an electrophysiological study of odour similarities of homologous

J. Physiol. (1966), 186, pp. 97,-109 97With 3 text-figuresPr inted in Great Britain

AN ELECTROPHYSIOLOGICAL STUDY OF ODOURSIMILARITIES OF HOMOLOGOUS SUBSTANCES

BY K. B. DOVING*From the Department of Physiology, Karolinska Institutet,

Stockholm 60, Sweden

(Received 7 February 1966)

SUMMARY

1. The activity of single bulbar units in the olfactory system of the frogwas recorded extracellularly by means of micro-electrodes. The electro-olfactogram was recorded simultaneously from the receptor epithelium.

2. The olfactory epithelium was stimulated with substances of homo-logous series of normal aliphatic alcohols, acetates and ketones.

3. The effect on a bulbar unit was classified as excitatory or inhibitory,and the chi-square values calculated with one degree of freedom for allpairs in a given series. The statistical values obtained indicated the degreeof similarity in olfactory stimulative properties between the odours.

4. The results show that the degree of similarity in olfactory propertiesis greatest among neighbouring substances and gradually decreases withincreasing separation in chain length.

5. Significant rank correlation coefficients were found between thechi-square values and the molecular weight ratios for alcohols and acetates.The results are discussed in relation to psychophysical findings obtainedwith the same substances.

INTRODUCTION

A challenging problem in olfactory research has for a long time been therelation between the chemical and physical properties of the odorouscompounds and their odour qualities. Available information on thissubject derives from two different approaches. The contributions of thechemists have mainly been restricted to description of the olfactory prop-erties of the odorous substances in terms of their similarities to variousknown chemicals or natural products (cf. Beets, 1964). The psychophysicalapproach to this problem has been concerned with developing methods usedfor describing the similarities in olfactory stimulative properties betweenodour pairs. One of the methods is based upon the phenomena of selectiveolfactory fatigue (Zwaardemaker, 1900, 1907; Backman, 1917; Le Magnen,

* Fellow of the Norwegian Research Council for Science and the Humanities.7 Physiol. i86

1948). Le Magnen (1948) showed, for example, that adaptation to any oneof the three odours, benzaldehyde, benzonitrile or safrole, affected thethreshold and quality of the others. These results indicate that the threeodours have some stimulative properties in common. This method, calledcross-adaptation, has been further developed by later authors (Cheesman& Mayne, 1953; Cheesman & Townsend, 1956; Engen, 1963; Koster, 1965)and they have worked out indexes for similarities of odour pairs. Otherpsychophysical methods have used scalar comparison of odour pairs andhave obtained similarity estimates for a large number of odours (Engen,1962; Wright & Michels, 1964; Schultz, 1964; Amoore & Venstrom, 1966).A possible electrophysiological approach to these problems would be to

determine the thresholds (sensitivities) of single olfactory receptors todifferent odours. However, the study of the activity of single olfactoryreceptors is hampered by the small dimensions of the sensory cells. Thepromising studies of Gesteland, Lettvin, Pitts & Rojas (1963) and Gesteland,Lettvin & Pitts (1965) have so far given little information about the relationbetween molecular properties and their stimulating properties. Usefulinformation may be obtained by studying the nervous activity at levelshigher than the olfactory receptors. Adrian (1953, 1956) showed that theunits in the olfactory bulb were specifically activated by various odours.Later studies have demonstrated that the activity of a large fraction of thesecondary units is inhibited during stimulation (Mancia, von Baumgarten& Green, 1962; D6ving, 1964). The classification of the effects of stimula-tion as excitatory or inhibitory allows for the data to be subjected toenumeration statistics (cf. Doving, 1965). The chi-square values calculatedfor the different odour pairs indicate whether or not there is an associationbetween the two odours. These statistical values can be interpreted as thedegree of similarity in olfactory stimulative properties, and might providea valuable complement to the psychophysical experiments.

In the present experiments the responses in the bulbar units of frog werestudied when stimulating with substances from homologous series ofaliphatic alcohols, acetates and ketones. The results showed that neigh-bouring compounds were more similar in their olfactory stimulative proper-ties than compounds which were more widely separated in chain length.

METHODSThe experiments were performed on frogs which were curarized and fastened to a cork

plate. The olfactory eminence on one side and both olfactory bulbs were uncovered, usinglocal anaesthesia. The inner and outer hard meninges were removed, the fluid above thebulbs was soaked away and the space filled with paraffin oil. A sheet of celluloid with a smallhole for the microelectrode was placed over the hole in the skull. This arrangement provedto be sufficient for eliminating the pulsation due to the heart beats. Calomel half-cellelectrodes were used for DC-recording of the electro-olfactogram (EOG). Glass micro-

K. B. DOVING98

SIMILARITIES OF HOMOLOGOUS SUBSTANCES 99

capillaries filled with 4 N-NaCl were used to pick up extracellular activity from single unitsin the olfactory bulb.

In a previous study (Doving, 1964) the odour stimuli were delivered by means of a pumpwhich gave short puffs of purified air which passed over an odorous solution in a bottle.The odorous air was presented through a pipette to the olfactory epithelium. When applyinga new odour, both the bottle and the pipette were changed. This took considerable time andwas combined with the possibility of touching the preparation, which sometimes led to lossof contact with the unit under observation. As a relatively large number of odours were usedit was found considerably easier to deliver the stimuli directly from a syringe. In the presentstudy 10 ml. glass syringes were used which were washed in soapy water, rinsed in water,alcohol and distilled water and then dried and kept in a box covered by activated charcoal. Adrop ofthe odorous material was placed in the syringe. Odorous puffs of less than 0.5 ml. weredelivered to the receptor epithelium by hand directly from the syringe. Reproduciblestimuli could be delivered by this method (see Figs. 1 and 3). The odorous puffs wereadjusted to give EOGs ofnearly equal amplitudes at about half the maximum response value.In this way the stimulating efficiency of each substance was monitored so that it was aboutthe same for all odours.The following substances were used: 1-propanol, 1-butanol, 1-hexanol, 1-heptanol,

I-octanol, methyl acetate, ethyl acetate, 1-propyl acetate, 1-butyl acetate, 1-pentyl acetate,2-butanone, 2-pentanone, 2-hexanone and 2-heptanone. Samples of the substances weretested in a gas chromotagraph for impurities. The ketones and alcohols contained less than1 % impurities except propanol and pentanol which contained 2-3 and 1-4 % impuritiesrespectively. Propyl acetate contained nearly 7 % contaminations. The other acetatescontained less than 1 7 % impurities. Both methyl and propyl acetate contained some ethylacetate and the pentyl acetate contained about 0-3% propyl acetate.Four series of experiments were made, one with each homologous series and one with two

substances from each of these series i.e., pentanol, hexanol, I-propyl acetate, 1-butylacetate, 2-pentanone and 2-hexanone.

In order to analyse the data the effects on the bulbar neurones were classified in threecategories: excitatory (+), inhibitory (-) and unaffected (0). The number of units excited,inhibited and unaffected was counted for all odours. When one of the three categories( +,-or 0) was obtained with only one of the odours in the series the unit was said to reactspecifically to that odour. The number of such specific responses was counted in all experi-mental series. The number of similar types of response was calculated for the combinationof the six alcohols taken 2, 3, 4 and 5 at a time.The chi-square values were calculated by a computer (IBM 1401) with one degree of

freedom, using Yates' formula for all odour pairs within each experimental series. The unitswhich were unaffected by one or both odours in the pairs were omitted from the chi-squareanalysis.

RESULTS

AlcoholsThe effect of the alcohols was studied in recordings from ninety-three

bulbar neurones. As seen from Table 1, about one third of the units wereexcited while inhibition of the spontaneous activity was obtained in 66%of the units. A small number of units was unaffected. It should be noticedthat the alcohols in the middle of the series inhibited more and excitedfewer units than did propanol and octanol. Frequently a unit was eitherexcited or inhibited by all alcohols used. An example of such a unit whichwas excited by the alcohols is shown in Fig. 1.

7-2

100 K. B. DOVING

A frequently observed feature of the effects on a neurone when stimu-lated with these alcohols was a change in type of response pattern withincreasing chain length, e.g. one unit could be excited by the short-chainedmolecules, unaffected by one or two intermediate alcohols and inhibitedby the rest. This type of change in response was obtained in about 60%of the units.

Fig. 1. Responses of a bulbar unit (upper trace) and the olfactory mucosa (lowertrace) to stimulation with alcohols. A, 1-propanol; B, 1-butanol; C, 1-pentanol;E, 1-heptanol; F, 1-octanol. Time bar 2 sec. Spikes retouched.

TABLE 1. The percentages of units excited, inhibited and unaffected by stimulation withdifferent alcohols. In this and the following tables the signs should be read: + = excitatory,- = inhibitory, 0 = unaffected. The last column in the table gives the number of specificresponses observed

Response No. ofA ^ > specific

Odour + - 0 responsesI-Propanol 38-7 527 8-6 13I-Butanol 32-3 65-5 2-:2 0I-Pentanol 30-1 67-7 2-2 41-Hexanol 26-8 70 0 3 2 21-Heptanol 24-6 74-3 1-1 01-Octanol 32-2 67-8 0 6Mean 30-8 66-3 2-9 Total 25

Specific respon8es. Some of the units studied were excited by only one ofthe odours and inhibited by the rest, in these cases the unit was said toreact specifically to that particular odour. For the six alcohols such specificresponses were distributed as shown in Table 1. It may be of interest tonotice that propanol and octanol evoked a greater number of specificresponses than did the other odours.

Similar responses. As mentioned above, there was a remarkably highproportion of units (40 0%) which gave similar types of response to all sixalcohols i.e. either inhibited by all six or excited by all six. Using thiscriterion, the number of units which gave similar responses were counted

SIMILARITIES OF HOMOLOGOUS SUBSTANCES

for all combinations of two, three, four and five odours that could be madeout of a total of six alcohols. As seen from the values plotted with opencircles in Fig. 2 the observed mean percentage increased from 42% for thecombination of five odours to 70% for the combination of two odours.The theoretical (expected) value for each combination, assuming indepen-dence between the different odours, was calculated from the followingformula for the probability (S) of obtaining similar responses

S = pn+qn+rnwhere n is the number of odours on each combination and p, q and r arethe mean values of units excited, inhibited and unaffected respectively.In the present experiments p = 0f31, q = 0*66, r = 0 03 (see Table 1).

80

70 F

60

A

0a

A

0

50 1 0

A0401 0

30 F

20 F 0

10 1

I I

2 3 4Number of odours compared

I I

5 6

Fig. 2. Incidence (in %) of similar types of response to combinations of differentnumbers of odours. Opencircles, observedmean values; filled circles, expectedvalues;triangles, neighbouring compounds (see text).

The filled circles in Fig. 2 show the theoretical values. The differencebetween the observed (open circles) and expected values increased withincreasing number of odours. If the probability of obtaining a givenresponse of a bulbar unit for any odour in the homologous series is inde-pendent of the other odours, the observed and expected values in Fig. 2

_-A

101

I

should coincide. Since this is not the case some odours must be dependentupon others. The triangles in Fig. 2 show the mean number of similartypes of response for neighbouring alcohols. The high values for theseselected combinations indicate that the neighbouring substances have moresimilar olfactory stimulative properties than those more widely separatedin chain length.

Comparison between pairs of odours. As pointed out above the resultsindicate that some alcohols were not independent of each other. None ofthe methods used above gives information about which odours wereindependent and which were not. This could be tested by the chi-squaremethod (cf. Doving, 1965). The chi-square values for the fifteen pairs ofalcohols are shown in Table 2. It should be noticed that the degree of

TABLE 2. The chi square values for different pairs of alcohols

SimilarityChi-square Significance estimates

Odour pairs values level (Engen, 1962)Propanol-butanol 46*885 77Propanol-pentanol 5-770 * 35Propanol-hexanol 2-456 46Propanol-heptanol 1-620 - 37Propanol-octanol 0-193 24Butanol-pentanol 17-061 *** 47Butanol-hexanol 7-000 ** 45Butanol-heptanol 3-862 * 36Butanol-octanol 1-979 35Pentanol-hexanol 29-821 53Pentanol-heptanol 17-879 *** 53Pentanol-octanol 7-390 ** 44Hexanol-heptanol 57-969 51Hexanol-octanol 27-664 58Heptanol-octanol 47-150 *** 81

In this and subsequent tables * indicates P = 0 05, **, P = 001 and ***, P = 0.001.

similarity gradually decreases as the odours in the pair are separated inchain length. Of the fifteen pairs only five were independent (chi-squarevalues lower than the significance level of 5 %) and in these pairs the odoursfrom the series were separated by three or more carbon atoms. The chi-square values were highest for the pairs C6-C7 and C7-C8.Homologous series have frequently been used in studies of olfactory

function (for review see Ottoson, 1963) because the physical and chemicalproperties are well known and the physical parameters change with themolecular weight. In an attempt to relate the molecular properties withthe present data the ratios between the moledular weights were calculatedfor each pair and the rank order compared to the experimental chi-squarevalues. For the alcohol series the rank correlation coefficient was found tobe 0-957 which is significant at a 0-1% level.

K. B. DOVING102


AcetatesAs seen from Table 3, the acetates, like the alcohols, inhibited the

spontaneous activity of the bulbar units in about 60% of the cases, while30% of the units were excited. Out of the total number of sixty-six unitsabout 27% gave the same type of response to all acetates. The typicalchange in response pattern with increasing chain length observed withalcohols was not so frequently observed when stimulating with the acetates.

Specific responses. The number of specific responses were in total thirty-two. The larger number of specific responses in this series than in theprevious one is reasonable in view of the smaller number of odours used.As seen from Table 3 half of the specific responses were elicited by methylacetate.

TABLE 3. The percentages of units excited, inhibited and unaffected by stimulation with thedifferent acetates. The last column gives the number of specific responses

Response No. ofA^ A specific

Odour + - 0 responsesMethyl acetate 21-2 69-7 9.1 16Ethyl acetate 21-2 71-3 7-5 9I-Propyl acetate 37-9 59-1 3-0 41-Butyl acetate 36-3 63-7 0 01-Butyl acetate 31-8 65-2 3 0 3Mean 29-7 65-8 4-5 Total 32

TABLE 4. The chi-square values for different pairs of acetates

Chi-square SignificanceOdour pairs value level

Methyl acetate-ethyl acetate 2-938 -Methyl acetate-propyl acetate 0-649Methyl acetate-butyl acetate 0 470Methyl acetate-pentyl acetate 0-254Ethyl acetate-propyl acetate 1-265Ethyl acetate-butyl acetate 0-019Ethyl acetate-pentyl acetate 0-012Propyl acetate-butyl acetate 31-528Propyl acetate-pentyl acetate 12-703Butyl acetate-pentyl acetate 22-496

Comparison between pairs of odours. The chi-square values obtained forthe different pairs of acetates are shown in Table 4. The methyl and ethylacetates were independent of each other as well as of the other acetates.The pairs between the three heaviest acetates gave the highest chi-squarevalues. The rank correlation coefficient between the chi-square values andthe molecular weight ratios was found to be 0-806, which is significant at a1% level.

K. B. DOVING

KetonesOnly four substances were used in this series with ketones. Out of the

eighty units investigated for these four odours, about 56% were inhibited,and 38% excited, while 5% remained unaffected (Table 5). Again a largenumber of units gave the same type of response to all odours. Relativelyfew units gave a change in response pattern with increasing chain lengthas they did for the acetates and alcohols.

Specific responses. In thirty-eight cases the units gave specific responses.This number is larger than in the previous series which is consistentwith the fact that a smaller number of odours was used in this series thanin the previous ones. As seen from Table 5 the 'end' substances in theseries, butanone and heptanone, evoked a greater number of specificresponses than did the two ketones with five and six carbon atoms.

TABLE 5. The percentage of units excited, inhibited and unaffected by the stimulation withdifferent ketones. The last column gives the number of specific responses

Response No. ofAt - 5 specific

Odour + - 0 responses2-Butanone 43-8 52-5 3X7 112-Pentanone 42-5 55-0 2-5 42-Hexanone 41-3 54-7 5-0 62-Heptanone 27-5 63-8 8-7 17Mean 38-8 56-2 5-0 Total 38

TABLE 6. The chi-square values for different pairs of ketones


Butanone-pentanone 34-336Butanone-hexanone 10-187 **Butanone-heptanone 3-86 *Pentanone-hexanone 24-018Pentanone-heptanone 7-736 **Hexanone-heptanone 28-685

Comparison between odour pairs. The chi-square values of the ketoneseries are given in Table 6 and show that all of the pairs of the four sub-stances used were associated. The values -decreased, however, withincreasing separation in chain length between the odours in the pairs, aswas seen in the alcohol series. For the ketones the rank correlation co-efficient between the chi-square values and the molecular weight ratios wasfound to be 0-772 which is not significant (P < 10-1).

'Cross-comaprison'To study the relation between the substances from different homo-

logous series, the effects on the bulbar units of two odours from each series

104


were observed. The compounds tested had five or six carbon atoms in thechain: 1-pentanol, 1-hexanol, 1-propyl acetate, 1-butyl acetate, 2-penta-none and 2-hexanone. As can be seen from Table 7 the distribution ofexcited, inhibited and unaffected units was found to be about the same asin the above-mentioned experiments. The acetates inhibited a smallerproportion of the seventy-six units investigated than did the alcohols andketones, consistent with the findings in the previous series. A statistical

TABLE 7. The percentage of units excited, inhibited and unaffected by stimulation with thealcohols, acetates and ketones with five and six carbon atoms. The last column gives thenumber of specific responses

Response No. ofA > specific

Odour + - 0 responses

1-Pentanol 21-1 67-1 11 8 61-Hexanol 19-7 67-1 13-2 31-Propyl acetate 26-3 59-2 14-5 21-Butyl acetate 237 61-8 14-5 52-Pentanone 21-1 72-3 6-6 42-Hexanone 21-1 69-8 9-2 3Mean 22-2 66-2 11-6 Total 23

Fig. 3. Responses of a bulbar unit (upper trace) and the olfactory mucosa (lowertrace). The unit is inhibited by stimulation with I-pentanol (A), 1-butyl acetate(C), and I-pentyl acetate (D), unaffected by stimulation with 1-hexanol (B) andexcited by 2-pentanone (E) and 2-hexanone (F). Time bar 2 sec. Spikes retouched.

treatment of the data obtained for the odours in this series and in the pre-vious ones did show that there was no significant difference in the pro-portion of units excited and inhibited. These results indicate that thesample size is representative for the population of bulbar units observed.

Frequently the two substances from the same homologous series elicitedthe same type of responses. An example of the responses to these sixodours is shown in Fig. 3. As seen ketones excited the bulbar unit while

pentanol and the acetates inhibited the unit. Hexanol did not affect thespontaneous activity in this unit.

Specific responses. The total number of specific responses was twenty-three for the experimental series (Table 7). It should be noticed that allodours evoked nearly the same number of specific responses. This is incontrast to the distribution of specific responses in the experiments abovewhere the substances at the end of the series gave the larger number ofsuch responses.

TABLE 8. The chi-square values for pairs of 1-pentanol, l-hexanol, 1-butyl acetate, 1-propylacetate, 2-pentanone and 2-hexanone


Pentanol-hexanol 37-044Pentanol-butyl acetate 0-380Pentanol-propyl acetate 2-332Pentanol-pentanone 0 804Pentanol-hexanone 1-911Hexanol-butyl acetate 0-670Hexanol-propyl acetate 4-379 *Hexanol-pentanone 4-137 *Hexanol-hexanone 3-823Butyl acetate-propyl acetate 42-492Butyl acetate-pentanone 3-454Butyl acetate-hexanone 4-472 *Propyl acetate-pentanone 2-054Propyl acetate-hexanone 3-332Pentanone-hexanone 25-442

Comparison between odour pairs. The chi-square values shown in Table 8demonstrate that the two odours selected from the same group were notindependent ofeach other. The chi-square values for these pairs of alcohols,ketones and acetates are about the same but are somewhat larger than thecorresponding values from the experiments mentioned above. These valuesare also considerably higher than for any other pairs in this study. Threechi-square values indicated an almost significant association between someof the odours. These were the pairs, hexanol-propyl acetate, hexanol-pentanone and hexanone-propyl acetate. Apart from these pairs all otherones gave chi-square values indicating that they were independent of eachother, i.e. had different stimulating properties.

DISCUSSION

As has been demonstrated (Adrian, 1953, 1956; Mancia et al. 1962;Doving, 1964) the neurones of the olfactory bulb maintain an irregularspontaneous activity. When the receptors are stimulated by odorous airthe activity of a particular bulbar unit affected may increase or decrease.Sometimes the inhibitory effect is followed by a post-inhibitory burst ofimpulses similar to the excitatory discharge. In studying the effect of

K. B. DOVING106

SIMILARITIES OF HOMOLOGOUS SUBSTANCES

various substances it is essential to be able to distinguish between thesetwo effects. This may be difficult or even impossible if the exact time courseof the stimulatory action of the odorant on the receptors is not known. Inthe present study the EOG was recorded simultaneously with the bulbaractivity. Since the EOG gives information about the onset and durationof the effect on the receptors (cf. Ottoson, 1956; 1959a, b) excitatoryeffects on the bulbar units could easily be distinguished from post-inhibi-tory effects. The EOG was also used as a measure of the stimulatorystrength of the substance tested and the stimulations were adjusted so thatapproximately equal stimulus strengths were used.The recordings in the present experiments were made from units in the

olfactory bulb. Three different cell types are present in the bulb, the mitralcells, the periglomerular cells and the granular cells. The mitral cells arethe largest ones and probably most of the recordings were made from these.The consistency between the proportion of units excited and inhibited forthe odours in the cross-comparison with the same odours in the other seriesindicated that the sample size was representative of the population ofunits.The classification of the responses makes the data able to be subjected

to enumeration statistics. If the responses of a sufficiently large number ofunits to stimulation with a given set of odours are observed, the chi-squarevalue indicates whether the odours of a particular pair are associated ornot associated (independent). A significantly high chi-square value indi-cates that the two odours are associated and may be interpreted as havinga high degree of similarity in olfactory stimulative properties. A highchi-square value, indicating that the two odours are not independent,does not necessarily imply that the two odours cannot be distinguished.Theoretically a single neurone giving different types of resporises to twosuch odours would be sufficient for discrimination. It should also be rea-lized that the classification of the responses into three types is a coarserepresentation of the true events in the olfactory bulb. Earlier experimentshave shown that there are variations in the patterns of the responseswithin each category (D6ving, 1964). These variations might also beutilized in the discrimination mechanisms as well as phenomena notobserved by the extracellular recording technique. These possibilities havenot been taken into account in the analysis of the present data.The traces of impurities present in the odorous substances used in these

experiments may have affected the results in two directions. If two sub-stances had some impurities in common the chi-square values wouldmost probably be higher than those obtained with the pure substances.On the other hand if a substance had a large amount of impurities notpresent among the other odours in the experimental series the chi-square

107

values could be lower. In such a case one would also expect the impuresubstance to evoke a larger number of specific responses than the otherodours in the series. The most contaminated substance used in the presentexperiments (propyl acetate) was used in two series, but in none of thesedid it elicit a greater number of specific responses than the other odours(see Tables 5 and 7). It does not seem likely that the results of theseexperiments were much influenced by the contaminations present in thesubstance used.The results of the experiments show that the neighbouring substances

in the homologous series had more similar olfactory stimulative propertiesthan those widely separated in chain length. The results from the threehomologous series provide evidence that the physical properties are im-portant in determining the stimulative properties. This is shown by thehigh rank correlation coefficient between the chi-square values and themolecular weight ratios of these three series. The steric configuration ofthe molecules is one of the properties which changes regularly with themolecular weight. The importance of the configuration of the odorousmolecules has been emphasized repeatedly (cf. Moncrieff, 1951; Beets,1957, 1964; Ruzicka, 1957; Stoll, 1957; Amoore, 1962). The chemicalproperties remain unaltered within each series that have the same 'func-tional group' (Beets, 1957, 1964) but are different for the different seriesof homologous substances. The results of the 'cross-comparison' of thepresent experiments demonstrate that the substances within one seriesgive much higher chi-square values than do the odours from differentseries, although they had nearly the same molecular weight. This indicatesthat any chemical and/or physical properties that change with the intro-duction of a new functional group are of significant importance.The same homologous alcohols used in the present study were used by

Engen (1962) who let a group of observers judge the degree of similaritybetween the fifteen pairs. The similarity estimates were expressed on ascale from zero to 100, 100 meaning identical. The last column in Table 2gives Engen's mean similarity estimates of ten observations. The rankcorrelation coefficient between Engen's psychological data and the chi-square values was found to be 0-868 which is significant at a 0'1 % level.This close correlation betweenthe results presented above and those obtainedfrom psychophysical experiments of similarity estimates for alcohols givessupport for the usefulness of these methods in odour research. It also indi-cates that the discriminatory mechanisms of the olfactory system aresimilar in frogs and humans.

The research reported in this paper has been sponsored by the Air Force Office of ScientificResearch under Grant AF EOAR 65-9 through the European Office of Aerospace Research(OAR), United States Air Force and by 'Nationalgaven til Chr. Michelsen' Bergen, Norway.

K. B. D6 V_[NG108


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an electrophysiological study of odour similarities of homologous

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