Chemical Senses Vol. 30 No. suppl 1 © Oxford University
Press 2005; all rights reserved
Odor and Taste Interaction on Brain Responses in Humans
1 Virginia Commonwealth University Health System, Richmond, USA, 2 University Hospital AKH, Vienna, Austria and 3 University Erlangen-Nuremberg, Erlangen, Germany
Correspondence to be sent to: Birgit Kettenmann, e-mail:bkettenmann2{at}vcu.edu
Key words: olfactory and gustatory pathways, olfactory event-related potentials, taste strips
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Introduction |
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 Top Introduction Materials and methodsResults and discussionReferences |
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Central neural integration of sensory input from different modalities is a prerequisite for flavor perception. Chemosensory information as part of flavor perception is mediated by (i) specific and (ii) general sensory systems. The mouth and the nose are areas housing specific sensory systems, i.e. the gustatory and the olfactory systems. In the gustatory and olfactory systems, as well as in the vomeronasal organ, information is mediated by chemoreceptors. In the somatosensory system, which is classified here as the general sensory system, chemoreceptors are mainly nociceptors. But there are other modalities present as well, such as mechano- and thermoreceptors.
It is commonly accepted that there are plenty of opportunities for the above-mentioned systems to interact. This can happen at several levels of the information processing before reaching the cortical level and even peripheral interactions seem possible.
In the psychophysical literature there are numerous demonstrations of taste/smell
interactions. However, our understanding of the CNS mechanisms and detailed
characteristics thereof are rather limited. In humans, some experiments have recently
been published addressing this phenomenon by using imaging technologies (Small et al., 1997
, 2004;
Cerf-Ducastel et al., 2001;
de Araujo et al., 2003;
Cerf-Ducastel and Murphy, 2004).
We wanted to know if the method of event-related potential (ERP) recording could be a
useful tool for the investigation of taste and smell interactions. The reason for this
approach is that ERPs have the highest possible time resolution compared to most imaging
techniques except magnetic source imaging (MSI). Although we do not report any results
here, it has now become possible to obtain information about areas of activation by
applying electrical source localization methods based on ERPs. In addition, we have
demonstrated that ERP components, namely the N1/P2 portion of the ERP, are generated in
the insular cortex (Kettenmann et al.,
1997) and also postulated that this area might play a crucial role in
integrating taste and smell information. This assumption has been recently supported by
Fu et al. (2004).
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Materials and methods |
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TopIntroductionMaterials and methodsResults and discussionReferences |
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Seventeen volunteers participated in six measurement days of recording olfactory event-related potentials (OERPs). In addition to the OERPs, hedonic and intensity ratings to the three different odorants [n-butylacetate (mild fruity smell); linalool (sweet floral smell, used as background for peach and apricot flavor); octanal (waxy citrus/orange/grapefruit smell)] were registered.
Odorants were applied by a vapor-dilution olfactometer (Burghart OM4b). Stimuli were applied at a constant flow rate of 140 ml/s, a humidity of 80% and a temperature of 37°C. Stimulus duration was 200 ms and the interstimulus interval was 40–50 s. Each volunteer participated in an additional training session prior to the actual measurement to become acquainted with the experimental procedures.
For taste stimulation, five different tastants and blank (sweet, sour, salty, bitter,
and umami) were applied using taste strips. Taste strips were made of paper soaked in
taste solutions and dried on a slowly rotating wheel. The length of a taste strip was 8
cm and an area of 2 cm2 was impregnated with a taste stimulant (Mueller et al., 2003) (Figure
1).
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In each session, one out of the three odorants was applied and OERPs were recorded while keeping the taste strips with one out of the five different tastants or blank at the center (not the tip) of the tongue. During one measurement day, one of the three odorants was combined with three out of the six different tastants in a randomized order. Odorants had to be rated for intensity and pleasantness after each of 16 olfactory stimuli necessary for obtaining an OERP. Endpoints for this study were intensity and hedonic ratings of odors and amplitudes and latencies of OERPs.
The quantities of tastants (dissolved in 100g double-distilled water) used to load the taste strips are given in Table 1. For olfactory stimuli, the three odorants were used at concentrations of 10% saturated vapor at 36°C.
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The EEG was recorded from 13 positions according to the 10/20 system referenced to both earlobes. Possible eye movement artifacts were recorded from an additional site. EEG segments of 2048 ms were recorded starting 500 ms prior to stimulus onset. The mean of this 500 ms pre-stimulus period served as a baseline for amplitude measurements.
Data analyses were performed using the BOMPE 03 program (Burghart Instruments, Germany). The 16 EEG segments linked to the olfactory stimulus of each recording were filtered and averaged. The resulting OERPs were further evaluated in measuring amplitudes and latencies of the P1, N1 and P2 components, separately for each recording site.
To stabilize vigilance during the intensity and hedonic ratings and during the OERP
measurements, volunteers performed a tracking task on a video monitor (Kobal et al., 1990).
SPSS 10.0 was employed for statistical evaluation.
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Results and discussion |
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TopIntroductionMaterials and methodsResults and discussionReferences |
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Although data are not fully evaluated and electric source localization has not been performed, there are some preliminary statistically significant results (P < 0.05) that are very promising. For the hedonic ratings, there were a couple of changes in the odor perception due to differences of the background taste stimulation. n-Butylacetate was perceived as most pleasant during sweet stimulation compared to umami and sour taste. Octanal was most pleasant with sour taste compared to salty, and linalool was most pleasant with the blank taste compared to salty and umami. In the intensity ratings we only found that octanal was most intensely perceived during umami stimulation compared to sour and sweet.
The five tastants modified OERP patterns differently and this interaction also seemed to be odor specific, e.g. the ‘early’ component N1 elicited by n-butylacetate was significantly reduced during sour stimulation, the later component P2 elicited by linalool was significantly enhanced during bitter stimulation and the same component elicited by octanal was significantly prolonged during salty stimulation.
Since we have not yet performed electric source localization, changes in amplitudes and latencies cannot be interpreted with the degree of certainty we would like to have. However, next to the possibility that changes in amplitudes and latencies can always be related to changes in the composition of neuronal populations, the fact that the N1 component elicited by n-butylacetate was significantly smaller during sour stimulation could either indicate a difference in intensity perception, which we could not find in the intensity estimates, or a change in the attention level towards the stimulus, as has been shown in other sensory modalities for this component. Changes in the P2 component of the linalool OERP during bitter stimulation and in the octanal OERP during salty stimulation could indicate changes in cognitive evaluation. Interestingly, the response to octanal as the major component of the grapefruit smell was influenced significantly by the salty taste in its more cognitive late positive component. In some regions, grapefruit juice is combined with salt to enhance the taste of the beverage.
The observed interactions between taste and the OERP components were mostly related
to potential components that are generated in the insular cortex. In MSI studies, we
found strong indications of an important representation of the insular cortex in the
processing of olfactory information. The vicinity of taste representation as a possible
location for interactions was recently stressed by findings of
Fu et al. (2004). They
interpret their results that the olfactory and gustatory pathways appeared to be
reciprocally connected through the insular cortex as the evidence that this area could
modulate mechanisms involved in food selection and emotional reactions.
In summary, we found that tastants can modify the pattern of OERPs, that different tastants modify OERP patterns differently and that tastants’ modifications of OERP seem to be odor-specific.
Hence, observed changes were clearly not uniform but followed specific and characteristic patterns. N1/P2 components seem to be useful in studying taste/smell interactions, because they are generated in the insular cortex.
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References |
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TopIntroductionMaterials and methodsResults and discussionReferences |
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Cerf-Ducastel, B. and Murphy, C. (2004) Validation of a stimulation protocol suited to the investigation of odor-taste interaction with fMRI. Physiol. Behav., 81, 389–396.[CrossRef][Medline]
Cerf-Ducastel, B., Van de Moortele, P.F., MacLeod, P., Le Bihan, D. and Faurion, A. (2001) Interaction of gustatory and lingual somatosensory perception at the cortical level in the human: a functional magnetic resonance imaging study. Chem. Senses, 26, 371–383.
de Araujo, I.E., Roll, E.T., Krigelbach, M.L., McGlone, F. and Phillips, N. (2003) Taste–olfactory convergence, and the representation of the pleasantness of flavour, in the human brain. Eur. J. Neurosci., 18, 2059–2068.[CrossRef][ISI][Medline]
Fu, W., Sugai T., Yoshimura, H. and Onoda, N. (2004) Convergence of olfactory and gustatory connections onto the endopiriform nucleus in the rat. Neuroscience, 126, 1033–1041.[Medline]
Kettenmann, B., Hummel, C., Stefan, H. and Kobal, G. (1997) Multiple olfactory activity in the human neocortex identified by magnetic source imaging. Chem. Senses, 22 , 493–502.
Kobal, G., Hummel, T., Nuernberg, B. and Brune, K. (1990) Effects of pentazocine and acetylsalicylic acid on pain-ratings, pain-related evoked potentials, and vigilance in relationship to pharmacokinetic parameters. Agents Actions, 29, 342–359.[CrossRef][ISI][Medline]
Mueller, C., Kallert, S., Renner, B., Stiassny, K., Temmel, A.F., Hummel, T. and Kobal, G. (2003) Quantitative assessment of gustatory function in clinical context using impregnated ‘taste strips’ . Rhinology, 41, 2–6.[Medline]
Small, D.M., Jones-Gotman, M., Zatorre, R.J., Petrides, M. and Evans, A.C. (1997) Flavor processing: more than a sum in its parts. Neuroreport, 22, 3913–3917.
Small, D.M., Voss, J., Mak, Y.E., Simmons, K.B., Parrish, T.R. and Gitelman, D.R. (2004) Experience dependent neural integration of taste and smell in the human brain. J. Neurophysiol., 92, 1892–1903.
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