Determinants of measured olfactory sensitivity

Abstract

Ten participants sought to detect four odorants: benzaldehyde, pyridine, and two alcohols, n-butyl and n-amyl alcohol, that smelled similar to each other. These were presented repeatedly on 3 successive days. The sequence of testing during a session made it possible to determine whether experience with one odorant would specifically facilitate the detectability of a similar-smelling odorant and whether any such facilitation would restrict itself to the nostril through which the experience was gained. Neither of these possibilities occurred. Instead, measured sensitivity increased rather uniformly both within and across days. Net gain from beginning to end exceeded an order of magnitude. Averaging across sessions gave a picture of smaller than usual individual differences, under 20 to 1, attributable mainly to general rather than odorant-specific differences in sensitivity. The results indicate that thresholds gathered in customary brief testing will underestimate olfactory sensitivity and overestimate individual differences. Incorporation of a reference odorant into threshold experiments should increase comparability among studies.Olfactory sensitivity as typically measured in the laboratory varies markedly from one person to another (Punter, 1983). Based on his own extensive data, Amoore (1980) developed the rule of thumb that about 96% of the population falls between lA6 and 16 times the mean threshold for an odorant, that is, within a 256-fold range.Does such high variation reflect only individual differences in absolute sensitivity? Substantial extraneous variation could occur from poor stimulus control, unsophisticated psychophysical methodology, or collection of insufficient data. Studies of olfactory threshold have often relied, for instance, on a single brief determination of threshold per participant. Even with more thorough measurements, however, reliability has received scant attention. Hence, we know little about whether variability between people commonly exceeds variability within a person tested at different times.Observers may differ in sensitivity in two ways: (1) One' observer may prove more sensitive than another to virtually all odorants. This situation could reflect the operation of some general factor, including something as simple as how many molecules can gain access to the receptors within the nasal cavity. (2) An observer may prove more sensitive than average to some odors and less sensitive to others. If observers show idiosyncratic yet stable disparities, then the possible explanations could prove numerous.The present study addressed the issue of individual variation and reliability, using standard threshold methodology to gather observations on four odorants for the same individuals both within a day and across days. The testing thereby allowed a glimpse of whether participants would show systematic changes both over time and across odorants. The sequence of testing within a day was designed to permit conclusions about the possible mechanism and specificity of any gains in performanc...

Variability of olfactory threshold and its role in assessment of aging

“…For Punter's three different assessments of butanol (the odorant used in the present study), they translate to 50/1, 750/1, and 10,000/1. Numbers on this order turn up routinely (see, e.g., Brown, & Robinette, 1968;Jones, 1957;Rabin & Cain, 1986;Yoshida, 1984). To explain (1) why intersubject variability itself varies so much across compounds, and (2) why it is so high compared with other sense modalities constitutes two major challenges to the student of olfaction .…”

“…Thus, the average of several short tests may yield a more stable impression of individual sensitivity and furnish a reliable index of individual olfactory aging. Rabin and Cain (1986) and Cain and Gent (1991) advocated frequent testing over time, arguing that the usual brief tests overestimate individual differences. Rabin and Cain's conclusion that "high person-to-person variability in olfactory sensitivity may be largely an illusion" (p. 284) foreshadows the present study.…”

Human olfactory detection of homologous n-alcohols measured via concentration–response functions

“…Some results indicate that interindividual variability can differ vastly among compounds, depending on chemical structure (Punter PH, 1983;Stevens JC, Cain WS, 1987). Other results favor a picture of general (rather than odorant-specific) and small (1 to 2 orders of magnitude) interindividual differences in sensitivity (Rabin MD, Cain WS, 1986). It is clear that a high enough amount of data per person is necessary in order to avoid an artificially high interindividual variability (Stevens JC et al, 1988).…”

Characteristic component odors emerge from mixtures after selective adaptation

“…Each of the 4 compounds served as X in 8 pairs per session, yielding 32 pairs that were presented in a random sequence for each subject. The stimulus pairs were presented at least 1 minute apart, sufficient time to re-establish vapor concentration in the head space [57]. Table 1 shows the adapt-test pairs for adaptation sessions in which subjects sniffed adapt items and then immediately sniffed and identified the odors of test items.…”