Chemical Senses Vol. 30 No. suppl 1 © Oxford University
Press 2005; all rights reserved
Flavor Processing: Perceptual and Cognitive Factors in Multi-modal Integration
Monell Chemical Senses Center, 3500 Market St., Philadelphia, PA 19104, USA
Correspondence to be sent to: Pamela Dalton, e-mail: pdalton{at}pobox.uppen.edu
Key words: integration, interaction, mixtures, multimodal, perceptual
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Introduction |
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 Top Introduction AcknowledgementsReferences |
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The chemical senses may represent the most fundamental integrative sensory modalities. For example, flavor perception is widely considered to be an integrative process whereby two distinct neural systems (olfactory and gustatory) are activated peripherally to give rise to a unitary oral sensation of flavor. Benefits of multimodal sensory integration in other modalities include increased sensitivity and reaction times (Krushel and van der Kooy, 1988
;
Fowler and Dekle, 1991;
Schifferstein and Verlegh, 1995).
Investigations into the integration of multisensory information at a neural level in cats
and primates have revealed that neurons of the superior colliculus are capable of
integrating cues from three sensory modalities: vision, audition and somatosensation
(Rolls and Baylis, 1994;
Schul et al., 1996;
Whalen, 1998). The close relationship
between smell and taste along with the evidence of the existence of neurons particularly
sensitive to multimodal inputs, led us to hypothesize that neural inputs for odor and
taste can be integrative and exhibit perceptual additivity at subthreshold intensities
such as that seen previously only with mixtures within a single modality.
In the studies presented here, we evaluated the summation of odor and taste mixtures
where each of the individual components were presented at sub-threshold levels. In the
first study, multiple olfactory thresholds were measured within a group of 10 volunteers
(five males and five females, ranging in age from 22 to 33 years) while holding an oral
stimulus in the mouth (Dalton et al.,
2000).
One each day of testing, nasal detection thresholds for benzaldehyde and oral detection thresholds for saccharin were measured in counter-balanced order. A modified staircase method was employed for collecting detection thresholds using a five-reversal criterion. Subjects were given a 15 min break before a second nasal detection threshold for benzaldehyde was measured while holding a saccharin solution in the mouth. As single thresholds always preceded mixtures, a control procedure was incorporated where subjects were tested for single benzaldehyde thresholds followed by another single benzaldehyde threshold without taste stimuli.
While holding the oral saccharin solution, sensitivity to benzaldehyde increased by an average of 28% over benzaldehyde threshold taken alone. This increase in sensitivity was significant (P = 0.01) with 9 of 10 subjects showing marked differences between benzaldehyde thresholds alone and in the cross-modal mixture. The incorporation of the control trials ruled out any effect of repeated testing since these trials resulted in a nonsignificant (P = 0.71) decrease in sensitivity.
The effect did not appear to be due to concomitant somatosensory input from an oral stimulus or differences in breathing or airflow, as a second experiment found that filtered deionized water did not lower the threshold for benzaldehyde. However, there did appear to be an effect of stimulus congruency: when benzaldehyde was paired with an incongruent taste stimulus (L-glutamic acid monosodium salt or MSG) in the same protocol, thresholds for benzaldehyde were not significantly lower in the presence of MSG than when tested alone (P = 0.57).
The specificity of the benzaldehyde–saccharin pairing in enhancing sensitivity
to benzaldehyde suggested that this phenomenon may not be evidence of a general
integration of taste and olfaction but rather an interaction that may be specific to
combinations of stimuli previously encountered. Accordingly, we evaluated the degree to
which this integration was a product of prior experience by having eight new volunteers
learn novel pairings of taste and smell stimuli (Belanger et al., 2002). Flavorless gum base was
infused with four incongruent flavor combinations: (i) phenylethyl alcohol (PEA) and
citric acid, (ii) PEA and quinine, (iii) L-carvone and citric acid, or (iv)
L-carvone and quinine (see Table
1). Each individual was given
3 weeks of daily exposure to one of the PEA or L-carvone gum flavor
combinations while the other served as their control. At the beginning and end of this
period, thresholds for the odor and taste were measured singly and in combination. After
a 2 week break the procedure was repeated for each subject with the other
odor–taste combination, such that each individual was exposed to only one of the
PEA and L-carvone combinations. Assignment of flavor combination to exposure
condition (learned versus novel) was counterbalanced across subjects. As predicted,
sensitivity to the odorant (PEA or L-carvone) increased significantly more
(P = 0.0025) when tested in combination with its exposure taste pairing
(M = 2.73, SEM = 0.51) than when tested in combination with its
control taste pairing (M = 0.66, SEM = 0.37).
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Additive response to combinations of sensory stimuli have been demonstrated within and across many sensory modalities (Fowler and Dekle, 1991
;
Wallace et al., 1996). For
within-modality stimulation these summations are reminiscent of proximal or temporal
additivity where the two stimuli are closely aligned in time or space and result in
activation of a common perceptual channel. However, crossmodal summation should require
the existence of a central point of integration sensitive to input from multiple
perceptual channels. The work presented here presents evidence for such an integrative
process and the forced-choice detection procedure employed provides a control for
attentional focus. The results provide additional support for, first, the functional
significance of neural responses to combinations of odor and taste stimuli (Rolls and Baylis, 1994) and, second, the
existence of central loci where neural inputs conveying olfactory and taste information
from our everyday chemosensory experience are integrated, in essence, a
‘flavor’ substrate (Schul et
al., 1996). Integration appears to enhance the detectability of weak
chemosensory signals and may yield novel sensations from the combined activation of
sensory modalities receptive to chemical stimulation. |
Acknowledgements |
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TopIntroductionAcknowledgementsReferences |
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This work was supported by NIH grants RO1-DC03704 (P.D.) and R29-DC02995 (P.B.).
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References |
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TopIntroductionAcknowledgementsReferences |
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Belanger, M.A., Tharp, C.D., Breslin, P.A.S. and Dalton, P. (2002) Role of prior associations in the sub-threshold integration of tastes and odors. Chem. Senses, 27, A66.
Dalton, P., Doolittle, N., Nagata, H. and Breslin, P.A.S. (2000) The merging of the senses: subthreshold integration of taste and smell. Nat. Neurosci., 3, 431–432.[CrossRef][Web of Science][Medline]
Fowler, C.A. and Dekle, J.A. (1991) Listening with the eye and hand: cross modal contributions to speech perception. J. Exp. Psychol. Hum. Percept. Perform., 17, 816–828.[Medline]
Krushel, L.A. and van der Kooy, D. (1988) Visceral cortex: integration of the mucosal senses with limbic information in the rat agranular insular cortex. J. Comp. Neurol., 270, 39–54.[CrossRef][Web of Science][Medline]
Rolls, E.T. and Baylis, L.L. (1994) Gustatory, olfactory and visual convergence within the primate orbitalfrontal cortex. J. Neurosci., 14, 5437–5452.[Abstract]
Schifferstein, H.N. and Verlegh, P.W. (1995) The role of congruency and pleasantness in odor-induced taste enhancement. Acta Psychol., 94, 87–105.
Schul, R., Slotnick, B.M. and Dudai, Y. (1996) Flavor and the frontal cortex. Behav. Neurosci., 110, 760–765.[Medline]
Wallace, M.T., Wilkinson, L.K. and Stein, B.E. (1996) Representation and integration of multiple sensory inputs in primate superior colliculus. J. Neurophysiol., 76, 1246–1266.
Whalen, P.J. (1998) Fear vigilance and ambiguity: initial neuroimaging studies of the human amygdala. Curr. Dir. Psychol. Sci., 7, 177–188.
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