chemosensory event-related potentials change with age

Chemosensory event-related potentials change with age

T. Hummela,c,*, S. Barza, E. Paulib, G. Kobala

aDepartment of Experimental and Clinical Pharmacology and Toxicology, University of Erlangen-Nu¨rnberg,Krankenhausstrasse 9, 91054 Erlangen, Germany

bDepartment of Neurology, University of Erlangen-Nu¨rnberg, Schwabachanlage 6, 91054 Erlangen, GermanycDepartment of Otorhinolaryngology, University of Dresden, Fetscherstr. 74, 01307 Dresden, Germany

Accepted for publication: 27 August 1997

Abstract

The study examined age-related changes in the perception of olfactory and trigeminal chemical stimuli using chemosensory event-relatedpotentials (CSERP). Three groups of healthy volunteers, each comprised of 8 men and 8 women, were tested (age ranges 15–34, 35–54,and 55–74 years). Subjects underwent extensive psychological testing focusing on impairments of memory and attention. In addition, odoridentification and discrimination ability was evaluated, as well as detection threshold sensitivity for two odorants. Odor discriminationscores exhibited a significant age-related decrease. Significant age-related changes were also observed for CSERP N1P2 and P2 amplitudes,and for the N1 peak latency. The age-related decrease of CSERP amplitudes appeared to follow a different time course for responses totrigeminal and olfactory stimulants. 1998 Elsevier Science Ireland Ltd.

Keywords:Olfaction; Nociception; Electrophysiology

1. Introduction

Early in this century it was shown that aging is accom-panied by a decrease in intranasal chemosensory sensitivity(Vaschide, 1904) to camphor. Numerous studies confirmedthis finding for various odorants (Venstrom and Amoore,1968; Schiffman et al., 1976; Stevens and Cain, 1987).Others have reported a decreased ability to identify odorantswith increasing age (Doty et al., 1984; Wood and Harkins,1987; Cain, 1989), as well as a greater tendency for olfac-tory adaptation and slower recovery of threshold sensitivity(Stevens et al., 1989). Clearly, the sense of smell undergoesa general blunting throughout life. Considerably less dataare available regarding age-related changes on trigeminalchemoreception. Elevated thresholds for menthol werereported in elderly subjects (Murphy, 1983); in addition, asteeper slope of the intensity function was found foryounger adults. Stevens et al. (1982) found an age-relateddecrease in the perceived intensity of CO2 (a trigeminal

stimulus with minimal or no odor qualities), and Stevensand Cain (1986) reported a strong age-related elevation ofthe threshold for transitory apnea in response to CO2. Thus,the trigeminal chemoreceptive system appears to exhibit anage-related functional decrease analogous to that of theolfactory system.

Chemosensory event-related potentials (CSERPs) havebecome an important tool not only in olfactory research(Livermore et al., 1992; Lorig et al., 1996), but also in thediagnosis of olfactory disorders (Kobal and Hummel, 1994).CSERPs reflect the intensity of a stimulus (Kobal and Hum-mel, 1988) and can discriminate between the excitation oftrigeminal or olfactory nerves (Hummel and Kobal, 1992).The present study compared CSERPs in response to trigem-inal and olfactory stimuli to psychophysical measures of thesubjects’ ability to discriminate, identify, and detect odors.In addition, subjects underwent extensive psychologicaltesting. Thus, the present investigation extended work ofMurphy et al. (1994) and Evans and Starr (Evans et al.,1995) both of whom used amyl acetate for olfactory stimu-lation, a stimulant that may produce mixed olfactory-tri-geminal sensations (Doty et al., 1978).

Electroencephalography and clinical Neurophysiology 108 (1998) 208–217

0168-5597/98/$19.00 1998 Elsevier Science Ireland Ltd. All rights reservedPII S0168-5597(97)00074-9 EEP 97058

* Corresponding author. Tel.: +49 351 4584420; fax: +49 351 4584326;e-mail: [email protected]

2. Methods and materials

Forty-eight healthy volunteers participated in the study.They were divided into 3 groups, termed A, B, and Cthroughout the text, comprised of 8 men and 8 women,respectively (mean ages (range) 24.8 (15–34), 43.9 (35–54), and 64.2 (55–74) years). None had a history or pre-sence of chronic nasal infections, severe head injuries, orneurological or psychiatric illnesses, and all reported a nor-mal sense of smell. Written informed consent was obtained;the study was performed in accordance to the Declaration ofHelsinki/Hong Kong.

Subjects participated in two sessions performed either onthe same day (separated by an interval of approximately 60min) or on two consecutive days. Electrophysiological mea-surements were performed during the first session, psycho-logical and psychophysical testing during the second.

During the first session 3 chemical stimulants were pre-sented at a single concentration: the odorants vanillin (0.8p.p.m.) and hydrogen sulfide (H2S; 2.1 p.p.m.), and the tri-geminal irritant carbon dioxide (CO2; 52% v/v). The plea-sant smell of vanillin is most often described as sweet andthe unpleasant smell of H2S as rotten eggs. The non-odorousCO2 produces sharp painful sensations (stinging). Stimuliwere applied to the left nostril (stimulus duration 200 ms,flow rate 145 ml/s). Due to a sophisticated stimulus presen-tation device, mechano- or thermoreceptors in the nasalmucosa were not activated by stimulus presentation(Kobal, 1985; Kobal and Hummel, 1988).

Subjects were comfortably seated in an electrically-shielded ventilated room, and were observed via a videocamera. White noise of approximately 50 dB SPL (ERAstimulator, Tonnies, Germany) prevented them from hear-ing clicks produced by the stimulator’s switching device.

CSERPs were obtained in response to randomized stimu-lation with CO2, vanillin and H2S. Each of the 3 stimuli waspresented 15 times (interstimulus interval approximately 40s). It was verified beforehand that the subjects were able toperceive the stimuli. Following each presentation, subjectsrated the intensity of each stimulus by moving a joystick toadjust a line on a visual analogue scale, displayed on a videomonitor in front of the subject. The left end of the scalesignified no sensation (0 estimation units= 0 EU), and the

right end an extremely strong sensation (100 EU). Intensityestimates were averaged off-line separately for the 3 stimu-lants. To monitor changes in vigilance, subjects had to per-form a tracking task on the video screen between stimuluspresentations (Kobal et al., 1990). They had to keep a smallsquare, which could be controlled by a joystick, inside alarger one, which moved around unpredictably. Perfor-mance was assessed by measuring for how long the subjectshad lost track of the independently moving square (rangefrom 0 to 100% success in tracking).

During the second session 3 psychological tests (durationapproximately 30 min) were administered to each subject.This was followed by 3 psychophysical tests of olfactoryperception; discrimination of odorants (20 min), thresholdtesting (20–30 min), and identification of odorants (10 min).During these tests, subjects were free to sample the odorantsas often and as long as convenient. Odorants were handledcarefully; they were stored in a dark, ventilated cabinet, andthe experimenter always wore deodorized disposablegloves. Solutions were replaced every fortnight. All mea-surements were conducted by the same experimenter.

Psychological testing was performed by means ofthe ‘Mehrfachwahl-Wortschatz-Intelligenztest’ (MWTB;Merz et al., 1975) (a short and simple measure of the sub-jects’ intellectual abilities), the ‘Wechsler-Memory-Scale’(WMS; Wechsler, 1945), and the ‘SKT-Kurztest’ (SKT;Erzigkeit et al., 1979) which was used to check for impair-ments of memory and attention.

2.1. Discrimination of odorants

In this test subjects were required to discriminate among8 pairs of odorants matched in overall intensity. The odor-ants (dissolved in propylene glycol, total volume 40 ml)were presented to the left nostril in 250 ml polyethylenesqueeze bottles (Table 1). For each discrimination trial, 3bottles were presented, two containing the same concentra-tion of an odorant and the other the target odorant. Subjectshad to indicate which one of the 3 odorants was different.

2.2. Detection thresholds of odorants

Detection thresholds were established for phenylethyl

Table 1

Pairs of odorants used in the olfactory discrimination task (concentrations in mg/ml propylene glycole)

Task number Odorant 1 Odorant 1 concentration Odorant 2 Odorant 2 concentration

1 Dimethyl disulfide 2 Phenyl ethanol 152 Butanol 19 Pyridine 13 Isoamylacetate 17 Cyclopentadecanoate 214 Anethole 10 Eugenol 625 Dihydrosenoxide 21 Octylacetate 216 Menthol 21 Eucalyptol 217 R(+)-limonene 41 S(−)-limonene 418 (+)-carvone 62 (−)-carvone 62

209T. Hummel et al. / Electroencephalography and clinical Neurophysiology 108 (1998) 208–217

alcohol (PEA) and pyridine (Fluka, Switzerland). Althoughit would have been desirable to use CO2 or H2S/vanillin forthreshold measurements, for technical reasons we elected toperform threshold testing using stable solvable odorants insqueeze bottles. The odorants were diluted in buffer (19.6 gsodium tetraborate/l distilled water; pH 7.0); the concentra-tion in stock solutions of 80 ml was 40 ml for both odorants.The next dilution was prepared by separating 40 ml of stocksolution and adding 40 ml buffer. This successive dilutionprocedure was performed 30 times (dilution ratio 2:1). Theresulting solutions of 40 ml were stored in polypropylenebottles (volume 250 ml). The odorant presentation was per-formed in a similar manner as described above for the dis-crimination task, with the notable exception that thesubjects were blindfolded. Detection thresholds were deter-mined by employing a method described by Wysocki andBeauchamp (1984). Three flasks, two of which containedthe solvent and one which contained the odorant in a certaindilution, were presented to each subject in a randomizedorder. Subjects had to indicate the bottle which presumablycontained the odorant. Within this triple-forced-choice-task,the odorants were presented to the subjects in rising con-centrations approximately every 20 s, until they had cor-rectly discerned the odorant in 3 successive trials withascending concentrations. The lowest of the 3 concentra-

tions was defined as detection threshold. No feed-backregarding decision accuracy was provided.

2.3. Identification of odorants

The subjects had to identify 8 common odorants, namelygravy, fish, raspberry, coffee, garlic, leather, orange andchocolate (Dragoco, Holzminden, Germany; Harmann andReimer, Holzminden, Germany; Chivaudan, Hannover,Germany). They were stored in opaque glass bottles of150 ml total volume, and presented one at a time to thesubject. The subject had to indicate one out of 4 colorphotographs depicted on a carton which best described theodorant. In addition, the photographs were subtitled with theitem’s name.

2.4. Recording of CSERPs

The EEG (bandpass 0.2–30 Hz) was recorded from 3midline positions (Fz, Cz, Pz) of the 10/20 system refer-enced to linked earlobes (A1+ A2; EEG amplifier SIR,Germany). Possible blink artifacts were registered from anadditional site (Fp2, referenced to A1+ A2). Analogue-to-digital conversion (CED1401, UK) of the stimulus-linkedEEG segments of 2048 ms started 500 ms prior to stimulus

Table 2

Means and standard deviations of chemosensory event-related potentials and intensity estimates

(A) Means and standard deviations of chemosensory event-related potentials at recording position CzAge (years) Stimulant A-N1P2 (mV) T-N1 (ms) T-P2 (ms) A-N1 (mV) A-P2 (mV)

15–34 CO2 Mean 52.1 362 546 −19.4 32.7SD 22.5 60 80 15.3 14.6

Vanillin Mean 24.1 410 608 −3.3 20.8SD 7.9 50 90 3.9 8.5

H2S Mean 25.3 381 623 −5.2 20.1SD 8.0 46 81 5.5 10.3

35–54 CO2 Mean 35.1 381 573 −14.9 20.2SD 18.2 87 117 11.1 16.9

Vanillin Mean 22.7 416 636 −7.4 15.2SD 15.0 52 61 12.3 9.7

H2S Mean 21.8 402 644 −7.8 13.9SD 12.4 62 83 6.2 9.7

55–74 CO2 Mean 33.7 395 590 −10.7 23.0SD 18.1 72 97 16.0 15.0

Vanillin Mean 16.0 432 639 −5.2 10.7SD 6.4 86 91 4.3 5.3

H2S Mean 14.0 443 649 −4.3 9.7SD 8.2 66 60 3.0 6.9

(B) Means and standard deviations of intensity estimates (in estimation units)Stimulant Age (years)

15–34 35–54 55–74

CO2 Mean 34.0 39.4 49.0SD 19.7 26.2 17.5

Vanillin Mean 14.8 20.9 22.3SD 17.7 20.6 14.8

H2S Mean 13.6 22.6 24.0SD 8.8 18.6 14.6

210 T. Hummel et al. / Electroencephalography and clinical Neurophysiology 108 (1998) 208–217

onset. The mean of this 500 ms pre-stimulus period servedas a baseline for amplitude measurements. All singleresponses contaminated by eye blinks were discardedfrom the average, yielding late nearfield event-relatedpotentials. Cortical generators of olfactory ERPs are foundin the temporal lobe (Kettenmann et al., 1996b; Aya-be-Kanamura et al., 1997), while generators of chemo-somatosensory ERPs in response to CO2 are found in thesecondary somatosensory cortex (Huttunen et al., 1986;Kettenmann et al., 1996a). The CSERP’s peaks N1 and P2were analyzed as they can be identified with the highestdegree of accuracy (Barz et al., 1997). Based on these mea-sures, the peak to peak amplitude A-N1P2, base to peakamplitudes A-N1 and A-P2, and latencies (in relation tostimulus onset) of T-N1 and T-P2 were submitted to statis-tical tests (Kobal and Hummel, 1991).

2.5. Statistical analyses

Results were analyzed by means of the SPSS/PC+ pro-gram package. The odor discrimination, odor identification,odor thresholds, tracking performance, and psychologicaltest data were analyzed by two-way ANOVAs (between-subject factors ‘age’ and ‘sex’). The odor intensity esti-

mates, and number of detected target-stimuli, were sub-mitted to three-way ANOVAs (between-subject factors‘age’ and ‘sex’, within-subject factor ‘stimulant’). Four-way ANOVAs (between-subject factors ‘age’ and ‘sex’,within-subject factors ‘stimulant’ and ‘recording site’)were used to evaluate the ERP data. The level of signifi-cance wasP , 0.05. Only in case a main effect of the factor‘age’ became significant weret tests computed.

Correlations were computed between ERP parametersand the corresponding intensity estimates, as well as thepsychophysical olfactory testing data. In case of correlationsthe level of significance was set atP , 0.01.

3. Results

3.1. CSERP

The 3 stimulants produced responses in all subjects. Dueto excessive blinking, however, they could not be analyzedin one subject of group A and two subjects of group B.Means and standard deviations of the CSERP at recordingposition Cz are given in Table 2A. Responses of the 3groups of subjects are presented in Fig. 1.

Fig. 1. CSERP at recording position Cz (see inset) of all individual subjects in response to stimulation with CO2, vanillin, and H2S separated for the 3 groupsinvestigated (group A: 15–34 years; group B: 35–54 years; group C: 55–74 years). Note the different scaling of CSERP to H2S and vanillin compared toresponses induced by CO2. The dotted line indicates onset of stimulation. While CSERP were largest for the youngest subjects, the smallest responses wereobtained for group C.


• N1P2 amplitudes decreased significantly in an age-related manner (factor ‘age’,F(2,39) = 4.49, P ,0.05; pwr= 0.73) (Fig. 2, Table 2A). For the tri-geminal stimulant, group A differed significantlyfrom the other two groups (A-B:t = 2.3, P ,0.05; A-C: t = 2.5, P , 0.05). For H2S, group Cdiffered significantly from the two other groups(A-C: t = 4.0, P , 0.001; B-C:t = 2.1, P, 0.05).For vanillin, only the oldest and the youngestgroups differed from one another (A-C:t = 3.1,P , 0.01). In general, CO2 produced the largestamplitudes, while responses to vanillin and H2Swere of approximately the same magnitude (factor‘stimulant’, F(2,78) = 53.4,P , 0.001; pwr= 1.0).CSERP to the 3 stimulants were of significantlydifferent size at the 3 recording positions (fac-tor ‘recording site’, F(2,78) = 41.9, P , 0.001;pwr = 1.0). As had been reported previously (Hum-mel and Kobal, 1992; Kobal et al., 1992), N1P2amplitudes in response to stimulation with CO2

were largest at Cz. In contrast, after stimulationwith the two odorants subjects exhibited largestamplitudes at Pz (interaction ‘stimulant’ by ‘record-

ing site’: F(4,156)= 6.05,P , 0.001; pwr= 0.98).The significant interaction between factors ‘age’and ‘recording site’ (F(4,78) = 3.94, P , 0.01;pwr = 0.89) indicated that the topographical differ-ences between CSERP amplitudes leveled off withincreasing age.

• N1 amplitude exhibited a mean decrease in relationto the subjects’ age (Table 2A) that did not reachthe level of significance. Largest responses wereobtained after stimulation with CO2 (factor ‘stimu-lant’, F(2,78) = 10.9,P , 0.001; pwr= 0.99). Sta-tistical analysis revealed differences in theCSERPs’ topographical distribution (factor ‘record-ing site’, F(2,78) = 7.41, P , 0.01; pwr= 0.93).While amplitudes N1 in response to CO2 were lar-gest at Cz and much smaller at Pz, this ratio chan-ged in favor of Pz when H2S or vanillin were usedas stimulants (interaction ‘stimulant’ by ‘recordingsite’: F(4,156)= 5.22,P , 0.001; pwr= 0.97).

• P2 amplitude decreased in an age-relatedmanner (factor ‘age’,F(2,39) = 5.63, P , 0.01;pwr = 0.83), reflecting, for the two odorants, differ-ences between the youngest and the oldest subjects

Fig. 2. CSERP amplitudes N1P2 (means, standard errors of means) at midline recording positions (Fz, Cz, Pz; see inset) in response to stimulation withCO2,vanillin, and H2S for the 3 groups of subjects (A: 15–34 years; B: 35–54 years; C: 55–74 years) separately for female and male subjects. Note the differentscaling of they-axes. There was a significant decrease of amplitudes A-N1P2 in relation to the subjects’ age. In general, female subjects exhibited largeramplitudes than males.


(t tests: vanillin, A-C:t = 3.9,P , 0.01; H2S, A-C,t = 3.3,P , 0.01); for CO2, differences were foundbetween the youngest and the middle-aged subjects(A-B: t = 2.2, P , 0.05). Thus, amplitude P2exhibited similar changes as amplitude N1P2. Simi-lar to the changes of amplitude N1P2 and amplitudeN1, amplitude P2 was largest after stimulation withCO2 (factor ‘stimulant’,F(2,78) = 21.4,P , 0.001;pwr = 1.0) when compared to the two olfactory sti-mulants (Table 2A). Largest amplitudes were gen-erally found at Pz (factor ‘position’,F(2,78) = 12.6,P , 0.001; pwr= 1.0).

• Latencies N1 increased with increasing age (factor‘age’, F(2,39) = 3.3,P , 0.05; pwr= 0.59). A sig-nificant difference for H2S was only observedbetween the youngest and the oldest group of sub-jects (t test: A-C: t = 3.1, P , 0.01). In general,responses were shortest when CO2 was used as astimulant compared to CSERP latencies after stimu-lation with H2S or vanillin (Table 2A).

• As with latencies N1, latencies P2 were longestafter stimulation with the two odorants comparedto stimulation with CO2 (factor ‘stimulant’,F(2,78) = 7.36, P , 0.01; pwr= 0.93). Statisticalanalysis revealed differences between the 3 record-ing positions (factor ‘position’, F(2,78) = 4.58,P , 0.05; pwr= 0.76). That is, latencies P2 were

generally shortest at Fz and longest at the parietalrecording position (Table 2A).

3.2. Intensity estimates

Mean intensity estimates of the chemosensory stimuliexhibited no significant changes in an age-related manner(Table 2B). There was a significant effect of the factor ‘sti-mulant’ (F(2,84) = 43.2, P , 0.001; pwr= 1.0); CO2 sti-muli were estimated to be stronger compared to the twoolfactory stimuli. After stimulation with CO2, N1 ampli-tudes increased with increasing intensity estimates (pos.Fz, r46 = 0.37,P , 0.01).

3.3. Odor discrimination

Mean discrimination of odorants decreased with age (fac-tor ‘age’: F(2,42) = 12.8, P , 0.001; pwr= 1.0; Fig. 3),largely reflecting the poor performance of the oldest sub-jects (A-C,t = 4.5,P , 0.001; B-C,t = 3.5,P , 0.01).

3.4. Odor identification

Mean odor identification exhibited a non-significantdecrease with increasing age of the subjects (Fig. 3). Acorrelation was observed between odor identification andP2 amplitudes after stimulation with vanillin (pos. Pz,r45 = 0.39,P , 0.01).

3.5. Thresholds for PEA and pyridine

There was no change of thresholds for either PEA andpyridine in relation to the subjects’ age (Fig. 3).

3.6. Tracking performance

The tracking performance decreased with age (group A:mean= 71, SD= 21; group B: mean= 70, SD= 16; groupC: mean= 54, SD= 19; factor ‘age’, F(2,42) = 4.34,P , 0.05; pwr= 0.72). The age-related change reflecteddifferences between group C and groups A and B (A-C,t = 2.4,P , 0.05; B-C,t = 2.6,P , 0.05).

3.7. Psychological tests

For all 3 tests employed (MWTB, WMS, SKT) no sig-nificant differences between the 3 groups could be found(Table 3) indicating that there were no major psychologicaldifferences between groups.

4. Discussion

The present study demonstrates an age-related decreasein chemosensory function, as indicated by electrophysiolo-gical measures. Specifically, the ERP’s P2 amplitude and

Fig. 3. Detection thresholds for PEA and pyridine, results of odor identi-fication and discrimination (means, standard error of means) for the 3groups (A: 15–34 years; B: 35–54 years; C: 55–74 years) separately formales and females. Detection thresholds are given in dilution steps. Highernumbers correlate with higher sensitivity, i.e. lower thresholds. Odor dis-crimination decreased in relation to the subjects’ age.


the composite N1P2 amplitudes decreased with increasingage while a prolongation of N1 peak latencies was observed.In this context it seems to be important to mention that theparticipants did not differ significantly in terms of theirmental states as it had been ascertained by means of differ-ent psychological tests.

Interestingly, the CSERP’s interindividual variabilityincreased in relation to the subject’s age. That is, whenthe data are expressed as percentage of the mean of therespective category (i.e. age group, recording position, sti-mulant), the standard deviations of these transformed dataare typically smallest in the young while they are largest inelderly subjects (Table 4). Thereby, interindividual varia-bility appears to be much larger compared to intraindividualvariability (compare Kobal and Hummel, 1988). Whencomparing the variation of CSERP measures, in generalboth latency and amplitude of the P2 peak exhibited lessvariability than N1. To address this question specificallydesigned studies are currently underway.

Murphy et al. (1994) investigated CSERP in elderly(mean age 66 years,n = 7) and young adults (mean age27 years,n = 7), respectively. In response to amyl acetatethey found differences in amplitudes at recording positionsCz and Pz, but not at the frontal site Fz. This compares to thepresent data which indicates a leveling of the topographicaldifferences between CSERP amplitudes, i.e. differencesbetween groups were largest at Cz while they were smallerat Fz and Pz. Murphy et al. (1994) also observed a non-significant prolongation of latencies N1 and P2. Usingamyl acetate Evans et al. (1995) reported a decrease of

peak-to-peak amplitudes N1P2 in relation to the subjects’age. They also reported a significant life-span prolongationof the P2 latency; this was accompanied by a trend of meanN1 latencies to increase with age. In addition, Hawkes andco-workers (personal communication) found both an agerelated decrease of CSERP amplitudes and an increase oflatencies for the stimulant H2S (n = 60). Thus, the presentlyobtained results compare to other research in that area.

The question remains, however, whether the observeddecrease of CSERP amplitudes is only the epiphenomenonof other age-related changes of the CNS. Although theremay be no definitive answer, there is no substantial slowingof background EEG (Duffy et al., 1993). In addition,research indicates that late ERP components do not uni-formly change as a function of age when assessed in healthysubjects (for review see Dustman et al., 1993). This is alsoillustrated by the non-significant change of CSERP ampli-tude N1 that was observed in the present study. Thus, it canbe assumed that the presently reported changes of chemo-sensory ERP rather reflect a decrease of sensory or cognitivefunction but a generalized, unspecific change of the CNS asexpressed in the EEG activity.

In contrast to CSERP, for intensity estimates of supra-threshold chemosensory stimuli, no age-related change wasfound. Therefore, it has to be kept in mind that subjectsestimated odor intensities in relation to a standard stimulus.Thus, a stronger decrease of intensity estimates in one groupwould reflect a more pronounced adaptation/habituation. Inaddition, Harkins and co-workers found elderly subjects(45–80 years of age) to overrate high-intensity stimuli

Table 3

Means and standard deviations of psychological tests

Age (years) MWTB WMS SKT

Mean SD Mean SD Mean SD

15–34 33.5 3.0 129.4 9.0 1.5 1.635–54 34.1 1.8 133.7 12.9 1.7 1.955–74 31.5 8.1 134.6 19.5 1.9 1.2

MWTB, Wortschatz-Intelligenztest Version B WMS; Wechsler-Memory-Scale SKT, short test for detection of impairments of memory or attention.

Table 4

Standard deviations of normalized chemosensory event-related potentials at recording position Cz

Age (years) Stimulant A-N1P2 T-N1 T-P2 A-N1 A-P2

15–34 CO2 SD 43 17 15 79 45Vanillin SD 33 12 15 119 41H2S SD 32 12 13 105 51



Individual data were recalculated as percentages of the mean of the respective category. T-N1, T-P2: latencies of N1 and P2, respectively; A-N1, A-P2: baseto peak amplitudes of N1 and P2, respectively; A-N1P2: peak to peak amplitude N1P2.


while stimuli of lower intensity were underrated (Harkinsand Chapman, 1976). Therefore, the lack of age-relatedchanges of intensity ratings to suprathreshold stimuli doesnot mean that there were no age-related differences in odorsensitivity.

Regarding other psychophysical measures of olfactorysensitivity (i.e., olfactory threshold, odor identification,odor discrimination), changes became only significant forthe odor discrimination task. The lack of a significant ageeffect on identification and threshold measures may in partrelate to weaknesses of the methods used (for review seeDoty and Kobal, 1995).

This present study confirmed previous results on differ-ences in the topographical distribution of CSERP ampli-tudes in response to olfactory or trigeminal stimulation(Hummel and Kobal, 1992; Hummel et al., 1992; Kobal etal., 1992; Livermore et al., 1992; Murphy et al., 1994; Evanset al., 1995). Specifically, olfactory stimuli produce largerN1 amplitudes at parietal recording sites while intranasalchemo-somatosensory stimuli produce largest responses atthe central recording position Cz. These differences appear

to be due to the existence of different cortical generators thatare activated either by trigeminal or olfactory stimulation.Olfactory stimulation seems to activate areas in the tem-poral lobe (Kettenmann et al., 1996b; Ayabe-Kanamura etal., 1997) whereas trigeminal stimuli activate areas in thesecondary somatosensory cortex (Huttunen et al., 1986;Kettenmann et al., 1996a). To explore CNS plasticity as aresult of age-related changes in olfactory function, furtherresearch is needed employing magneto-encephalographictechniques or functional magnetic resonance imaging(fMRI).

Results of the present study also pointed to gender-relateddifferences in response to olfactory stimuli (compareBecker et al., 1993; Evans et al., 1995); i.e. in generalfemales exhibited larger mean CSERP amplitudes whencompared to male subjects possibly indicating a higherolfactory sensitivity. However, although not statisticallysignificant, for responses of female subjects to trigeminalstimulation with CO2 there was a strong decrease of CSERPamplitudes N1P2 between groups A (age 15–34 years) andB (age 35–54 years). The results obtained in different

Fig. 4. Results of a cross-sectional study of an individual female subject over 10 years. CSERP were recorded in response to stimulation with 52% v/v CO2.Graphs in the upper half demonstrate the results of 6 consecutive measurements. Examples of individual CSERP are shown in the lower half; the N1 and P2peaks are indicated by pin-like markers. These intra-individual results compare to the inter-individual decrease of CSERP amplitudes (Fig. 2) which was mostpronounced for female subjects of groups A (15–34 years) and B (35–54 years).


groups of subjects compare to changes of responses in afemale subject where CSERP were recorded over a periodof 10 years (Fig. 4). Although on an anecdotal level, it isinteresting to see that, similar to inter-individual changes, anintraindividual decrease of response amplitudes occursbetween the ages of 28 and 38 years.

To summarize, the present results indicate that CSERPamplitudes and latencies changed in an age-relatedmanner; for amplitudes this appeared to follow a differenttime course for responses to trigeminal and olfactory stimu-lants.

Acknowledgements

We would like to thank Dr. Frithjof Kruggel, Munich,Germany, for his help in setting up the psychophysicalolfactory testing. This research was supported by the Mar-ohn-Stiftung, Germany, and grant P01 DC 00161 from theNational Institute on Deafness and Other CommunicationDisorders, USA. We appreciate the help of Dragoco, andHarmann and Reimer (both Holzminden, Germany), andChivaudan, Hannover, Germany, in donating some of thenatural aromas.

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