Perception & Psychophysics1998.60 (4).650-661

The influence of chemical complexity on theperception of multicomponent odor mixtures

ANDREWLIVERMORECharles Sturt University, Bathurst, New South Wales, Australia

and

DAVID G. LAINGUniversity ofWestern Sydney, Richmond, New South Wales, Australia

The present study investigates the hypothesis that complex object odors (odors that emanate fromflowers, foods, sewage, etc.) that consist of dozens of odorants are processed and encoded as discreteentities, as if each was a single chemical odor. To test this hypothesis, the capacity of trained subjectsto discriminate and identify the components of stimuli consisting of one to eight object odors was de­termined. The results indicated that subjects could only identify up to four object odors in a mixture,which is similar to earlier findings with mixtures that contained only single chemical odors. The lim­ited capacity was also reflected in the number of odors selected, regardless of whether the choiceswere correct or incorrect, in confidence ratings, and in decision times. The identification of a limitednumber of object odors in every mixture that was presented suggests that both associative (synthetic)and dissociative (analytic) processes are involved in the perceptual analysis of odor mixtures.

A major function of the olfactory system is to discrim­inate and identify behaviorally relevant odors from a com­plex background that often consists of tens or hundredsof less relevant odorous stimuli. Accordingly, in order tobetter understand olfactory perception and informationprocessing, it is essential to investigate how the systemprocesses complex olfactory stimuli. Recent research hasindicated that humans can only discriminate and identifyup to four odorants in multicomponent mixtures (Laing& Francis, 1989; Livermore & Laing, 1996) and that thislimited capacity is independent of the type (quality) ofodors (Livermore & Laing, 1998) and test method (Laing& Glemarec, 1992). For a system that must operate in anenvironment in which a large number of odorants are si­multaneously present, this limit is surprising.

Although a major function of the olfactory system ap­pears to be the analysis of odors in the environment, ol­factory responses are often characterized by the associa­tion of large numbers of odorous components into aunitary percept. That is, object odors (i.e., the odors ofreal­world objects rather than of single chemicals) that arethemselves complex mixtures consisting ofup to hundredsof single compounds are commonly recognized and iden­tified, within a second or two, as being single undivided

This research was supported in part by a CSIRO Australia Postgrad­uate Student Fellowship to A.L. We gratefully acknowledge Dragoco,Australia, for the provision of a number of the odorants and for their ad­vice in selecting the odor sets used in the study. We would also like tothank Andrew Eddy for upgrading the software. Requests for reprintsshould be sent to A. Livermore, School of Social Sciences and LiberalStudies, Charles Sturt University-Mitchell, Bathurst, NSW, Australia,2795 (e-mail: alivermore@csu.edu.au).

entities, with their individual components generally notbeing identified. Examples of these odors include coffee,chocolate, smoke, and kerosene, each containing multi­ple components, none of which, in isolation, smells likethe mixture.

At least two major questions are raised by the percep­tion of complex object odors as unitary stimuli. First, isit necessary for the olfactory system to discriminate andidentify the individual components of complex odors inorder to identify the whole mixture? Secondly, if the an­swer to the first question is no, how are object odors rec­ognized and identified as unitary percepts? The presentstudy investigates these questions-in particular, whethercomplex object odors are encoded and processed withinthe olfactory system as unitary percepts, rather than asmulti component stimuli.

The mechanism by which the olfactory system extractsmeaningful information from its complex environmentis not well understood. Studies utilizing intra- and extra­cellular recording techniques with the spiny lobster (see,e.g., Derby, Girardot, & Daniel, 1991a, 1991b; Gleeson& Ache, 1985; Michel, McLintock, & Ache, 1991; seeAche, 1989, and Derby, Girardot, Daniel, & Fine-Levy,1989, for reviews) and with insects (see, e.g., De Jong &Visser, 1988a, 1988b; Getz & Smith, 1990; O'Connell &Akers, 1989) have indicated that the initial site of inter­action between individual chemicals in a complex odorstimulus odors (i.e., mixture interactions) is within indi­vidual receptor cells in the peripheral olfactory region.This work suggests that this may occur through the inter­action ofsecond messenger pathways or receptor mediatedevents. This interaction may subsequently lead to the lossof odor-specific information in the olfactory system; as

Copyright 1998 Psychonomic Society, Inc. 650

CHEMICAL COMPLEXITY IN MULTICOMPONENT ODOR MIXTURES 651

a result, the pattern ofneural activity in response to a mix­ture may be qualitatively different from the response toits components or to the average ofthese responses (Derbyet aI., 1991a, 1991b).

This proposal is supported by other studies, using theradioactive cell marker 2-deoxyglucose, that indicate thatneural activity patterns at the surface ofthe olfactory bulbare no more complex for the multicomponent odors ofrat cages (Stewart, Kauer, & Shepherd, 1979) or scentglands (Skeen, 1977) than they are for single compounds,such as limonene (Bell, Laing, & Panhuber, 1987) or amylacetate (Skeen, 1977). Complexity in the latter studies re­fers to the number ofradioactive foci induced in the bulbby the odors. Support also comes from other findings thatindicate that there is little relationship between the chem­ical complexity of an odor, as measured by the numberofdifferent chemical compounds that it contains, and itsperceived complexity, defined as the number of "im­pressions" that subjects perceive the odor to contain (Jel­linek & Koster, 1979, 1983).

However, recent modeling studies involving extracel­lular recordings from olfactory receptor neurons in theantennules of the spiny lobster have indicated that thecomponents of binary mixtures may be processed inde­pendently (Daniel, Burgess, & Derby, 1996; Steullet &Derby, 1997). These results indicate that, at least for mix­tures of two single chemical components, the pattern ofactivity across the population of peripheral neurons issimilar to that expected from a combination of the com­ponents. Thus, separate representations ofthe two compo­nents appear to exist, which suggests that discriminationbetween them may be possible at higher neural levels.

Our study investigates the hypothesis that the neuralrepresentation of a complex object odors can be storedand processed in olfactory memory as a unique and sin­gular entity rather than as a cluster of relatively indepen­dent chemical odors. The experimental design was sim­ilar to that of other studies (Livermore & Laing, 1996,1998), in which subjects were required to identify thecomponents of stimuli containing between one and eightsingle component odors, except that, in the present study,the stimuli were all object odors consisting of tens oreven hundreds of components rather than single chemi­cals. It was expected that, if the odors were encoded andprocessed as single discrete entities, from three to fourobject odors would be identified, the same number asthat found for single chemical odors. However, ifthe ob­ject odors were encoded and processed as an assemblageoftens or hundreds ofcomponents, mixing might result inthe vast number of single components interacting to cre­ate a modified pattern of neural activity. This new pat­tern might bear little resemblance to existing odorantrepresentations in memory and might result in the iden­tification of fewer or none of the none of the complexodors.

As in the previous studies of the capacity of humansfor analyzing mixtures, the perceptual independence ofthe odorants in the present study, as measured by sam-

piing independence or dimensional orthogonality (Ashby& Townsend, 1986), was not rigorously evaluated. Suchan evaluation cannot be conducted with odors at present,because of the lack of information about the physico­chemical factors that determine the smell of a particularmolecule. However, perceptual independence was opti­mized by the use offamiliar and meaningful odorants thatwere rated by subjects as being highly dissimilar.

METHOD

SubjectsThe subjects were 26 students at Macquarie University. The 10

male and 16 female subjects were between 18 and 32 years of age.All of the subjects provided written informed consent prior to thecommencement of the study.

Materials and ApparatusThe odorants and their concentrations and perceived intensity

ratings are listed in Table I. With the exception of kerosene, theodorants were commercial preparations donated by Dragoco, Aus­tralia. They were all complex mixtures, consisting of tens, or pos­sibly hundreds, of odorous components, and were designed so thatthey did not contain any major components that would dominatetheir character. They represented common and familiar environ­mental stimuli that were readily distinguishable from each otherwhen presented in isolation.

Stimulus delivery was via an eight-channel computer (Apple lie)controlled air-dilution olfactometer, the same as that used by Liver­more and Laing (1996), which was capable of delivering a fixedconcentration ofa single odorant or mixtures containing up to eightodorants. The specific concentration ofeach odorant was producedby first passing a known flow of ultrahigh-purity nitrogen across itssurface while it was contained in a glass vessel (Laing, Panhuber,& Baxter, 1978). This stream of saturated odor vapor was dilutedby a stream of medical air (100 ml/min :"). The stimuli were thendirected, via one of eight solenoid valves, to a fan-driven exhaustor into a glass manifold, where it was diluted with another streamof medical air (2,500 ml/min " '). The odorized air then passedthrough a mixing chamber and to a sampling outlet (150 em long,55 cm in diameter), for presentation to a subject. A large exhaustinlet was positioned above the outlet to prevent contamination ofthe subjects' environment. The subjects were instructed to "pushthe exhaust tube aside and to position their nose over the centralsection ofthe outlet, as close to it as possible without touching, andto sniff normally." During the interstimulus intervals and betweensubjects, the olfactometer was continually flushed with nonodor­ous air. Odorant concentrations were calculated from the flow ratesofthe diluting nitrogen and air streams, which have been shown, inprevious studies in this laboratory with similar olfactometers, to beclose to those obtained with a gas chromatograph. The concentra-

Table 1Complex Object Odors

Odorant Description Concentration* Perceived Intensityt

Smoky 0.16 77.8Strawberry 0.15 77.7Lavender 0.12 87.8Kerosene 2.60 71.9Rose 2.40 73.8Honey 3.00 81.8Cheese 0.12 84.3Chocolate 3.00 76.3

*Expressedas the fraction of odor saturated vapor at 20°C (X 10-2)."Measured in millimeterson a 130-mmscale.

652 LIVERMORE AND LAING

tion ofeach odor was not altered when presented in mixtures; thus,an odor presented at 3% of saturated vapor on its own remained at3% of saturated vapor in the mixture.

ProcedureThe test procedure was the same as that used previously by Liv­

ermore and Laing (1996). The computer was programmed to selectand deliver stimuli, to instruct, monitor, and give feedback to sub­jects, and to record the data.

Subjects were seated before a computer monitor, a pressure­sensitive Apple graphics tablet, and the odor outlet. Sampling in­structions were displayed on the monitor. Situated on the left-handside of the graphics tablet was a column of eight labels. Each labelwas 5.8 em long, 1.5 em wide, and had the common description ofone of the eight odorants written on it (Table I). Across the top ofthe tablet were labels designating different subject age groups-forexample, 20-29, 30-39 years, and so forth-and two other labelsindicating gender. On the lower right-hand side was a label, Selec­tion Finished.

To initiate a trial, a subject touched an age and gender label withan electronic pen. This double action triggered the opening of oneto eight solenoids and resulted in either a single odor or a mixtureof odors being delivered to the sampling outlet. Five seconds aftera solenoid had opened and the odor had been delivered, an instruc­tion on the monitor screen indicated to subjects that they shouldcommence sniffing at the outlet. Subjects indicated which odorswere present by touching the appropriate label(s) with the pen. Atrial ended when a subject touched the Selection Finished label or50 sec after the instruction to commence sampling was given. After50 sec, a tone sounded and a light flashed, and subjects were re­quired to touch the Selection Finished label, regardless of whetherthey had completed their selection ofodors. Touching the SelectionFinished label also closed all the solenoid valves and allowed themain air stream to flush out the odors from the previous trial. Thesubjects then waited 53 sec before commencing the next trial,which was automatically initiated by the computer.

Each time a subject touched a label, signifying his or her choiceofodor, the word on the label appeared on the monitor-for exam­ple, kerosene. Once a trial had ended and the Selection Finished labelhad been touched, the correct answer(s) appeared on the monitor,so that subjects had immediate feedback. The data recorded werethe age and gender of the subjects, the stimuli delivered on eachtnal, the odors that were correctly identified (hits), the odors missed(misses), the odors selected but not present (false alarms), and theodors absent and not selected (correct rejections).

Label training was conducted over 5 days. On Day I, the subjectssampled each of the eight odorants at the outlet of the olfactometer.They were told the veridical label by the experimenter as soon asthey had finished sampling the stimulus and were asked to rate itsintensity on a line scale 130 mm in length. The left-hand end of thescale was marked no odor and the right extremely strong. They werethen instructed to rate the goodness offit of the label to the odor andthe familiarity and meaningfulness (the number of associations orlabels that can be linked to a stimulus) of the odor on 7-point cate­gory scales. In addition, they were asked to think of a time, place,or event with which they associated the odor and to write down allof the life associations that they could recall for each of the odor­ants. At the end of each day, the intensities of the individual odorswere adjusted up or down by the experimenter in accordance withthe average of these ratings, so as to achieve approximately similarperceived intensity levels for each odorant. The final task on Day Iwas an identification test. Different random sequences of the odor­ants were presented, and each subject was required to identify eachodor, using the veridical label supplied. At the completion of thetest, the subjects were informed oftheir mistakes, and the odors thatthey had incorrectly identified were presented for sampling again.

On Days 2 5, the subjects were first required to familiarizethemselves with each of the odors by sampling them from the 01-

factometer. At this time, they were again asked to rate the intensityof each odor, to think ofa time, place, or event with which they as­sociated the odor, and to make a note ofany new associations. Theywere then given an odor identification test. The criterion set forentry into the main experiment with mixtures was the correct iden­tification of at least six of the eight odors by Day 5. Two subjectshad to be excluded because of nonattendance or illness. Of the re­maining subjects who entered the mixture experiment, 4 identifiedseven odors, and the remaining 20 correctly identified all eight.

Two days after completion ofthe familiarization period, the sub­jects were tested once per day over 3 consecutive days. Three test ses­sions were chosen because Livermore and Laing (1996) had found,in a series of 10 sessions, that there was no improvement in perfor­mance above that reached after I test session. Before commencingdaily sessions, the subjects sampled and identified the individualodors from the olfactometer. Corrective feedback was provided forany odors that were incorrectly identified. Sixteen test trials werethen given, with each trial consisting ofa single object odor or of amixture containing up to eight object odors. The composition ofeachof the 16 stimuli presented in each session was randomly deter­mined by the computer, with the restriction that, within each testsession, two ofeach mixture type-that is, from one to eight objectodors-was delivered. On each trial, after completion oftheir odorselections, the subjects were required to rate the confidence withwhich they had selected each odor from the mixture on a 7-pointcategory scale.

Prior to sampling, the subjects were told that they would "be re­ceiving an odor which could be either one of the single odors thatthey had learned (kerosene, chocolate, rose, etc.) or a mixture of2,3,4,5,6,7, or 8 ofthe odors. Your task is to identify as many odorsas you can recognize and to mark your choices on the computer." Atthe end ofeach trial, they received corrective feedback via the com­puter, indicating the odors selected and the actual odors presentedon that trial.

The variables recorded for analysis were the odorants presentedin a mixture, the odorants that were selected by subjects, the orderin which the odors were selected, the time taken by the subjects tomake their first selection and to press the selection finished label;confidence, meaningfulness, familiarity, and goodness of fit rat­ings; and session number.

Statistical AnalysisExcept where indicated, analyses ofvariance (ANOVAs) were of

a mixed model format containing both independent groups (session[3], hits [2], order of selection [6], and confidence rating [6]) andrepeated measures (number of odors presented or mixture type [8]and odor type [8]) factors. Odor type refers to the eight types ofob­ject odors; mixture type refers to the number of object odors in amixture. The results were investigated by means ofthe SPSS-X pro­gram package. With the exception of the t tests, where p < .0 I wasused in order to avoid overinflation ofthe overall significance level(Bonferroni adjustment, e.g., Stevens, 1986), a significance level ofp < .05 was set for all analyses. A Greenhouse-Geisser correctionofthe degrees of freedom was carried out for all of the main effectsand interactions that involved repeated measures factors. The p val­ues reported, but not the degrees offreedom, were corrected. Wheremeasurements ofvariability in the data are quoted, they refer to val­ues ofomegas. Because ofthe large number oftests that would haveresulted and, therefore, the inflated probability of Type I errors,multiple comparisons of the means were not performed for all sig­nificant effects.

RESULTS

Inorder to analyze the relative performance ofthe sub­jects, a variable described here as correct was chosen,which was defined as the sum of the number of hits and

CHEMICAL COMPLEXITY IN MULTICOMPONENT ODOR MIXTURES 653

correct rejections on anyone trial (see Figure 1). To de­termine whether training may improve the ability ofsub­jects to discriminate mixture components and to estab­lish whether it was necessary to include session as afactor in subsequent analyses, the performance of thesubjects across test sessions was analyzed (Figure 1;mixedmodel ANOVA, correct score by subject and session andcorrect by subject and mixture type). Although there wasno change in performance across sessions, there was ahighly significant decrease in accuracy as the number ofodors in the mixture increased [Figure 1;F(7,161) = 93.66,p < .001].

The results of applying a stricter criterion of perfor­mance-namely, the percentage of absolutely correctidentifications for each type of mixture-are shown inFigure 2. As in Figure 1, these results indicate that a sig­nificant and extremely sharp decrease in correct responsesoccurred as the number of odors in a mixture increased[repeated measures ANOVA, percent absolutely correctresponses X mixture type [F(7,161) = 46.16,p < .001,(jJ2 = 66.7%], with the percentage of absolutely correctresponses decreasing from approximately 50% with sin­gle odors to 15%, 5%, and 3% with binary, trinary, andquaternary mixtures, respectively.

The hit rate for single odor stimuli was extremely high,with the subjects correctly identifying the presented odoron 97% of the trials (Figures 3 and 4), which indicatesthat the training program was successful in establishingstrong links between the odors and their names. How­ever, the mean number ofhits per trial did not exceed 3.5,

even for mixtures containing eight odorants. Figure 3 alsoshows that the mean number offalse alarms per trial wasvery low, being less than or equal to one per trial, for allmixture types. Accordingly, the data for hits and falsealarms indicated that the low level of identifications wasdue to the limited capacity ofthe olfactory system to dis­criminate and identify mixture components rather than tothe inability of subjects to name odors or to a tendencyfor them to guess.

An important point, illustrated in Figure 4, is that themost frequent number of correct identifications did notexceed four for any type ofmixture. For example, in mix­tures that contained five to eight components, there wasonly a marginal increase in the number of componentsidentified, with four odors being correctly identified on21%, 29%, 28%, and 35% of the trials, respectively,whereas five odors were correctly identified on 5%, 7%,10%, and 13% ofthe trials. Although the increasing fre­quency with which five odors were identified may sug­gest that a ceiling was imposed on the number ofcorrectidentifications by restricting the maximum number ofcomponents in the mixture to eight, the extremely low fre­quency with which six, seven, and eight components wereidentified suggests that the limit of identification hadbeen reached.

In order to test whether the results were independentofthe type ofodors used for testing, the data were analyzedto determine whether some odors were more readilyidentified than others. As with the correct score gener­ated for each type of mixture, a correct score was gener-

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Number of odors present7 8

Figure 1. Mean correct score for each type of mixture presented. Correct is thesum of the number of hits (correct selections) and correct rejections on anyonetrial. Thus, the maximum obtainable score on any trial was eight, which, for sin­gle odors, represents one hit and seven correct rejections. The bars indicate thestandard errors ofthe mean.

654 LIVERMORE AND LAING

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Figure 2. The percentage of absolutely correct responses for each number of odorspresented. Absolutely correct indicates that aU the components of the mixture werecorrectly identified and that there were no false alarms. The bars indicate the stan­dard errors of the mean.

ated that represented the ability ofthe subjects to correctlyidentify and to reject odors. Although some variation be­tween odors occurred, the accuracy with which eachodor was identified decreased as the number of compo­nents in the mixture increased [Figure 5; repeated mea­sures ANOVA, correct score X mixture type and odor

type,F(7,161) = 92.59,p<.001,ro2 = 79.1%]. Further­more, in addition to a significant difference betweenodors [F(7,161) = 11.62,p < .001, ro2 = 30.6%], therewas a significant interaction between mixture type andodor [F(7,1127) = 2.88,p < .001, ro2 = 7.3%]. The lat­ter result reflects the observation that lavender odor was

..... Hits -+- False Alarms4,------------------------,

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1 2 3 4 5 6 7 8

Numberof odors presentFigure 3. Mean number of hits (correct identifications, upper line), and false alarms

(incorrect identifications, lower line) for each type of mixture. The bars indicate thestandard errors of the mean.

CHEMICAL COMPLEXITY IN MULTICOMPONENT ODOR MIXTURES 655

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Number of hits for each number of odors presentFigure 4. Percentage of correct judgments (hits) of mixture components. Zero

hits signifies that the indicated judgments were entirely incorrect. See the text forexplanation.

more accurately identified than were the others, particu­larly in the more complex five- to eight-component mix­tures, and that, relative to the other odors, strawberrywas poorly identified.

The relationship between the number ofodors presentin a mixture and the number ofodors selected by the sub­jects, regardless ofwhether their choices were correct orincorrect, is illustrated in Figure 6. The latter measure wasincluded, as it provides an indication of the perceptualcomplexity ofa mixture. The figure indicates that, as the

number ofcomponents in a mixture increased, there wasa steady increase in the number of odors selected, up tothe point where four were selected in five-componentmixtures. However, the most frequent number of odorsselected did not rise above four in six- to eight-componentstimuli. Subjects were relatively accurate with their re­sponses to single odors and binary mixtures, in that themost frequent number of odors selected were one andtwo, respectively. In contrast, they were less accurate withthree-, four-, and five-component mixtures, in which two,

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Figure 5. Mean correct score for each odorant in each type of mixture. For eachodor, a hit or a correct rejection on anyone trial resulted in a correct score of I,whereas misses or false alarms resulted in a score of O. Thus, each point on thegraph represents the sum ofthe mean number of hits and correct rejections for a par­ticular odor when a particular number of components were present in the mixture.

656 LIVERMORE AND LAING

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12345678Numberof odors .elected for eachnumber presentIn the mixture

Figure 6. Complexity of stimuli, as measured by the number of odor selectionsmade, regardless oftheir correctness, when each type ofmixture was presented.Numhers of responses are expressed as percentages. For example, when onlyone odor was presented, 51% ofthe responses indicated that one odor was pre­sent, 23% that two were present, and 15% that three were present, and so forth.

three, and four were the most common number of odorsselected. These findings support the results reported inprevious studies by Laing and Francis (1989; Laing &Glemarec, 1992; Livermore & Laing, 1996) in that theyindicate that the number ofcomponents that subjects feelthey can sense and identify in olfactory mixtures reachesa plateau at four odors, regardless ofthe number ofodorsin the mixture. A repeated measures ANOVA (numberofodors selected X mixture type) verified that there wasa highly significant difference between mixture types interms ofthe number ofodors selected [F(7,140) = 46.31,P < .001, Q)2 = 69.8%]. Figure 7 reveals the relationshipbetween the order in which odors were selected and theconfidence ratings ofthe subjects and indicates that therewas a steady decline in the confidence with which thesubjects made their selections, up to their fourth choice.To aid interpretation, the scale is reversed in Figure 7, sothat 1 signifies extremely unconfident, and 7 extremelyconfident. Also, as the selection of more than six odorswas very rare, for purposes of analysis the scale was re­duced from eight to six categories, the categories com­prise 1, 2, 3, 4, 5, and 6+ ratings. These observationswere supported by a repeated measures ANOVA (confi­dence rating X accuracy and order of selection), whichrevealed a significant association between confidencerating and order of selection [F(5,3526) = 200.38, P <.001, Q)2 = 19.8%] and also that subjects were signifi­cantly more confident about odors that were correctlyidentified (mean rating = 3.69) than about those thatwere not [mean rating = 4.43; F(l,3562) = 303.21,p <.001, Q)2 = 6.0%].

Table 2 lists the intercorrelations between the accuracyofidentification and rejection for each odor, as measuredby correct score, by odor meaningfulness and familiar­ity,and by the goodness offit ofan odor to its given name.There was no significant association between the accu­racy with which odors were identified and any of theother variables, which indicates that the identification ofmixture components was not affected by the prior expe­rience of the subjects with the odors or by the inade­quacy of the names of the odors. However, there werehighly significant correlations between the three cogni­tive variables-meaningfulness, familiarity, and good­ness of fit. Once again, the range ofratings was relativelynarrow. On scales where 0 indicated extremely meaning­ful orfami/iar, and 7 indicated extremely meaningless orunfamiliar, meaningfulness ranged between 2.50 and4.25, familiarity between 2.09 and 3.13, and goodness offit between 2.00 and 3.43.

Figure 8 shows that the time taken to select the firstodor in one- to three-component mixtures was shorterthan for the five- to eight-component stimuli. Similarly,the total decision time increased from the one- to the

Table 2Correlation Matrix for Correct, Meaningfulness,

Familiarity, and Goodness of Fit (Fit)

Factor Correct Meaningfulness Familiarity

Meaningfulness .042Familiarity .072 .594*Fit .056 .433* .551*

Note--One-tail level of significance: tp < .0I, df = 158.

CHEMICAL COMPLEXITY IN MULTICOMPONENT ODOR MIXTURES 657

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Figure 7. Mean confidence rating and the order in which odors were selected.Low ratings indicate a low level of confidence in a given selection. The bars indi­cate the standard errors ofthe mean.

four-component stimuli, with little difference being ob­served between four- and eight-component mixtures.Analysis of the two decision time measures (repeatedmeasures ANOVAs, time to first selection X mixturetype and time to completion of selections X mixturetype) revealed that there were significant differences be­tween the various mixture types, as measured by the timeto first odor selection [F(7,161) = 2.62, P < .05, 0)2 =10.22%] and also by the time to completion ofselections[F(7,161) = 14.78,p < .001, 0)2 = 39.1%].

DISCUSSION

The most important finding of the present study wasthat complex object odors are encoded and processed asdiscrete entities within the olfactory system, as if theywere odors from single chemicals. Clearly, object odorsare not stored and interpreted as conglomerates of inde­pendently classified single chemical components. If ob­ject odors were encoded in this manner, the mixing of anumber of them would most likely have resulted in theloss of the code for individual object odors, and the abil­ity to discriminate and identify each object odor wouldhave decreased considerably. As the results show, this wasnot the case; object odors were discriminated and iden­tified as if they were from single chemicals. Thus, the re­sults support a Gestalt encoding hypothesis.

The results indicated that four and five object odorswere identified, without error, on only 3% and 2% ofthetrials, respectively. The most frequent number of odorsthat were correctly identified and the most frequent num­ber ofodors selected did not exceed four in five- to eight­component mixtures. In addition, the confidence ratingsduring decision making suggest that discriminating among

components in one- to four-component stimuli is a taskthat is qualitatively different from that for stimuli con­taining five to eight components and that there was a steadydecrease in the confidence with which odors were se­lected as the number of components selected increasedfrom one to four. However, this decrease leveled offwhenmore than four odors were presented, which suggeststhat it had reached a minimal acceptable level for selec­tions. Similarly, total decision time increased as the num­ber of components in the mixture increased from oneto five, but did not increase further for five- to eight­component mixtures. Finally, the subjects took signifi­cantly less time to make their first odor selection withone- to three-component stimuli than they did with five­to eight-component mixtures, responses to four-compo­nent stimuli being intermediate between the two groups.Furthermore, the similarity with which identification de­creased for each odor as the number of components inthe mixture increased suggests that the results were largelyindependent of the type of odors used. With every mea­sure conducted, the results ofthe present study confirmedthose of four previous studies (Laing & Francis, 1989;Laing & Glemarec, 1992; Livermore & Laing, 1996,1998), which showed that humans can only discriminateand identify four components in odor mixtures.

It is significant that mixtures containing four odors­the same number of odors found to be the limit for mix­ture component discrimination-serve as pivotal pointsfor all ofthe above effects. The subjects identified odorsrelatively quickly and confidently with moderate accu­racy when one to four odors were present. However,whenfive or more components were in the mixture, only asmall percentage of the subjects were successful in iden­tifying more than four components, the confidence that

658 LIVERMORE AND LAING

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Numberof odors present7 8

Figure 8. (a) Mean time to first odor selection. (b) Time to completion ofse­lections. The bars indicate the standard errors of the mean.

subjects placed in their decisions remained at a constantminimal level, the time taken to complete odor selectionsleveled out, and the time taken to discriminate and iden­tify the first odor increased. All of these effects suggestthat the discrimination task became substantially moredifficult when four or more odors were presented simul­taneously. A probable reason for this is the occurrence ofblending, or synthesis, between a number of the stimuli.This suggestion is compatible with Berglund's (1974)proposal that additive processes dominate odor percep­tion in one to three component mixtures and interactiveprocesses dominate when four or more odors are present.However, because ofthe consistent ability ofthe subjects

to discriminate three to four odors in four- to eight-com­ponent stimuli, the present results imply that additiveand interactive processes may coexist in mixtures offouror more odors.

Thus, the data indicate that the perceptual capacity forodorant discrimination in mixtures is 4 ± 1, which maycorrespond to Miller's (1956) "magic number" of? ± 2,which was found to be the capacity for immediate mem­ory in other senses. Using a similar logic, it may be thattraining with chemically complex object odors and theirveridical names results in the chunking of their odorcomponents into a single entity. This would result in theirrepresentation in memory as a single unit that can be

CHEMICAL COMPLEXITY IN MULTICOMPONENT ODOR MIXTURES 659

stored and retrieved as a whole, allowing subjects tomake optimal use of a limited capacity, in the same wayas the one that Miller found for strings ofnumbers. It mayalso reflect the importance oforganization in making op­timal use of memory.

The very high hit rate of97% with single (object odor)component stimuli indicates that the training and encod­ing regimes undertaken prior to testing were successfulin establishing strong associations between odors andtheir names, so that the poor ability to discriminate mix­ture components was not due to an inability to name theodors. In addition, the low false alarm rate, which, on av­erage, was less than one per trial, indicated that the sub­jects could clearly perceive the odors they selected anddid not guess when responding. Although the subjectswere relatively accurate in their identification ofthe odorsthat they selected, they were unable to use the process ofelimination to accurately select all of the odors present.

In the present study, there was no association betweenthe accuracy with which odors were discriminated andtheir meaningfulness, familiarity, or goodness of fit totheir given names. However, this does not necessarilyimply that these variables do not influence odor memory.The results may be explained in at least two ways. First,as the odors were initially chosen to be common and fa­miliar, the range of ratings may have been too small toobtain a significant difference between them. Second, thesubjects received extensive training with each ofthe sin­gle object odors; hence, differences in familiarity, mean­ingfulness, and goodness offit may have been eliminatedby the time testing occurred.

The process whereby a mixture of single odorants (in­dividual chemicals) blend to produce a unique smell (e.g.,an object odor such as coffee) is known as synthetic or as­sociative processing and is not well understood. This iscontrasted with analytic or dissociative processing, whichoccurs when the individual components ofa mixture standout and can be separately identified (see, e.g., Berglund,1974; Erickson, 1982; Westbrook & Charnock, 1985).The ability to discriminate three to four components inmixtures, be they single chemicals or complex object odors,indicates dissociative processing. In contrast, the reactionto complex object odors as unique stimuli rather than asa jumble of components supports the notion of associa­tive (or synthetic) processing.

Although the precise nature of the perceptual changesthat occur with learning are beyond the scope of the pre­sent study, it is worthwhile examining how they mayoccur. Studies ofodor and taste have indicated that an an­imal will perceive a mixture associatively, unless it haspreviously been made aware of the single components.However, if the animal is made aware of the existence ofthe individual components in isolation or in combinationwith a different odor or taste, it will perceive the mixtureanalytically (Staubli, Fraser, Faraday, & Lynch, 1987;Stevenson, Prescott, & Boakes, 1995; Westbrook &Charnock, 1985). Similar results have been found for thevisual system (Lamb & Robertson, 1989; Pomerantz,

1983; Tremain & Kiernan, 1995), although face recog­nition may be a special case (Tanaka & Farah, 1993).

Recently, it was found that spiny lobsters could betrained to perceive the same odor mixtures either asso­ciatively or dissociatively, depending on the order inwhich the mixtures were presented to them (Livermore,Hutson, Ngo, Hadjisimos, & Derby, 1997). As learningcannot take place in the nose in such a short period oftime, these findings couldn't have resulted from chemi­cal interactions at this level. Therefore, the perceptual ef­fects must have been produced by the way in which theodors were analyzed in the brain.

It has been proposed that the process that underlies thedissociative and associative perception ofmixtures is theformation ofspecific memories. When a person or an ani­mal learns about a stimulus, a smell, a taste, a sight, or asound, the stimulus is stored as a representation in mem­ory (McLaren, Kaye, & Mackintosh, 1989; Rescorla,Grau, & Durlach, 1985; Tanaka & Farah, 1993; West­brook & Charnock, 1985). Similarly, when they learnabout a mixture, that mixture is stored as a representationin memory. Once associative learning has taken place, acomplex mixture may be perceived and processed in thesame way as its single components-that is, as a singleunique representation or, alternatively, as separate butstrongly linked representations for each component. Ex­periments are currently underway to investigate this pro­posal with olfactory mixtures.

Rabin and Cain (1984) suggested that familiarity fa­cilitates olfactory discrimination by enhancing the per­ceptual boundaries or unique characteristics possessedby an odor. High levels of familiarity with object odorsin the everyday environment and the frequent simultane­ous presentation of their components may result in asso­ciative processing dominating their perception. As a re­sult, the components of the mixture are synthesized intoa new and unique perceptual and, possibly, neural repre­sentation. Alternatively, a lack of familiarity may lead tothe perceptual boundaries ofan object odor being poorlydefined, so that its components are not associated, and,hence, it may not be discriminated as a single entity. Thus,the way in which animals and humans learn about a com­plex sensory event may influence the way in which thecomponents of that event are perceived-that is, as sep­arate odors or as a single unique stimulus. Familiarityand exposure to components in isolation may be the basison which odor mixtures are perceived either as Gestaltsor as an assortment ofcomponents. Experiments currentlyunderway are directed at testing this possibility.

The question then arises as to whether there is a phys­iological basis for the association of some odors and thedissociation of others, as appears to have occurred inprevious studies of the limited olfactory capacity ofhu­mans. Evidence-in particular, that of Walter Freemanand his coworkers-indicates that, with repeated rein­forced exposure, odor-specific bursts of neural activityare produced from the surface of the olfactory bulb (see,e.g., Carmi & Leon, 1991; Chaput & Holley, 1985; Coop-

660 LIVERMORE AND LAING

ersmith & Leon, 1984; Freeman & Grajski, 1987; Grajski& Freeman, 1986, 1989; Sullivan, Wilson, & Leon, 1989).Furthermore, learning has been shown to produce odor­related synaptic changes in both the bulb (Gervais, Hol­ley, & Keverne, 1988; Grajski & Freeman, 1989; Gray,Freeman, & Skinner, 1986; Nicoll & Jahr, 1982; Sullivanet aI., 1989; Wilson & Sullivan, 1991) and olfactory cor­tex (Haberly & Bower, 1989; Kauer, 1987; Schild, 1988).It is possible, therefore, that, through learning and asso­ciation, these changes are the basis on which object odorscome to be recognized as single entities. That is, frequentreinforced exposure may lead to the emergence of a sta­ble pattern ofneural activation that is characteristic ofthecomplex mixture rather than ofits individual components.

In conclusion, the finding that humans can discrimi­nate and identify four object odors in mixtures contain­ing up to eight object odors indicates that the olfactorysystem processes and encodes complex object odors asdiscrete entities, in a manner that is similar to that forsingle chemical odors. Although the mechanisms bywhich mixtures are processed neurally remain to be re­solved, it appears that the encoding of single and complex(object) odors involves both associative and dissociativeprocesses that have evolved to provide a rapid process fordiscriminating and identifying environmentally or be­haviorally relevant odors.

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