Chem. Senses 24: 161-170,
1999
© Oxford University Press
Olfactory Discrimination Ability of Human Subjects for Ten Pairs of Enantiomers
Institut für Medizinische Psychologie, Ludwig-Maximilians-Universität München, Goethestraße 31, D-80336 München, Germany
Correspondence to be sent to: Matthias Laska, Institut für Medizinische Psychologie, Ludwig-Maximilians-Universität, Goethestraße 31, D-80336 München, Germany. e-mail: Laska{at}imp.med.uni-muenchen.de
 |
Abstract |
|---|
 Top Abstract IntroductionExperiment 1: discrimination of...Experiment 2: trigeminality of...Experiment 3: detection...DiscussionReferences |
|---|
We tested the ability of human subjects to distinguish between enantiomers, i.e. odorants which are identical except for chirality. In a forced-choice triangular test procedure 20 subjects were repeatedly presented with 10 enantiomeric odor pairs and asked to identify the bottle containing the odd stimulus. We found (i) that as a group, the subjects were only able to significantly discriminate the optical isomers of
-pinene, carvone and limonene, whereas
they failed to distinguish between the (+) and (-)-forms of menthol, fenchone, rose
oxide, camphor, -terpineol, ß-citronellol and 2-butanol; (ii) marked individual
differences in
discrimination performance, ranging from subjects who were able to significantly discriminate
between 6 of the 10 odor pairs to subjects who failed to do so with 9 of the 10 tasks; (iii) that
with none of the 10 odor pairs were the antipodes reported to differ significantly in subjective
intensity when presented at equal concentrations; and (iv) that error rates were quite stable and
did not differ significantly between sessions, and thus, we observed a lack of learning or training
effects. Additional tests of the degree of trigeminality and threshold measurements of the optical
isomers of -pinene, carvone and limonene suggest that the discriminability of these three
enantiomeric odor pairs is indeed due to differences in odor quality. These findings support the
assumption that enantioselective molecular odor receptors may only exist for some but not all
volatile enantiomers and thus that chiral recognition of odorants may not be a general
phenomenon but is restricted to some substances. |
Introduction |
|---|
|
TopAbstractIntroductionExperiment 1: discrimination of...Experiment 2: trigeminality of...Experiment 3: detection...DiscussionReferences |
|---|
Chiral recognition of substances, i.e. the ability to distinguish a molecular structure from its mirror image, is one of the most important and widespread principles of biological activity (Holmstedt et al., 1990
).
Discrepant enantiomer effects are well-established, with numerous examples in drug
effectiveness
(e.g. Caldwell, 1996),
taste perception
(e.g. Siertsema et al., 1998)
and insect chemical communication
(e.g. Silverstein, 1979).
The first molecular event in odor perception is the interaction of an odorant with a receptor.
As olfactory receptors have been identified as proteins, i.e. chiral molecules
(Buck and Axel, 1991;
Hildebrand and Shepherd, 1997),
this interaction should also be enantioselective, meaning that odor receptors should react
differently with the two enantiomeric forms of a chiral odorant, leading to differences in odor
strength and/or quality
(Pickenhagen, 1989).
A variety of optical isomers have been described as having different odor qualities and/or
different odor intensities for humans
(e.g. Ohloff, 1994),
although the number of cases reported in which the differences are small seems inconsistent
with the large differences found in other biological interactions between body tissues and dextro-
and levo- forms of the same compounds. There are also reports of identically smelling
enantiomeric odor pairs
(Theimer et al., 1977)
which seem inconsistent with the assumption that optically active olfactory receptors should be
enantioselective. The situation is even more complicated by findings of chiral isomers in which
one form has a distinct odor quality whereas the other form is odorless
(Simmons et al., 1992).
Most of the studies reporting qualitative and/or quantitative differences between
enantiomers, however, have employed odor profiling or scaling procedures which are presumed
to be particularly susceptible to cognitive influences
(Corwin, 1992).
Surprisingly few studies, on the other hand, have directly tested the discriminability of chiral
odorants, although this method largely avoids the disadvantages of comparatively poor
resolution, subjectivity, likely context dependence and semantic ambiguity
(Cain and Olsson, 1995).
Even fewer studies using discrimination procedures have assessed whether inter- or
intraindividual variability in discrimination performance rather than perceptual differences
between antipodes may at least partly account for the sometimes widely differing findings with
the same chiral odor pairs. Further, studies on discriminability of enantiomers have so far largely
been restricted to testing the ability of subjects to distinguish between (+)- and (- )-
carvone
(Jones and Velasquez, 1974;
Pike et al., 1987,
1988;
Cowart, 1990;
Hormann and Cowart, 1993),
one of the first substances for which both chiral isomers could be synthesized selectively and
with high purity rather than extracted from plant matter, thereby excluding the possibility of trace
impurities as a source of qualitative differences
(Friedman and Miller, 1971;
Russell and Hills, 1971).
To the best of our knowledge, only one study so far has investigated the discrimination
performance of humans for an array of enantiomeric odorants
(Jones and Elliot, 1975).
Unfortunately, the authors of this study reported only the total number of correct discriminations
pooled from all their subjects—drawing statistically invalid conclusions as to
discriminability of a given chiral odor pair due to an inflated number of observations—and
gave only cursory information with regard to inter- or intraindividual variability of
performance.
Given the continuing uncertainty in the field of chiral recognition of odorants and the possible importance of enantioselectivity for our understanding of the molecular mechanisms underlying the interaction between odor stimulus and olfactory receptor, we decided to test the ability of human subjects to distinguish between 10 pairs of enantiomers.
|
Experiment 1: discrimination of enantiomers |
|---|
|
TopAbstractIntroductionExperiment 1: discrimination of...Experiment 2: trigeminality of...Experiment 3: detection...DiscussionReferences |
|---|
In this experiment, we assessed the ability of human subjects to distinguish between 10 enantiomeric odor pairs. Substances were chosen on the basis of earlier studies which reported qualitative attributes of antipodes to range from `identical' to `very different', allowing us to (i) present odor pairs presumed to differ in their degrees of perceptual similarity and thus discriminability and (ii) test whether reported differences in qualitative attributes assigned to substances predict discriminability.
Materials and methods
Subjects
Twenty healthy, unpaid volunteers (14 females and 6 males), 22–37 years of age,
participated in the study. All were non-smokers and none had any history of olfactory
dysfunction. All subjects had previously participated in a clinical test of olfactory function and
were found to be normosmic. All subjects had also previously served in olfactory discrimination
tests and were familiar with the basic test procedure. They were informed about the aim of the
experiment and provided written consent. The study was performed in accordance with the
Declaration of Helsinki/Hong Kong.
Odorants
A set of 20 odorants comprising 10 pairs of enantiomers was used (Table 1). All substances
had a nominal purity of at least 99%. They were diluted using diethyl phthalate (Merck,
Darmstadt, Germany) as the solvent. The enantiomers of a given pair were presented at equal
concentrations in order to assess whether differences in perceived intensity rather than
differences in perceived odor quality contributed to discrimination performance (cf. Test
procedure). In an attempt to ensure that the different enantiomeric odor pairs were of
approximately equal strength when presented in squeeze bottles, intensity matching was
performed by a panel of six subjects adopting a standardized psycho-physical procedure (ASTM, 1975).
|
Test procedure
A 40 ml aliquot of each odorant was presented in a 250 ml polyethylene squeeze bottle equipped with a flip-up spout which for testing was fitted with a handmade Teflon nose-piece. Subjects were instructed as to the manner of sampling and at the start of the first session were allowed time to familiarize themselves with the bottles and the sampling technique. Care was taken to ensure that the nose-piece was only a short distance (1–2 cm) from the nasal septum during sampling of an odorant in order to allow the stimulus to enter both nostrils.
In a forced-choice triangular test procedure 20 subjects were asked to compare three bottles and to identify the one containing the odd stimulus. Additionally, after each decision, subjects were asked whether their choice was predominantly based on perceived differences in odor quality or on perceived differences in odor intensity. Each bottle could be sampled twice with an inter-stimulus interval of at least 10 s. Sampling duration was restricted to 1 s per presentation in order to minimize adaptation effects. The sequence of presenting the stimulus pairs was systematically varied between sessions and individual subjects while ensuring that the presentation of a given odorant as odd or even stimulus was balanced within and between sessions. In order to control for possible cross-adaptation effects, the order in which the stimuli of a given triad were sampled was systematically varied between sessions. The inter-trial interval was ~30 s and no feedback regarding the correctness of the subjects' choice was given.
The 10 stimulus pairs were presented twice per session and testing was repeated in four more sessions, each 1–3 days apart, enabling 10 judgements per stimulus pair and panelist to be collected.
Data analysis
The criterion for an individual subject to be regarded as capable of discriminating a given
odor pair was set at 7 or more out of 10 decisions correct (two-tailed binomial test, P< 0.05). Accordingly, the criterion for the group of subjects to be regarded as capable of
discriminating a given odor pair was set at 12 or more out of 20 subjects performing
significantly above chance (two-tailed binomial test, P < 0.05).
Comparisons of group performance across tasks or sessions were made using the Friedman
two-way analysis of variance. When ANOVA detected differences between tasks, this was then
followed by pairwise Wilcoxon signed-rank tests for related samples to evaluate which tasks
were responsible
(Siegel and Castellan, 1988).
All data are reported as means ± SD.
Results
Figure 1 summarizes the mean performance of 20 subjects in
discriminating between the 10 enantiomeric odor pairs. As a group, the human subjects
performed significantly above chance in only three tasks—involving the discrimination of
the enantiomers of -pinene, carvone and limonene—whereas they failed to do so
with the seven other tasks.
|
Interindividual variability was high, particularly in tasks that were not significantly discriminated at the group level (cf. SDs in Figure 1). However, ANOVA detected significant differences in the group's performance between tasks (Friedman, P < 0.001) and subsequent pairwise tests revealed that the enantiomers of ß-citronellol, menthol, fenchone, rose oxide, camphor,
-terpineol and 2-butanol were
significantly more difficult to discriminate than -pinene, carvone and limonene (Wilcoxon, P < 0.01). Accordingly, between 12 and 19 out of 20 subjects failed to significantly
distinguish between the antipodes of the former group of substances, whereas only 2 or 3 out of
20 subjects were unable to discriminate the enantiomers of the latter group of substances. Discrimination scores within these two groups of substances did not differ significantly from each other (Wilcoxon, P> 0.05).
Figure 2 shows the distribution of individual performance in
discriminating between the 10 enantiomeric odor pairs. The percentage of errors ranged from
32% for the subject performing best up to 65% for the worst. Accordingly, the best panelists
were able to significantly distinguish 6 out of 10 enantiomeric odor pairs whereas the
poorest-performing subject failed to do so with all tasks but one. Nevertheless, the across-task
patterns of performance were very similar between subjects, with virtually all individuals scoring
better with -pinene, carvone and limonene than with the other tasks.
|
Figure 3 shows the mean performance of the 20 subjects across the five test sessions. Error rates were quite stable and did not differ significantly between sessions (Friedman, P > 0.05), and thus no significant learning or training effects at the group level were found.
|
With all 10 odor pairs <17% of decisions were reported to be based upon perceived differences in odor intensity rather than odor quality (cf. Test procedure). The three enantiomeric odor pairs that were significantly discriminated at the group level yielded the lowest percentages of perceived intensity as the choice criterion, with 3.5, 5.0 and 5.5% for limonene, carvone and
-pinene respectively, whereas the percentages with the seven odor pairs that
were not significantly distinguished at the group level ranged from 7.5% for -terpineol to
16.0% for fenchone. Thus, a negative correlation between discriminability of the enantiomeric
odor pairs and the frequency of perceived differences in odor intensity as the choice criterion was
found (r = - 0.76). With none of the 10 odor pairs did discriminability differ as a function of whether the (+)-form or the (-)-form of an odorant was presented as the odd stimulus in a given triad (Wilcoxon, P > 0.05 for all pairs).
|
Experiment 2: trigeminality of enantiomers |
|---|
|
TopAbstractIntroductionExperiment 1: discrimination of...Experiment 2: trigeminality of...Experiment 3: detection...DiscussionReferences |
|---|
The results of experiment 1 showed that human subjects are able to discriminate between the enantiomers of
- pinene, carvone and limonene when presented at equal concentrations. In
order to elucidate whether the nasal trigeminal system contributed to this performance, we
assessed whether the antipodes of these substances differ in their degree of trigeminality by
testing subjects' ability to localize the side of monorhinal stimulation. This simple
method has been shown to reliably quantify the trigeminal impact of odorants
(Berg et al., 1998). Materials and methods
Subjects
Ten healthy, unpaid volunteers (seven females and three males), 22–37 years of age,
participated in the study. Two of the subjects had already participated in experiment 1.
Odorants
A set of six odorants comprising the enantiomers of - pinene, carvone and limonene
was used (Table 1). The substances were diluted, using diethyl phthalate
as the solvent, to the
same concentrations as in experiment 1.
Test procedure
Using a custom-made squeezer, air from two 250 ml polyethylene squeeze bottles was
applied to the right and to the left nostril of a subject. One bottle contained 40 ml of an odorant
whereas the other bottle contained 40 ml of the odorless solvent. Both bottles were equipped with
a flip-up spout which for testing was fitted with a handmade Teflon nose-piece. Care was taken
that the nose-pieces were in direct contact with the nostrils during sampling in order to ensure
that each stimulus entered one nostril only. Presentation of an odorant was synchronized with a
subject's inhalation and the squeezer was calibrated to deliver 20 ml of air to each nostril.
In a forced-choice test procedure 10 subjects were asked to identify the side of stimulation with an odorant. The sequence of presenting the stimuli was systematically varied between sessions and individual subjects while ensuring that the presentation of a given odorant to the left or the right nostril was balanced within and between sessions. The inter-trial interval was ~30 s and no feedback regarding the correctness of the subjects' choice was given. The six stimuli were presented five times per session and testing was repeated in three more sessions, each 1–3 days apart, enabling 20 judgements per stimulus and panelist to be collected.
Data analysis
The criterion for an individual subject to be regarded as capable of localizing the side of
monorhinal stimulation with a given odorant was set at 14 or more out of 20 decisions correct
(two-tailed binomial test, P < 0.05). Accordingly, the criterion for the group of
subjects
to be regarded as capable of localizing a given odorant was set at 8 or more out of 10 subjects
performing significantly above chance (two-tailed binomial test, P < 0.05).
Comparisons of group performance across sessions were made using the Friedman two-way
analysis of variance, and comparisons of group performance between tasks involving the
antipodes of a given substance were made using the Wilcoxon signed-rank test for related
samples (Siegel and Castellan, 1988). All data are reported as means
± SD.
Results
Figure 4 summarizes the mean performance of 10 subjects in
localizing the side of monorhinal stimulation with the enantiomers of -pinene, carvone and
limonene when presented at the same concentrations as in experiment 1. As a group, the human
subjects failed to perform significantly above chance in all six tasks, with between 5 and 10 out
of 10 individuals not reaching the criterion of at least 14 out of 20 decisions correct.
|
Interindividual variability was low (cf. SDs in Figure 4) and altogether there were only two cases of individual subjects scoring 80% correct choices (corresponding to a 1% level of significance), one with (-)-
-pinene and one with (+)-limonene.
Pairwise comparisons of performance between the two antipodes of a substance revealed that
the enantiomers of -pinene, carvone and limonene did not differ significantly in their
degree of trigeminality at the concentrations tested (Wilcoxon, P > 0.10).
Figure 5 shows the distribution of individual performance in
localizing the side of monorhinal stimulation with the (+)- and (- )-forms of
-pinene, carvone and limonene. The percentage of correct choices ranged from 64% for the
best-performing subject to 47% for the worst. Even the best panelists were only able to
significantly localize 3 out of 6 enantiomers at a 5% level of significance whereas the
poorest-performing subject failed to do so with all six tasks.
|
Figure 6 shows the mean performance of the 10 subjects across the four test sessions. Localization scores were quite stable and did not differ significantly between sessions (Friedman, P > 0.05), and thus no significant learning or training effects at the group level were found.
|
|
Experiment 3: detection thresholds of enantiomers |
|---|
|
TopAbstractIntroductionExperiment 1: discrimination of...Experiment 2: trigeminality of...Experiment 3: detection...DiscussionReferences |
|---|
The results of experiment 2 showed that the nasal trigeminal system is unlikely to contribute to the ability of human subjects to discriminate between the enantiomers of
- pinene,
carvone and limonene at the concentrations tested. In order to get a further indication of whether
differences in perceived odor intensity rather than odor quality of the discriminants contributed to
this performance—despite the subjects' self-reports in experiment 1, which suggest
this not to be the case—we determined olfactory detection thresholds for the optical
isomers of these three substances. Materials and methods
Subjects
Ten healthy, unpaid volunteers (seven females and three males), 22–37 years of age,
participated in the study. All subjects had already participated in experiment 1 and/or in
experiment 2.
Odorants
A set of six odorants comprising the enantiomers of - pinene, carvone and limonene
was used (Table 1). For each stimulus, a geometric dilution series using
diethyl phthalate as the
solvent was prepared, starting at a concentration of 1.0 g/l and progressing by a factor of 5. Stem
dilutions were designated step 1, and subsequent dilutions step 2, 3 and so forth.
Test procedure
A 40 ml aliquot of each odorant was presented in a 250 ml polyethylene squeeze bottle
equipped with a flip-up spout which for testing was fitted with a handmade Teflon nose-piece.
Bottles containing the pure diluent served as blanks. Subjects were instructed as to the manner of
sampling and at the start of the first session were allowed time to familiarize themselves with the
bottles and the sampling technique. Care was taken that the nose-piece was only a short distance
(1–2 cm) from the nasal septum during sampling of an odorant in order to allow the
stimulus to enter both nostrils.
Detection thresholds were determined using a triangular test procedure in which panelists
were presented with three randomly arranged bottles, two of which contained pure diluent and
the third the stimulus (Laska and Hudson, 1991; Laska et
al., 1996, 1997). In order to minimize adaptation effects,
testing followed an ascending staircase procedure. At the first testing, stimuli were presented two
concentration steps below the investigator's threshold and in subsequent sessions one
concentration step below the threshold previously determined for the panelist.
Each bottle could be sampled twice per trial, with an inter-stimulus interval of at least 10 s. Sampling duration was restricted to 1 s per presentation in order to minimize adaptation effects. Panelists were required to decide whether there was no difference between the bottles or identify one as containing the stimulus. In the case of `no difference', testing proceeded to the next dilution step, otherwise the bottles were rearranged and the panelist was allowed to sample a second time. If both choices were correct, this was provisionally recorded as the threshold dilution. However, if these had been preceded by one correct and one incorrect choice, the previous dilution was again tested, and if both choices were then correct this was taken as the threshold. In this way, thresholds for the six odorants were determined for each panelist. Testing was repeated in four more sessions, each 1–3 days apart, taking care to systematically vary the order in which the six odorants were presented across sessions.
Data analysis
Comparisons of group performance across sessions were made using the Friedman two-way
analysis of variance. When ANOVA detected differences between tasks, this was then followed
by pairwise Wilcoxon signed-rank tests for related samples to evaluate which sessions were
responsible. Comparisons of group performance between tasks involving the antipodes of a
given substance were made using the Wilcoxon signed-rank test for related samples
(Siegel and Castellan, 1988). All data are reported as means ±
SD.
Results
Figure 7 shows the mean detection thresholds of 10 subjects for each of the six odorants tested across five sessions. With the exception of (+)-carvone, for which a significant increase in performance from the first to the third session was found (Wilcoxon P < 0.05), threshold values were quite stable and did not differ significantly across sessions (Friedman P > 0.1).
|
Interindividual variability was comparatively low, as can be inferred from the SDs in Figure 7, which ranged from 0.52 dilution steps (i.e. a factor of 2.3) for (+)-limonene in session 4 to 2.72 dilution steps (i.e. a factor of 80) for (-)-
-pinene in session 5.
Detectability of the (+)- and the (-)-form of -pinene did not differ
significantly from each other in any session (Wilcoxon P > 0.05). The same is true
for the antipodes of limonene. In contrast, detectability of the enantiomers of carvone was found
to differ significantly in three of the five sessions, with the (-)-form yielding lower
threshold values than the (+)-form (Wilcoxon P < 0.05 in session 2, and P < 0.01 in sessions 1 and 5).
|
Discussion |
|---|
|
TopAbstractIntroductionExperiment 1: discrimination of...Experiment 2: trigeminality of...Experiment 3: detection...DiscussionReferences |
|---|
The results of this study demonstrate that the ability of human subjects to discriminate between enantiomeric odor pairs is substance-specific and thus not a generalizable phenomenon. Whereas almost all subjects had few difficulties in distinguishing the (+)- and the (-)-forms of
-pinene, carvone and limonene, most panelists failed to discriminate
between the antipodes of ß-citronellol, menthol, fenchone, rose oxide, camphor,
-terpineol and 2-butanol when presented at equal concentrations.
These findings are in accordance with earlier reports which assigned different verbal
descriptors to the en- antiomers of carvone (Russell and Hills, 1971; Friedman and Miller, 1971; Leitereg et al., 1971a, b; Pickenhagen, 1989; Koppenhoefer et al., 1994; Ohloff, 1994), limonene
(Koppenhoefer et al., 1994; Ohloff, 1994) and -pinene (Beets, 1978).
They are also in line with reports which assigned the same verbal labels to the antipodes of
menthol (Doll and Bournot, 1949; Beets, 1978; Eccles, 1990), citronellol (Maas et al., 1993), camphor (Theimer et al., 1977;Simmons et al., 1992; Ohloff, 1994),
fenchone (Ohloff, 1994) and 2-butanol (Ohloff, 1994).
On the contrary, our findings do not agree with reports which assigned different verbal labels
to the enantiomers of menthol and -terpineol
(Beets, 1978;
Koppenhoefer et al., 1994),
and to the optical isomers of citronellol
(Ohloff, 1972,
1994)
and rose oxide
(Ohloff, 1972;
Pickenhagen, 1989).
They also differ from reports which assigned the same verbal labels to the antipodes of
-pinene
(Ohloff, 1994).
The fact that different authors came to contradictory conclusions with regard to the equality
or inequality of qualitative attributes assigned to several of the enantiomeric odor pairs employed
here (-pinene, menthol and citronellol) reflects the fundamental problem of semantic
ambiguity in the verbal description of odor quality and illustrates the need for more unequivocal
means of assessing qualitative similarities and differences between odorants.
The few studies which have so far used discrimination procedures to assess the ability of
humans to detect differences between enantiomeric odor pairs are generally in agreement with
our findings.
Jones and Velasquez (1974),
Pike et al. (1987,
1988),
Cowart (1990)
and Hormann and Cowart (1993)
all reported the (+)- and (-)-forms of carvone to be readily discriminable both when
presented at equal concentrations and when stimulus intensity of one of the discriminants was
intentionally altered. Using a triangular test procedure similar to the one employed here,
Cowart (1990)
also found that humans are unable to discriminate between the antipodes of fenchone.
In the only study so far that has employed an array of chiral odor pairs,
Jones and Elliot (1975)
reported the ability of human subjects to discriminate between enantiomers to be both
substance-specific and subject-specific. In line with our results, the majority of their subjects
were able to distinguish the antipodes of carvone and of -pinene. Their finding of
2-butanol—which was significantly discriminated by only 1 out of 20 subjects in our
study—to be discriminable from its mirror image, however, was based on invalid
statistics as the authors applied binomial tests to the total number of correct responses pooled
from all subjects. Converted to percentages, their summed score for this odor pair corresponds
to 40.3% decisions correct, which compares favorably with our finding of an average score of
37.5%.
The same authors reported large differences in discrim- ination performance between
subjects. Unfortunately, they gave no detailed information but only stated that 7 of their 31
subjects failed to reach a significant overall score which the authors discussed as a
`general chiral anosmia'
(Jones and Elliot, 1975).
We also found considerable interindividual variability both with individual odor pairs (cf. SDs
in Figure 1) and across tasks (cf. Figure 2).
However, the across-task patterns of performance
were very similar between subjects, with virtually all individuals scoring better with
-pinene, carvone and limonene than with the other tasks. This suggests that the
substance-specificity of the ability to discriminate between enantiomeric odor pairs is a robust
phenomenon.
It is well-established that both the olfactory and trigeminal systems contribute to the
perception of the majority of odorants
(Doty, 1995).
This raises the possibility that the nasal trigeminal system might have contributed to the
discrimination of the enantiomers of -pinene, carvone and limonene, a possibility which is
supported by the finding that congenitally anosmic subjects possess at least a coarse ability to
distinguish between odorants using sensory information provided by their fifth cranial nerve
(Laska et al., 1997).
The results of experiment 2, however, strongly suggest that the substances used here had little if
any trigeminal-stimulating properties at the concentrations tested and that in any case the
antipodes of a given substance did not differ in their degree of trigeminality. Thus, the possibility
of trigeminal involvement in the discrimination of the three enantiomeric odor pairs in question
can be excluded.
The possibility that differences in perceived odor intensity might have contributed to the
discrimination performance also seems quite unlikely as >90% of the subjects'
decisions
involving the three odor pairs that were significantly discriminated at the group level in
experiment 1 were reported to be based on perceived differences in odor quality rather than
odor intensity (cf. Test procedure). Further, the comparatively few instances in which perceived
differences in odor intensity were reported seem to reflect a subject's difficulty to
discriminate at all, as error rates in such cases tended to be higher compared with the regular case
of reported differences in odor quality. The same tendency for higher error rates with reports of
perceived differences in odor intensity rather than odor quality as a choice criterion has been
found in studies assessing the discriminability of members of homologous series of aliphatic
alcohols (Laska and Trolp, 1998) and carboxylic acids
(Laska and Teubner, 1998).
The results of experiment 3 lend additional support to the assumption that possible differences in
odor intensity did not contribute to discrimination performance as detection thresholds for the
enantiomers of -pinene and the antipodes of limonene did not differ from each other (cf.
Figure 7). Our finding that (-)-carvone yielded significantly lower
threshold values than
(+)-carvone in three of the five test sessions is in line with earlier studies
(Leitereg et al., 1971a,
b;
Cowart, 1990;
Hormann and Cowart, 1993)
reporting the same discrepancy with these stimuli. However,
Cowart (1990)
also reported suprathreshold concentrations of (+)-carvone to be more intense than its
mirror
image and discriminability to be largely unaffected by changes in the concentration of one of the
discriminants.
Taken together, the results of experiments 2 and 3 suggest that the discrimination scores
found with -pinene, carvone and limonene reflect the ability of the human olfactory system
to distinguish the odor qualities of these enantiomeric odor pairs.
A final aspect of the present study is the finding that no generalizable conclusions can be
drawn from our data as to odor structure–activity relationships which would allow us to
predict whether or not a given pair of enantiomers can be olfactorily discriminated. However, it
was apparent that two of the three substances whose optical isomers were significantly
distinguished (carvone and limonene) share a propenyl group at the chiral center and thus it
would be worthwhile to include other enantiomeric odor pairs which show this structural feature
in future studies of olfactory discrimination performance. Our finding that the antipodes of
-pinene were also discriminable despite their lack of a propenyl group, on the other hand,
illustrates that the presence or absence of a certain functional group at the chiral carbon atom is
not a sufficient predictor of enantio-selectivity. Similarly, membership of a certain chemical
class is not a predictor of whether or not the antipodes of a substance are discriminable as, for
example, carvone, fenchone and camphor are all carbonyl compounds but differ significantly in
their discriminability (cf. Figure 1).
A more biological explanation of why some enantiomeric odor pairs can be discriminated
whereas others cannot is that enantioselectivity of the human olfactory system may be restricted
to substances for which both optical isomers are widely present in our natural odor world. There
is accumulating evidence that the mammalian olfactory system, analogous to the immune system,
may be capable of increasing the expression of molecular receptors that are responsive to a given
odorant after repeated exposure to that stimulus
(Wang et al., 1993;
Semke et al., 1995).
Thus it might be that chiral odorants for which only one of their antipodes is naturally occurring
cannot be discriminated from their mirror images due to a lack of an appropriate enantioselective
receptor. Analytical studies of essential oils
(König et al., 1990;
Mosandl et al., 1990b)
and fruit flavours
(Gessner et al., 1988;
Mosandl et al., 1990a)
have shown that the relative amounts found with the optical isomers of a chiral substance can
vary widely. With menthol, for example, the levo-form prevails in all essential oils containing
this compound whereas the dextro-form is found only in trace amounts
(Eccles et al., 1988).
Carvone, -pinene and limonene, on the other hand, are widely distributed with both their
enantiomeric forms—although in different ratios—in a wide variety of plant
extracts
(König et al., 1990;
Mosandl et al., 1990b).
Our finding that the optical isomers of the latter three substances were discriminable while those
of menthol were not supports the hypothesis that a widespread occurrence of both enantiomeric
forms of a substance in our odorous environment is a prerequisite for our ability to distinguish
between these. However, in order to further corroborate this hypothesis it is clearly important to
include other enantiomeric odor pairs in studies of olfactory discrimination performance and to
compare these findings with the natural occurrence and distribution of such substances.
So far, the results of the present study provide evidence that the ability of humans to discriminate between enantiomeric odor pairs is substance-specific and thus support the assumption that enantioselective molecular odor receptors may only exist for some but not all volatile enantiomers.
|
Acknowledgments |
|---|
We thank our panelists for their willingness to participate in this study, Thomas Hummel for his assistance in building the custom-made squeezer and the Deutsche Forschungsgemeinschaft for financial support (La 635/6-1 and 6-2).
|
References |
|---|
|
TopAbstractIntroductionExperiment 1: discrimination of...Experiment 2: trigeminality of...Experiment 3: detection...DiscussionReferences |
|---|
ASTM (1975) Standard recommended practice for referencing suprathreshold odor intensity. Am. Soc. Testing Materials (Phil.), E, 544–575.
Beets, M.G.J. (1978) Structure–Activity Relationships in Human Chemoreception. Applied Science, London.
Berg, J., Hummel, T., Huang, G. and Doty, R.L. (1998) Trigeminal impact of odorants assessed with lateralized stimulation. Paper presented at the 20th Annual Meeting of AChemS, Sarasota, Florida.
Buck, L. and Axel, R. (1991) A novel multigene family may encode odorant receptors. Cell , 65 , 175 –187.[Web of Science][Medline]
Cain, W.S. and Olsson, M.J. (1995) How shall we measure odor quality? Chem. Senses , 20 , 674.
Caldwell, J. (1996) Chirality and Analgesia. Adis, Auckland.
Corwin, J. (1992) Assessing olfaction: cognitive and measurement issues. In Serby, M.J. and Chobor, K.L. (eds), Science of Olfaction. Springer-Verlag, Berlin, pp. 335–354.
Cowart, B.J. (1990) Olfactory responses to enantiomers. Chem. Senses , 15 , 562 –563.
Doll, W. and Bournot, K. (1949) Über den Geruch optischer Antipoden. Die Pharmazie , 4 , 224 –227.
Doty, R.L. (1995) Intranasal trigeminal chemoperception. In Doty, R.L. (ed.) Handbook of Olfaction and Gustation. Marcel Dekker, New York, pp. 821–833.
Eccles, R. (1990) Effects of menthol on nasal sensation of airflow. In Green, B.G., Mason, J.R. and Kare, M.R. (eds), Chemical Senses, Vol.2. Irritation. Marcel Dekker, New York, pp. 275–292.
Eccles, R., Griffiths, D.H., Newton, C.G. and Tolley, N.S. (1988) The effects of menthol isomers on nasal sensation of airflow. Clin. Otolaryngol. , 13 , 25 –29.[Web of Science][Medline]
Friedman, L. and Miller, J.G.
(1971)
Odour incongruity and chirality. Science
, 172
, 1044
–1046.
Gessner, M., Deger, W. and Mosandl, A. (1988) Stereoisomeric flavour compounds. XXI. Chiral aroma compounds in foods. Z. Lebensm.-Unters. Forsch. , 186 , 417 –421.
Hildebrand, J.G. and Shepherd, G.M. (1997) Mechanisms of olfactory discrimination: converging evidence for common principles across phyla. Annu. Rev. Neurosci. , 20 , 595 –631.[Web of Science][Medline]
Holmstedt, B., Frank, H. and Testa, B. (1990) Chirality and Biological Activity. Alan R. Liss, New York.
Hormann, C.A. and Cowart, B.J. (1993) Olfactory discrimination of carvone enantiomers. Chem. Senses , 18 , 573.
Jones, F.N. and Elliot, D.
(1975)
Individual and substance differences in the discriminability of optical isomers. Chem. Senses Flavor
, 1
, 317
–321.
Jones, F.N. and Velasquez, V. (1974) Effect of repeated discrimination on the identifiability of the enantiomers of carvone. Percept. Motor Skills, 38 , 1001 –1002.[Web of Science][Medline]
König, W.A., Krebber, R., Evers, P. and Bruhn, G. (1990) Stereochemical analysis of constituents of essential oils and flavor compounds by enantioselective capillary gas chromatography. J. High Res. Chromatogr. , 13 , 328 –332.
Koppenhoefer, B., Behnisch, R., Epperlein, U., Holzschuh, H., Bernreuther, A., Piras, P. and Roussel, C. (1994) Enantiomeric odor differences and gas chromatographic properties of flavors and fragrances: a selected review. Perfum. Flav. , 19 , 1 –14.
Laska, M. and Hudson, R.
(1991)
A comparison of the detection thresholds of odour mixures and their components. Chem. Senses
, 16
, 651
–662.
Laska, M. and Teubner, P. (1998) Odor structure– activity relationships of carboxylic acids correspond between squirrel monkeys and humans. Am. J. Physiol. , 274 , R1639 – R1645.
Laska, M. and Trolp, S. (1998) Olfactory discrimination ability of human subjects for aliphatic alcohols. Paper presented at the 20th Annual Meeting of AChemS, Sarasota, FL.
Laska, M., Koch, B., Heid, B. and Hudson, R. (1996) Failure to demonstrate systematic changes in olfactory perception in the course of pregnancy: a longitudinal study. Chem. Senses , 21 , 567 –571.
Laska, M., Distel, H. and Hudson, R. (1997) Trigeminal perception of odorant quality in congenitally anosmic subjects. Chem. Senses , 22 , 447 –456.
Leitereg, T.J., Guadagni, D.G., Harris, J., Mon, T.R. andTeranishi, R. (1971a) Chemical and sensory data supporting the difference between the odors of the enantiomeric carvones. J. Agric. Food Chem. , 19 , 785 – 787.
Leitereg, T.J., Guadagni, D.G., Harris, J., Mon, T.R. and Teranishi, R. (1971b) Evidence for the difference between the odours of the optical isomers (+) and (- ) carvone. Nature , 230 , 455 –456.[Medline]
Maas, B., Dietrich, A. and Mosandl, A. (1993) Enantioselective capillary gas chromatography—olfactometry in essential oil analysis. Naturwissenschaften , 80 , 470 –472.
Mosandl, A., Deger, W., Gessner, M., Günther, C., Singer, G., Kustermann, A. and Schubert, V. (1990a) Chiral fruit flavour compounds: stereodifferentiation and fruit-specific distribution of enantiomers. In Holmstedt, B., Frank, H. and Testa, B. (eds), Chirality and Biological Activity. Alan R. Liss, New York, pp. 119– 127.
Mosandl, A., Hener, J., Kreis, P. and Schmarr, H.G. (1990b) Enantiomeric distribution of alpha-pinene, beta-pinene and limonene in essential oils and extracts. Part I. Rutaceae and Gramineae. Flav. Fragr. J. , 5 , 193 –199.
Ohloff, G. (1972) Odorous properties of enantiomeric compounds. In Schneider, D. (ed.), Olfaction and Taste IV. Wissenschaftliche Verlags- gesellschaft, Stuttgart, pp.156–160.
Ohloff, G. (1994) Scent and Fragrances. The Fascination of Odors and their Chemical Perspectives. Springer, Berlin.
Pickenhagen, W. (1989) Enantioselectivity in odor perception. In Teranishi, R., Buttery, R.G. and Shahidi, F. (eds), Flavor Chemistry. Trends and Developments. American Chemistry Society, Washington, DC, ACS Symposium Series 388, pp. 151–157.
Pike, L.M., Enns, M.P. and Hornung, D.E. (1987) Intensity effects on the odor of the enantiomers of carvone. Chem. Senses , 12 , 689.
Pike, L.M., Enns, M.P. and Hornung, D.E.
(1988)
Quality and intensity differences of carvone enantiomers when tested separately and in
mixtures. Chem. Senses
, 13
, 307
–309.
Russell, G.F. and Hills, J.I.
(1971)
Odour differences between enantiomeric isomers. Science
, 172
, 1043
–1044.
Semke, E., Distel, H. and Hudson, R. (1995) Specific enhancement of olfactory receptor sensitivity associated with fetal learning of food odors in the rabbit. Naturwissenschaften , 82 , 148 –149.[Web of Science][Medline]
Siegel, S. and Castellan, N.J. (1988) Nonparametric Statistics for the Behavioral Sciences. McGraw Hill, New York.
Siertsema, R.W., Birch, G.G. and Merlini, L. (1998) Chirality of sweetness and sweetness inhibition. Paper presented at the 20th Annual Meeting of AChemS, Sarasota, Florida.
Silverstein, R.M. (1979) Enantiomeric composition and bioactivity of chiral semiochemicals in insects. In Ritter, F.J. (ed.), Chemical Ecology: Odour Communication in Animals. Elsevier, Amsterdam, pp. 133–146.
Simmons, D.P., Reichlin, D., Skuy, D. and Margot, C. (1992) Stereo- selectivity of odor perception: odorless enantiomers of strong perfumes. Chem. Senses , 17 , 881.
Theimer, E.T., Yoshida, T. and Klaiber, E.M. (1977) Olfaction and molecular shape: chirality as a requisite for odor. J. Agric. Food Chem. , 25 , 1168 –1177.[Web of Science][Medline]
Wang, H.W., Wysocki, C.J. and Gold, G.H.
(1993)
Induction of olfactory receptor sensitivity in mice. Science
, 260
, 998
–1000.
Accepted December 9, 1998
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  D. Stephenson and B. P. Halpern No Oral-Cavity-Only Discrimination of Purely Olfactory Odorants Chem Senses, February 1, 2009; 34(2): 121 - 126. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 W. Li, J. D. Howard, T. B. Parrish, and J. A. Gottfried Aversive Learning Enhances Perceptual and Cortical Discrimination of Indiscriminable Odor Cues Science, March 28, 2008; 319(5871): 1842 - 1845. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. C. Daly, L. A. Carrell, and E. Mwilaria Characterizing Psychophysical Measures of Discrimination Thresholds and the Effects of Concentration on Discrimination Learning in the Moth Manduca sexta Chem Senses, January 1, 2008; 33(1): 95 - 106. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Joshi, M. Volkl, G. M. Shepherd, and M. Laska Olfactory Sensitivity for Enantiomers and Their Racemic Mixtures--A Comparative Study in CD-1 Mice and Spider Monkeys Chem Senses, September 1, 2006; 31(7): 655 - 664. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. I Rupp, W. W. Fleischhacker, G. Kemmler, H. Oberbauer, A. W Scholtz, C. Wanko, and H. Hinterhuber Various Bilateral Olfactory Deficits in Male Patients With Schizophrenia Schizophr Bull, January 1, 2005; 31(1): 155 - 165. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. E. Reisenman, T. A. Christensen, W. Francke, and J. G. Hildebrand Enantioselectivity of Projection Neurons Innervating Identified Olfactory Glomeruli J. Neurosci., March 17, 2004; 24(11): 2602 - 2611. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Laska Olfactory Discrimination Ability of Human Subjects for Enantiomers with an Isopropenyl Group at the Chiral Center Chem Senses, February 1, 2004; 29(2): 143 - 152. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Laska and N. Grimm SURE, Why Not? The SUbstitution-REciprocity Method for Measurement of Odor Quality Discrimination Thresholds: Replication and Extension to Nonhuman Primates Chem Senses, February 1, 2003; 28(2): 105 - 111. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Linster, B. A. Johnson, A. Morse, E. Yue, and M. Leon Spontaneous versus Reinforced Olfactory Discriminations J. Neurosci., August 15, 2002; 22(16): 6842 - 6845. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. Yoshii, Y. Yamada, T. Hoshi, and H. Hagiwara The Creation of a Database of Odorous Compounds Focused on Molecular Rigidity and Analysis of the Molecular Features of the Compounds in the Database Chem Senses, June 1, 2002; 27(5): 399 - 405. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Laska Olfactory Discrimination Ability for Aromatic Odorants as a Function of Oxygen Moiety Chem Senses, January 1, 2002; 27(1): 23 - 29. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Linster, B. A. Johnson, E. Yue, A. Morse, Z. Xu, E. E. Hingco, Y. Choi, M. Choi, A. Messiha, and M. Leon Perceptual Correlates of Neural Representations Evoked by Odorant Enantiomers J. Neurosci., December 15, 2001; 21(24): 9837 - 9843. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Laska Perception of Trigeminal Chemosensory Qualities in the Elderly Chem Senses, July 1, 2001; 26(6): 681 - 689. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. J. Olsson and W. S. Cain Psychometrics of Odor Quality Discrimination: Method for Threshold Determination Chem Senses, October 1, 2000; 25(5): 493 - 499. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. M. Wise, M. J. Olsson, and W. S. Cain Quantification of Odor Quality Chem Senses, August 1, 2000; 25(4): 429 - 443. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. W. Scott, T. Brierley, and F. H. Schmidt Chemical Determinants of the Rat Electro-Olfactogram J. Neurosci., June 15, 2000; 20(12): 4721 - 4731. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. M. Wise and W. S. Cain Latency and Accuracy of Discriminations of Odor Quality between Binary Mixtures and their Components Chem Senses, June 1, 2000; 25(3): 247 - 265. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Laska, S. Ayabe-Kanamura, F. Hubener, and S. Saito Olfactory Discrimination Ability for Aliphatic Odorants as a Function of Oxygen Moiety Chem Senses, April 1, 2000; 25(2): 189 - 197. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Laska, C. G. Galizia, M. Giurfa, and R. Menzel Olfactory Discrimination Ability and Odor Structure–Activity Relationships in Honeybees Chem Senses, August 1, 1999; 24(4): 429 - 438. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||