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Selective attention and the perceptual analysis of odor mixtures
Physiology&Behavior,Vol. 52, pp. 1047-1053, 1992
0031-9384/92 $5.00 + .00 Copyright© 1992PergamonPressLtd.
Printed in the USA.
Selective Attention and the Perceptual Analysis of Odor Mixtures D. G. L A I N G .1 A N D A. G L E M A R E C t
*Faculty of Science and Technology, University of Western Sydney, Bourke Street, Richmond N S W 2753, Australia and i'Ecole Nationale Supdrieure De Biologic, Appliqu£e A La Nutrition Et A L'Alimentation, Centre Universitaire Montmuzard, 21100 Dijon, France R e c e i v e d 16 S e p t e m b e r 1991 LAING, D. G. AND A. GLEMAREC. Selectiveattention and the perceptualanalysisof odormixtures. PHYSIOL BEHAV 52(6) 1047-1053, 1992.--Two psychophysical methods were used to investigate the capacity of humans to identify the constituents of odor mixtures consisting of up to six components. With one method subjects were required to identify all the components present in each stimulus; with the other, a selective attention procedure was used where subjects had to identify only one component at each trial. Little difference was found between the levels of identification obtained with both methods, reinforcing the finding that humans have great difficulty in identifying more than three components in an odor mixture and indicating that it is unlikely that olfactory adaptation influenced the identification process. Odor mixtures
Human olfaction
Olfactory adaptation
OLFACTORY stimuli in our environment usually consist of many different odorous molecules and often appear to be perceived as a single odor, e.g., chocolate, suggesting that odors may be combined perceptually to form a homogeneous stimulus. Nevertheless, anecdotal information, particularly as regards the capabilities of perfumers and flavorists, suggests that humans are capable of analyzing odor mixtures and identifying significant numbers of the components. Several years ago, for example, one of the authors (D.G.L.) on casual questioning of such experts regarding their respective analytical abilities, obtained answers varying between 5 and 30 components. However, few studies have investigated the capacity of humans to identify components in odor mixtures, and of these, most have been concerned with binary mixtures. Such studies have shown that depending on the quality and intensity of the components, one or both components are perceived (4,13). In addition, it has been shown that the familiarity and pleasantness of odorants can affect the ability to detect components of binary mixtures (17). Currently, there is no method for predicting which odors will be perceived in a mixture, or for predicting the overall aroma of a mixture, without prior experience with the set of odorants used. This situation is likely to continue unless an understanding of how mixtures are perceived is obtained. Accordingly, as part of a program aimed at resolving this impasse, a series of experiments has been initiated in this laboratory to determine: I. the capacity of humans to analyze and identify odors in mixtures, and 2. the physiological factors that determine the capacity. Requests for reprints should be addressed to D. G. Lalng.
1047
Selective attention
Neural capacity
With this information, it may then be possible to provide an objective basis for predicting the aroma of a mixture. Recently, using untrained subjects, Laing and Francis (10) reported that it was very difficult for humans to identify more than three components in mixtures containing up to five dissimilar but familiar odors, and Laing and Livermore (1 l) reported a similar outcome with trained subjects and a group of perfumers and flavorists. A question arising from these studies concerns the origin of the apparent limited capacity of the olfactory system to process information from mixtures. The limitation may arise from physiological constraints related to the restricted capacity of the system to process simultaneously perceived information on odor identity, or it may be an artefact of the psychophysical method employed, which was the same in the earlier studies. As regards physiological constraints, limited neural processing capacity is known to limit identification of tones to about seven (15), and it was reported to be increasingly difficult to identify different taste stimuli when the perceived intensity of tastants was adjusted to produce similar reaction (identification) times (7). A likely interfering factor arising from the methodology used in the earlier studies by Laing and co-workers was olfactory adaptation arising from the considerable time subjects spent sampling each stimulus. In those studies, the majority of subjects spent most of each 50 s trial sniffing the stimulus, and from the author's experience with this test paradigm, considerable adaptation during a trial would have occurred. In support of this observation, self-adaptation as a result of a few inhalations has
1048
I AIN(i AND (.It.EMARt:('
been reported to substantially reduce odor intensity (3). It is unlikely that self or cross adaptation would have occurred across trials to any great extent since the intertrial interval was 45 s, a substantial duration. Unfortunately, no studies of adaptation to components in mixtures have been reported that might indicate whether adaptation occurs equally or at different rates for different components, or whether the rate of adaptation to highand low-intensity components in mixtures is different. To further complicate the outcome it is also possible that if selective adaptation to a mixture component had occurred, this could have resulted in the release of other components from suppression during a trial and thereby increased detection of the released component. Since the phenomenon of suppression release (14) has been demonstrated to occur across trials with intertrial intervals of up to 60 s duration (Laing, unpublished data), it is possible that the lengthy sampling times observed earlier by Laing and co-workers could have resulted in suppression release of a component(s) in a subsequent trial. This latter action, however, would favor rather than hinder identification of components. The present study investigates the possible influence of olfactory adaptation on the capacity of humans to identify mixture components. It compares the capacity of humans in a task which minimizes adaptation, namely a selective attention task where the presence or absence of only one target component need be indicated, with the capacity recorded using the multiidentification task described earlier (10,11), when all the components of each stimulus had to be identified during each trial. The aim in focussing a subject's attention on only one component during a trial was to reduce the time spent sniffing the stimulus and, hence, reduce the effects of adaptation on identification. It has been demonstrated, for example, that subjects only require a sniff of about 0.4 s duration to identify an odorant of moderate intensity (9). The study was conducted in two parts. During the first part, subjects were asked to identify all the components in each stimulus, and during the second part, the selective attention procedure was used. In brief, the study employs two very different test procedures, one aimed at minimizing adaptation, to investigate the capacity of humans to identify the components of stimuli consisting of up to six odorants. METHOD
General Eleven subjects, four males and seven females, aged between 19 and 48 (mean, 27.5 years), participated in the study. All were staff of this Institute and most had previously participated in sensory studies. The six odorants used during this study, their concentratiqns, mean perceived intensity, and the words used by subjects to identify each odor are given in Table 1. All odorants were of the highest purity available from Fluka AG, except cis-3-hexenol which was donated by Dragoco (Australia) and ethyl-n-butyrate from BDH Chemicals (Australia). This group of odorants differs from the group used previously in this laboratory (10,11) with the inclusion of ethyl-n-butyrate as a better representative of a fruity odorant than ethyl-n-caprylate, cis-3-hexenol as a more discrete odor than c~-pinene, i.e., was easier to discriminate from the other odors listed than apinene, and acetic acid was not used because it was often identified by subjects previously by its sharp note, and identification may have been primarily through its trigeminal action. All the odorants were chosen because they are common and dissimilar in character and were of similar perceived intensity.
IABLE f OD(.)RA NI S Chemical
Common Name
('*
Mean lntensll? (ram)
( )-Carvone Benzaldehyde Cis-3-hexenol (+)-Limonene Eugenol Ethyl-n-butyrate
Spearmint Almond Cut grass Orange Cloves Fruity
1.04 0.16 0.22 2.01 1.16 0.07
¢,(i.4 60.6 68.2 53.5 54.6 59.3
* Concentration expressed as the dilution of saturated odor vapor at 20°C (× 10-2).
The latter was established with the present subjects using a graphic rating scale (130 ram) which had the words "no odor" and "extremely strong" at the ends. Odorants were presented singly and in different random sequences to different subjects in duplicate. Several sessions were required to adjust the perceived intensities to a moderate level. Subjects indicated their estimate of intensity by placing a mark on the line scale. An analysis of variance and least significant difference test showed that only cis-3-hexenol was significantly stronger than some of the other odorants (Table 1), but as shown in Fig. 4, this did not appear to enhance its identification. An air-dilution olfactometer with multiple odor channels (12) was used to deliver a single concentration of each odorant or mixtures containing up to six odorants to subjects. This instrument was almost identical to that previously described for the production of binary mixtures (13), except that it had been modified slightly to allow mixing of up to six odorants. As with the previous design, large mixing chambers were used to ensure complete mixing of odors before delivery to Perspex sniffing outlets (12). Before commencing the identification tests, subjects were trained to use an appropriate label for each odorant presented singly over four sessions. During the first session subjects provided their own label for each odor and then were asked to choose between their own label and the common name for each odor. Subjects preferred the common name, and this was used in the following three training sessions. At each of these sessions the six odorants were presented twice in different random sequences to each of the subjects, and the latter were required to identify each odor. Corrective feedback was given after each trial. During the fourth session all subjects correctly labelled the six odorants. This training procedure was similar to that used earlier (11) which allowed subjects to obtain almost perfect identification scores during test sessions when the stimulus was a single odorant. Because of technical limitations with the olfactometer it was not possible to present subjects with a totally randomized set of stimuli at each test session or the same number of stimuli at each session. Therefore, to compare the two test methodologies a set of mixtures was selected so that each odor appeared twice in each type of mixture, i.e., in one, two, t h r e e . . , six-component mixtures. The set of 24 stimuli which included the six single odorants are listed in Table 2.
Multi-Identification Task Test procedure. The test method was similar to that reported (10, l 1) to allow comparison of the results with those from the latter studies and with the selective attention task (see below).
ODOR MIXTURE PERCEPTION
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TABLE 2 TEST STIMULI A B C D E F
AB AD BE CE CF DF
ACD AEF BDF BCE
ABDE ACEF BCDF
TABLE 3 PERCENTIDENTIFICATIONOF MIXTURECOMPONENTS ABCDE ABCDF ABCEF
ABCDEF ABCDEF
Numberof Odors Identified 0 1 2 3 4 5 6
A: (-)-carvone; B: benzaldehyde;C: cis-3-hexenol;D: (+)-limonene; E: eugenol; F: ethyl n-butyrate. All 24 stimuli were presented once over four test sessions. During a session, the same four to seven stimuli were presented to each subject, but in different random sequences. At the beginning of each test session before subjects commenced the identification task they were required to sniff the six odorants contained in labelled bottles to remind them of the characteristics of each odor. Before commencing the first test session, subjects were advised that each stimulus could contain one or up to six of the odorants. With each stimulus presentation subjects were provided with a scoresheet which indicated the six common names of the odorants. The subjects had 50 s to sample a stimulus and to indicate which odors were present. At the end of each trial subjects were asked to indicate whether they thought there was an odor(s) present that they had not identified due to the time available or had been unable to propose an appropriate description. Subjects were then told which odors had been present. The results indicated that lack of time for identification occurred on only 7.6% of trials. In the calculation of results this outcome was considered to be a false alarm when all the odors present had been identified, and was counted as an odor selected when assessing how many odors a subject indicated were present, regardless of the correctness of identifications. Between successive trials there was a 1-min interval, the beginning and end of which was signified by the sounding of a buzzer. Results. The percentage of correct odor identifications recorded when one or more odorants were presented is displayed in two ways in Fig. 1 and more detail is given in Table 3. Function A shows the percentage of times that the correct odor(s) and no other was selected. Function B represents a more liberal criterion of correctness and shows the percentage of times that the correct odor(s) was selected but others were also (incorrectly) selected. Figure 1 clearly shows that regardless of the criterion for correctness, both functions indicate that there is a dramatic reduction in the correct identification of odorants when the stimuli are mixtures. For example, the figure shows that percent correct judgements fell from 57.6 and 95.4% for single odorants for functions A and B, respectively, to 0% for both functions with mixtures containing four components. Chi-square analysis of correct and incorrect judgements confirmed there was a significant difference in judgements of the identity of single and m ulticomponent stimuli [function A: Xz(3) = 60.9, p < 0.001; function B: X2(3) = 138.9, p < 0.001)]. Using a linear contrast (18) to partition these chi-square values showed that for both functions A and B the proportions of correct judgements decreased linearly (p < 0.001) as the number of components in a mixture increased. In these analyses, because of the low number of correct judgements with four-, five- and six-component mixtures, the data from these stimuli were combined. The results, therefore, show that humans have great difficulty in identifying more than three components in a mixture and are in agreement with those reported (10,11). The data were also analyzed to determine
Numberof Odors Presented 1
2
3
4
5
6
4.5 95.4
0 42.4 57.6
0 13.6 56.8 29.5
0 6 48.5 45.5 0
0 12.1 24.2 45.4 9.1 9.1
0 0 31.8 40.9 27.3 0 0
whether some odors were more readily identified than others when the odorants were presented singly or in mixtures. A chisquare analysis of these data showed that no odor was more readily identified than another when it was presented as a single odor or in mixtures, except with the six-component mixture, x2(5) = 16.5, p < 0.01, where the scores for (+)-limonene (orange) (27.3%) and eugenol (cloves) (31.8%) were significantly lower than, for example, the score for (-)-carvone (spearmint) (77.3%) (Table 4). Interestingly, the cut grass odor ofcis-3-hexenol, which had a significantlyhigher intensity than the other odorants when sniffed alone, was not the most readily identified overall, indicating that the relatively small intensity differences between odorants did not have a significant effect on the identification of odors. A further assessment of the data was conducted to determine whether subjects found mixtures with increasing numbers of components to become more complex. In other words, regardless of whether their judgements were correct or incorrect, did subjects report that more odorants were present as the number of components in stimuli increased. The relevant data are presented 100-
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2
3
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NO. OF COMPONENTS IN STIMULUS
FIG. 1. Percentage of judgements correctly identifying the components of stimuli consistingof one to six odorants. Functions A and A 1 indicate the percentage of times the correct odor(s) and no other was selected with the multiidentification and selective attention tasks, respectively. Functions B and B, are the corresponding functions for the two tasks which show the percentage of times that the correct odor(s) was selected but others were also (incorrectly)selected.
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I~AING AND ( i I . E M A R E (
TABLE 4 P E R C E N T I D E N T I F I C A T I O N O F I N D I V I D U A L O D O R S IN S T I M U L I
N u m b e r of Components Odorants
I
2
3
4
5
6
Almond Cloves Grass Fruity Orange Spearmint
100 100 100 90.9 81.8 100
81.8 77.3 95.5 81.8 81.8 77.3
81.8 59.1 81.8 68.2 59.1 81.8
54.4 68.2 63.6 54.5 68.2 50.0
54.5 59.1 60.6 59.1 40.9 57.6
63.6 31.8 54.5 40.9 27.3 77.3
in Fig. 2 and show that one component is the most c o m m o n number chosen with single odor stimuli, that two is the most c o m m o n for binary mixtures and three-component stimuli, and three is the most c o m m o n number chosen with four-, five-, and six-component mixtures. These results are very similar to those reported earlier (10,11), and as before, indicate that with mixtures not only do humans have difficulty in identifying more than three components, they also have difficulty in perceiving more than this number.
Selective Attention Task Test procedure. As in the procedure for the multiidentification task, four to seven stimuli were presented at each session, and all 24 stimuli were presented over four sessions. At the commencement of each set of four sessions each subject was randomly allotted a particular odorant as the target to be sought in each test stimulus over the four sessions. O n completing the four sessions, a subject would be allotted a different target odorant and the next set of sessions commenced. As before, the four to seven test stimuli at each session were sampled in different random orders by different subjects. Testing proceeded in this manner over 6 weeks until each of the six odorants had been the target for each subject. The procedure followed at each session was similar to that in the procedure for the multiidentification task, with subjects being given one labelled bottle containing their target odorant to sniff before c o m m e n c i n g the first trial, a 50 s sampling period was allowed, and there was a l-min intertrial interval. Results. To directly compare the results from both tasks, the data collected over 6 weeks during the selective attention task for each target odorant were combined for each stimulus. For example, with the mixture ABED, the data collected when A, B, C, and D were the target odorants were combined and analyzed as for the multiidentification task. Accordingly, the combined data are plotted in Fig. 1, A~ representing the function where subjects chose only the correct odorant(s) and no others and function B~ shows the percentage of judgements where subjects chose the correct odorant(s) but chose others incorrectly. An analysis of variance with subjects, odorants, and tasks as the factors, indicated that there was no difference between functions A and A~, but that functions B and B~ were different with a significantly better performance being recorded using the selective attention task. A further analysis of functions A and A~ was conducted to determine if there was a difference in the rates at which the two functions changed in level as the number of stimulus components increased. First, an arc sine transformation of the proportions of correct and incorrect judgements was made to stabilize the variance. A model to compare functions A and A~ was used which fitted linear and quadratic trends as well as
the interaction between task type and the linear and quadratic trends. The analysis showed that there was no difference between the two tasks (main effect, p -: 0.75): there was a significant overall linear trend (p < 0.01), indicating that as the number of constituents in a mixture increased the number of correct judgements decreased; the task • linear interaction (p :: 0.30) and task × quadratic interaction (p ~ 0.85) were nol significant. Thus, there was no significant differences between functions A and A~ in trend or overall scores. In practical terms, whichever criterion of correctness is adopted, the advantage gained by employing the selective attention task is small, and clearly had little effect on the outcome when the more rigorous criterion of correctness was applied. Nevertheless, function A1 indicates that at least a small number of subjects (10%) were able to identify all the components in five- and six-component mixtures, whilst function Bj shows a slightly better outcome with tbur-, five-, and six-component mixtures. Since the probability of correctly choosing combinations of six odors twice is approximately 5~ (p = 0.047), the 10% level of correct judgements obtained with the selective attention task is just above the chance level, suggesting the identification of six odors was not a chance event. Despite this finding, in no instance did any of the subjects who correctly identified all the odorants in four-, five-, or six-component mixtures achieve this on more than one occasion, indicating that identifying more than three to four odors in a mixture is a difficult task. The data were also analyzed in terms of hits [correct odor(s) identified] and false alarms [incorrect odor(s) selected] to more closely compare the performance of subjects in both tasks. A three-way analysis of variance of hits, with odors, tasks, and numbers of components as the factors, after an arc sine transformation of the data, confirmed the earlier result that the number of components correctly identified decreased significantly with increasing numbers of components in a stimulus, G~o~(5, 25) = 43.1 (Fig. 3). The analysis also indicated that there was an odor-task interaction, [o.o~(5, 25) = 3.86. A least significant difference test revealed that the odor-task interaction was primarily due to a significant difference between the identification levels for two of
60-
50-
40o~
z O ft. ,~, 30.
20.
10.
0. 1
2 3 4 5 NO. OF COMPONENTS IN STIMULUS
(~
FIG. 2. Relationship between the percentage of responses (correct and incorrect) that indicated a specific number of components were present (number at the top of each bar) when stimuli consisting of between one and six components were presented (abscissa). Subjects used a multiidentification task when evaluating each stimulus.
ODOR MIXTURE PERCEPTION
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I00-
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90-
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70:
~oi
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6o!
so! ~5o.
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2 3 4 5 NO. OF COMPONENTSIN STIMULUS
i
6
0Almond
Cloves
Grass
Fruity
Orange Spearmint
FIG. 3. Percentageof hits (correct selections)recorded with stimuli consistingof one to six components using the multiidentification(dark bars) and selective attention (light bars) tasks.
FIG. 4. Percentage of hits recorded for individual odorants with stimuli consistingof one to six components using multiidentification(dark bars) and selective attention (light bars) tasks.
the six odors, namely the fruity odors of ethyl n-butyrate and (+)-limonene. For these odors, improved performances were obtained with the selective attention task (Fig. 4). The improved performance appears to have had its genesis in the confusion subjects encountered when discriminating and identifying these two odorants. For example, when both were present in the same stimulus the percentage of times both were identified was different for the two tasks, correct scores of 25.7 and 56. 1% being recorded for the multiidentification and selective attention tasks, respectively. This latter result contrasts with the correct scores recorded when only one of these two odorants was present in a stimulus where the respective scores were 58.8 and 63.0%. Thus, the selective attention procedure assisted identification when both odorants were present in the same stimulus but produced little improvement when only one was present. Furthermore, no such finding occurred with any of the other odorants, suggesting that where there is some general similarity between two odorants the selective attention procedure could be the method of choice; otherwise, little is gained with this procedure, or at least the variant used here. A similar analysis of false alarms revealed that the false alarm rate recorded with the selective attention procedure was nearly twice that found with the multiidentification task, ff0t(1, 15) = 43.03 (Fig. 5) with each of the stimuli. The false alarm rate was calculated from the ratio
As regards stimulus complexity, which was defined above as the number of odors selected regardless of whether the choice was correct or incorrect, comparison of data in Figs. 2 and 6 indicate that when subjects used the selective attention task they tended to select more odorants than with the multiidentification task. Four components, for example, was the most common number chosen using the selective attention task when five- or six-component mixtures were presented, compared to three selected when the other procedure was used. The higher number of components chosen with the selective attention task appears to be due to the higher number of false alarms recorded with this procedure rather than to any other factor. DISCUSSION The data from both test methods indicate that humans have very great difficulty in identifying more than three components
43.
m
---2.
Number of Times a Component Was Incorrectly Selected Number of Possible False Alarms The maximum number of possible false alarms with a binary mixture, for example, was four. The analysis also indicated that although there was no odor-test interaction, the false alarm rate increased significantly with both tasks as the number of components increased, F.ol(3, 15) = 9.79. In the case of the multiidentification task, it appears from Fig. 5 that little change in the false alarm rate occurs with stimuli containing more than three components. In contrast, Fig. 5 shows that there was a steady rise in false alarms when the selective attention task was employed that did not appear to have reached an asymptote with five component mixtures. The significantly higher false alarm rate with the latter test is clearly a negative aspect of this procedure.
<, < uJ
1
2
3
NO. OF COMPONENTS
4
5
IN STIMULUS
FIG. 5. Relationship between the false alarm rate and number of components in stimuli consisting of one to six components recorded during the multiidentification (dark bars) and selective attention (light bars) tasks.
1052
i \ l N ( i ANI; ~,,I.t..MARt.t
5° l
2
45 40. 35.
30. O ,,~ 25. n20. 15. 10. 5. O. 1
2
3 4 5 NO. OF COMPONENTS IN STIMULUS
6
FIG. 6. Relationship between the percentage of responses (correct and incorrect) that indicated a specific number of components were present (number at the top of each bar) when stimuli consistingof between one and six components were presented (abscissa). Subjects used a selective attention task when evaluating each stimulus.
in a mixture and support the earlier findings from this laboratory (10,11). The data also indicate that olfactory adaptation, which was thought to be limiting the ability of subjects in the previous studies to perceive more than three components, does not appear to be a critical factor, because little improvement in performance was gained by using the selective attention procedure. This conclusion is tempered with the comment that the extent of adaptation during each trial with either method was not determined. It was not possible for technical reasons, for example, to count the number of sniffs employed by each subject, and sampling times were not measured. However, the shorter times spent sampling with the selective attention procedure were obvious to the experimenters and should have reduced adaptation within and across trials and aided identification. Furthermore, the limited capacity does not appear to be due to the similarity of the odors or unfamiliarity of the odors to subjects, since in addition to the stimuli being common dissimilar odors regularly encountered during daily life, all subjects were trained for four sessions to discriminate and identify each odor prior to commencing the multiidentification task, and all subjects had completed eight sessions of familiarization and testing prior to commencing the selective attention task where at the beginning of each session they refamiliarized themselves with the target odorant. The result obtained with the selective attention task is contrary to that found in some studies with the other senses where improved performance was the usual outcome. For example, use of a selective attention task in studies of the neural processing of shapes (5,8), color, and velocity (5) resulted in improvement of both accuracy and reduced reaction time. An unexpected outcome of the use of the selective attention task in the present study, however, was the significantly higher number of false alarms recorded, indicating a shift in the response criterion of individuals which, as might be expected, produced a rather more flattering level of correct identifications. Thus, the higher number of correct judgements with mixtures of four-, five-, and six-components with the latter task can, in part, be attributed to the shift in response criterion which is also reflected in the tendency of subjects to select more odors (Fig. 6) than with the multiidentification task (Fig. 2). The reason for the shift in response criterion is not apparent from the data.
l h e results of the present stud3, theretbrc, support the vlev~ that the limited capacity of humans to identify components in mixtures is not an artefact of the methodology used, whilst other studies in this laboratory (11) with a wide variety of odorants and mixtures containing up to eight components have indicated that the type of odorants used in this study was not a limiting factor. The inability of humans to identify more than about three components in mixtures and the rapid fall off in performance with two- and three-component stimuli suggest that physiological/biochemical events in the olfactory system rather than cognitive factors are the major influences on the processing oi" information from odor mixtures. These events appear to produce at least two outcomes; odor suppression, where one odor reduces the perceived intensity of another (13), and blending, where two or more odorants fuse to form an odor that generally does not resemble the component odors, e.g., chocolate. In the latter case. the complex aroma is viewed as a single entity, i.e., holistically. Little is known about the mechanisms of odor suppression or blending, but from physiological studies, suppression has been shown to occur at the peripheral odor receptor cells (1) most likely through competition for sites on cells or through allosteric effects, and in the olfactory, bulb (2). Odor suppression appears to have been the major cause for loss of information about mixture components in the present study since there were very few instances (7.6%) where subjects indicated they had insut~cient time to complete the identification task and believed there was at least one component present they had not identified. Whether this component was indeed a blend of several components or one of the test odorants is not known, but the small number of instances this occurred does indicate that failure to identify an odor was primarily due to the odor being suppressed below its identification threshold. Recently it was suggested that the limited capacity to identify odors in mixtures is due to both spatial and temporal neural filtering effects (11 ), and that spatial filtering may occur at the periphery through competition for receptor sites by mixture constituents, resulting in the normal input of many of the individual constituents being suppressed. The normal pattern of receptor cells activated by an individual constituent of a mixture, for example, could be dramatically altered and reduced through competition and inhibition, with perhaps relatively few cells being activated by a particular constituent. The resuiting pattern of activated receptor cells following stimulation by an aroma such as coffee, therefore, is likely to be very different to the patterns of activation produced when each constituent is presented individually. Further alteration of the resulting pattern is possible through lateral inhibition in the glomerular layer or between mitral cells in the olfactory bulb (16,19) and perhaps by other unknown mechanisms at other olfactory centers before the pattern of activation is stored in memory. Identification of an odor from the spatial pattern in memory, however, is likely to have a second dimension involving a temporal component. Electrophysiological studies, for example, indicate that the time taken to activate a receptor cell differs between odors by as much as several hundred milliseconds (6). Fast odorants would, therefore, be in a favored position to occupy receptor sites or simply act as antagonists and block activation of a cell by a slow odorant. Evidence for temporal filtering has been obtained recently in psychophysical studies in this laboratory (Laing, Eddy, and Francis, in preparation) where differences in latencies between two odor-
ODOR MIXTURE PERCEPTION
1053
ants under conditions where strong suppression occurs, is of the order of several h u n d r e d milliseconds in favor of the suppressor. In summary, the present study represents yet another example that demonstrates humans have great difficulty in identifying more than three components in an odor mixture. Together with a recent study of temporal processing in this laboratory, the results reported here suggest the limiting factors are physiological
and biochemical in origin rather than to cognitive or methodological factors. ACKNOWLEDGEMENTS The authors extend their thanks to Mr. D. J. Best and Ms. L. Stephens of the CSIRO Institute of Animal Production and Processing Biometrics Unit for assistance with the statistical analyses and to the panellists for their excellent cooperation.
REFERENCES 1. Ache, B. W. Central and peripheral bases for mixture suppression in olfaction: A crustacean model. In: Laing, D. G.; Cain, W. S.; McBride, R. L.; Ache, B. W., eds. Perception of complex smells and tastes. Sydney: Academic Press; 1989:10 l - l 14. 2. Bell, G. A.; Laing, D. G.; Panhuber, H. Odor mixture suppression: Evidence for a peripheral mechanism in human and rat. Brain Res. 426:8-18; 1987. 3. Berglund, B.; Berglund, U.; Lindvall, T. Olfactory self- and crossadaptation: Effects of time of adaptation on perceived odor intensity. Sensory Proc. 2:191-197; 1978. 4. Cain, W. S.; Drexler, M. Scope and evaluation of odor counteraction and masking. Ann. NY Acad. Sci. 237:427--439; 1974. 5. Corbetta, M.; Miezin, F.; Dobmeyer, S.; Shulman, S.; Petersen, S. Attentional modulation of neural processing of shape, color and velocity in humans. Science 248:1556-1559; 1990. 6. Getchell, T. V.; Margolis, F. L.; Getchell, M. L. Perireceptor and receptor events in vertebrate olfaction. Prog. Neurobiol. 23:317345; 1984. 7. Kuznicki, J. T.; Turner, L. S. Reaction time in the perceptual processing of taste quality. Chem. Senses 11:183-201; 1986. 8. La Berge, D.; Brown, V. Theory of attentional operations in shape identification. Psychol. Rev. 96:104-124; 1989. 9. Laing, D. G. Identification of single dissimilar odors is achieved by humans with a single sniff. Physiol. Behav. 37:163-170; 1986. 10. Laing, D. G.; Francis, G. W. The capacity of humans to identify odors in mixtures. Physiol. Behav. 46:809-814; 1989.
11. Laing, D. G.; Livermore, B. A. Perceptual analysis of complex chemical signals by humans. In: Doty, R. L.; Muller-Schwartze, D., eds. Chemical signals in vertebrates VI. New York: Plenum Press; 1992:587-593. 12. Laing, D. G.; Panhuber, H.; Baxter, R. I. Olfactory properties of amines and butanol. Chem. Senses Flavor 3:149-166; 1978. 13. Laing, D. G.; Panhuber, H.; Willcox, M. E.; Pittman, E. A. Quality and intensity of binary odor mixtures. Physiol. Behav. 33:309-319; 1984. 14. Lawless, H. T. An olfactory analogy to release from mixture suppression in taste. Bull. Psychon. Soc. 25:266-268; 1987. 15. Miller, G. A. The magical number seven, plus or minus two: Some limits on our capacity for processing information. Psychol. Rev. 63: 81-97; 1956. 16. Pinching, A. J.; Powell, T. P. S. The neuropil of the glomeruli of the olfactory bulb. J. Cell Sci. 9:347-377; 1977. 17. Rabin, M. D.; Cain, W. S. Attention and learning in the perception of odor mixtures. In: Laing, D. G.; Cain, W. S.; McBride, R. L.; Ache, B. W. Perception of complex smells and tastes. Sydney: Academic Press; 1989:173-178. 18. Snedecor, G. W.; Cochran, W. G. Statistical methods. Ames, IA: Iowa State University Press; 1989:208. 19. White, E. L. Synaptic organization of the mammalian olfactory glomerulus: New findings including an intraspecific variation. Brain Res. 60:299-313; 1979.
0031-9384/92 $5.00 + .00 Copyright© 1992PergamonPressLtd.
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Selective Attention and the Perceptual Analysis of Odor Mixtures D. G. L A I N G .1 A N D A. G L E M A R E C t
*Faculty of Science and Technology, University of Western Sydney, Bourke Street, Richmond N S W 2753, Australia and i'Ecole Nationale Supdrieure De Biologic, Appliqu£e A La Nutrition Et A L'Alimentation, Centre Universitaire Montmuzard, 21100 Dijon, France R e c e i v e d 16 S e p t e m b e r 1991 LAING, D. G. AND A. GLEMAREC. Selectiveattention and the perceptualanalysisof odormixtures. PHYSIOL BEHAV 52(6) 1047-1053, 1992.--Two psychophysical methods were used to investigate the capacity of humans to identify the constituents of odor mixtures consisting of up to six components. With one method subjects were required to identify all the components present in each stimulus; with the other, a selective attention procedure was used where subjects had to identify only one component at each trial. Little difference was found between the levels of identification obtained with both methods, reinforcing the finding that humans have great difficulty in identifying more than three components in an odor mixture and indicating that it is unlikely that olfactory adaptation influenced the identification process. Odor mixtures
Human olfaction
Olfactory adaptation
OLFACTORY stimuli in our environment usually consist of many different odorous molecules and often appear to be perceived as a single odor, e.g., chocolate, suggesting that odors may be combined perceptually to form a homogeneous stimulus. Nevertheless, anecdotal information, particularly as regards the capabilities of perfumers and flavorists, suggests that humans are capable of analyzing odor mixtures and identifying significant numbers of the components. Several years ago, for example, one of the authors (D.G.L.) on casual questioning of such experts regarding their respective analytical abilities, obtained answers varying between 5 and 30 components. However, few studies have investigated the capacity of humans to identify components in odor mixtures, and of these, most have been concerned with binary mixtures. Such studies have shown that depending on the quality and intensity of the components, one or both components are perceived (4,13). In addition, it has been shown that the familiarity and pleasantness of odorants can affect the ability to detect components of binary mixtures (17). Currently, there is no method for predicting which odors will be perceived in a mixture, or for predicting the overall aroma of a mixture, without prior experience with the set of odorants used. This situation is likely to continue unless an understanding of how mixtures are perceived is obtained. Accordingly, as part of a program aimed at resolving this impasse, a series of experiments has been initiated in this laboratory to determine: I. the capacity of humans to analyze and identify odors in mixtures, and 2. the physiological factors that determine the capacity. Requests for reprints should be addressed to D. G. Lalng.
1047
Selective attention
Neural capacity
With this information, it may then be possible to provide an objective basis for predicting the aroma of a mixture. Recently, using untrained subjects, Laing and Francis (10) reported that it was very difficult for humans to identify more than three components in mixtures containing up to five dissimilar but familiar odors, and Laing and Livermore (1 l) reported a similar outcome with trained subjects and a group of perfumers and flavorists. A question arising from these studies concerns the origin of the apparent limited capacity of the olfactory system to process information from mixtures. The limitation may arise from physiological constraints related to the restricted capacity of the system to process simultaneously perceived information on odor identity, or it may be an artefact of the psychophysical method employed, which was the same in the earlier studies. As regards physiological constraints, limited neural processing capacity is known to limit identification of tones to about seven (15), and it was reported to be increasingly difficult to identify different taste stimuli when the perceived intensity of tastants was adjusted to produce similar reaction (identification) times (7). A likely interfering factor arising from the methodology used in the earlier studies by Laing and co-workers was olfactory adaptation arising from the considerable time subjects spent sampling each stimulus. In those studies, the majority of subjects spent most of each 50 s trial sniffing the stimulus, and from the author's experience with this test paradigm, considerable adaptation during a trial would have occurred. In support of this observation, self-adaptation as a result of a few inhalations has
1048
I AIN(i AND (.It.EMARt:('
been reported to substantially reduce odor intensity (3). It is unlikely that self or cross adaptation would have occurred across trials to any great extent since the intertrial interval was 45 s, a substantial duration. Unfortunately, no studies of adaptation to components in mixtures have been reported that might indicate whether adaptation occurs equally or at different rates for different components, or whether the rate of adaptation to highand low-intensity components in mixtures is different. To further complicate the outcome it is also possible that if selective adaptation to a mixture component had occurred, this could have resulted in the release of other components from suppression during a trial and thereby increased detection of the released component. Since the phenomenon of suppression release (14) has been demonstrated to occur across trials with intertrial intervals of up to 60 s duration (Laing, unpublished data), it is possible that the lengthy sampling times observed earlier by Laing and co-workers could have resulted in suppression release of a component(s) in a subsequent trial. This latter action, however, would favor rather than hinder identification of components. The present study investigates the possible influence of olfactory adaptation on the capacity of humans to identify mixture components. It compares the capacity of humans in a task which minimizes adaptation, namely a selective attention task where the presence or absence of only one target component need be indicated, with the capacity recorded using the multiidentification task described earlier (10,11), when all the components of each stimulus had to be identified during each trial. The aim in focussing a subject's attention on only one component during a trial was to reduce the time spent sniffing the stimulus and, hence, reduce the effects of adaptation on identification. It has been demonstrated, for example, that subjects only require a sniff of about 0.4 s duration to identify an odorant of moderate intensity (9). The study was conducted in two parts. During the first part, subjects were asked to identify all the components in each stimulus, and during the second part, the selective attention procedure was used. In brief, the study employs two very different test procedures, one aimed at minimizing adaptation, to investigate the capacity of humans to identify the components of stimuli consisting of up to six odorants. METHOD
General Eleven subjects, four males and seven females, aged between 19 and 48 (mean, 27.5 years), participated in the study. All were staff of this Institute and most had previously participated in sensory studies. The six odorants used during this study, their concentratiqns, mean perceived intensity, and the words used by subjects to identify each odor are given in Table 1. All odorants were of the highest purity available from Fluka AG, except cis-3-hexenol which was donated by Dragoco (Australia) and ethyl-n-butyrate from BDH Chemicals (Australia). This group of odorants differs from the group used previously in this laboratory (10,11) with the inclusion of ethyl-n-butyrate as a better representative of a fruity odorant than ethyl-n-caprylate, cis-3-hexenol as a more discrete odor than c~-pinene, i.e., was easier to discriminate from the other odors listed than apinene, and acetic acid was not used because it was often identified by subjects previously by its sharp note, and identification may have been primarily through its trigeminal action. All the odorants were chosen because they are common and dissimilar in character and were of similar perceived intensity.
IABLE f OD(.)RA NI S Chemical
Common Name
('*
Mean lntensll? (ram)
( )-Carvone Benzaldehyde Cis-3-hexenol (+)-Limonene Eugenol Ethyl-n-butyrate
Spearmint Almond Cut grass Orange Cloves Fruity
1.04 0.16 0.22 2.01 1.16 0.07
¢,(i.4 60.6 68.2 53.5 54.6 59.3
* Concentration expressed as the dilution of saturated odor vapor at 20°C (× 10-2).
The latter was established with the present subjects using a graphic rating scale (130 ram) which had the words "no odor" and "extremely strong" at the ends. Odorants were presented singly and in different random sequences to different subjects in duplicate. Several sessions were required to adjust the perceived intensities to a moderate level. Subjects indicated their estimate of intensity by placing a mark on the line scale. An analysis of variance and least significant difference test showed that only cis-3-hexenol was significantly stronger than some of the other odorants (Table 1), but as shown in Fig. 4, this did not appear to enhance its identification. An air-dilution olfactometer with multiple odor channels (12) was used to deliver a single concentration of each odorant or mixtures containing up to six odorants to subjects. This instrument was almost identical to that previously described for the production of binary mixtures (13), except that it had been modified slightly to allow mixing of up to six odorants. As with the previous design, large mixing chambers were used to ensure complete mixing of odors before delivery to Perspex sniffing outlets (12). Before commencing the identification tests, subjects were trained to use an appropriate label for each odorant presented singly over four sessions. During the first session subjects provided their own label for each odor and then were asked to choose between their own label and the common name for each odor. Subjects preferred the common name, and this was used in the following three training sessions. At each of these sessions the six odorants were presented twice in different random sequences to each of the subjects, and the latter were required to identify each odor. Corrective feedback was given after each trial. During the fourth session all subjects correctly labelled the six odorants. This training procedure was similar to that used earlier (11) which allowed subjects to obtain almost perfect identification scores during test sessions when the stimulus was a single odorant. Because of technical limitations with the olfactometer it was not possible to present subjects with a totally randomized set of stimuli at each test session or the same number of stimuli at each session. Therefore, to compare the two test methodologies a set of mixtures was selected so that each odor appeared twice in each type of mixture, i.e., in one, two, t h r e e . . , six-component mixtures. The set of 24 stimuli which included the six single odorants are listed in Table 2.
Multi-Identification Task Test procedure. The test method was similar to that reported (10, l 1) to allow comparison of the results with those from the latter studies and with the selective attention task (see below).
ODOR MIXTURE PERCEPTION
1049
TABLE 2 TEST STIMULI A B C D E F
AB AD BE CE CF DF
ACD AEF BDF BCE
ABDE ACEF BCDF
TABLE 3 PERCENTIDENTIFICATIONOF MIXTURECOMPONENTS ABCDE ABCDF ABCEF
ABCDEF ABCDEF
Numberof Odors Identified 0 1 2 3 4 5 6
A: (-)-carvone; B: benzaldehyde;C: cis-3-hexenol;D: (+)-limonene; E: eugenol; F: ethyl n-butyrate. All 24 stimuli were presented once over four test sessions. During a session, the same four to seven stimuli were presented to each subject, but in different random sequences. At the beginning of each test session before subjects commenced the identification task they were required to sniff the six odorants contained in labelled bottles to remind them of the characteristics of each odor. Before commencing the first test session, subjects were advised that each stimulus could contain one or up to six of the odorants. With each stimulus presentation subjects were provided with a scoresheet which indicated the six common names of the odorants. The subjects had 50 s to sample a stimulus and to indicate which odors were present. At the end of each trial subjects were asked to indicate whether they thought there was an odor(s) present that they had not identified due to the time available or had been unable to propose an appropriate description. Subjects were then told which odors had been present. The results indicated that lack of time for identification occurred on only 7.6% of trials. In the calculation of results this outcome was considered to be a false alarm when all the odors present had been identified, and was counted as an odor selected when assessing how many odors a subject indicated were present, regardless of the correctness of identifications. Between successive trials there was a 1-min interval, the beginning and end of which was signified by the sounding of a buzzer. Results. The percentage of correct odor identifications recorded when one or more odorants were presented is displayed in two ways in Fig. 1 and more detail is given in Table 3. Function A shows the percentage of times that the correct odor(s) and no other was selected. Function B represents a more liberal criterion of correctness and shows the percentage of times that the correct odor(s) was selected but others were also (incorrectly) selected. Figure 1 clearly shows that regardless of the criterion for correctness, both functions indicate that there is a dramatic reduction in the correct identification of odorants when the stimuli are mixtures. For example, the figure shows that percent correct judgements fell from 57.6 and 95.4% for single odorants for functions A and B, respectively, to 0% for both functions with mixtures containing four components. Chi-square analysis of correct and incorrect judgements confirmed there was a significant difference in judgements of the identity of single and m ulticomponent stimuli [function A: Xz(3) = 60.9, p < 0.001; function B: X2(3) = 138.9, p < 0.001)]. Using a linear contrast (18) to partition these chi-square values showed that for both functions A and B the proportions of correct judgements decreased linearly (p < 0.001) as the number of components in a mixture increased. In these analyses, because of the low number of correct judgements with four-, five- and six-component mixtures, the data from these stimuli were combined. The results, therefore, show that humans have great difficulty in identifying more than three components in a mixture and are in agreement with those reported (10,11). The data were also analyzed to determine
Numberof Odors Presented 1
2
3
4
5
6
4.5 95.4
0 42.4 57.6
0 13.6 56.8 29.5
0 6 48.5 45.5 0
0 12.1 24.2 45.4 9.1 9.1
0 0 31.8 40.9 27.3 0 0
whether some odors were more readily identified than others when the odorants were presented singly or in mixtures. A chisquare analysis of these data showed that no odor was more readily identified than another when it was presented as a single odor or in mixtures, except with the six-component mixture, x2(5) = 16.5, p < 0.01, where the scores for (+)-limonene (orange) (27.3%) and eugenol (cloves) (31.8%) were significantly lower than, for example, the score for (-)-carvone (spearmint) (77.3%) (Table 4). Interestingly, the cut grass odor ofcis-3-hexenol, which had a significantlyhigher intensity than the other odorants when sniffed alone, was not the most readily identified overall, indicating that the relatively small intensity differences between odorants did not have a significant effect on the identification of odors. A further assessment of the data was conducted to determine whether subjects found mixtures with increasing numbers of components to become more complex. In other words, regardless of whether their judgements were correct or incorrect, did subjects report that more odorants were present as the number of components in stimuli increased. The relevant data are presented 100-
\
90.
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70-
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O
\
4oi
AI~"~
\
~ ~\
~2
O: 1
2
3
4
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6
NO. OF COMPONENTS IN STIMULUS
FIG. 1. Percentage of judgements correctly identifying the components of stimuli consistingof one to six odorants. Functions A and A 1 indicate the percentage of times the correct odor(s) and no other was selected with the multiidentification and selective attention tasks, respectively. Functions B and B, are the corresponding functions for the two tasks which show the percentage of times that the correct odor(s) was selected but others were also (incorrectly)selected.
1050
I~AING AND ( i I . E M A R E (
TABLE 4 P E R C E N T I D E N T I F I C A T I O N O F I N D I V I D U A L O D O R S IN S T I M U L I
N u m b e r of Components Odorants
I
2
3
4
5
6
Almond Cloves Grass Fruity Orange Spearmint
100 100 100 90.9 81.8 100
81.8 77.3 95.5 81.8 81.8 77.3
81.8 59.1 81.8 68.2 59.1 81.8
54.4 68.2 63.6 54.5 68.2 50.0
54.5 59.1 60.6 59.1 40.9 57.6
63.6 31.8 54.5 40.9 27.3 77.3
in Fig. 2 and show that one component is the most c o m m o n number chosen with single odor stimuli, that two is the most c o m m o n for binary mixtures and three-component stimuli, and three is the most c o m m o n number chosen with four-, five-, and six-component mixtures. These results are very similar to those reported earlier (10,11), and as before, indicate that with mixtures not only do humans have difficulty in identifying more than three components, they also have difficulty in perceiving more than this number.
Selective Attention Task Test procedure. As in the procedure for the multiidentification task, four to seven stimuli were presented at each session, and all 24 stimuli were presented over four sessions. At the commencement of each set of four sessions each subject was randomly allotted a particular odorant as the target to be sought in each test stimulus over the four sessions. O n completing the four sessions, a subject would be allotted a different target odorant and the next set of sessions commenced. As before, the four to seven test stimuli at each session were sampled in different random orders by different subjects. Testing proceeded in this manner over 6 weeks until each of the six odorants had been the target for each subject. The procedure followed at each session was similar to that in the procedure for the multiidentification task, with subjects being given one labelled bottle containing their target odorant to sniff before c o m m e n c i n g the first trial, a 50 s sampling period was allowed, and there was a l-min intertrial interval. Results. To directly compare the results from both tasks, the data collected over 6 weeks during the selective attention task for each target odorant were combined for each stimulus. For example, with the mixture ABED, the data collected when A, B, C, and D were the target odorants were combined and analyzed as for the multiidentification task. Accordingly, the combined data are plotted in Fig. 1, A~ representing the function where subjects chose only the correct odorant(s) and no others and function B~ shows the percentage of judgements where subjects chose the correct odorant(s) but chose others incorrectly. An analysis of variance with subjects, odorants, and tasks as the factors, indicated that there was no difference between functions A and A~, but that functions B and B~ were different with a significantly better performance being recorded using the selective attention task. A further analysis of functions A and A~ was conducted to determine if there was a difference in the rates at which the two functions changed in level as the number of stimulus components increased. First, an arc sine transformation of the proportions of correct and incorrect judgements was made to stabilize the variance. A model to compare functions A and A~ was used which fitted linear and quadratic trends as well as
the interaction between task type and the linear and quadratic trends. The analysis showed that there was no difference between the two tasks (main effect, p -: 0.75): there was a significant overall linear trend (p < 0.01), indicating that as the number of constituents in a mixture increased the number of correct judgements decreased; the task • linear interaction (p :: 0.30) and task × quadratic interaction (p ~ 0.85) were nol significant. Thus, there was no significant differences between functions A and A~ in trend or overall scores. In practical terms, whichever criterion of correctness is adopted, the advantage gained by employing the selective attention task is small, and clearly had little effect on the outcome when the more rigorous criterion of correctness was applied. Nevertheless, function A1 indicates that at least a small number of subjects (10%) were able to identify all the components in five- and six-component mixtures, whilst function Bj shows a slightly better outcome with tbur-, five-, and six-component mixtures. Since the probability of correctly choosing combinations of six odors twice is approximately 5~ (p = 0.047), the 10% level of correct judgements obtained with the selective attention task is just above the chance level, suggesting the identification of six odors was not a chance event. Despite this finding, in no instance did any of the subjects who correctly identified all the odorants in four-, five-, or six-component mixtures achieve this on more than one occasion, indicating that identifying more than three to four odors in a mixture is a difficult task. The data were also analyzed in terms of hits [correct odor(s) identified] and false alarms [incorrect odor(s) selected] to more closely compare the performance of subjects in both tasks. A three-way analysis of variance of hits, with odors, tasks, and numbers of components as the factors, after an arc sine transformation of the data, confirmed the earlier result that the number of components correctly identified decreased significantly with increasing numbers of components in a stimulus, G~o~(5, 25) = 43.1 (Fig. 3). The analysis also indicated that there was an odor-task interaction, [o.o~(5, 25) = 3.86. A least significant difference test revealed that the odor-task interaction was primarily due to a significant difference between the identification levels for two of
60-
50-
40o~
z O ft. ,~, 30.
20.
10.
0. 1
2 3 4 5 NO. OF COMPONENTS IN STIMULUS
(~
FIG. 2. Relationship between the percentage of responses (correct and incorrect) that indicated a specific number of components were present (number at the top of each bar) when stimuli consisting of between one and six components were presented (abscissa). Subjects used a multiidentification task when evaluating each stimulus.
ODOR MIXTURE PERCEPTION
1051
I00-
90-:
90-
80-
8o~
70:
~oi
-I-
6o!
so! ~5o.
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~40-
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3020. 10.
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i
1
......
I
I
i
2 3 4 5 NO. OF COMPONENTSIN STIMULUS
i
6
0Almond
Cloves
Grass
Fruity
Orange Spearmint
FIG. 3. Percentageof hits (correct selections)recorded with stimuli consistingof one to six components using the multiidentification(dark bars) and selective attention (light bars) tasks.
FIG. 4. Percentage of hits recorded for individual odorants with stimuli consistingof one to six components using multiidentification(dark bars) and selective attention (light bars) tasks.
the six odors, namely the fruity odors of ethyl n-butyrate and (+)-limonene. For these odors, improved performances were obtained with the selective attention task (Fig. 4). The improved performance appears to have had its genesis in the confusion subjects encountered when discriminating and identifying these two odorants. For example, when both were present in the same stimulus the percentage of times both were identified was different for the two tasks, correct scores of 25.7 and 56. 1% being recorded for the multiidentification and selective attention tasks, respectively. This latter result contrasts with the correct scores recorded when only one of these two odorants was present in a stimulus where the respective scores were 58.8 and 63.0%. Thus, the selective attention procedure assisted identification when both odorants were present in the same stimulus but produced little improvement when only one was present. Furthermore, no such finding occurred with any of the other odorants, suggesting that where there is some general similarity between two odorants the selective attention procedure could be the method of choice; otherwise, little is gained with this procedure, or at least the variant used here. A similar analysis of false alarms revealed that the false alarm rate recorded with the selective attention procedure was nearly twice that found with the multiidentification task, ff0t(1, 15) = 43.03 (Fig. 5) with each of the stimuli. The false alarm rate was calculated from the ratio
As regards stimulus complexity, which was defined above as the number of odors selected regardless of whether the choice was correct or incorrect, comparison of data in Figs. 2 and 6 indicate that when subjects used the selective attention task they tended to select more odorants than with the multiidentification task. Four components, for example, was the most common number chosen using the selective attention task when five- or six-component mixtures were presented, compared to three selected when the other procedure was used. The higher number of components chosen with the selective attention task appears to be due to the higher number of false alarms recorded with this procedure rather than to any other factor. DISCUSSION The data from both test methods indicate that humans have very great difficulty in identifying more than three components
43.
m
---2.
Number of Times a Component Was Incorrectly Selected Number of Possible False Alarms The maximum number of possible false alarms with a binary mixture, for example, was four. The analysis also indicated that although there was no odor-test interaction, the false alarm rate increased significantly with both tasks as the number of components increased, F.ol(3, 15) = 9.79. In the case of the multiidentification task, it appears from Fig. 5 that little change in the false alarm rate occurs with stimuli containing more than three components. In contrast, Fig. 5 shows that there was a steady rise in false alarms when the selective attention task was employed that did not appear to have reached an asymptote with five component mixtures. The significantly higher false alarm rate with the latter test is clearly a negative aspect of this procedure.
<, < uJ
1
2
3
NO. OF COMPONENTS
4
5
IN STIMULUS
FIG. 5. Relationship between the false alarm rate and number of components in stimuli consisting of one to six components recorded during the multiidentification (dark bars) and selective attention (light bars) tasks.
1052
i \ l N ( i ANI; ~,,I.t..MARt.t
5° l
2
45 40. 35.
30. O ,,~ 25. n20. 15. 10. 5. O. 1
2
3 4 5 NO. OF COMPONENTS IN STIMULUS
6
FIG. 6. Relationship between the percentage of responses (correct and incorrect) that indicated a specific number of components were present (number at the top of each bar) when stimuli consistingof between one and six components were presented (abscissa). Subjects used a selective attention task when evaluating each stimulus.
in a mixture and support the earlier findings from this laboratory (10,11). The data also indicate that olfactory adaptation, which was thought to be limiting the ability of subjects in the previous studies to perceive more than three components, does not appear to be a critical factor, because little improvement in performance was gained by using the selective attention procedure. This conclusion is tempered with the comment that the extent of adaptation during each trial with either method was not determined. It was not possible for technical reasons, for example, to count the number of sniffs employed by each subject, and sampling times were not measured. However, the shorter times spent sampling with the selective attention procedure were obvious to the experimenters and should have reduced adaptation within and across trials and aided identification. Furthermore, the limited capacity does not appear to be due to the similarity of the odors or unfamiliarity of the odors to subjects, since in addition to the stimuli being common dissimilar odors regularly encountered during daily life, all subjects were trained for four sessions to discriminate and identify each odor prior to commencing the multiidentification task, and all subjects had completed eight sessions of familiarization and testing prior to commencing the selective attention task where at the beginning of each session they refamiliarized themselves with the target odorant. The result obtained with the selective attention task is contrary to that found in some studies with the other senses where improved performance was the usual outcome. For example, use of a selective attention task in studies of the neural processing of shapes (5,8), color, and velocity (5) resulted in improvement of both accuracy and reduced reaction time. An unexpected outcome of the use of the selective attention task in the present study, however, was the significantly higher number of false alarms recorded, indicating a shift in the response criterion of individuals which, as might be expected, produced a rather more flattering level of correct identifications. Thus, the higher number of correct judgements with mixtures of four-, five-, and six-components with the latter task can, in part, be attributed to the shift in response criterion which is also reflected in the tendency of subjects to select more odors (Fig. 6) than with the multiidentification task (Fig. 2). The reason for the shift in response criterion is not apparent from the data.
l h e results of the present stud3, theretbrc, support the vlev~ that the limited capacity of humans to identify components in mixtures is not an artefact of the methodology used, whilst other studies in this laboratory (11) with a wide variety of odorants and mixtures containing up to eight components have indicated that the type of odorants used in this study was not a limiting factor. The inability of humans to identify more than about three components in mixtures and the rapid fall off in performance with two- and three-component stimuli suggest that physiological/biochemical events in the olfactory system rather than cognitive factors are the major influences on the processing oi" information from odor mixtures. These events appear to produce at least two outcomes; odor suppression, where one odor reduces the perceived intensity of another (13), and blending, where two or more odorants fuse to form an odor that generally does not resemble the component odors, e.g., chocolate. In the latter case. the complex aroma is viewed as a single entity, i.e., holistically. Little is known about the mechanisms of odor suppression or blending, but from physiological studies, suppression has been shown to occur at the peripheral odor receptor cells (1) most likely through competition for sites on cells or through allosteric effects, and in the olfactory, bulb (2). Odor suppression appears to have been the major cause for loss of information about mixture components in the present study since there were very few instances (7.6%) where subjects indicated they had insut~cient time to complete the identification task and believed there was at least one component present they had not identified. Whether this component was indeed a blend of several components or one of the test odorants is not known, but the small number of instances this occurred does indicate that failure to identify an odor was primarily due to the odor being suppressed below its identification threshold. Recently it was suggested that the limited capacity to identify odors in mixtures is due to both spatial and temporal neural filtering effects (11 ), and that spatial filtering may occur at the periphery through competition for receptor sites by mixture constituents, resulting in the normal input of many of the individual constituents being suppressed. The normal pattern of receptor cells activated by an individual constituent of a mixture, for example, could be dramatically altered and reduced through competition and inhibition, with perhaps relatively few cells being activated by a particular constituent. The resuiting pattern of activated receptor cells following stimulation by an aroma such as coffee, therefore, is likely to be very different to the patterns of activation produced when each constituent is presented individually. Further alteration of the resulting pattern is possible through lateral inhibition in the glomerular layer or between mitral cells in the olfactory bulb (16,19) and perhaps by other unknown mechanisms at other olfactory centers before the pattern of activation is stored in memory. Identification of an odor from the spatial pattern in memory, however, is likely to have a second dimension involving a temporal component. Electrophysiological studies, for example, indicate that the time taken to activate a receptor cell differs between odors by as much as several hundred milliseconds (6). Fast odorants would, therefore, be in a favored position to occupy receptor sites or simply act as antagonists and block activation of a cell by a slow odorant. Evidence for temporal filtering has been obtained recently in psychophysical studies in this laboratory (Laing, Eddy, and Francis, in preparation) where differences in latencies between two odor-
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ants under conditions where strong suppression occurs, is of the order of several h u n d r e d milliseconds in favor of the suppressor. In summary, the present study represents yet another example that demonstrates humans have great difficulty in identifying more than three components in an odor mixture. Together with a recent study of temporal processing in this laboratory, the results reported here suggest the limiting factors are physiological
and biochemical in origin rather than to cognitive or methodological factors. ACKNOWLEDGEMENTS The authors extend their thanks to Mr. D. J. Best and Ms. L. Stephens of the CSIRO Institute of Animal Production and Processing Biometrics Unit for assistance with the statistical analyses and to the panellists for their excellent cooperation.
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