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Classic psychophysical studies have provided significant information on the psychophysical functions for taste stimuli. With the advent of fMRI, studies are being conducted that provide insight into central processing of gustation in humans. However, fMRI experiments impose physical limitations on stimulus delivery. In the present study, we compared psychophysical functions relating perceived intensity to concentration, derived from previous studies that used the traditional sip-and-spit and dorsal flow delivery techniques, to psychophysical functions generated in this study using a simulated stimulus delivery technique (SSDT). The SSDT delivered minute quantities of taste stimuli to the dorsal surface of the tongue, just as in an fMRI scanner. As was hypothesized, the results indicated that slopes of intensity functions were dependent on the type of stimulus delivery technique. The SSDT resulted in slopes that were more similar to those generated by dorsal flow than by sip-and-spit stimulus delivery techniques, suggesting the importance of considering the influence of stimulus delivery on psychophysical response in designing and interpreting experiments.

This event-related functional magnetic resonance imaging (er-fMRI) study investigated BOLD signal change in response to a series of pure gustatory stimuli that varied in stimulus quality when subjects were hungry and sated with a nutritional pre-load. Group analyses showed significant differences in activation in the hunger minus satiety condition in response to sucrose, caffeine, saccharin, and citric acid within the thalamus, hippocampus, and parahippocampus. When examining the hunger and satiety conditions, activation varied as a function of stimulus, with the majority of the stimuli exhibiting significantly greater activation in the hunger state within the insula, thalamus, and substantia nigra, in contrast to decreased activation in the satiated state within the parahippocampus, hippocampus, amygdala, and anterior cingulate. Region of interest (ROI) analysis revealed two significant interactions, ROI by physiology and ROI by physiology by stimulus. In the satiety condition, the primary (inferior and superior insulae) and secondary (OFC 11 and OFC 47) taste regions exhibited significantly greater brain activation in response to all stimuli than regions involved in processing eating behavior (hypothalamus), affect (amygdala), and memory (hippocampus, parahippocampus and entorhinal cortex). These same regions demonstrated significantly greater activation within the hunger condition than the satiety condition, with the exception of the superior insula. Furthermore, the patterns of activation differed as a function taste stimulus, with greater activation in response to sucrose than to the other stimuli. These differential patterns of activation suggest that the physiological states of hunger and satiety produce divergent activation in multiple brain areas in response to different pure gustatory stimuli.

An experimental method for event-related functional magnetic resonance imaging that allows for the presentation of several chemosensory stimuli in the oral cavity during the same run, the collection of psychophysical measures (intensity or pleasantness) during the presentation of the stimuli, and the analysis of the data in an event-related fashion are described. The automatic pumps used to present taste stimuli allowed for multiple tastes to be delivered in small amounts under computer control. Psychophysical ratings of pleasantness or intensity were collected after each presentation of a taste stimulus and water, with the general labeled magnitude scale, using a joystick that controlled the movement of an arrow on the visual display. Performing these cognitive tasks required that the participant remained focused, and aided in the interpretation of the data collected. The perceived pleasantness differed across stimuli for all conditions; however, pleasantness ratings for the same stimulus displayed consistency, over the duration of the run and before each scan on separate days. Activation in response to sucrose and caffeine while the participant rated pleasantness was found in the insula, frontal operculum, rolandic operculum and orbitofrontal cortex which is consistent with previous taste fMRI studies.

A protocol suited to the investigations of odor-taste interactions in a functional magnetic resonance imaging (fMRI) scanner is described and results that confirm the validity of that protocol are presented. Stimuli were dissolved in water and presented to the mouth of the participant through plastic syringes and plastic tubes as 50 microl every 3 s for 18 s. Participants were lying on the back to simulate the conditions in a fMRI scanner and gave pleasantness and intensity ratings in response to each stimulus using a Labeled Magnitude Scale (LMS). Stimuli were three concentrations of two tastants (i.e., sucrose and NaCl) and two odorants [i.e., ethyl butyrate (fruity) and citral (citrus)] and all possible odor-taste mixtures. Results showed that additivity of intensity in odor-taste mixtures was linear and did not depend on the pleasantness of the mixture. This study showed the feasibility of acquiring reliable psychophysical data with a stimulation device compatible with the fMRI environment and provided the foundation for the interpretation of future fMRI investigations of odor-taste interactions in flavor.

The present study has investigated interaction at the cortical level in the human between two major components of flavor perception, pure chemical gustatory and lingual somatosensory perception. Twelve subjects participated in a functional magnetic resonance imaging study and tasted six stimuli, applied on the whole tongue, among which four were pure gustatory stimuli (NaCl, aspartame, quinine and HCl, pH 2.4 or 2.2) and two were both taste and lingual somatosensory stimuli, i.e. somato-gustatory stimuli (HCl, pH 1.6 or 1.5, and aluminum potassium sulfate). Functional images were acquired with an echo planar sequence on a 3 T system and were individually processed by correlation with the temporal perception profile. Both sets of stimuli showed activation in the same cortical areas, namely the insula, the rolandic operculum (base of the pre- and post-central gyri), the frontal operculum and the temporal operculum, confirming a wide overlap of taste and lingual somatosensory representations. However, the relative activation across areas and the analysis of co-activated areas across all runs for each set of stimuli allowed discrimination of taste and somatosensory modalities. Factor analysis of correspondences indicated different patterns of activation across the sub-insular and opercular regions, depending on the gustatory or somato-gustatory nature of the stimuli. For gustatory stimuli different activation patterns for the superior and inferior parts of the insula suggested a difference in function between these two insular sub-regions. Furthermore, the left inferior insula was co-activated with the left angular gyrus, a structure involved in semantic processing. In contrast, only somato-gustatory stimuli specifically produced a simultaneous and symmetrical activation of both the left and right rolandic opercula, which include a part of the sensory homunculus dedicated to the tactile representation of oral structures.

Whole-brain functional magnetic resonance imaging was used to detect local hemodynamic changes reflecting cortical activation in five left handed and five right handed human subjects during bilateral stimulation of the tongue with various tastes. Activation was found bilaterally in the insula and the perisylvian region. These regions correspond to the primary taste cortical areas identified with electrophysiological recordings in monkeys and suggested from former clinical observations in human subjects. Moreover, a unilateral projection was described for the first time in the inferior part of the insula of the dominant hemisphere, according to the subject's handedness.

Ten healthy subjects aged 20-25 including five right-handed and five left-handed according to the Dellatolas test participated in this functional magnetic resonance imaging (fMRI) study. A 3 Tesla whole-body MR scanner allowed echo planar imaging (EPI)-64 x 64 pixels, repetition time (TR) = 6 s, field of view (FOV) = 20 x 20 cm2--associated to acute anatomical localization of activated foci (256 x 256 pixels). Subjects were bilaterally stimulated with NaCl 85 mM, aspartame 2 mM, quinine hydrochloride 1 mM, glycyrrhizic acid 0.5 mM, guanosine monophosphate 1 mM and D-threonine 250 mM alternating with water. Stimuli and rinse were continuously pushed as bolus of 50 microliters every 3 s to the subject's mouth through microsyringes. We detected brain activated areas by correlation of the MR signal to an on-line perception profile recorded for each experiment and each subject with the finger-span method. We found most activations in the insula and the perisylvian region in agreement with previous electrophysiological studies on monkeys and clinical reports in humans. The superior part of the insula was bilaterally activated, in accordance with a whole-mouth stimulation. A striking lateralization related to handedness was found in a lower part of the insula. This projection in the dominant hemisphere, located in the same coronal plane as the upper insular activation, is the first evidence of a functional lateralization of brain processing involved in taste perception.

We used 3 Tesla functional magnetic resonance imaging (fMRI) with echo planar imaging to map taste projections in the human. Activations were found surrounding and buried in the sylvian fissure; the upper part of insula, the frontal operculum, and the foot of the pre- and postcentral gyri were usually activated. Moreover, we could describe, for the first time, a lateralized associative projection located in the lower part of the dominant hemisphere of the subject (n = 10). We also observed activations in the anterior cingulate gyrus, the centromedial thalamus, and other areas related to emotional or cognitive processes. Thirty subjects were submitted to a familiarization experiment sampling neophobic and nonneophobic stimuli. Measurements of isointense concentrations, magnitude estimates and hedonic values were assessed repetitively for 10 weeks. Five subjects performed 3 fMRI experiments, before, during and after familiarization. Psychophysical data showed a relationship between the evolution of the hedonic assessment and the intensity of the perception, and fMRI results showed a relationship between the evolution of the hedonic assessment and the evolution of the percent of activated pixels in taste cortical area.