fMRI study of taste cortical areas in humans

Cerebral cortical representation of automatic and volitional swallowing in humans

Although the cerebral cortex has been implicated in the control of swallowing, the functional organization of the human cortical swallowing representation has not been fully documented. Therefore, the present study determined the cortical representation of swallowing in fourteen healthy right-handed female subjects using single-event-related functional magnetic resonance imaging (fMRI). Subjects were scanned during three swallowing activation tasks: a naïve saliva swallow, a voluntary saliva swallow, and a water bolus swallow. Swallow-related laryngeal movement was recorded simultaneously from the output of a bellows positioned over the thyroid cartilage. Statistical maps were generated by computing the difference between the magnitude of the voxel time course during 1) a single swallowing trial and 2) the corresponding control period. Automatic and volitional swallowing produced activation within several common cortical regions, the most prominent and consistent being located within the lateral precentral gyrus, lateral postcentral gyrus, and right insula. Activation foci within the superior temporal gyrus, middle and inferior frontal gyri, and frontal operculum also were identified for all swallowing tasks. In contrast, activation of the caudal anterior cingulate cortex was significantly more likely in association with the voluntary saliva swallow and water bolus swallow than the naïve swallow. These findings support the view that, in addition to known brain stem areas, human swallowing is represented within a number of spatially and functionally distinct cortical loci which may participate differentially in the regulation of swallowing. Activation of the insula was significantly lateralized to the right hemisphere for the voluntary saliva swallow, suggesting a functional hemispheric dominance of the insula for the processing of swallowing.

Cortical activation induced by intraoral stimulation with water in humans

Studies of gustatory processing frequently utilize water as a control stimulus. However, the neural representations of intraoral stimulation with water have received little attention. We report a series of positron emission tomography studies involving intraoral stimulation with deionized distilled water. Attempting to taste water produced large, bilateral activations in insular, opercular, Rolandic and cerebellar cortices relative to resting with eyes closed or 'smelling' odorless air. The magnitude and volume of activation was substantially reduced when tasting water was contrasted with voluntary swallowing. This indicates that much of the activity induced by water reflects intraoral somatosensory or motor processing. Nevertheless, portions of the insula, operculum, post-central gyrus and cerebellum remained significantly activated in the contrast between 'tasting' water and swallowing. This activity appears to represent a specific neural correlate of fluid stimulation, and may reflect aspects of trigeminal, gustatory or thermal coding. These findings emphasize the large volume of cortex dedicated to intraoral processing, and highlight the importance of controlling for nongustatory factors in studies of gustation.

Taste memory induces brain activation as revealed by functional MRI

PURPOSE

Our goal was to use functional MRI (fMRI) to measure brain activation in response to imagination of tastes in humans.

METHOD

fMR brain scans were obtained in 31 subjects (12 men, 19 women) using multislice FLASH MRI and echo planar imaging (EPI) in response to imagination of tastes of salt and sweet in coronal sections selected from anterior to posterior temporal brain regions. Activation images were derived using correlation analysis, and ratios of areas of brain activated to total brain areas were calculated. Total activated pixel counts were used to quantitate regional brain activation.

RESULTS

Activation was present in each section in all subjects after imagination of each tastant. Activation was similar in response to imagination of either salt or sweet and was quantitatively similar to that previously reported in response to imagination of odors of banana and peppermint. Activation was similar in both men and women as opposed to previous results of odor memory in which activation in men was greater than in women. However, subjective responses of intensity of imagined tastes were significantly greater than those previously obtained for odor memory and were consistently, albeit not significantly, greater in women than in men, similar to results previously reported for odor memory. Brain regions activated in response to taste imagination were consistent with regions previously described as involved with actual taste perception in both humans and animals. Regional brain localization for salt and sweet memories could not be differentiated.

CONCLUSION

These studies indicate that (a) tastes can be imagined, (b) brain regions activated for taste imagination are consistent with regions previously described for actual taste perception, and (c) similar to odor memory for banana and menthone, regional brain localization for salt or sweet taste memories could not be differentiated.