and 149 as overweight. During the first visit, the height and weight of 387 subjects were measured to calculate their body mass index (B.M.I.=mass), calculated as (kg)/[height (m)]2
. We failed to obtain height and weight measures of 4 subjects. Subjects that self-identified as underweight had an average B.M.I. of 21.9±0.5, those that self-identified as of “just right” weight had a B.M.I. of 24.3±0.4, and those that self-identified as overweight had a B.M.I. of 30.8±0.6. The self-reported body weight data were well matched to typical boundaries used for clinical classifications of body weight. Subjects with a B.M.I. under 18.5 are considered underweight; a B.M.I. between 18.5 and 24.9 is considered normal weight; and a B.M.I. between 25 and 29.9 is considered overweight. People with a B.M.I. over 30 are considered to be obese. 290 subjects were non-smokers (defined as those who stated that they do not smoke or smoke very rarely; e.g. one cigarette per week). 92 subjects were smokers. 267 subjects used perfume at least once a week whereas 112 never used perfume. 366 of the subjects were from New York State, 19 from New Jersey, and 6 from elsewhere (Texas, Illinois, Pennsylvania, Florida, the United Kingdom) and on a short-term visit to the New York City area. Of the 366 subjects from New York State, 160 were from Manhattan, 75 from Brooklyn, 56 from Queens, 51 from the Bronx, 3 from Staten Island, and 21 from outside New York City. In some cases, demographic data do not add up to the number of subjects (391) because subjects were given the option not to answer any given demographic question. These missing data are indicated as “Do Not Wish To Specify” or “N/A” in the “demographics” tab of Additional file
. All subjects gave their informed consent to participate and all procedures were approved by the Rockefeller University Institutional Review Board.
The data in Figure
are based on a subset of 56 subjects (35 women, 21 men; 28 Caucasians, 16 African-Americans, 12 Other) who were reinvited for a third and fourth visit more than one year after the first visit (Figure
b). For Figure
, we quantified within-individual variability by having these 56 subjects rate intensity and pleasantness of fifteen stimuli on eight occasions: twice within 30 minutes on four visits (Figure
b). The first two visits were about one week apart and the third visit was scheduled more than one year later. Visit four was about one week after visit three.
All other figures are based on the data collected from 391 subjects (including the 56 subjects evaluated for Figure 3) who participated in the first two visits that were about one week apart. We attempted to eliminate the effects of within-individual variability by averaging the responses from these visits.
In total, 77% of enrolled subjects (N=412 subjects) completed the study, meaning that they completed two visits and provided a blood sample for genetic analysis. As in our previous analysis of these data
], the 21 subjects (5%) with the lowest olfactory acuity were excluded from the analysis to avoid inclusion of malingerers and subjects with general anosmia. Methods to determine general olfactory acuity are described below.
Intensity and valence rating
The intensity and valence of 66 odours at two concentrations (high and low) and two solvents (paraffin oil and propylene glycol) (Figure
) were rated using a 7-point scale (Figures
). The odours tested (in alphabetical order) were: (−)-menthol, (+)-menthol, 1-butanol, 2-butanone, 2-decenal, 2-ethylfenchol, 2-methoxy-4-methylphenol, 4-methylvaleric acid, ambrette, androstadienone, androstenone, anise, banana, bourgeonal, butyl acetate, butyric acid, cedarwood oil, cineole, cinnamon, cis
-3-hexen-1-ol, citral, citronella, decyl aldehyde, diacetyl, diallyl sulphide, diphenyl ether, ethyl vanillin, ethylene brassylate, eugenol, eugenol acetate, eugenol methyl ether, fenchone, fir, galaxolide, geranyl acetate, guaiacol, heptaldehyde, heptyl acetate, hexanoic acid, hexyl butyrate, isobornyl acetate, isobutyraldehyde, isobutyric acid, isoeugenol, isovaleric acid, jasmine, lime, linalool, methanethiol, methyl salicylate, nonyl aldehyde, nutmeg, octyl acetate, octyl aldehyde, orange, pentadecalactone, phenyl acetaldehyde, pyrazine, (r
)-limonene, sandalwood oil, spearmint oil, terpineol, terpinyl acetate, undecanal, and vanillin. Odours were diluted in paraffin oil, except for (−)-menthol, (+)-menthol, androstadienone, androstenone, ethyl vanillin, pentadecalactone, pyrazine, and vanillin which were diluted in propylene glycol and methanethiol, which was diluted in water. Most odours were obtained from Sigma and of the highest purity available. The odour qualities that have been reported to be associated with these odours have been reported earlier
]. Odour dilutions, solvent, and Chemical Abstracts Service (C.A.S.) numbers for all odours can be found in the “odours and sequence of stimuli” tab in Additional file
For pleasantness, the rating scale was: “extremely unpleasant,” “very unpleasant,” “slightly unpleasant,” “neither unpleasant nor pleasant,” “slightly pleasant,” “very pleasant,” and “extremely pleasant.” For intensity, the rating scale was: “extremely weak,” “very weak,” “slightly weak,” “neither weak nor strong,” “slightly strong,” “very strong,” and “extremely strong.” In addition to the 7-point scale, there was a button on the screen labeled “I can’t smell anything” and a button labeled ”Don’t Know.“ If the ”Don’t Know” button was pressed, no rating was recorded. If the “I can’t smell anything” button was pressed, a 0 was recorded for the intensity rating and no rating was recorded for pleasantness.
Prior to these ratings, six stimuli that represented the spectrum of intensity and pleasantness of the stimuli used in the study were presented to allow the subjects to calibrate their usage of the scale (Figure
c; grey ovals). These six calibration stimuli were terpineol (high); garlic (high); pyrazine (low); methanethiol (high); methyl salicylate (low), undecanal (high): (see the “odours and sequence of stimuli” tab in Additional file
for concentration and solvent information). The subjects were unaware that the first six stimuli served this purpose. After subjects had rated the solvents and 66 odours at two concentrations, 15 stimuli that were presented earlier in the experiment were repeated (Figure
c; orange ovals). Odour stimuli were presented in the same order in all visits to facilitate comparisons between subjects. The complete sequence of all presented odours, their dilution, and solvent can be found in the “odours and sequence of stimuli” tab in Additional file
To reduce olfactory adaptation or fatigue, the computer application for the intensity and valence rating was programmed to enforce a mandatory 15 second inter-stimulus interval. However, most subjects took longer than 15 seconds to move from one stimulus to the next, so this was rarely enforced. Although there was some variability between the first and second presentation of these stimuli, there was no indication of a systematic difference between the intensity rating at the beginning and end of the visit. Eight of the 15 stimuli were rated on average as more intense at the end of the visit, whereas seven were rated as less intense. This indicates that adaptation and olfactory fatigue during the testing did not systematically influence the results.
Prior to the study, the concentrations used for each odorant were determined in intensity-matching experiments in which subjects rated the intensity of stimuli. Odours were considered “low” intensity when the intensity rating was within one standard deviation of the intensity rating for an arbitrary low concentration odour standard, a 1:10,000 dilution of 1-butanol. Odours were considered “high” intensity when the intensity rating was within one standard deviation of an arbitrary high concentration odour standard, a 1:1,000 dilution of 1-butanol. For ethylene brassylate, eugenol methyl ether, (−)-menthol, (+)-menthol, and vanillin, the pure odour or the saturated dilution was rated less intense than the criteria for “high” intensity and these odours were therefore presented at the highest possible concentration. Androstenone and androstadienone could not be intensity matched in any meaningful way because of the high perceptual variability across subjects. Ten subjects participated in a pilot study aimed at intensity matching all stimuli and six visits for each subject were necessary to match all stimuli.
For the comparison between demographic groups in Figure
d-h and Figure
, the mean of the two visits was calculated for each subject. The stimuli were then ranked according to intensity (Figure
d-h) or pleasantness (Figure
) for each subject. The difference in mean rank of a stimulus between two demographic groups is shown.
Determining general olfactory acuity
We devised a measure of general olfactory acuity based on the data collected in this study. This measure of general olfactory acuity served two purposes. First, the 21 subjects (5%) with the lowest olfactory acuity were excluded from the analysis to avoid inclusion of malingerers and subjects with general anosmia. The prevalence of olfactory impairment in the United States is approximately 3.8%
], so our exclusion criteria will exclude those suffering from damage to the olfactory system. Second, we used this measure to compare the olfactory acuity of demographic groups (Figure
). Six performance indicators were ranked and the average of these six ranks was calculated as the general olfactory acuity, which is expressed as a rank from 1 (lowest acuity) to 391 (highest acuity). The six performance indicators were:
1. vanillin detection threshold
2. pentadecalactone detection threshold
3. isovaleric acid detection threshold
4. percentage of odours for which the “low” concentration was rated higher than the solvent
5. percentage of odours for which the “high” concentration was rated higher than the solvent
6. percentage of odours for which the “high” concentration was rated higher than the “low” concentration.
These six indicators are weakly correlated. The average Pearson’s correlation coefficient (r) between two of the indicators is 0.25. The notable exceptions are indicators 4 and 5, which are both strongly influenced by how the subjects rate the intensity of the solvent and are therefore strongly correlated (r=0.81). Despite this weak correlation between the indicators, the resulting measure of general olfactory acuity is stable. If the olfactory acuity is calculated using only five of the six indicators, the average Pearson’s correlation coefficients between the resulting six measures that are based on five indicators is 0.93. This shows that no single indicator contributes disproportionally to the measure of general olfactory acuity used here.